ElectrochemistryChapters 18-19

Defining Oxidation and Reduction

• Oxidation – old definition – reacts with oxygen- current definition – loss of electrons

• Reduction - old definition – formation of metals from its compounds

- current definition – gain of electronsZn (s) + Cu2+ (aq) → Cu (s) + Zn2+ (aq)

As the reaction proceeds, the blue color of the Cu2+ solution fades and Cu is deposited on the metal strip.

The Zn strip is in direct contact with the Cu2+ solution and the exchange of electrons between the reactants can occur.

Equation:Zn (s) + CuSO4 (aq) → Cu (s) + ZnSO4 (aq)

Ionic Equation: Zn (s) + Cu2+

(aq) + SO42-

(aq)→ Cu (s) + Zn2+ (aq) + SO4

2-(aq)

Net Ionic Equation: Zn (s) + Cu2+

(aq) → Cu (s) + Zn2+ (aq)

By looking at the charges – there is obviously a change...

Zinc atoms lose electrons undergoes oxidationCopper ions gain electrons undergoes reduction

When both oxidization and reduction occurs in a reaction, it is known as an Oxidation-Reduction Reaction, or a Redox Reaction.

LEO the lion goes GERLoss of Electrons is OxidationGain of Electrons is Reduction

OIL RIGOxidation Is LossReduction Is Gain

In a redox reaction, both reactants have specific names:

Oxidizing Agent – the reactant that oxidizes another reactant.

The oxidizing agent is reduced.

Reducing Agent – the reactant that reduces the other reactant.

The reducing agent is oxidized.

Practice Problems #1-4 Page 715

Half Reactions• To monitor the transfer of electrons in a redox

reaction, you can represent the oxidation and reduction separately.

• A half-reaction is a balanced equation that shows the number of electrons involved in either oxidation or reduction.

• It requires two half-reactions to represent a redox reaction. One that shows oxidation; one that shows reduction.

Redox Reaction:Zn (s) + Cu2+

(aq) → Cu (s) + Zn2+ (aq)

Zn (s) is oxidized Cu2+(aq) is reduced.

Oxidation Half-Reaction: Zn (s) → Zn2+ (aq) + 2 e-

Reduction Half Reaction: Cu2+

(aq) + 2e- → Cu (s)

Note: Each side balances.... We use lowest whole numbers.

There is one special type of redox reaction.

A disproportionation reaction is a redox reaction where one element undergoes both oxidation and reduction.

Ex. 2 Cu+(aq) → Cu (s) + Cu2+

The two half reactions are:Oxidation: Cu+

(aq) → Cu2+ + 1 e-

Reduction: Cu+(aq) + 1 e- → Cu (s)

Practice Problems #5-8 Page 716

Oxidation Numbers

• Many redox reactions involve reactants and products with covalent bonds –▫ Including elements that exist as covalent compounds,

such as O2

▫ covalent compounds, such as H2O▫ Including polyatomic ions that are not spectator ions,

such as MnO4-, NO3

-, etc.• For reactions involving covalent compounds, we

need to use oxidation numbers to determine whether or not a compound has gained or lost electrons.

Oxidation Number – an actual or hypothetical charge that is assigned by a set of rules.

Not all oxidation numbers are integers – you can still use them.

Charges are2-, 3-, 2+

Oxidation Numbers are-2, -3, +4

Practice Problems #9-12 Page 726

Identifying Redox Reactions

• A Redox reaction must contain an oxidation and a reduction.

• If it does not contain both, it is not a redox reaction.

When looking at any reaction,1. Determine the oxidation numbers of all elements.2. Identify any elements with an increase in oxidation number and a decrease in oxidation number.3. If you have one of each – REDOX!!!

Sample Problem Page 727Practice Problems #13-16 Page 728

Balancing Half Reactions

• As redox reactions may not take place under neutral situations, one must consider the H+ and the OH- ions.

• As acidic and basic solutions are different, there are specific steps in balancing both acidic and basic conditions.

Balancing Half-Reactions in Acidic Solution

1. Write an unbalanced half-reaction for the given reactants and products.

2. Balance any atoms except Oxygen and Hydrogen.

3. Balance any Oxygen atoms by adding water molecules.

4. Balance and Hydrogen atoms by adding hydrogen atoms.

5. Balance the charges by adding electrons.

Ex. Write a balanced half-reaction that shows the reduction of permanganate, MnO4

-, to manganese (II) ions in acidic solution.

Step One: MnO4- → Mn2+

Step Two: 1 MnO4- → 1 Mn2+

Step Three: 1 MnO4- → 1 Mn2+ + 4 H2O

Step Four: 1 MnO4- + 8 H+ → 1 Mn2+ + 4 H2O

Step Five 7+ → 2+ So add 5e- to balance

Therefore: 1 MnO4- + 8 H+ + 5e- → 1 Mn2+ + 4 H2O

Practice Problems 17-20 on Page 732

Balancing Half-Reactions For Basic Solutions

1. Write an unbalanced half-reaction for the given reactants and products.

2. Balance any atoms except Oxygen and Hydrogen.3. Balance any Oxygen atoms by adding water

molecules.4. Balance and Hydrogen atoms by adding hydrogen

atoms.5. Adjust for basic conditions... Add the same

number of hydroxide ions to both sides of the equation.

6. Combine and H+ ions with OH- ions to make water.7. Remove any water molecules present on both

sides of the equation.8. Balance the charges by adding electrons.

Write a balanced half-reaction that shows the oxidation of thiosulfate ions, S2O3

2-, to SO32-, in basic solution.

Step One: S2O32- → SO3

2-

Step Two: S2O32- → 2 SO3

2-

Step Three: S2O32- + 3 H2O → 2 SO3

2-

Step Four: S2O32- + 3 H2O → 2 SO3

2- + 6 H+

Step Five: S2O32- + 3 H2O + 6 OH- → 2 SO3

2- + 6 H+ + 6 OH-

Step Six: 6 H2O

Step Seven: S2O32- + 6 OH- → 2 SO3

2- + 3 H2O

Step Eight: 8- → 4-

So add 4e-

Therefore: S2O32- + 6 OH- → 2 SO3

2- + 3 H2O + 4e-

Practice Problems #21-24 Page 734

Half-Reaction Method For Balancing Redox Rxns

1. Write an unbalanced net ionic equation – if it is not already given.

2. Split the unbalanced net ionic equation into its two half-reactions.

3. Balance the oxidation half separately.4. Balance the reduction half separately.5. Determine the Least Common Multiple (LCM) for the

electrons from both half-reactions.6. Multiply the half-reactions by the appropriate numbers to

eliminate the electrons from both sides.7. Add the two half-reactions.8. Remove any identical species from both sides of the

equations.9. Add any spectator ions and any states if necessary.

Sample Problem on Page 737

ProblemsBalance the following redox reactions:

1. Cr (s) + Sn4+(aq) → Cr3+ (aq) + Sn2+ (aq) Acidic

Soln

2. Al (s) + H+(aq) → Al3+ (aq) + H2 (g) Acidic Soln

3. Zn (s) + Ag+(aq) → Zn2+ (aq) + Ag (s) Acidic Soln

4. Br2 (l) + SO32-

(aq) → Br- (aq) + SO42- (aq) Basic

Soln

5. IO3- (aq) + H2S(g)→ I2 (g) + SO3

2- (aq) Basic Soln

6. NO2 (g) + ClO-(aq) → NO3

- (aq) + Cl- (aq) Basic Soln

Do Practice Problems on Page 738-739 #s 25 - 28