11

Solutions: TermsSolutions: Terms

Solute, solvent, dissolution, precipitation, Solute, solvent, dissolution, precipitation, solubility, solubility, molaritymolarity, , molalitymolality, mole fraction., mole fraction.SoluteSolute: substance in smaller proportion.: substance in smaller proportion.SolventSolvent: substance in greater proportion.: substance in greater proportion.DissolutionDissolution: process of dissolving.: process of dissolving.PrecipitationPrecipitation: reverse of dissolution.: reverse of dissolution.SolubilitySolubility: amount solute to saturate solvent.: amount solute to saturate solvent.

22

Solutions: TermsSolutions: TermsMolarityMolarity: moles of solute per : moles of solute per literliter of solution.of solution.MolalityMolality: moles of solute per : moles of solute per kgkg of solvent.of solvent.Mole fractionMole fraction: fraction of moles of given : fraction of moles of given substance relative to total moles in solution.substance relative to total moles in solution.•• XXaa = = nnaa//ΣΣnn

What is an What is an electrolyteelectrolyte??Free ions in a solution Free ions in a solution conducts electricity.conducts electricity.What is the What is the van’tvan’t Hoff factor, Hoff factor, ii??Number of ions existing after dissolution of Number of ions existing after dissolution of one unit of substance: one unit of substance: NaClNaCl, CaCl, CaCl22

33

ColligativeColligative Properties: Properties: Important!Important!

What does a What does a colligativecolligative property depend property depend upon?upon?Number of solute particles in solution (Number of solute particles in solution (not not identityidentity of solute) of solute) –– van’tvan’t Hoff factor comes Hoff factor comes up big!up big!There are three for the MCAT There are three for the MCAT –– name ‘name ‘emem::

•• vaporvapor--pressure depression.pressure depression.•• boilingboiling--point elevation.point elevation.•• freezingfreezing--point depression.point depression.

44

Vapor PressureVapor PressureDefinition?Definition?Sometimes a tough concept Sometimes a tough concept –– pressure pressure exerted by the gaseous phase of a liquid that exerted by the gaseous phase of a liquid that evaporated from exposed surface of liquid.evaporated from exposed surface of liquid.My definition: pressure exerted by My definition: pressure exerted by molecules leaving liquid phase on the molecules leaving liquid phase on the atmosphere atmosphere –– let’s draw what I mean!let’s draw what I mean!

55

Vapor PressureVapor Pressure

66

Vapor PressureVapor PressureThink about my definition…what property of Think about my definition…what property of the liquid will determine vapor pressure?the liquid will determine vapor pressure?Intermolecular forces! Intermolecular forces! High ones will hold liquid in liquid form and High ones will hold liquid in liquid form and decrease pressure exerted by molecules leaving decrease pressure exerted by molecules leaving liquid.liquid.Low ones will allow liquid to escape into Low ones will allow liquid to escape into gaseous form and increase pressure exerted by gaseous form and increase pressure exerted by molecules as they change phase molecules as they change phase –– volatile.volatile.

77

Vapor Pressure DepressionVapor Pressure DepressionNow for VP depression…Now for VP depression…Given a solution of two liquids, A and B:Given a solution of two liquids, A and B:•• Total VP is equal to sum of partial pressures of Total VP is equal to sum of partial pressures of

each liquid.each liquid.•• Partial pressure of A is equal to proportion of A Partial pressure of A is equal to proportion of A

in solution.in solution.

What is the law called that describes this What is the law called that describes this situation?situation?Raoult’sRaoult’s LawLaw: : PPaa = = XXaaPPaa*…where * is pure A.*…where * is pure A.

88

Vapor Pressure DepressionVapor Pressure DepressionFrom From Raoult’sRaoult’s Law, the presence of liquid B Law, the presence of liquid B will lower will lower PPaa by lowering by lowering XXaa..Vapor Pressure Depression: Vapor Pressure Depression: ∆∆PPaa = = ––XXbbPPaa**Some deviations from Some deviations from Raoult’sRaoult’s Law are Law are possible.possible.Conceptually, what is going on?Conceptually, what is going on?Generally, presence of liquid B will add Generally, presence of liquid B will add intermolecular forces to solution A and make intermolecular forces to solution A and make phase change to gas harder…interference!phase change to gas harder…interference!

99

Boiling Point ElevationBoiling Point ElevationWhat happens to a liquid when it boils?What happens to a liquid when it boils?Input of energy overcomes intermolecular Input of energy overcomes intermolecular forces of liquid to cause phase change.forces of liquid to cause phase change.Relation to VP Relation to VP boiling point is boiling point is temperature at which VP = temperature at which VP = PPatmosphereatmosphere

•• equilibrium between liquid pushing on air and equilibrium between liquid pushing on air and vice versavice versa…free movement of molecules …free movement of molecules between phases.between phases.

What causes BP change (elevation)?What causes BP change (elevation)?

1010

Boiling Point ElevationBoiling Point ElevationIntermolecular forces!Intermolecular forces!Addition of solute introduces extra Addition of solute introduces extra intermolecular forces between solute and intermolecular forces between solute and liquid, making it harder for phase change.liquid, making it harder for phase change.Real life example Real life example –– cooking pasta.cooking pasta.What equation quantifies the temp change?What equation quantifies the temp change?∆∆TTbb = = KKbbimim…define the terms!…define the terms!A constant, A constant, van’tvan’t Hoff factor, Hoff factor, molalitymolality..Why does Why does van’tvan’t Hoff factor matter?Hoff factor matter?

1111

Freezing Point DepressionFreezing Point DepressionWhat happens to liquid when it freezes?What happens to liquid when it freezes?Molecules assemble into an orderly, tightlyMolecules assemble into an orderly, tightly--packed array.packed array.What might the addition of a different What might the addition of a different molecule have on this lattice formation?molecule have on this lattice formation?Interference! More difficult to achieve solid Interference! More difficult to achieve solid state state FP will go down (need to be colder FP will go down (need to be colder to achieve the solid state).to achieve the solid state).

1212

Freezing Point DepressionFreezing Point Depression

What is the equation that quantifies this What is the equation that quantifies this concept?concept?∆∆TTff = = ––KKffimim …define the terms!…define the terms!A constant, A constant, van’tvan’t Hoff factor, Hoff factor, molalitymolality..What is the effect of What is the effect of van’tvan’t Hoff factor here?Hoff factor here?Higher number of ion species will cause Higher number of ion species will cause more interference!more interference!

