Chapter 4: RXN TYPES and SOLN STOICHIOMETRY (2)

Aqueous Solutions and Electrolytes

Precipitation, Acid-Base, Oxidation-Reduction (Redox)

Reactions

CHEMICAL RXNS IN AQUEOUS SOLNS (4.4)

• Precipitation is the formation of solid from two aqueous solutions.

• Acid-Base neutralization is the formation of water.

• Oxidation-Reduction or redox is the transfer of electrons from one reactant atom to another reactant atom.

PRECIPITATION (4.5, 4.7)

• The formation of a solid (precipitate) when two aqueous solutions are mixed; we say that the product cmp is insoluble or only slightly soluble in water.

• Soluble salt I (aq) + Soluble salt II (aq) Solid cmp (s) + Soluble salt III

• Table 4.1 Solubility Rules for Salts in Water (memorize)

• Ion interchange or metathesis (switch cation/anion partners)

T4.1 SOLUBILITY RULES Practice Prob 35, 37, 4, 43

1. Most nitrates are soluble2. Most salts with Grp 1A ions and NH4

+ are soluble.

3. Most salts with Cl-, Br-, I- are soluble EXCEPT those with Ag+, Pb2+, Hg2

2+

4. Most sulfates are soluble EXCEPT those with Ba2+, Pb2+, Hg2

2+, Ca2+.5. Most hydroxides are slightly soluble EXCEPT

the strong bases (Ca, Sr, Ba).6. Most sulfides, carbonates, chromates and

phosphates are slightly soluble.

Fig 4.13 K2CrO4(aq) + Ba(NO3)2(aq) KNO3(aq) + BaCrO4(s)

CHEMICAL EQUATION (4.6)

• Identify reactants, products, states of matter [g, s, aq, ℓ].

• Balance equation to conserve mass (and charge).

• Calculate quantitative or stoichiometric relationships (mol ratios) between rxn participants (R or P) based on balanced chemical rxn.

CHEMICAL EQUATIONS (2)

• Formula Equation: write all reactants and products as “neutral molecules”, show state of each.

• Complete Ionic Equation: write strong electrolytes as ions (aq).

• Net Ionic Equation: cancel out spectator ions. This eqn expresses the basic chemical rxn.

Figure 4.15 a&b The Reaction of K2CrO4 and Ba(NO3)2

Chapter 4 Problems

• 36, 38, 44, 46

SOLUTION STOIOCHIOMETRY (Ch 4 + Ch 3 + Ch 2)

• Typical stoichiometric calculation for reactions taking place in aq soln.

1. Write balanced net ionic eqn to identify the chem reaction

2. Calculate mols of known A from VA and MA

3. Calculate mols of unknown B, then VB

4. VA, MA #mol A #mol B VB if MA and MB are known

5. Determine LR after Step 1 if appropriate.

STOICHIOMETRIC PROBLEMS

• 48, 50

ACID + BASE RXNS

• Acids donate protons, i.e. provide H+(aq) or H3O+ (aq, hydronium) ions in water (Arrhenius).

• Polyprotic acids: sulfuric, phosphoric.

• Bases accept protons.

• Review Ch 2 nomenclature and Ch 4 strong acids (SA) and bases (SB).

ACID + BASE RXN: NEUTRALIZATION

• Acid + Base → Salt + Water• SA + SB: HCl (aq) + NaOH(aq) →

NaCl(aq) + H2O(ℓ)– Net ionic: H+(aq) + OH-(aq) → H2O(ℓ)

• WA + SB: HF(aq) + KOH(aq) → KF(aq) + H2O(l)– Net ionic: HF(aq) + OH-(aq) → F-(aq) + H2O(ℓ)

• SA + WB: Problem 4.57c

ACID + BASE NEUTRALIZATION

• The key in a neutralization reaction is that one H+ ion reacts with one OH- ion.

• It is NOT that one mol acid reacts with one mol base. – Be careful with acids that provide 2 or 3 H+

ions per mol acid (sulfuric or phosphoric).– Similarly for bases like barium hydroxide.

ACID-BASE TITRATION (volumetric analysis)

• Exptal technique for determining quantity of an unknown substance (analyte in beaker) by reacting a measured volume of it with another reactant (titrant in buret) of known concentration.

• This method works when the rxn is 100% complete (reaches equivalence pt) and that there is an indicator (color change, pH) that signals the rxn completion (endpoint).

