mcat - general chemistry overview
TRANSCRIPT
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Atoms, Molecules, and Quantum Mechanics
Avogadro’s Number 6.022 x 1023 molecules in a mole
Moles = Grams / Molecular Weight
Group 1A Alkali Metals (+1)
Reacts with hydrogen to form hydrides such as NaH
Reacts exothermically with water to produce metal hydroxide and hydrogen gas
Group 2A Alkaline Earth Metals (+2)
Harder, denser, and higher MP than alkali metals
Group 4A Can form 4 covalent bonds with nonmetals
All but CARBON can form 2 additional bonds with Lewis bases (6 total)
Only CARBON can form strong pi bonds for double/triple bonds
Group 5A can form 3 covalent bonds (NITROGEN can form 4th covalent bonds with lone pair)
All but NITROGEN can form 2 additional bonds by using d orbitals
1 additional bond can be formed with Lewis bases (6 total)
NITROGEN can form strong pi bonds for double/triple bonds
PHOSPHOROUS can form weak pi bonds for only double bonds
Group 6A Chalcogens (‐2)
OXYGEN – 2nd most electronegative / Can form strong pi bonds for double bonds
SULFUR – exists as pure S8 / can form up to 6 bonds / can double bond
Group 7A Halogens (‐1)
FLUORINE always has oxidation number of ‐1 (only one bond possible)
Cl / Br / I can take on oxidation states as high as +7 / forms more than one bond
Group 8A Noble Gases (0)
Inert / nonreactive
Small Atoms – make STRONG PI bonds
Large Atoms – UNABLE to make STRONG PI bonds / have d orbitals allowing for MORE than 4 bonds
PREDICTING THE ION CHARGE:
1) Atoms lose electrons from the highest energy shell first. In transition metals, this means that electrons are lost
from the s subshell first, and then from the d subshell
2) Ions are looking for symmetry. Representative elements form noble gas electron configurations when they
make ions. Transition metals try to ‘even‐out’ their d orbitals, so each orbital has the same number of
electrons
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PERIODIC TRENDS
GREATER Energy of Ionization
Electron Affinity
Electronegativity
GREATER Atomic Radius
Metallic Character
Mass Kilogram kg Length Meter m Time Second s
Electric Current Ampere A Temperature Kelvin K
Luminous Intensity Candela cd Amount of Substance Mole mol
Mega‐ M 106 Kilo‐ k 103 Deci‐ d 10‐1 Centi‐ c 10‐2 Milli‐ m 10‐3 Micro‐ μ 10‐6 Nano‐ n 10‐9 Pico‐ p 10‐12 Femto‐ f 10‐15
ENERGY IS REQUIRED TO BREAK A BOND
ENERGY IS RELEASED WHEN A NEW BOND FORMS
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NAMING INORGANIC COMPOUNDS
CATIONS Transition metal cation – use roman numerals in parentheses after element
Nonmetal cation – use suffix –ium
ANIONS Monatomic / simple polyatomic anions – use suffix –ide
Complex polyatomic anions – more oxygenated species uses –ate suffix
Less oxygenated species uses –ite suffix
If necessary ‐ Least oxygenated uses hypo‐ prefix
If necessary – most oxygenated uses per‐ prefix
Binary molecular compounds – compounds with only two elements
Physical Reaction – undergoes reaction and maintains its molecular structure and identity
Melting, evaporating, dissolving, rotation of polarized light
Chemical Reaction – undergoes reaction and changes its molecular structure to form a new compound
Combustion, metathesis, and redox
ANSWER IS ALWAYS A BALANCED EQUATION UNLESS OTHERWISE SPECIFIED
Percent Yield = Actual Yield / Theoretical yield x 100
FUNDAMENTAL REACTION TYPES
Combination A + B C