1313

Kinetics: Some TermsKinetics: Some TermsDefine kinetics (Don’t confuse with Define kinetics (Don’t confuse with thermodynamics!!!).thermodynamics!!!).How fast a reaction occurs…this says How fast a reaction occurs…this says absolutely nothingabsolutely nothing about spontaneity!!about spontaneity!!Mechanism, intermediates, rateMechanism, intermediates, rate--determining determining step…step…MechanismMechanism is a sequence, is a sequence, intermediatesintermediates are are not reactants or products, not reactants or products, raterate--determining stepdetermining stepputs limit on maximum speed.puts limit on maximum speed.

1414

Kinetics: Some TermsKinetics: Some Terms

RateRate--determining stepdetermining step: the slowest step in a : the slowest step in a process determines the overall reaction rate.process determines the overall reaction rate.In a two step reaction, if step two is slow, In a two step reaction, if step two is slow, then the speed of step one is irrelevant:then the speed of step one is irrelevant:

bimolecular

termolecularunlikely!

•• 2NO 2NO NN22OO22 (fast)(fast)•• NN22OO22 + O+ O22 2NO2NO22 (slow)(slow)•• overall reaction: 2NO + Ooverall reaction: 2NO + O22 2NO2NO22

1515

Kinetics: Reaction RateKinetics: Reaction Rate

What three factors determine reaction rate?What three factors determine reaction rate?They are…They are…•• frequency of collisions.frequency of collisions.•• orientation of colliding molecules.orientation of colliding molecules.•• energy of molecules.energy of molecules.

The concept of The concept of activation energyactivation energy……what is what is it and why does it matter?it and why does it matter?

1616

Activation Energy, EActivation Energy, Eaa

Activation energy is the extra “kick” of Activation energy is the extra “kick” of energy that a reaction needs to proceed.energy that a reaction needs to proceed.MCAT loves MCAT loves EEa a diagrams, but first, what diagrams, but first, what are are exothermicexothermic and and endothermicendothermic reactions?reactions?Two diagrams, one for each above type:Two diagrams, one for each above type:

Ea

∆H _R

P

Ea

∆H +RP

1717

Activation Energy, EActivation Energy, EaaFrom the activation energy concept, three From the activation energy concept, three statements can be made:statements can be made:

•• more reactants more reactants fast rate fast rate (more collisions possible).(more collisions possible).

•• higher temperature higher temperature fast fast rate (more reactant molecules rate (more reactant molecules have sufficient kinetic energy have sufficient kinetic energy to overcome to overcome EEaa).).

•• low low EEaa fast reaction rate.fast reaction rate.Ea1 Ea2

1818

CatalystsCatalystsWhat is a catalyst?What is a catalyst?Substance that makes reaction go faster by Substance that makes reaction go faster by speeding up ratespeeding up rate--determining step or providing determining step or providing an optimized route to products.an optimized route to products.

Reactant Reactant vsvs. catalyst . catalyst ––catalyst unchanged at catalyst unchanged at end of reaction.end of reaction.Catalysts change rate, Catalysts change rate, notnot thermodynamics thermodynamics ((∆∆GG, , ∆∆HH, , ∆∆SS, , etcetc.)..).

EaEa,cat

1919

Rate LawsRate Laws

What does a rate law tell us?What does a rate law tell us?The rate at which a reactant disappears.The rate at which a reactant disappears.Consists of Consists of rate constantrate constant, and , and concentrations of reactantsconcentrations of reactants in slow step in slow step only!!only!!Rate laws determined Rate laws determined experimentallyexperimentally..aaAA ++ bbBB ccCC + + ddDD……rate law?rate law?

2020

Rate LawsRate LawsRate = Rate = kk[A][A]xx[B][B]yy

xx order of order of rxnrxn with respect to A.with respect to A.yy order of order of rxnrxn with respect to B.with respect to B.xx++yy overall order of reaction.overall order of reaction.kk rate constantrate constant..NoteNote:: cannot get orders of reactants or rate cannot get orders of reactants or rate constant from balanced equation.constant from balanced equation.Need to look at experiment to get rate law.Need to look at experiment to get rate law.

2121

Rate LawsRate LawsA + B + C A + B + C D + ED + EExperimentExperiment [A][A] [B][B] [C][C] Initial Rate, Initial Rate,

MsMs––11

11 0.2 M0.2 M 0.1 M0.1 M 0.05 M0.05 M 1 1 × × 1010––33

22 0.4 M0.4 M 0.1 M0.1 M 0.05 M0.05 M 2 2 × × 1010––33

33 0.2 M0.2 M 0.2 M0.2 M 0.05 M0.05 M 4 4 × × 1010––33

44 0.2 M0.2 M 0.1 M0.1 M 0.1 M0.1 M 1 1 × × 1010––33

2222

Rate LawsRate LawsLook at two experiments where only one Look at two experiments where only one reactant concentration changes.reactant concentration changes.Determine the factor by which it changes, Determine the factor by which it changes, and compare to factor by which rate and compare to factor by which rate changes.changes.[A] [A] doublesdoubles

order 0.order 0.

[B] [B] doublesdoubles

[C] [C] doublesdoubles

rate rate doublesdoubles

rate rate quadruplesquadruples

rate rate uneffecteduneffected

order 1.order 1.

order 2.order 2.

2323

Rate LawsRate LawsWhen order is 0, rate does not depend on When order is 0, rate does not depend on concentration of reactant.concentration of reactant.Rate law for this example: Rate law for this example: kk[A][B][A][B]22

Determining Determining kk…anyone remember the …anyone remember the formula for finding formula for finding kk??Solve the rate law…Solve the rate law…k k = rate= rate//[A][B][A][B]22

Substitute numbers from any experiment for Substitute numbers from any experiment for [A] and [B].[A] and [B].•• k k = = 1 1 × × 1010––33 / / 0.20.2 × × (0.1)(0.1)22 = 0.5 Ms= 0.5 Ms––11

2424

The Equilibrium ConstantThe Equilibrium Constant

First, what is the definition of an equilibrium?First, what is the definition of an equilibrium?The rate of the forward reaction equals reverse.The rate of the forward reaction equals reverse.Given this generic reaction, what is the Given this generic reaction, what is the KKeqeq??•• aaAA + + bbBB ccCC + + ddDD

MassMass--action ratio action ratio ratio of products to ratio of products to reactants:reactants:

Gases and aqueous molecules only!!Gases and aqueous molecules only!!

