4.9 Oxidation-Reduction (Redox) Reactions

Charges of chromium:

2+ and 3+

Charges of copper:

1+ and 2+

Charges of iron:

2+ and 3+

Charges of manganese:

2+ and 4+

Charges of mercury:

1+ and 2+

Charges of lead:

2+ and 4+

Charges of tin:

2+ and 4+

Redox is the:

Transfer of electrons from one element to another

Can oxidation reaction occur without reduction?

No

Why can’t oxidation occur without reduction?

One substance must gain the electrons lost by the other substance

Which types of chemical reactions are redox reactions?

All except for double replacement (precipitation reactions) and neutralization reactions

The substance that undergoes oxidation

Reducing agent

The substance that undergoes reduction

Oxidizing agent

Oxidation occurs when:

An atom undergoes an increase in oxidation number

How does an atom undergo an increase in oxidation number?

By losing electrons

Reduction occurs when:

An atom undergoes a decrease in oxidation number

How does an atom undergo a decrease in oxidation number?

By gaining electrons

Oxidation Numbers are a method of:

Tracking the “ownership” of electrons by atoms

Rules for assigning oxidation numbers:

1. An atom in its elemental form has an oxidation number of 0

2. Any monoatomic ion has the oxidation number equal to the charge on the ion.

3. Oxygen in a compound has an oxidation number of -2 (exceptions are peroxides, O2^2- = -1).

4. Hydrogen in a compound has an oxidation number of +1 (exceptions are hydrides where H = -1).

5. In binary compounds, the more electronegative atom is given the oxidation state of its species (ie. the common ion of its group)

6. Fluorine is always -1.

7. Atoms in groups 1 and 2 are +1 and +2, respectively.

8. The sum of the oxidation state must be equal to the overall charge of the species

Half reactions show:

Either oxidation or reduction

How to construct a balanced redox equation?

By combining half reactions and making sure that the number of electrons that are lost are equal to the number of electrons gained

What is conserved in a balanced redox reaction?

Charge and atoms

2 Na (s) + Cl2 (g) → 2NaCl (s)

What are the oxidation numbers?

Na: 0 to +1

Cl: 0 to -1

CaO (s) → Ca (s) + O2 (g)

What are the oxidation numbers?

Ca: +2 to 0

O: -2 to 0

What is oxidized in single replacement reactions?

Metal

In single replacement reactions, metals are oxidized by:

Acids and salts

How to determine if a single replacement reaction will occur?

By using an activity series table

How to use an activity series table to determine if a single replacement reaction will occur?

If the elemental metal is higher or more reactive than the one it is replacing then a reaction will occur

CH4 + O2 → CO2 + H2O

What are the oxidation numbers?

C: -4 to +4

H: +1 to +1

O: 0 to -2

Methods for balancing equations:

Guess and check method and Red-Ox method

When can the Red-Ox method for balancing equations be used?

When there is a transfer of electrons

Rules for Balancing using the RedOx method: (Standard method)

1) Assign Oxidation Numbers

2) Write Oxidation and Reduction half reactions.

a. Balance Half Reactions for ATOMS

b. Balance Half Reactions for CHARGE by adding electrons.

3) Make electrons lost equal to the electrons gained by multiplying the entire half reactions

4) Put the coefficients into the reaction; then balance the remaining atoms. Do not change the coefficients for the species involved in the redox.

5) Check to make sure that the total charge is the same on both side of the equation.

Rules for Balancing redox reactions in acidic solution:

1) Assign Oxidation Numbers

2) Write Oxidation and Reduction half reactions.

a. Balance Half Reactions for ATOMS

b. Balance Oxygen by adding H2O as needed.

c. Balanced Hydrogen by adding H+ as needed.

d. Balance Half Reactions for CHARGE by adding electrons.

3) Make electrons lost equal to the electrons gained by multiplying the entire half reactions

4) Put the coefficients into the reaction; then balance the remaining atoms. Do not change the coefficients for the species involved in the redox.

5) Check to make sure that the total charge is the same on both side of the equation.

Rules for Balancing redox reactions in basic solution:

1) Complete the balancing process as if it is an acid. Then add OH- to both sides of the equation to form water with all of the hydrogen ions.

2) Cancel as many water molecules as possible.

3) CHECK!!