Decomposition C A + B
Single Displacement A + BC B + AC
Double Displacement AB + CD AD + CB
BONDING IN SOLIDS
Crystalline (individual molecules cannot be distinguished)
Ionic – positively charges ions held together by electrostatic forces (salts)
Network Covalent – infinite network of atoms held together by polar and nonpolar bonds (diamonds)
Metallic – single metal atoms bonded together by delocalized electrons
Molecular – individual molecules held together by intermolecular bonds (ice)
Amorphous – no characteristic shape and melts over a temperature range (glass)
Polymers – solids with repeated structural units
Rapid cooling of liquid polymers results in amorphous solid
Slow cooling of liquid polymers results in crystalline solids
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QUANTUM NUMBERS
Principal quantum number (n) – designates shell level
Azimuthal quantum number (l) – designates sub‐shell
l = n – 1
l = 0 = s subshell l = 1 = p subshell l = 2 = d subshell l = 3 = f subshell
Magnetic quantum number (m) – designates orbital
Range of magnetic quantum numbers from (–l to +l)
Electron Spin quantum number (m) – designates spin
½ or ‐½
HEISENBERG UNCERTAINTY PRINCIPLE
The more known about position, the less can be known about momentum and vice versa
AUFBAU PRINCIPLE
With each new proton added to create a new element, a new electron is added as well
Electrons look for an available orbital with the lowest energy state whenever they add to an atom
HUND’S RULE
Electrons will not fill any orbital in the same subshell until all orbitals in that subshell contain at least one electron
Unpaired electrons will have parallel spins
PLANCK’S QUANTUM THEORY
Electromagnetic energy is quantized
ΔE = hƒ (h = Planck’s constant = 6.6 x 10‐34 J s)
PHOTOELECTRIC EFFECT
Energy of photoelectrons depends on FREQUENCY of light, NOT INTENSITY
When an electron falls from a higher energy rung to a lower energy rung,
Energy is released from the atom in the form of a photon
The photon MUST have a frequency which corresponds to the change in energy of the electron
If the photon doesn’t have enough energy to bump the electron to the next rung,
The electron will not move from its present run and the photon is reflected away
KINETIC ENERGY OF ELECTRONS INCREASES ONLY WHEN
INTENSITY IS INCREASED BY INCREASING THE FREQUENCY OF
EACH PHOTON, THE QUANTITY OF PHOTONS IS IRRELEVENT
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Gases, Kinetics, and Chemical Equilibrium
Standard Temperature and Pressure (STP) 0 degrees Celsius / 1 atmosphere
ALL GASES ARE MISCIBLE WITH EACH OTHER
GIVEN TIME AND LOW TEMPS, HEAVIER GASES TEND TO SETTLE BELOW LIGHTER GASES
IDEAL GASES
1) Gas molecules have zero volume
2) Gas molecules exert no forces other than repulsive forces due to collisions
3) Gas molecules make completely elastic collisions
4) Average kinetic energy of gas molecules is DIRECTLY PROPORTIONAL to the TEMPERATURE of the gas
IDEAL GAS LAW
PV = nRT (R = universal gas constant = 0.08206 L atm / K mol = 8.314 J / K mol)
V is proportional to T at constant P
V is inversely proportional to P at constant T
V is proportional to n at constant T and P
Caveat – increasing V can lower T, because increasing V requires work which needs kinetic energy, lowering T
STANDARD MOLAR VOLUME
ANY GAS BEHAVING IDEALLY ‐ 22.4 liters at STP
PARTIAL PRESSURE
Pa = χ
a Ptotal
DALTON’S LAW
PTOTAL
