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CHEM 180/181 Chapter 20

Chapter 20 Electrochemistry

CHEM 180/181 Chapter 20

Topics in Electrochemistry • Oxidation-Reduction (Redox) Numbers • Balancing Redox Reactions • Voltaic Cells • Cell EMF • Spontaneity of Redox Reactions • Batteries • Corrosion • Electrolysis CHEM 180/181 Chapter 20

Oxidation and Reduction Oxidation (loss of e ) Na Na + + e Reduction (gain of e ) Cl + e Cl Oxidation-reduction (redox) reactions occur when electrons are transferred from an atom that is oxidized to an atom that is reduced.

Electron transfer can produce electrical energy spontaneously, but sometimes electrical energy is used to make them occur (nonspontaneous).

CHEM 180/181 Chapter 20

CHEM 180/181 Chapter 20

Oxidation-Reduction (Redox) Reactions BOTH reduction and oxidation must occur.

A substance that gives up electrons is oxidized and is called a reducing agent or reductant (causes another substance to be reduced).

A substance that accepts electrons is reduced and is therefore called an oxidizing agent or oxidant (causes another substance to be oxidized). CHEM 180/181 Chapter 20

Oxidation-reduction (Redox) Reactions Determine if it’s a redox reaction by keeping track of the oxidation states of all elements involved.

Zn(s) + 2H + (aq) Zn 2+ (aq) + H 2 (g) (0) (+1) (+2) (0) Quick hint: If a reaction includes an ELEMENT that turns into an ion, it’s a redox reaction.

CHEM 180/181 Chapter 20

Oxidation Number Guidelines

1. Atoms in elemental form, oxidation number is zero.

(Cl 2 , H 2 , P 4 , Ne are all zero)

2. Monoatomic ion, the oxidation number is the charge on the ion.

(Na + : +1; Al 3+ : +3; Cl : -1)

3. O is usually -2. But in peroxides (like H 2 O 2 has an oxidation number of -1.

and Na 2 O 2 ) it 4. H is +1 when bonded to nonmetals and -1 when bonded to metals.

(+1 in H 2 O, NH 3 and CH 4 ; -1 in NaH, CaH 2

5. The oxidation number of F is -1.

and AlH 3 )

6. The sum of the oxidation numbers for the molecule is the charge on the molecule (zero for a neutral molecule).

CHEM 180/181 Chapter 20

Example Determine the oxidation state of all elements in ammonium thiosulfate (NH 4 ) 2 (S 2 O 3 ).

sum = overall charge on ion CHEM 180/181 (NH 4 ) 2 (S 2 O 3 ) NH 4 + S 2 O 3 2 -3 +1 +2 -2 NH 4 + S 2 O 3 2 -3 +4 total +4 -6 Chapter 20

Determining Oxidation States What is the oxidation state of Mn in MnO 4 ?

Answer: +7 CHEM 180/181 Chapter 20

Balancing oxidation-reduction equations Balancing chemical equations follows law of conservation of mass.

AND, gains and losses of electrons must also be balanced.

CHEM 180/181 Chapter 20

Half-Reactions Separate oxidation and reduction processes in equation, Sn 2+ (aq) + 2Fe 3+ (aq) Sn 4+ (aq) + 2Fe 2+ (aq) Oxidation: Sn 2+ (aq) Sn 4+ (aq) + 2e Reduction: 2Fe 3+ (aq) + 2e 2Fe 2+ (aq) Overall, the number of electrons lost in the oxidation half reaction must equal the number gained in the reduction half reaction.

CHEM 180/181 Chapter 20

Balancing Equations by the Method of Half-Reactions: Acidic Consider : MnO 4 (aq) + C 2 O 4 2 (aq) Mn 2+ (aq) + CO 2 (g) (acidic) Unbalanced half-reactions: MnO 4 (aq) Mn 2+ (aq) C 2 O 4 2 (aq) CO 2 (g) First, balance everything EXCEPT hydrogen and oxygen.

Deal with half-reactions SEPARATELY.

CHEM 180/181 Chapter 20

Balancing Equations by the Method of Half-Reactions: Acidic To balance O: Add 4H 2 O to products to balance oxygen in reactants.

Mn O 4 (aq) Mn 2+ (aq) + 4H 2 O (l) To balance H: Add 8H + to reactant side to balance the 8H in water.

8H + (aq) + MnO 4 (aq) Mn 2+ (aq) + 4H 2 O(l) CHEM 180/181 Chapter 20

Balancing Equations by the Method of Half-Reactions: Acidic Balance charge: Add up charges on both sides. Add 5 electrons to reactant side.

