Redox Reactions

Oxidation Numbers

Oxidation numbers are sometimes known as charges or oxidation states.

For most parts, you can use the groups of the periodic table to tell what the oxidation number is.

• Magnesium ion = +2

• Chlorine ion = -1

Sometimes it can get confusing especially when you have multiple negatively charged atoms and/or transition metals.

• When a transition metal is present, solve the anions first. Transition metals will always be positively charged but can have a variety of oxidation numbers

• When you have more than 1 possible negatively charged atom,

  1. Identify the more electronegative element

  2. The charge of this atom will be following the periodic table

  3. Solve the oxidation states for the remaining atoms using Total charge = Total cation + Total anion

Redox Reactions

A redox reaction is when a reduction reaction and an oxidation reaction happens simultaneously which results in the transfer of electrons between two chemicals.

Oxidation is the reaction in which electrons are lost.

• Mg → Mg⁺² + 2e–

• Zn → Zn⁺² + 2e-

Reduction is the reaction in which electrons are gained.

• Cu⁺² + 2e- → Cu

• O₂ + 4e– → 2O⁻²

A common acronym for this is:

Oxidant and Reductant

An oxidising agent or oxidant enables or causes another chemical to be oxidised.

A reducing agent, or reductant, enables or causes another chemical to be reduced.

Redox reactions always involve an oxidising agent and a reducing agent that react together.

A common acronym for this is:

Oxidants and reductants are derived from the reactants of a redox reaction.

Writing Half Equations

When writing half equations, always use the KOHES method.

K: Balance key elements (anything not H or O)

O: Balance Oxygen by adding H₂O on opposite side

H: Balance Hydrogens by adding H⁺ on opposite side

E: Balance charges by adding electrons on the more positive side

S: Write the correct states

The equations that you come across in Unit 2 are often under the assumption that it is in a acidic solution.

If it is a basic solution, use the KOHES-OH method

K: Balance key elements (anything not H or O)

O: Balance Oxygen by adding H₂O on opposite side

H: Balance Hydrogens by adding H⁺ on opposite side

E: Balance charges by adding electrons on the more positive side

S: Write the correct states

OH: Add OH⁻ to the side that has H⁺ ions. This forms water.

Oxidation Numbers Practice & Solutions

Redox Rules

Writing Redox Reaction Equations Practice & Solutions