= P1 + P
2 + P
3…
(each gas behaves like it is in the container alone, so total pressure is sum of them all)
AVERAGE KINETIC ENERGY for any gas
K.E.AVG
= 3/2 R T (R = universal gas constant = 0.08206 L atm / K mol = 8.314 J / K mol)
GRAHAM’S LAW (rms velocity varies according to mass)
Average SPEED of molecules is INVERSELY PROPORTIONAL to sq rt. of MASS of molecules
EFFUSION – spreading of gas from high pressure to low pressure through pinhole
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DIFFUSION – spreading of one gas into another gas or into empty space
EFFUSION and DIFFUSION follow GRAHAM’S LAW
REAL GASES
VREAL
is GREATER than VIDEAL
PREAL
is LESS than PIDEAL
COLLISION MODEL
The reacting molecules MUST COLLIDE
1) Relative kinetic energies (due to relative velocity only) of colliding molecules must reach a threshold energy
called ACTIVATION ENERGY
2) Colliding molecules must have the proper spatial orientation
RATE OF A REACTION INCREASES WITH TEMPERATURE
EQUATIONS FOR REACTION RATES
MOLECULARITY – the number of molecules colliding at one time to make a reaction
UNIMOLECULAR / BIMOLECULAR / TERMOLECULAR
ELEMENTARY REACTIONS ( the question has to inform you that a reaction is elementary)
Reaction that occurs in a single step
aA + bB cC + dD Molecularity is given by (a+b)
Coefficient tells you how many molecules participate in a reaction producing collisions
INTERMEDIATES – Species that are products of one reaction and reactants of a later reaction in a reaction chain
RATE LAW FOR FORWARD REACTIONS
rateFORWARD = kf [A]
α [B]
β
α and β are the order of each respective reactant
α + β is the overall order of the reaction
SLOW STEP = rate determining step
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Catalyst – creates a new reaction pathway which typically includes an intermediate
Increases the rate of a reaction without being consumed or permanently altered
Lowers energy of activation & DOES NOT change equilibrium
CHEMICAL EQUILIBRIUM
The RATE at EQUILIBRIUM is zero (a net reaction rate) Forward rate = Reverse rate
At Equilibrium, there is no change in the concentration of the products or reactants
EQUILIBRIUM is the point of GREATEST ENTROPY
kf / kr = products / reactants
kf / k
r = EQUILIBRIUM CONSTANT K
LAW OF MASS ACTION
K = [C]c[D]
d / [A]
a[B]
b = products
coeff. / reactants coeff.
EQUILIBRIUM CONSTANT DEPENDS UPON TEMPERATURE ONLY
DON’T USE SOLIDS OR PURE LIQUIDS (water) IN THE LAW OF MASS ACTION
NON‐EQUILIBRIUM REACTION QUOTIENTS
Q = Products coeff. / Reactants coeff.
Q is NOT a constant & ALWAYS changes TOWARD K; can be used to predict the direction of the reaction
Q / K COMPARISONS
IF Q = K reaction is at equilibrium
IF Q > K Too Many PRODUCTS – will shift to the LEFT
IF Q < K Too Many REACTANTS – will shift to the RIGHT
LE CHATELIER’S PRINCIPLE
When a system at equilibrium is stressed,
The reaction will shift in a direction that will reduce that stress
1) Addition or removal of product or reactant
2) Changing the pressure of the system
3) Heating or cooling the system
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Thermodynamics Three Types of Systems and their interactions with their Surroundings
Exchanging Energy with surroundings Exchanging Mass with surroundings Open System Yes Yes Closed System Yes No Isolated System No No
STATE FUNCTIONS
State – physical condition of a system described by a specific set of thermodynamic property values