[C][C]cc[D][D]dd

[A][A]aa[B][B]bbKKeqeq ==

2525

The Equilibrium ConstantThe Equilibrium ConstantThe value of The value of KKeq eq for a reaction is a constant at a for a reaction is a constant at a given temperature given temperature temp change causes temp change causes KKeq eq change.change.Equilibrium constants, in general, may have Equilibrium constants, in general, may have different subscripts (different subscripts (eqeq, sp, a, b, , sp, a, b, etcetc.) but they all .) but they all obey the same rules of calculation and temp obey the same rules of calculation and temp dependence.dependence.The value of K The value of K what values favor react. what values favor react. vsvs. prod.. prod.•• KKeq eq < 1 favors reactants.< 1 favors reactants.•• KKeq eq = 1 balances reactants and products.= 1 balances reactants and products.•• KKeq eq > 1 favors products.> 1 favors products.

2626

The Reaction QuotientThe Reaction QuotientWhat is the reaction quotient? Symbol?What is the reaction quotient? Symbol?Ratio of concentrations of products to reactants, Ratio of concentrations of products to reactants, QQ, , but NOT at equilibrium but NOT at equilibrium differs from differs from KK..How do the values of How do the values of KK relate to values of relate to values of QQ??When When QQ < < KK proceeding in proceeding in forwardforward direction.direction.When When QQ = = KK at equilibriumat equilibrium..When When QQ > > KK proceeding in proceeding in reversereverse direction.direction.

2727

Le Le Châtelier’sChâtelier’s PrinciplePrincipleWhat does Le What does Le Châtelier’sChâtelier’s principle state?principle state?A system at equilibrium will try to neutralize any A system at equilibrium will try to neutralize any imposed change (stress) in order to reestablish imposed change (stress) in order to reestablish equilibrium.equilibrium.The effects of different stresses:The effects of different stresses:•• Addition of product or reactant.Addition of product or reactant.•• Removal of product or reactant.Removal of product or reactant.•• Changing volume and temperature.Changing volume and temperature.•• Adding inert gas and adding catalyst.Adding inert gas and adding catalyst.

2828

Le Le Châtelier’sChâtelier’s PrinciplePrincipleNN22 (g) + 3 H(g) + 3 H22 (g) (g) 2 NH2 NH33 (g) + heat(g) + heatAdding or removing Adding or removing NN22

Adding or removing NHAdding or removing NH33

Reducing volume? Increasing volume?Reducing volume? Increasing volume?•• cut volume, raise pressure cut volume, raise pressure favor fewer molesfavor fewer moles•• raise volume, decrease pressure raise volume, decrease pressure favor greater moles.favor greater moles.

Changing temp? Adding or removing heat.Changing temp? Adding or removing heat.Inert gas and catalyst Inert gas and catalyst no change (catalyst no change (catalyst changes rate of forward and reverse equally).changes rate of forward and reverse equally).

2929

Solubility ProductSolubility ProductHow is the solubility product constant defined?How is the solubility product constant defined?The extent to which a salt will dissolve in water.The extent to which a salt will dissolve in water.KKspsp is determined just like is determined just like KKeqeq..Solubility product constant is also temperature Solubility product constant is also temperature dependent.dependent.MMnnXXmm(s(s) ) nnMMxx++(aq(aq) + ) + mmXXyy––((aqaq))KKspsp = [= [MMxx++]]nn[X[Xyy––]]mm

3030

Solubility Product: ExampleSolubility Product: ExampleThe value of the solubility product for copper (I) The value of the solubility product for copper (I) chloride is chloride is KKspsp = 1.2 = 1.2 × × 1010––66. Under normal . Under normal conditions, the maximum concentration of an conditions, the maximum concentration of an aqueous aqueous CuClCuCl solution will be:solution will be:•• less than 10less than 10––66 MM•• greater than 10greater than 10––66 M and less than 10M and less than 10––4 4 M M •• greater than 10greater than 10––44 M and less than 10M and less than 10––2 2 MM•• greater than 10greater than 10––22 M and less than 10M and less than 10––1 1 M M

3131

Ion Product and Common Ion EffectIon Product and Common Ion EffectWhat is an What is an ion production product??Actually just the reaction quotient (Actually just the reaction quotient (QQ) for ) for solubility reactions.solubility reactions.Allows us to make predictions just as before…Allows us to make predictions just as before…When When QQspsp < < KKspsp more salt can be more salt can be dissolveddissolved..When When QQspsp = = KKspsp solution is solution is saturatedsaturated..When When QQspsp > > KKspsp excess salt will excess salt will precipitateprecipitate..Think of Think of KKspsp as the “saturation ceiling”.as the “saturation ceiling”.

3232

Ion Product and Common Ion EffectIon Product and Common Ion Effect

What is the What is the common ion effectcommon ion effect??Disturbing of a solubility equilibrium by adding a Disturbing of a solubility equilibrium by adding a common ion, one that already exists in the common ion, one that already exists in the equilibrium.equilibrium.Follows Le Follows Le Châtelier’sChâtelier’s principle, causing a principle, causing a decrease in solubility (equilibrium favors the decrease in solubility (equilibrium favors the solid and not the aqueous).solid and not the aqueous).Mg(OH)Mg(OH)22(s) (s) MgMg2+ 2+ ((aqaq)) + 2OH+ 2OH––((aqaq))……after after OHOH–– added (added (e.ge.g. soluble . soluble NaOH(aqNaOH(aq)).)).

3333

Acids and Bases: DefinitionsAcids and Bases: DefinitionsArrheniusArrhenius acids/bases, acids/bases, BrønstedBrønsted--Lowry Lowry acids/bases, Lewis acids/bases…differentiate!acids/bases, Lewis acids/bases…differentiate!ArrheniusArrhenius: acids ionize in water to produce H: acids ionize in water to produce H++

and bases ionize to produce OHand bases ionize to produce OH––..BrønstedBrønsted--LowryLowry: acids are proton (H: acids are proton (H++) donors ) donors and bases are proton (Hand bases are proton (H++) acceptors.) acceptors.LewisLewis: acids are electron pair acceptors and : acids are electron pair acceptors and bases are electron pair donors.bases are electron pair donors.

3434

Conjugate Acids and BasesConjugate Acids and Bases

What are conjugate acids and bases, or how What are conjugate acids and bases, or how would you recognize them?would you recognize them?When When BrønstedBrønsted--Lowry acid donates an HLowry acid donates an H++ the the remaining structure is conjugate base.remaining structure is conjugate base.When When BrønstedBrønsted--Lowry base accepts an HLowry base accepts an H++ the the new species is the conjugate acid.new species is the conjugate acid.Name the acid, base, and conjugates:Name the acid, base, and conjugates:•• NHNH3 3 + H+ H22O O NHNH44

++ + OH+ OH––

base acid conj. acid conj. base

3535

Strengths of Acids and BasesStrengths of Acids and BasesBrønstedBrønsted--Lowry acids/bases placed in two Lowry acids/bases placed in two broad broad catagoriescatagories: : strongstrong and and weakweak..What is a What is a strongstrong acid?acid?Strong acids dissociate nearly completely. Their Strong acids dissociate nearly completely. Their KKaa > 1, favoring products considerably.> 1, favoring products considerably.Examples (memorize!) For the MCAT, all Examples (memorize!) For the MCAT, all others not listed here are considered weak:others not listed here are considered weak:•• HI, HI, HBrHBr, , HClHCl, HClO, HClO44, HClO, HClO33, H, H22SOSO44, HNO, HNO33

3636

Strengths of Acids and BasesStrengths of Acids and Bases

What is a What is a weakweak acid?acid?Beyond the opposite of a strong acid, a weak Beyond the opposite of a strong acid, a weak acid has a acid has a KKaa < 1 because reactants are favored.< 1 because reactants are favored.Ranking acid strength: higher Ranking acid strength: higher KKaa means means stronger acid…same for base strength.stronger acid…same for base strength.How does this all work for conjugates?How does this all work for conjugates?

3737

Strengths of Conjugate PairsStrengths of Conjugate PairsGiven a Given a strong acidstrong acid, is conjugate base strong , is conjugate base strong or weak? More importantly, why?or weak? More importantly, why?It is weak, because complete forward reaction It is weak, because complete forward reaction means essentially no reverse (conjugate cannot means essentially no reverse (conjugate cannot take up Htake up H++) and therefore is weak.) and therefore is weak.Same holds true for strong base.Same holds true for strong base.For For weak acidweak acid??Conjugate is weak base, but the smaller the Conjugate is weak base, but the smaller the KKaa, , the stronger the conjugate base will be, relative the stronger the conjugate base will be, relative to other weak bases.to other weak bases.

3838

Strengths of Conjugate Pairs: Strengths of Conjugate Pairs: ExampleExample

Of the following anions, which is the strongest Of the following anions, which is the strongest base?base?•• II––

•• CNCN––

•• NONO33––

•• BrBr––

3939

Strengths of Conjugate Pairs: Strengths of Conjugate Pairs: ExampleExample

Of the following, which acid has the weakest Of the following, which acid has the weakest conjugate base?conjugate base?•• HClOHClO44

•• HCOOHHCOOH•• HH33POPO44

•• HH22COCO33

4040

AmphotericAmphoteric SubstancesSubstancesWhat is an What is an amphotericamphoteric substance?substance?A substance that can act as either an A substance that can act as either an acidacid or a or a basebase..Where does this happen? Where does this happen? PolyproticsPolyprotics!!When a substance has more than one proton to When a substance has more than one proton to donate, the conjugate base can either donate donate, the conjugate base can either donate another or accept a free proton.another or accept a free proton.HH33POPO4 4 HH22POPO44

–– + H+ H+ + HPOHPO4422–– + 2H+ 2H+ +

4141

IonIon--Product Constant of WaterProduct Constant of Water

Water is an Water is an amphotericamphoteric substance.substance.HH22OO + + HH22OO HH33OO++ + + OHOH––

•• reacts with itself in reacts with itself in BrønstedBrønsted--Lowry reaction.Lowry reaction.

Anyone know the term for this equilibrium?Anyone know the term for this equilibrium?AutoionizationAutoionization of water of water KKww = [H= [H++][OH][OH––]]Value of Value of autoionizationautoionization constant at 25 constant at 25 °°C?C?KKww = 1.0 = 1.0 ×× 1010––1414…as in …as in pH + pH + pOHpOH = 14= 14

4242

pH Calculations: A ShortcutpH Calculations: A ShortcutNormal formula: pH = Normal formula: pH = ––log[Hlog[H++]…also,]…also,pOHpOH = = ––log [OHlog [OH––]]Great when [HGreat when [H++] = 1 ] = 1 × × 1010––22 pH = pH = 22But how about when [HBut how about when [H++] = 2.3 ] = 2.3 ×× 1010––44 M?M?Use this shortcut…Use this shortcut…•• if [Hif [H++] = ] = yy ×× 1010––nn M (where M (where nn is a whole number).is a whole number).•• then pH is between (then pH is between (nn––1) and 1) and nn..

For above example, pH is between 3 and 4!For above example, pH is between 3 and 4!

4343

pH CalculationspH Calculations

Remember the shortcut for estimating pH Remember the shortcut for estimating pH from [Hfrom [H++].].When looking at weak acid (base), given its When looking at weak acid (base), given its initial concentration, use the initial concentration, use the IICCEE method to method to find [Hfind [H++] and pH.] and pH.What is pH of 0.2 M solution of HCN?What is pH of 0.2 M solution of HCN?•• KKaa = 4.9 = 4.9 × × 1010––1010

The The KKaa < 10< 10––44 rule…the usual on the MCAT.rule…the usual on the MCAT.

4444

pKpKaa and and pKpKbb

We know pH is We know pH is ––log [Hlog [H++], so what do you ], so what do you think think ppKKaa or or ppKKbb is?is?Any “p” function represents the Any “p” function represents the ––log!log!ppKKaa = = ––log log KKaa & & ppKKbb = = ––log log KKbb

High High KKaa or or KKbb (closer to 1) means strength, so (closer to 1) means strength, so how about for how about for ppKKaa or or ppKKbb??Lower is stronger! Think of it like pH Lower is stronger! Think of it like pH lower is more concentrated.lower is more concentrated.Don’t forget Don’t forget ppKKaa + + ppKKbb = 14= 14

4545

Neutralization ReactionsNeutralization ReactionsWhat is a What is a neutralizationneutralization reaction?reaction?When an acid and base react to form a When an acid and base react to form a saltsaltand and waterwater..Like when you take antacid (Tums, Like when you take antacid (Tums, etcetc.)..).A handy formula for complete neutralization:A handy formula for complete neutralization:•• aa ×× [A][A] ×× VVaa = = bb ×× [B][B] ×× VVbb

•• aa is # acidic H, is # acidic H, bb is # H’s base can accept.is # H’s base can accept.•• [X] is acid/base [X] is acid/base concenconcen. and . and VV is volume.is volume.

4646

Neutralization ReactionsNeutralization ReactionsHow much 0.1 M How much 0.1 M NaOHNaOH solution needed to solution needed to neutralize 40 neutralize 40 mLmL of 0.3 M of 0.3 M HClHCl??HCl(aqHCl(aq) + ) + NaOH(aqNaOH(aq) ) NaCl(aqNaCl(aq) + H) + H22O(l)O(l)aa ×× [A] [A] ×× VVaa = = bb ×× [B] [B] ×× VVbb

1 1 ×× 0.3 0.3 ×× 40 = 1 40 = 1 ×× 0.1 0.1 ×× VVbb

Solve for Solve for VVbb to get 120 to get 120 mLmL..Remember the formula to get you out of Remember the formula to get you out of jams…works with neutralization reactions jams…works with neutralization reactions only to save time over the only to save time over the IICCEE method.method.

