9.2 – Redox Equations

9.2 – Redox Equations

9.2.1 – Deduce simple oxidation and reduction half-equations given the species involved in

a redox reaction

 

The half equations break up the whole equation to show what is happening with each element or molecule. Since one element or molecule will be oxidised while the other is reduced, one half equation will show reduction and the other will show oxidation.

Using these half-equations, it is easy to see where the electron transfer is taking place, and thus determine which species is undergoing reduction or oxidation. The equations must be balanced for the number of electrons being transferred.

9.2.2 – Deduce redox equations using half-equations

You may be given two half equations for a complete reaction and be asked to use them to find the whole equation. The individual half-equations allow us to see what is happening to each substance, but can be put back together to give the whole equation. In doing so, the electrons present in these equations will cancel out.

The Seven Steps to Deducing Redox Equations

1. Identify what is oxidised and what is reduced, and to what.
2. Balance for elements other than hydrogen and oxygen
3. Balance for oxygen by adding water (H₂O) to the side that is deficient in oxygen
4. Balance for hydrogen by adding H⁺ to the side that is deficient in hydrogen
5. Balance for charge by adding electrons (e^–) to the more positive side of the equation
6. Equate the electrons lost and gained by multiplying by the small whole integer
7. Add the two half equations, cancelling the electrons, H₂O and H⁺ if possible. All the e^–should cancel out.

These reactions often take place in acidified solutions, accounting for the presence of H⁺ions.

Example:

 

 

 

 

 

 

 

 

 

 

 

 

9.2.3 – Define the terms oxidising agent and reducing agent

Oxidising Agent – A substance that causes another reactant to be oxidised

Reducing Agent – A substance that causes another reactant to be reduced

 

9.2.4 – Identify the oxidising and reducing agents in redox equations

Looking at a redox equation, you will need to determine what is happening to the electrons. The species that loses electrons is the reducing agent, whilst the species that gains electrons is the oxidising agent.

Species being reduced = Oxidising agent

Species being oxidised = Reducing agent

In the example before, the following reaction was used:

In this equation, zinc is oxidised and copper is reduced. Therefore, zinc is the reducing agent and copper is the oxidising agent.