EXTENSIVE PROPERTIES – proportional to the size of the system (ex. Volume & Moles)
INTENSIVE PROPERTIES – independent of the size of the system (ex. Pressure & Temperature)
STATE FUNCTIONS – VALUE FORWARD IS SAME AS VALUE REVERSED (regarding reaction direction)
Pathway independent
Change in a state property is the same regardless of the process via which the system was changed
3 State Properties (1 extensive) describe a system unambiguously
HEAT
Movement of energy via CONDUCTION, CONVECTION, or RADIATION
Rate of Heat Flow is like Volume Flow Rate or Electric Current (Resistance always exists)
Thicker conduits allow for greater flow
Longer conduits impede flow
Flow rate depends on the difference in a property of the reservoirs at either end of the conduit
CONDUCTION – thermal energy transfer via molecular collisions
CONVECTION – thermal energy transfer via fluid movements
RADIATION – thermal energy transfer via electromagnetic waves
PV work – constant pressure / volume changes
Work = P ΔV (constant pressure) (IF volume is constant, no PV WORK is done)
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0th Law of Thermodynamics
Temperature exists as a state function
1st Law of Thermodynamics
Energy of the system and surroundings is ALWAYS conserved
Any energy change to a system must equal the heat flow into the system plus the work done on the system
ΔE = q + w
2nd Law of Thermodynamics
Total entropy of an isolated system tends to increase over time
Heat cannot be changed completely into work in a cyclical process (some heat energy is always lost)
3rd Law of Thermodynamics Assigns ZERO entropy value to any pure substance at absolute zero and in internal equilibrium ABSOLUTE ZERO IS UNATTAINABLE
THERMODYNAMIC FUNCTIONS
Internal Energy U Temperature T
Pressure P Volume V
Enthalpy H Entropy S
Gibbs energy G
INTERNAL ENERGY – all the possible forms of energy imaginable on a molecular scale
In a closed system at rest, the only energy change will be in internal energy… ΔU = q + w
For a reaction in such a system with NO CHANGE IN VOLUME, there is no work… ΔU = q
Temperature is measurement of how fast the molecules are moving or vibrating, as substance gets hotter, it is because
their molecules move faster
The average kinetic energy of a single molecule in a fluid is … K.E.AVG = 3/2 k T
VIRTUALLY ALL PHYSICAL PROPERTIES CHANGE WITH TEMPERATURE
THE GREATER THE RANDOM TRANSLATIONAL KINETIC ENERGY OF GAS
MOLECULES PER VOLUME, THE GREATER THE PRESSURE
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ENTHALPY – property that accounts for the extra capacity of a substance to do PV work
H = U + PV ΔH = ΔU + PΔV Measured at 25 degrees Celsius and 1 atmosphere STANDARD ENTHALPY OF FORMATION ΔH°ƒ Change in enthalpy for a reaction that creates 1 mol of a compound from its raw elements in their standard state
IF NO CHANGE IN PRESSURE (essentially no gas in reaction), then ΔH = q ΔH°REACTION = ΔH°ƒ PRODUCTS ‐ ΔH°ƒ REACTANTS
WHEN YOU ADD REACTIONS, YOU CAN ADD THEIR ENTHALPIES
EXOTHERMIC (‐ΔH) – release heat, making reaction system hot
ENDOTHERMIC (+ΔH) – absorb heat making reaction system cold Activation Energy Transition State – peak of the hill where old bonds break and new bonds form Intermediates – products of the first step in a two step reaction (in trough between two hills) Catalyst