4747

IndicatorsIndicatorsAn indicator marks the endpoint of a titration, An indicator marks the endpoint of a titration, but why does it change color?but why does it change color?Indicator is actually weak acid whose Indicator is actually weak acid whose protonatedprotonated form is one color and form is one color and deprotonateddeprotonatedis another.is another.How does this help us?How does this help us?Don’t forget that the indicator is a weak acid!Don’t forget that the indicator is a weak acid!

4848

IndicatorsIndicatorsHInHIn HH++ + + InIn––

KKaa = [H= [H++][In][In––]/[]/[HInHIn]]……now rearrange:now rearrange:[H[H++]/]/KKaa = [= [HInHIn]/[In]/[In––]]……and look at ratios:and look at ratios:•• If [HIf [H++] » ] » KKaa, [, [HInHIn] » [In] » [In––]…see ]…see color 1color 1•• If [HIf [H++] = ] = KKaa, [, [HInHIn] = [In] = [In––]…mix of two ]…mix of two ccoolloorrss•• If [HIf [H++] « ] « KKaa, [, [HInHIn] « [In] « [In––]…see ]…see color 2color 2

So indicator turns colors over short pH range So indicator turns colors over short pH range near its near its ppKKaa..

4949

Picking an IndicatorPicking an IndicatorWhen picking an indicator, use one whose When picking an indicator, use one whose ppKKaa value lies within the pH range that you value lies within the pH range that you want to detect.want to detect.For indicator, For indicator, ppKKaa +/+/–– 1 represents effective 1 represents effective range for color change.range for color change.So don’t expect to detect a pH change from So don’t expect to detect a pH change from 3 to 4 if your indicator has a 3 to 4 if your indicator has a ppKKaa of 8 or 9!of 8 or 9!

5050

Hydrolysis of SaltsHydrolysis of SaltsThe reaction of a substance (salt/ion) with The reaction of a substance (salt/ion) with waterwater is a hydrolysis reaction.is a hydrolysis reaction.Question…will the hydrolysis result in a Question…will the hydrolysis result in a neutral, acidic, or basic salt?neutral, acidic, or basic salt?NaClNaCl in water in water neutral because neither Naneutral because neither Na++

nor nor ClCl–– will react with water:will react with water:•• ClCl–– is conjugate base of a strong acid.is conjugate base of a strong acid.•• NaNa++ is conjugate acid of a strong base.is conjugate acid of a strong base.

NHNH44Cl in water…acidic because NHCl in water…acidic because NH44++ will will

react with water (it is a weak acid).react with water (it is a weak acid).

5151

Hydrolysis of SaltsHydrolysis of Salts

NHNH44CN in water…NHCN in water…NH44++ is weak acid, but is weak acid, but

CNCN–– is good base, so which wins??is good base, so which wins??Need to know Need to know KKaa//KKbb values!values!KKaa for NHfor NH44

++ = 6.3 = 6.3 ×× 1010––1010

KKbb for CNfor CN–– = 1.6 = 1.6 ×× 1010––55

Solution will be basic because CNSolution will be basic because CN–– is better is better base than NHbase than NH44

++ is an acid.is an acid.

5252

What is a buffer?What is a buffer?A solution that resists changing pH when a A solution that resists changing pH when a small amount of acid or base is added.small amount of acid or base is added.The resistance comes from the presence of a The resistance comes from the presence of a weak acid or base and its conjugate in weak acid or base and its conjugate in roughly equal concentrations.roughly equal concentrations.An expression to remember:An expression to remember:

KKaa = = [H[H++]] [conj. base][conj. base][acid][acid]

5353

Buffer CompensationBuffer Compensation

A biochemical example: the blood.A biochemical example: the blood.The main buffer: carbonic acid:The main buffer: carbonic acid:COCO22 + H+ H22O O HH22COCO3 3 HH++ + HCO+ HCO33

––

5454

Buffer CompensationBuffer CompensationCompensation critical to prevent medical Compensation critical to prevent medical disorders like acidosis and disorders like acidosis and alkylosisalkylosis..Essentially Le Essentially Le Châtelier’sChâtelier’s principle at work.principle at work.Addition of small amount of acid like Addition of small amount of acid like HClHClcauses added causes added HH++ to react with present to react with present HCOHCO33

––

to formto form HH22COCO33..Equilibrium has shifted to the left, but existing Equilibrium has shifted to the left, but existing HH22COCO3 3 prevents large change in pH prevents large change in pH (compensation reverses effects).(compensation reverses effects).

5555

Buffer CompensationBuffer CompensationProvided that acid and its conjugate are in Provided that acid and its conjugate are in similar concentration, the pH change is similar concentration, the pH change is minimal.minimal.Addition of small amount of base like KOH Addition of small amount of base like KOH causes added OHcauses added OH–– to react with present Hto react with present H22COCO33to form HCOto form HCO33

––..Equilibrium has shifted to the right, but Equilibrium has shifted to the right, but existing HCOexisting HCO33

–– prevents large change in pH prevents large change in pH (compensation reverses effects).(compensation reverses effects).

5656

Dealing with BuffersDealing with Buffers

If [AIf [A––] = [HA], then pH = ] = [HA], then pH = ppKKaa

An ideal buffer works under these An ideal buffer works under these conditions. Small changes to the ratio are conditions. Small changes to the ratio are acceptable, but the ratio should stay near 1.acceptable, but the ratio should stay near 1.

HendersonHenderson--HasselbalchHasselbalch Equation:Equation:

pHpH = = ppKKaa + log+ log pOHpOH = = ppKKbb + log+ log[[AA––]][[HAHA]] [[AA––]]

[[HAHA]]

5757

Dealing with Buffers Dealing with Buffers –– ExampleExample

Which of the following compounds could be Which of the following compounds could be added to a solution of HCN to create a buffer?added to a solution of HCN to create a buffer?•• HNOHNO33

•• CaClCaCl22

•• NaCNNaCN•• KOHKOH

5858

AcidAcid--Base Titrations Base Titrations –– Terms Terms TitrationTitration experimental technique to experimental technique to determine concentration or identity of determine concentration or identity of unknown weak acid or base by determining unknown weak acid or base by determining ppKKaa or or ppKKbb..TitrantTitrant strong acid or base of known strong acid or base of known concentration that is added to solution of the concentration that is added to solution of the unknown base or acid.unknown base or acid.Titration curveTitration curve a plot of pH a plot of pH vsvs. volume of . volume of titranttitrant added.added.