5e + 8H + (aq) + MnO 4 (aq) Mn 2+ (aq) + 4H 2 O(l) Mass balance of C in oxalate half-reaction.

C 2 O 4 2 (aq) 2CO 2 (g) Balance charge by adding two electrons to the products.

C 2 O 4 2 (aq) 2CO 2 (g) + 2e Last step: Cancel electrons and add reactions together.

CHEM 180/181 Chapter 20

Balancing Equations by the Method of Half-Reactions: Acidic 5e + 8H + (aq) + MnO 4 (aq) Mn 2+ (aq) + 4H 2 O(l) C 2 O 4 2 (aq) 2CO 2 (g) + 2e Top reaction times 2. Bottom reaction times 5. ALL PARTS!!

10e + 16H + (aq) + 2MnO 4 (aq) 2Mn 2+ (aq) + 8H 2 O(l) 5C 2 O 4 2 (aq) 10CO 2 (g) + 10e 16H + (aq) + 2MnO 4 (aq) + 5C 2 O 4 2 (aq) 2Mn 2+ (aq) + 8H 2 O(l) + 10CO 2 (g) .

**Electrons have cancelled out** CHEM 180/181 Chapter 20

Balancing Equations by the Method of Half-Reactions: Acidic Summary 1. Divide equation into two incomplete half-reactions.

2. Balance each half-reaction (a) balance elements other than H and O.

(b) balance O atoms by adding H require further work at this step).

2 O.

(c) balance H atoms by adding H+ (basic conditions will (d) balance charge by adding e overall positive charge.

to the side with greater CHEM 180/181 Chapter 20

Balancing Equations by the Method of Half-Reactions: Summary 3. Multiply each half-reaction by an integer so that the number of electrons lost in one half-reaction equals the number gained in the other half-reaction.

4. Add the two half-reactions and cancel out all species appearing on both sides of the equation.

5. Check equation to make sure there are same number of atoms of each kind and the same total charge on both sides. Errors can be caught!!

CHEM 180/181 Chapter 20

Balancing Equations by the Method of Half-Reactions: Basic Balancing process is started using H with OH + and H 2 O, then adjusting to uphold reaction conditions (H + does not exist in basic solutions).

Balance the following reaction: H 2 O 2 (aq) + ClO 2 (aq) ClO 2 (aq) + O 2 (g) (basic) CHEM 180/181 Chapter 20

Balancing Equations by the Method of Basic Redox Reactions Half-Reactions: Basic Split into two half-reactions.

H 2 O 2 (aq) O 2 (g) ClO 2 (aq) ClO 2 (aq) Balance elements, then oxygen by adding H 2 O. Then, add H + to balance H, just like an acidic redox reaction.

H 2 O 2 (aq) O 2 (g) + 2H + ClO 2 (aq) ClO 2 (aq) CHEM 180/181 Chapter 20

Basic Redox Reactions Half-Reactions: Basic Add OH to both sides , enough to neutralize all H + reactions cannot support H + ).

2OH + H 2 O 2 (aq) O 2 (g) + 2H ClO 2 (aq) ClO 2 (aq) + + 2OH (basic Combine H 2OH + and OH to form H 2 O.

+ H 2 O 2 (aq) O 2 (g) + 2H 2 O CHEM 180/181 Chapter 20

Basic Redox Reactions Half-Reactions: Basic Balance charge by adding e .

2OH 1e + H 2 O 2 (aq) O 2 (g) + 2H 2 O + 2e + ClO 2 (aq) ClO 2 (aq) Multiply each reaction so both have same e . Then add them together, cancelling where possible.

2OH + H 2 O 2 (aq) + 2ClO 2 (aq) O 2 (g) + 2H 2 O + 2ClO 2 (aq) Double-check your answer!

CHEM 180/181 Chapter 20

Balance this redox equation Cu(s) + NO 3 (aq) Cu 2+ (aq) + NO 2 (g) (acidic) Ans: Cu(s) + 2NO 3 (aq) + 4H + (aq) Cu 2+ (aq) + 2NO 2 (aq) + 2H 2 O(l) Solution posted on notice board.

CHEM 180/181 Chapter 20

Balance this redox equation NO 2 (aq) + Al(s) NH 3 (aq) + Al(OH) 4 (aq) (basic) 2Al(s) + NO 2 (aq) +OH (aq) + 5H 2 O(aq) 2Al(OH) 4 (aq) + NH 3 (aq) Solution posted on notice board.

CHEM 180/181 Chapter 20