Lowers activation energy for both forward and reverse reactions EQUILIBRIUM / ENTHALPY CHANGE IS UNAFFECTED BY CATALYSTS RATE IS AFFECTED BY CATALYSTS ENTROPY (S) – measure of a system’s disorder, Nature likes to spread energy evenly between systems Nature’s tendency to create the most probable situation that can occur within a system To DECREASE entropy, WORK is REQUIRED
ΔSSYSTEM + ΔSSURROUNDINGS = ΔSUNIVERSE ≥ 0 Entropy (not energy) dictates the direction of a reaction / Entropy increases with number, volume, and temperature REACTIONS AT EQUILIBRIUM HAVE ACHIEVED MAXIMUM UNIVERSAL ENTROPY GIBBS FREE ENERGY (G)
ΔG = ΔH – TΔS NEGATIVE ΔG indicates a SPONTANEOUS REACTION ΔG ΔH ΔS + + ‐ ‐ ‐ + + / ‐ ‐ ‐ + / ‐ + +
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Solutions Solution – homogeneous mixture of 2+ compounds in a single phase Solvent – indicates predominant compound in a solution Solute – the compound that is less concentrated IDEAL SOLUTIONS Solutions made from compounds that have similar properties IDEALLY DILUTE SOLUTIONS Solute molecules are completely separated by solvent molecules so they have no interaction with each other MOLE FRACTION of SOLVENT is approximately ONE NONIDEAL SOLUTIONS Violate the two conditions above COLLOIDS Like a solution, only the solute particles are larger Too small to filter, but large enough to be separated by a semi‐permeable membrane LIKE DISSOLVES LIKE Nonpolar dissolves nonpolar Polar dissolves polar LONDON DISPERSION FORCES Weak intermolecular bonds resulting from instantaneous dipole moments SOLVATION When ionic compounds dissociate into their cations and anions
They are SURROUNDED by the oppositely charged ends of the polar solvent HYDRATION
Solvation in water When several water molecules attach to one side of an ionic compound, they can overcome the strong ionic bond
AQUEOUS PHASE – something that is hydrated is in this phase ELECTROLYTE – compound which forms ions in aqueous solutions
WATER IS A POOR CONDUCTOR OF ELECTRICITY UNLESS IT CONTAINS ELECROLYTES
MOLARITY
M = moles of SOLUTE / volume of TOTAL SOLUTION MOLALITY
m = moles of SOLUTE / kilograms of SOLVENT
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MOLE FRACTION
χ = moles of SOLUTE / total moles of ALL SOLUTES and SOLVENT MASS PERCENTAGE
mass % = mass of SOLUTE / mass of TOTAL SOLUTION x 100 PARTS PER MILLION
ppm = mass of SOLUTE / mass of TOTAL SOLUTION x 1000000 NORMALITY – measure of the number of dissociated protons in acid/base solution H2SO4 is a 2‐normal solution because 2 protons can be donated for each single molecule SOLUTION FORMATION
1) The breaking of the intermolecular bonds between solute molecules (ENDOTHERMIC) 2) The breaking of the intermolecular bonds between solvent molecules (ENDOTHERMIC) 3) The formation of intermolecular bonds between the solvent and the solute molecules
(EXOTHERMIC)
ENERGY IS REQUIRED TO BREAK A BOND HEAT OF SOLUTION
ΔHSOL = ΔH
1 + ΔH
2 + ΔH
3
NEGATIVE HEAT OF SOLUTION – results in STRONGER intermolecular bonds / LOWER vapor pressure POSITIVE HEAT OF SOLUTION – results in WEAKER intermolecular bonds / HIGHER vapor pressure
WHEN SOLUTIONS FORM, ENTROPY INCREASES VAPOR PRESSURE Some surface molecules can break the forces holding them in liquid and launch out into open space When rate of molecules leaving and entering liquid solution is equal, equilibrium is established At this point, the pressure created by the molecules in the open space is the VAPOR PRESSURE of the LIQUID