5959

AcidAcid--Base Titrations Base Titrations –– Terms Terms Buffering domainBuffering domain the section of the the section of the titration curve where the pH changes titration curve where the pH changes graduallygradually before the equivalence point.before the equivalence point.Equivalence pointEquivalence point the point during a the point during a drastic pH change in which there is drastic pH change in which there is complete neutralization of acid and base.complete neutralization of acid and base.End pointEnd point one or two drops of one or two drops of titranttitrant past past equivalence point!equivalence point!•• indicator color flip visually detectable:indicator color flip visually detectable:

HInHIn InIn––HInHIn InIn––

6060

Titration Curves Titration Curves –– The Basics The Basics

Equivalence point and pH: not necessarily Equivalence point and pH: not necessarily at pH of 7 at pH of 7 salt hydrolysis?salt hydrolysis?Looking at the pH of the equivalence point Looking at the pH of the equivalence point allows you to determine if unknown is weak allows you to determine if unknown is weak or strong.or strong.Some quick rules for determining the pH at Some quick rules for determining the pH at the equivalence point.the equivalence point.

6161

Titration Curves Titration Curves –– The BasicsThe Basics

weak acidweak acid + + strong basestrong base::•• pH > 7pH > 7 because product contains because product contains basicbasic salt.salt.•• HF + HF + NaOHNaOH NaNaFF + H+ H22OO

strong acidstrong acid + + weak baseweak base::•• pH < 7pH < 7 because product contains because product contains acidicacidic salt.salt.•• NHNH33 + + HClHCl NHNH44ClCl

strong acidstrong acid + + strong basestrong base::•• pH = 7pH = 7 because product contains neutral salt.because product contains neutral salt.•• HClHCl + + NaOHNaOH NaClNaCl + H+ H22OO

6262

Titration Curves Titration Curves –– The BasicsThe Basics

Buffer!Buffer!

HalfHalf--equivalence?equivalence?The volume of The volume of titranttitrant at halfat half--equiv. is half equiv. is half that at equivalence.that at equivalence.More importantly, the concentration of More importantly, the concentration of unknown at half equiv is equal to the unknown at half equiv is equal to the concentration of the unknownconcentration of the unknown’’s conjugate at s conjugate at half equivalence.half equivalence.•• NHNH33 + + HClHCl NHNH44ClCl

6363

Titration Curves Titration Curves –– The Basics The Basics In that case, the following must be true:In that case, the following must be true:•• [[HA]HA]half equivhalf equiv = [A= [A––]]half equivhalf equiv = [HA]= [HA]ii

By HBy H--H equation, the pH at half equiv. must H equation, the pH at half equiv. must be equal to be equal to ppKKaa this allows identification of this allows identification of unknown.unknown.Types of Curves:Types of Curves:•• Slope to upper right Slope to upper right SASA++SBSB, , WAWA++SBSB•• Slope to lower right Slope to lower right WBWB++SASA•• PolyproticPolyprotic acids have multiple equiv points (why?)acids have multiple equiv points (why?)

6464

Titration Curves Titration Curves –– SASA + + SBSB

6565

Titration Curves Titration Curves –– WAWA + + SBSB

6666

Titration Curves Titration Curves –– WBWB + + SASA

6767

Titration Curves Titration Curves –– Examples Examples

Methyl red is an indicator that changes from Methyl red is an indicator that changes from redred to to yellowyellow in pH range in pH range 4.4 4.4 –– 6.26.2. For which . For which of the following titrations would methyl red be of the following titrations would methyl red be useful for indicating the equivalence point?useful for indicating the equivalence point?•• HCN with KOHHCN with KOH•• NaOHNaOH with HIwith HI•• CC66HH55COOH with COOH with LiOHLiOH•• CC66HH55NHNH22 with HNOwith HNO33

6868

Titration Curves Titration Curves –– Examples Examples

Draw the following titration curves, paying Draw the following titration curves, paying particular attention to shape while labeling particular attention to shape while labeling the relevant parts:the relevant parts:•• CHCH33COOH titrated with KOHCOOH titrated with KOH•• CC66HH55NHNH22 titrated with HNOtitrated with HNO33

•• HH33POPO44 titrated with KOHtitrated with KOH

6969

System and SurroundingsSystem and Surroundings

The The systemsystem is the thing we’re looking at, for is the thing we’re looking at, for example a melting ice cube or a solid dissolving example a melting ice cube or a solid dissolving into water.into water.Everything else is known collectively as the Everything else is known collectively as the surroundingssurroundings, for example the table the ice cube , for example the table the ice cube sits on and the surrounding air or the beaker that sits on and the surrounding air or the beaker that the solution sits in.the solution sits in.SystemSystem + + surroundingssurroundings = = thermodynamic thermodynamic universeuniverse..

7070

System and Surroundings: Energy System and Surroundings: Energy FlowFlow

Remember conservation of energy?Remember conservation of energy?When energy flows into a system from the When energy flows into a system from the surroundings (+), the energy of the system surroundings (+), the energy of the system increases and energy of surroundings decreases.increases and energy of surroundings decreases.When energy flows out of a system into the When energy flows out of a system into the surroundings (surroundings (––), the energy of the system ), the energy of the system decreases and energy of surroundings increases.decreases and energy of surroundings increases.

7171

First Law of ThermodynamicsFirst Law of ThermodynamicsThe First Law The First Law anybody know it?anybody know it?Total energy of the universe is constant, so Total energy of the universe is constant, so energy can be transferred but not created or energy can be transferred but not created or destroyed.destroyed.Transfer of energy can occur as heat or work:Transfer of energy can occur as heat or work:∆∆EE = = qq + + ww ww = = PP∆∆VVEnergy input Energy input positive positive qq (heat (heat absorbedabsorbed) and ) and ww(work done (work done onon system).system).Energy out Energy out negative negative qq (heat (heat releasedreleased) and ) and ww(work done (work done byby system).system).

7272

First Law of ThermodynamicsFirst Law of ThermodynamicsEnergy transfer in car air conditioner.Energy transfer in car air conditioner.Air taken in through hose and moves over fins of Air taken in through hose and moves over fins of evaporator.evaporator.This hot air evaporates refrigerant inside (heat This hot air evaporates refrigerant inside (heat transfer) which travels to condenser and is transfer) which travels to condenser and is changed back into liquid (heat transfer).changed back into liquid (heat transfer).Air which was cooled when heating the fins of Air which was cooled when heating the fins of the evaporator flows into the car’s interior.the evaporator flows into the car’s interior.No net change in energy for system…refrigerant No net change in energy for system…refrigerant heated but then cooled to its original state.heated but then cooled to its original state.