NEUTRALIZATION TITRATION

• Write the balanced acid (assume to be analyte) + base (use strong base) rxn for the titration.

• Use an indicator (e.g. phenolphthalein) that signals the equivalence point.

• The molarity and volume of the titrant (SB) must be known accurately.

• Fig 4.18

NEUTRALIZATION TITRATION

• Prob 4.64 and 4.66 (This is what you will do in lab)

OXIDATION-REDUCTION REACTIONS

• A redox reaction involves the transfer of electrons between atoms in the reactants.

• Electrons gained by one atom must equal electrons lost by another. (conservation of e-s)

• Oxidation states or numbers are assigned to atoms and they change in a redox rxn.

• Both oxidation and reduction must occur simultaneously. (or e-s would not be conserved)

2

OXIDATION STATES OR NUMBERS (OX#)

• Actual or imaginary charge on atom: single atom, atom in molecule or atom in polyatomic ion

• We use these OX#s to keep track of electrons in redox rxns.

• We will study rules for assigning OX# and then use this information to balance redox equations

DETERMINING OX# (T4.2)

• OX# of an atom in an element is 0 [Fe, O2]

• If the species is neutral, sum of OX# is 0 [NaCl, MnO2]

• If the species is charged, sum of OX# is value of overall charge (NH4

+; SO42-)

• OX# of a monatomic ions is its charge: 1A atoms have OX# = +1; 2A atoms have OX# = +2; 7A atoms have OX# = -1, etc

OX# (2)

• In molecular (covalent) cmps O has OX# = -2; sometimes -1 (with metal)

• In molecular (covalent) cmps H has OX# = +1; sometimes -1 (peroxide)

• F always has OX# = -1; other halides can have other OX#s

• There are exceptions

OXIDATION

• If atom X in compound A loses electrons and becomes more positive (OX# increases), we say X (with charge) or A is oxidized.

• Also, we say that A is the reducing agent (RA) or is the electron donor.

REDUCTION

• If atom Y in compound B gains electrons and becomes more negative (OX# decreases), we say Y (with charge) or B is reduced.

• Also, we say that B is the oxidizing agent (OA) or is the electron acceptor.

Figure 4.20 A Summary of Oxidation-Reduction Process

Redox Basics

• Prob 68, 72

ACTIVITY SERIES (Expt 7)

• Redox participants have varying capacities to gain or lose electrons.

• The Activity Series lists metal elements in order of decreasing strength as a reducing agent; ie. ability to lose electrons and undergo oxidation.

• A particular rxn in the list will cause the reduction of any rxn below it.

Activity Series of Metals in Aqueous Solution

INTERPRETATION OF ACTIVITY SERIES

• What rxn will occur between lithium and calcium? The choices are– Li(s) + Ca(s) Li+(aq) + Ca2+(aq) – 2Li(s) + Ca2+(aq) 2Li+(aq) + Ca(s)– 2Li+(aq) + Ca(s) Ca2+(aq) + 2Li(s)– 2Li+(aq) + Ca2+(aq) 2Li(s) + Ca(s)

• The strongest RA is at the top (Li) meaning that Li loses e-s and is oxidized. And Ca2+ gains e-s and must be reduced.

INTERPRETATION OF ACTIVITY SERIES

• Therefore when Lithium and Calcium react,

• 2Li(s) + Ca2+(aq) 2Li+(aq) + Ca(s)

• We say that Li displaces calcium ion from soln. Li(s) dissolves and Ca(s) forms

BALANCING REDOX EQNS Half-Rxn Method (acid)

• Write half chem eqn for reduction– This requires determining what atom is

reduced; use OX#s

• Write half chem eqn for oxidation– same

• Balance all atoms except H and O

• Balance O with H2O and H with H+

Half-Reaction Method (acid, 2)

• Add electrons to balance charge (I.e. show loss or gain of electrons)

• Balance the number of electrons between the two half-rxns by multipying by appropriate factor – #e- gained by atom Y = #e- lost by atom X)

• Add two half-rxns and cancel identical species.

• Check for atom and charge balance

The Half-Reaction Method (Acidic Solution)

Half-Reaction Method (base)

• Follow steps for balancing in acid

• Add OH- ions to cancel out the H+ ions, thus forming water.

• Cancel out water molecules

• Check for atom and charge balance. Make sure there are no H+ ions remaining.

Balancing Redox Equations

• Prob 74, 76