VAPOR PRESSURE INCREASES WITH TEMPERATURE
BOILING OCCURS WHEN VAPOR PRESSURE OF A LIQUID EQUALS THE ATMOSPHERIC PRESSURE
MELTING OCCURS WHEN VAPOR PRESSURE OF A SOLID EQUALS VAPOR PRESSURE OF THE LIQUID
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NONVOLATILE SOLUTES (have no vapor pressure of their own)
RAOULT’S LAW PSOLUTION
= χa PLIQUID
VOLATILE SOLUTES (have vapor pressure of their own)
RAOULT’S LAW PSOLUTION
= χa Pa + χ
b Pb
SOLUBILITY – solute’s tendency to dissolve in a solvent EX. Salts dissolve in water in FORWARD DIRECTION Dissolved Salts PRECIPITATE in the REVERSE DIRECTION (slower than forward) When rates of DISSOLUTION and PRECIPITATION are equal, the solution is SATURATED
SOLUBILITY PRODUCT Ksp
Use just like any other equilibrium constant (excluding solids and pure liquids) SET Ksp EQUAL TO PRODUCTS over REACTANTS raised to the POWER OF THEIR COEFFICIENTS
If ratio is 1 : 1 : 1 (regarding coefficients of solid AND dissolved species) Solubility = square_root (Ksp) If ratio is 1 : 2 : 1 (regarding coefficients of solid AND dissolved species) Solubility = cube_root (Ksp / 4) Solubility product – a constant found in a book Solubility – maximum number of moles of the solute that can dissolve in solution
SOLUBILITY PRODUCT CHANGES ONLY WITH TEMPERATURE SOLUBILITY DEPENDS UPON TEMPERATURE and IONS IN SOLUTION
COMMON IONS ADDED TO A SATURATED SOLUTION WILL SHIFT the
EQUILIBRIUM (DOES NOT AFFECT Ksp)
COMMON IONS ADDED TO NON‐SATURATED SOLUTION WILL NOT SHIFT EQUILIBRIUM BECAUSE NO EQUILIBRIUM EXISTS
AS THE TEMPERATURE INCREASES, THE SOLUBILITY OF SALTS INCREASES AS THE TEMPERATURE INCREASES, THE SOLUBILITY OF GASES DECREASES
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Heat Capacity, Phase Change, and Colligative Properties REMEMBER – phases are not just SOLID / LIQUID / GAS There can be multiple solid phases of the same element (rhombic sulfur vs. monoclinic sulfur) Heat Capacity – Measure of the energy change needed to change the temperature of a substance Think of it as the “internal energy capacity”
IF NO PV WORK IS DONE BY A SYSTEM AT REST, ALL HEAT ENERGY GOES INTO INCREASING TEMPERATURE
WHEN SYSTEM IS HEATED AT CONSTANT VOLUME,
NO PV WORK IS DONE, and TEMP INCREASES A LOT
WHEN SYSTEM EXPANDS AT CONSTANT PRESSURE,
PV WORK IS DONE, and TEMP INCREASE A LITTLE BIT
HEAT CAPACITY is the amount of energy a substance can absorb per unit of temperature change FOR HEAT CAPACITIES OF ENTIRE SYSTEMS
q = C ΔT FOR SPECIFIC HEAT CAPACITIES (divided by mass)
q = m c ΔT TYPES OF PHASE CHANGES: Melting‐freezing Vaporization‐condensation Sublimation‐deposition Each Phase of a substance has its own specific heat
CONSTANT PRESSURE MEANS NO PV WORK IS DONE, and q = ΔH
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BOILING – Vapor pressure of a liquid equals the atmospheric pressure above the liquid EVAPORATION – When the partial pressure above a liquid is less than the liquid’s vapor pressure, but the atmospheric pressure is greater than the vapor pressure Phase Diagram – Indicates the phases of a substance at different pressures and temperatures Triple Point – Where a substance can exist in equilibrium as a solid, liquid, and gas Critical Point – The critical temperature and the critical pressure ENERGY enters a substance as HEAT or PV WORK
During a PHASE CHANGE, ENERGY breaks BONDS
