Redox Experiments

A key part of the assessment of this unit is to carry out some experiments and infer what is happening. We then need to write balanced half-equations and overall equations, as well as identifying what is acting as the oxidant and what is the reductant.

Some ions are very difficult to identify due to their colours. For example, both Fe³⁺ and Cr₂O₇^2- are orange; Fe²⁺ is such a pale green that it often looks colourless. We need to use some other reagents/tests to confirm what has been produced:

Oxidation Numbers

Oxidation Numbers are a very convenient way to determine what has been oxidised and what has been reduced, particularly if you don't have half-equations (for example, if you have an overall equation).

There are some simple rules for assigning oxidation numbers:

1. All elements = 0   e.g. Fe, O₂
2. Monatomic ions = their charge   e.g. Fe²⁺ = +2, Fe³⁺ = +3
3. Oxygen (in a compound or ion) = -2, except in hydrogen peroxide (H₂O₂) = -1
4. Hydrogen (in a compound or ion) = +1, except in a metal hydride = -1 e.g. LiH
5. The sum of all oxidation numbers in a compound = 0
6. The sum of all oxidation numbers in a polyatomic ion = its charge

Here is the video from 2013, which covers it in a slightly different way: