Oxidation, Reduction and Redox Equations: Half-equations

Question 1

What is a half-equation?

Answer 1

• An ionic equation that shows oxidation or reduction - one half of a redox equation

Question 2

How do you write a half-equation from a given formula?

Answer 2

• Write equation as an ionic equation (this shows just the ions which are reacting and ignores any spectator ions which don’t change)
• Write a half-equation for each of the remaining elements
• The donated/accepted electrons should cancel out when combining both half-equations

Question 3

Has a substance been oxidised or reduced if there has been an increase in the oxidation state?

Answer 3

• Increase in oxidation state is oxidation

Question 4

Has a substance been oxidised or reduced if there has been an decrease in the oxidation state?

Answer 4

• Decrease in oxidation state means reduction

Question 5

How do you write a half-equation for conversion formulas?

Answer 5

• Write down formula with reactants and products
• Balance the atoms undergoing redox
• Balance any oxygen atoms by adding water
• Balance any hydrogen atoms by adding H+ ions
• Balance the charge by adding electrons
• The total number of electrons transferred will be equal to the total change in oxidation state

Question 6

How do you combine half-equations? These questions ask for an overall reaction, stating reactants and products and the change in their oxidation states

Answer 6

• Write half-equations for each formula and balance using water, H+s and electrons if needed
• Multiply formulas so number of electrons in both half-equations cancels out (is equal)
• Combine the half-equations