and does NOT CHANGE TEMPERATURE
When the PHASE IS NOT CHANGING, energy INCREASES MOLECULAR MOVEMENT,
increasing TEMPERATURE COLLIGATIVE PROPERTIES DEPEND ON NUMBER, not kind
1) Vapor pressure a. Adding nonvolatile solutes will lower vapor pressure of the solution in proportion to the # of
particles
2) Boiling point a. Boiling point RISES with addition of nonvolatile solutes
3) Freezing point a. Freezing point DECREASES with addition of nonvolatile solutes
4) Osmotic pressure a. Measure of the tendency of water to move into a solution via osmosis b. INCREASED OSMOTIC PRESSURE on the SOLUTION SIDE OF THE water‐solution
division by a non‐permeable membrane – pure water enters solution side to try to equalize concentrations
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Acids and Bases
Arrhenius Acid – any species that produces HYDROGEN IONS in aqueous solutions Arrhenius Base – any species that produces HYDROXIDE IONS in aqueous solutions
Bronsted & Lowry Acid – any species that DONATES a PROTON Bronsted & Lowry Base – any species that ACCEPTS a PROTON
Lewis Acid – any species that ACCEPTS an ELECTRON PAIR Lewis Base – any species that DONATES an ELECTRON PAIR ACIDIC SOLUTION – contains more H+ than OH‐ BASIC SOLUTION – contains more OH‐ than H+
pH = ‐ log [H+]
THE STRONGER THE ACID, the WEAKER THE CONJUGATE BASE THE STRONGER THE BASE, the WEAKER THE CONJUGATE ACID
AMPHOTERIC – a substance that can act as either an ACID or a BASE
3 = ‐ log [10‐3] OR 3 = ‐ log [.003] VERY ACIDIC
10 = ‐ log [10‐10] OR 10 = ‐ log [.0000000001] VERY BASIC
HYDRONIUM ION ( H3O+ ) STRONG ACIDS (stronger than H3O+) STRONG BASES (stronger than OH‐) Hydroiodic acid HI Hydrobromic acid HBr Hydrochloric acid HCl Nitric Acid HNO3 Perchloric Acid HClO4 Chloric Acid HClO3 Sulfuric Acid H2SO4
Sodium Hydroxide NaOH Potassium Hydroxide KOH Amide Ion NH2‐ Hydride Ion H‐ Calcium Hydroxide Ca (OH)2 Sodium Oxide Na2O Calcium Oxide CaO
ACID DISSOCIATION DECREASES as ACID CONCENTRATION INCREASES
ACID STRENGTH INCREASES as ACID CONCENTRATION INCREASES
INCREASING POLARITY / BOND STRENGTH = DECREASING ACIDITY
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H‐I is STRONGER than H‐F
In a series of OXYACIDS, MORE OXYGENS is a STRONGER ACID
HYPOCHLOROUS acid < CHLOROUS acid < CHLORIC acid < PERCHLORIC acid EQUILIBRIUM FOR AUTO‐IONIZATION OF WATER ACID‐BASE REACTIONS Kw is the equilibrium constant for this reaction
Kw = [H+][OH‐] = 10
‐14
pH + pOH = pKw
For Aqueous Solution at 25 degrees Celsius
pH + pOH = 14 ACID DISSOCIATION CONSTANT Ka
HA + H2O ‐> H
3O+ + A‐
Ka = [H+][A‐] / [HA]
EQUILIBRIUM FOR CONJUGATE BASE
A‐ + H2O ‐> OH‐ + HA
Kb = [OH‐][HA] / [A‐]
(Ka)(K
b) = K
w
pK
a + pK
b = pK
w
pKa + pK
b = 14
pKa = pH – log( [ A‐] / [HA] )
If pH > pK
a, then molecule will LOSE PROTON (‐ charge overall)
If pH = pKa, then molecule has 50% CHANCE of LOSING PROTON
If pH < pKa, then molecule will probably GAIN A PROTON (+ charge overall )
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STRONG ACID has Ka > 1 OR A pKa < 0 FOR STRONG ACIDS / BASES
The entire concentration dissociates, so [H+] or [OH‐] is the same as the original concentration of the acid / base So 0.1M HCL has 0.1M [H+]
