Chapter 5 – Analysing Oxidants & Reductants Week 3, Lesson 1 Chapter 5 – Analysing Oxidants & Reductants

Redox Reactions Redox reactions involve the transfer of electrons. Two half equations occur during a redox reaction. One reactant loses electrons – this is oxidation. One reactant gains electrons – this is reduction. These two processes occur simultaneously.

Redox Reactions Substances that cause oxidation are called oxidants, or oxidising agents. While, substances that cause reduction are called reductants, or reducing agents.

Oxidation Numbers To determine whether a reaction is a redox reaction, oxidation numbers are assigned to elements involved in the reaction. These numbers are determined by applying a set of rules that treat molecular substances as if they were composed of ions. Oxidation numbers have no physical meaning. Oxidation numbers are also known as oxidation states.

Oxidation Number Rules Free elements have an oxidation number of 0. In ionic compounds composed of simple ions, the oxidation number is the same as the charge on that ion. Some elements have oxidation numbers that are regarded as fixed, except in a few exceptional circumstances: Oxygen usually takes -2 in compounds, except in peroxides it is -1. Hydrogen takes +1 in compounds, except in metal hydrides if is -1. The sum of the oxidation number in a neutral compound is zero. In a polyatomic ion the sum of the oxidation numbers is equal to the charge on that ion. The most electronegative element in a compound has the negative oxidation number.

Variable Oxidation Number In all their compounds, the group I metals only have an oxidation number of +1 and the group II metals only have an oxidation number of +2. The oxidation numbers of transition metals vary depending on the compound. The oxidation numbers of non-metallic elements may also vary from compound to compound.

Changing Oxidation Numbers Once oxidation numbers are assigned to the elements involved in a chemical reaction, we can look for a change in these numbers during a reaction. An increased oxidation number means the element has been oxidised. A decreased oxidation number means the element has been reduced.

Writing Half Equations Half equations that involve atoms or simple ions can be written with little difficulty, as you just need to add electrons to the appropriate side. Half equations involving polyatomic ions is a little more difficult. The process is as follows: Balance all elements except hydrogen and oxygen in the half equation. Balance the oxygen atoms by adding water. Balance the hydrogen atoms by adding H+ ions. Balance the charge by adding electrons.

Volumetric Analysis This process can also be used to find the concentration of solutions of substances in a redox reaction. Instead of reacting an acid with a base, a redox titration involves reaction of an oxidant with a reductant. To find the concentration, the same process is followed as in an acid-base reaction.