7373

First Law of ThermodynamicsFirst Law of ThermodynamicsAdiabatic and Adiabatic and isothermicisothermic??IsothermicIsothermic: expansion or compression of gas at : expansion or compression of gas at constant temperature (heat input or output).constant temperature (heat input or output).AdiabaticAdiabatic: expansion or compression of gas : expansion or compression of gas without a heat exchange (temperature changes; without a heat exchange (temperature changes; qq = 0 = 0 ∆∆EE = = ww))Adiabatically expanding gases cool while Adiabatically expanding gases cool while adiabatically compressing gases warm.adiabatically compressing gases warm.Adiabatic principle at work: snowAdiabatic principle at work: snow--making making machine.machine.

7474

First Law of ThermodynamicsFirst Law of Thermodynamics

SnowSnow--making machines contain mixture of making machines contain mixture of compressed air and water vapor (20 compressed air and water vapor (20 atmatm).).When sprayed into the air, the mixture When sprayed into the air, the mixture undergoes a huge pressure change and expands undergoes a huge pressure change and expands so rapidly that essentially no heat is exchanged so rapidly that essentially no heat is exchanged between system and surroundings (adiabatic between system and surroundings (adiabatic expansion).expansion).Adiabatic expanding gases cool, so the water Adiabatic expanding gases cool, so the water vapor changes almost instantaneously to snow.vapor changes almost instantaneously to snow.

7575

Second Law of Second Law of ThermodynamicsThermodynamics

The Second Law The Second Law what does it state?what does it state?All processes tend to run in a direction that leads All processes tend to run in a direction that leads to maximum disorder. If a process is to maximum disorder. If a process is spontaneous in one direction, then the reverse spontaneous in one direction, then the reverse cannot be spontaneous cannot be spontaneous makes sense!makes sense!From here, we can discuss From here, we can discuss entropyentropy, , enthalpyenthalpy, , and and free energyfree energy..

7676

Entropy: Disorder or RandomnessEntropy: Disorder or Randomness∆∆SS = = SSproductsproducts –– SSreactantsreactants•• Increasing randomness (decreasing order) results in a Increasing randomness (decreasing order) results in a

positive positive ∆∆SS..•• Decreasing randomness (increasing order) results in Decreasing randomness (increasing order) results in

negative negative ∆∆SS..What are some situations in which entropy is What are some situations in which entropy is predictable?predictable?Liquids have more than solids, gases more than Liquids have more than solids, gases more than liquids, particles in solution more than solids, liquids, particles in solution more than solids, two moles more than one mole.two moles more than one mole.

7777

Enthalpy: Heat EnergyEnthalpy: Heat EnergyTwo important principles Two important principles bond formation bond formation releases energy (not needed to ensure stability), releases energy (not needed to ensure stability), energy must be added to break a bond.energy must be added to break a bond.∆∆HH = = HHproductsproducts –– HHreactantsreactants

ExothermicExothermic: heat released because products of : heat released because products of reaction have stronger, more stable bonds than reaction have stronger, more stable bonds than reactants.reactants.EndothermicEndothermic: heat absorbed because products of : heat absorbed because products of reaction have weaker bonds and energy needed reaction have weaker bonds and energy needed to transition to this more unstable state.to transition to this more unstable state.

7878

Enthalpy and Heat of Enthalpy and Heat of FormationFormation

Amount of energy required to make Amount of energy required to make one moleone mole of of a compound from pure elements in their natural a compound from pure elements in their natural or standard state.or standard state.If If ––∆∆HH, heat given off…if , heat given off…if ++∆∆HH, heat required., heat required.What is standard state?What is standard state?TT = 298 K, = 298 K, PP = 1 = 1 atmatm, [X] = 1 M., [X] = 1 M.Designated by Designated by °° superscript, standard state is superscript, standard state is condition for determining heat of formation, condition for determining heat of formation, enthalpies, constants, enthalpies, constants, etcetc..

7979

More EnthalpyMore Enthalpy

Each reaction has own Each reaction has own ∆∆HH doubling reactants doubling reactants will double the heat required or released.will double the heat required or released.Hess’s LawHess’s Law of Heat Summation, Remember?of Heat Summation, Remember?If a reaction occurs in several steps, the sum of If a reaction occurs in several steps, the sum of the energies absorbed or released in all steps will the energies absorbed or released in all steps will be the same as the overall reaction.be the same as the overall reaction.If reaction is reversed, the sign of If reaction is reversed, the sign of ∆∆HH reverses reverses too.too.If equation multiplied by constant, so too is If equation multiplied by constant, so too is ∆∆HH..

8080

Hess’s Law ExampleHess’s Law Example

C(s) + C(s) + 22HH22(g)(g)

CHCH44(g)(g)

COCO22(g) + (g) + 22HH22O(g)O(g)

Direct pathDirect path ∆∆HHnetnet= = ∆∆HH11 + + 22∆∆HH22 + (+ (––∆∆HH33))Indirect pathIndirect path

+ 2O+ 2O22(g)(g)∆∆HHrxrx = = ∆∆HH11 + + 22∆∆HH22

–– 2O2O22(g)(g)∆∆HHrxrx = = ––∆∆HH33

8181

Gibbs Free EnergyGibbs Free Energy

The energy available to do useful work from a The energy available to do useful work from a chemical reaction.chemical reaction.Spontaneity determined by enthalpy and entropy, Spontaneity determined by enthalpy and entropy, so spontaneity can be accounted for by so spontaneity can be accounted for by ∆∆GG..∆∆GG = = ∆∆HH –– TT∆∆SSWhen…When…•• ∆∆G < 0G < 0 spontaneousspontaneous•• ∆∆G = 0G = 0 at equilibriumat equilibrium•• ∆∆G > 0G > 0 nonnon--spontaneousspontaneous

8282

∆∆G and TemperatureG and Temperature∆∆GG = = ∆∆HH –– TT∆∆SS

∆∆HH ∆∆SS ∆∆GG Reaction is…Reaction is…

–– ++ –– SpontaneousSpontaneous

++ ++ –– at high at high TT

+ + at low at low TT

SpontaneousSpontaneousNonNon--spontaneousspontaneous

–– –– + + at high at high TT

–– at low at low TT

NonNon--spontaneousspontaneousSpontaneousSpontaneous

++ –– ++ NonNon--spontaneousspontaneous

8383

Reaction Energy Diagrams RevisitedReaction Energy Diagrams RevisitedTwo different diagrams Two different diagrams endothermic and endothermic and exothermic.exothermic.∆∆GG ≈ ≈ ∆∆HH when when TT∆∆SS is small.is small.Do notDo not confuse confuse ∆∆GG, , ∆∆HH, or , or ∆∆SS with rate or with rate or EEaa!!•• catalysts lower catalysts lower EEaa, but don’t touch , but don’t touch ∆∆G!G!

