AQA Chemistry Unit 2: 10 Redox Reactions

Redox: Reaction where both reduction and oxidation take place

Oxidation: The gain of oxygen, loss of hydrogen and loss of electrons

Reduction: The loss of oxygen, gain of hydrogen and gain of electrons

Oxidising Agent: Causes oxidation, causes gain of oxygen and loss of hydrogen and electrons. The agent itself is reduced in the reaction

Reducing Agent: Causes reduction, causes loss of oxygen and gain of hydrogen and electrons. The agent itself is oxidised in the reaction

• Half equations are used to show the gain or loss of electrons
• In a reaction if an ion takes no part in the actual reaction it is known as a spectator ion
• In a chemical reaction if one species is oxidised another must be reduced

Oxidising and Reducing Agents

• Reducing agents give away electrons so are electron donors
• Oxidising agents accept electrons
• Reducing agentscauses an increase in oxidation number thus their oxidation number decreases
• Oxidising agents cause a decrease in oxidation number thus their oxidation number increases

• Oxidation State: The number of electrons lost/gained by an atom in a compound compared to the uncombined atom
• It is the charge on a simple ion
• For an atom in a group which makes an ion it is the charge it would have if all other atoms were removed
• The more electronegative an element the more negative its oxidation state

Rules

• Every uncombined element has an oxidation state of zero
• A positive number shows the element has lost electrons so has been oxidised
• A negative number shows the element has gained electrons so has been reduced
• The sum of all oxidation states in a compound is zero
• The sum of oxidation states in an ion equals the charge on the ion
• The most electronegative element has a negative oxidation state

Uncombined elements: Zero

Oxygen in compounds: -2

Oxygen in peroxides: -1

Hydrogen in compounds: +1

Hydrogen in metal hydrides: -1

Group 1 metals: +1

Group 2 metals: +2

Group 6: -2

Group 7: -1

• An equation for a redox reaction can be split into 2 equations
• One shows oxidation and the other shows reduction

Rules:

• Only one element in a half equation changes
• It must balance for atoms
• It must balance for charge

Additional Rules for Aqueous Solutions

• Water provides a source of oxygen
• Any surplus oxygen is converted to water by a reaction with acidic hydrogen ions

• Multiply whole half equations so the number of electrons given by the oxidised species equals the number needed by the species being reduced
• No electrons therefore appear in any overall redox equation

Strategy:

• Build the first half equation
• Build the second half equation
• Multiply (so no electrons are equal) and combine
• Cancel species like H+ and H20 that appear on both sides of the redox equation