TITRATION drop by drop mixing of an acid and a base Performed to find the concentration of some unknown by comparing it with the concentration of the TITRANT EQUIVALENCE POINT / STOICHIOMETRIC POINT – the midpoint of the graph line that is almost vertical If the BASE and the ACID are equivalent in strength, the equivalence point is probably pH 7, unless one species is diprotic If the BASE is STRONGER than the ACID, the point will be above pH 7 If the ACID is STRONGER than the BASE, the point will be below pH 7 HALF‐EQUIVALENCE POINT – point where ½ of the acid has been neutralized by the base The concentration of the acid is equal to the concentration of its conjugate base This point shows the point in the titration where the solution is the most well‐buffered HENDERSON‐HASSELBALCH EQUATION ‐ the pH of the solution is equal to the pKa of the acid
THE END POINT is where the INDICATOR CHANGES COLOR (ENDICATOR)
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Electrochemistry Oxidation‐Reduction The TRANSFER of electrons from one atom to another
OXIDATION IS LOSING ELECTRONS; REDUCTION IS GAINING ELECTRONS
OXIDATION STATES Possible CHARGE VALUES that an atom may hold within a molecule
Oxidation State Atom0 Atoms in their elemental form ‐1 Fluorine +1 Hydrogen (except when bonded to a metal: then ‐1) ‐2 Oxygen (except when it is in a peroxide like H2O2)
Oxidation State Group on Periodic Table
+1 Group 1 Elements (alkali metals) +2 Group 2 Elements (alkaline earth metals) ‐3 Group 15 Elements (nitrogen family) ‐2 Group 16 Elements (oxygen family) ‐1 Group 17 Elements (halogens)
The Reducing Agent is OXIDIZED
The Oxidizing Agent is REDUCED
Potentials There is an ELECTRIC POTENTIAL E associated with any redox reaction Each component is a HALF‐REACTION
REDUCTION POTENTIAL E° must be POSITIVE for it to occur.
Take E° of substance BEING REDUCED, subtract E° for the substance BEING OXIDIZED
E°SUBSTANCE BEING REDUCED
‐ E°SUBSTANCE BEING OXIDIZED
> 0
Half‐reaction that occurs at a standard HYDROGEN ELECTRODE is arbitrarily assigned as ZERO VOLTS
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The GALVANIC CELL (a.k.a. VOLTAIC CELL) Chemical Energy INTO Electrical Energy
• Multiphase Series of components with no component occurring in more than one phase • All phases must conduct electricity • At least ONE PHASE must be IMPERMEABLE TO ELECTRONS • IONIC CONDUCTOR is used to carry current in form of IONS • This IONIC CONDUCTOR is usually a SALT BRIDGE
Terminal1 – Electrode
1 – Ionic conductor – Electrode
2 – Terminal
2
The emf is the ELECTRIC POTENTIAL DIFFERENCE between Terminal
1 & Terminal
2
Anode – NEGATIVE Cathode – POSITIVE (think cat‐ion)
RED CAT; AN OX Reduction cathode; anode oxidation
IUPAC Convention Example
Pt’ (s) | Zn (s) | Zn2+ (aq) || Cu
2+ (aq) | Cu (s) | Pt (s)
FREE ENERGY & Chemical Energy ΔG = ‐n F E
MAX
ΔG = ΔG° + R T ln(Q)
ΔG° = ‐ R T ln(K)
If K = 1; then ΔG° = 0 If K > 1; then ΔG° < 0 If K < 1; then ΔG° > 0
The CONCENTRATION CELL • Limited form of a galvanic cell with a reduction half reaction taking place in one half cell and the exact reverse of
that half reaction taking place in the other half • If concentrations on both sides are equal, then E = 0 • One side must have a higher concentration of the reactants than the other side for this to work
The ELECTROLYTIC CELL • Put a POWER SOURCE across a galvanic cell’s resistance and force it to run BACKWARDS
Galvanic Cell Electrolytic Cell Positive CATHODE Negative Cathode Negative Anode Positive ANODE
POSITIVE EMF POTENTIAL NEGATIVE EMF POTENTIAL Electrons flow toward CATHODE Electrons flow toward ANODE