ReversibilityReversibility: the reverse of any reaction has the : the reverse of any reaction has the same magnitude for all thermodynamic values same magnitude for all thermodynamic values (opposite signs) and the same reaction pathway:(opposite signs) and the same reaction pathway:•• Coming from products side, Coming from products side, EEaa is now forward is now forward EEaa

plus plus ∆∆GG…look at the graph.…look at the graph.

8484

Thermodynamics and EquilibriumThermodynamics and EquilibriumThe two concepts are related by this equation:The two concepts are related by this equation:•• ∆∆GG°° = = ––2.3RTlog2.3RTlogKKeqeq

Should you know this equation? Probably not, Should you know this equation? Probably not, but know this for sure…but know this for sure…•• ∆∆GG°° < 0 < 0 spontaneous spontaneous KKeqeq > 1> 1•• ∆∆GG°° = 0 = 0 at equilibrium at equilibrium KKeqeq = 1= 1•• ∆∆GG°° > 0 > 0 nonnon--spontaneous spontaneous KKeqeq < 1< 1

Quick summary: for a spontaneous reaction, Quick summary: for a spontaneous reaction, products are favored; for nonproducts are favored; for non--spontaneous spontaneous reaction, reactants are favored.reaction, reactants are favored.

8585

OxidationOxidation--Reduction ReactionsReduction Reactions

Oxidation numberOxidation number: how many electrons an atom : how many electrons an atom is donating or accepting in is donating or accepting in moleculaarmoleculaar bonding.bonding.OxidationOxidation--reductionreduction ((redoxredox) reaction: reaction in ) reaction: reaction in which oxidation numbers of reactants change.which oxidation numbers of reactants change.LEO says GER, LEO the LEO says GER, LEO the GERmanGERman, OIL RIG, OIL RIG•• OOxidation xidation iis s lloss, oss, RReduction eduction iis s ggain of electrons.ain of electrons.

Reducing agent causes reduction and is oxidized.Reducing agent causes reduction and is oxidized.Oxidizing agent causes oxidation and is reduced.Oxidizing agent causes oxidation and is reduced.

8686

Galvanic (Voltaic) CellsGalvanic (Voltaic) Cells

Harnessing flow of electrons from Harnessing flow of electrons from redoxredox reaction reaction to generate an electric current.to generate an electric current.Red catRed cat, , an oxan ox Reduction @ cathode; oxidation Reduction @ cathode; oxidation @ anode.@ anode.

8787

Galvanic (Voltaic) CellsGalvanic (Voltaic) CellsHalfHalf--reactions and potentials reactions and potentials positive positive EE is is spontaneous.spontaneous.∆∆GG = = ––nFEnFE…where …where FF is Faraday’s constant is Faraday’s constant (96500 (96500 CmolCmol––11), ), nn is number of electrons is number of electrons transferred, transferred, EE is halfis half--reaction potential.reaction potential.Flipping halfFlipping half--reaction changes sign on reaction changes sign on EE..More More negativenegative reduction potential reduction potential product product strong strong reducing agentreducing agent…more …more positivepositivereduction potential reduction potential reactant strong reactant strong oxidizing oxidizing agentagent..

8888

RedoxRedox Electrode PotentialsElectrode Potentials

8989

NernstNernst EquationEquation

EE = = EE°° –– 2.3032.303RTRTnFnF loglogQQ = = EE°° –– 0.0592 V0.0592 V

nn loglogQQ

Electrode potentials for Electrode potentials for nonnon--standardstandardconditions, but conditions, but TT still = 298 K.still = 298 K.Zn(s) + CuZn(s) + Cu2+2+(aq) (aq) ZnZn2+2+(aq) + Cu(s)(aq) + Cu(s)

EE = = EE°° ––0.0592 V0.0592 V

22loglog [Zn[Zn2+2+]]

[Cu[Cu2+2+]]

9090

Electrolytic CellsElectrolytic Cells

Unlike a galvanic cell, electrolytic cells use an Unlike a galvanic cell, electrolytic cells use an external voltage source like a battery to create an external voltage source like a battery to create an electric current that forces a nonelectric current that forces a non--spontaneous spontaneous redoxredox reaction to occur reaction to occur electrolysiselectrolysis..Electroplating: plating a thin layer of metal on top Electroplating: plating a thin layer of metal on top of another material of another material application of electrolytic application of electrolytic cells, often done in coin mints or with jewelry.cells, often done in coin mints or with jewelry.Red catRed cat, , an oxan ox still applies!still applies!

9191

Faraday’s Law of ElectrolysisFaraday’s Law of ElectrolysisDetermine amounts of species produced at Determine amounts of species produced at electrolytic electrodes using this law.electrolytic electrodes using this law.The amount of chemical change is proportional to The amount of chemical change is proportional to the amount of electricity flowing through cell.the amount of electricity flowing through cell.Three steps:Three steps:•• Determine amount of electricity in coulombs from Determine amount of electricity in coulombs from

QQ = = ItIt•• Convert Convert QQ to moles of electrons with Faraday to moles of electrons with Faraday

constant using constant using QQ = = nFnF..•• Use Use stoichiometrystoichiometry of reaction to finish the calculation.of reaction to finish the calculation.

9292

Faraday’s Law of ElectrolysisFaraday’s Law of Electrolysis

A piece of steel is the cathode in a hot solution A piece of steel is the cathode in a hot solution of chromic acid to electroplate it with of chromic acid to electroplate it with chromium metal. How much Cr deposited after chromium metal. How much Cr deposited after 48250 C of electricity was forced through cell?48250 C of electricity was forced through cell?•• 1/12 mol1/12 mol•• 1/6 mol1/6 mol•• 1 mol1 mol•• 3 mol3 mol

9393

Concentration CellsConcentration CellsA galvanic cell that has identical electrodes but A galvanic cell that has identical electrodes but whose halfwhose half--cells have different ion cells have different ion concentrations.concentrations.Electric current from potential difference arising Electric current from potential difference arising from unequal concentrations.from unequal concentrations.Electrons will flow in direction of highest Electrons will flow in direction of highest concentration of positive ions.concentration of positive ions.When concentrations of solutions become equal, When concentrations of solutions become equal, the reaction will stop.the reaction will stop.

9494

Concentration CellsConcentration Cells

EE = = EE°° ––0.0592 V0.0592 V

22loglog

[Ni[Ni2+2+]]dilutedilute

[Ni[Ni2+2+]]concentratedconcentrated