The use of oxidising agents as antiseptics, bleaches and in rocket fuels, the action of oxidising agents in killing bacteria and breaking down coloured compounds, redox titrations, and combining half-equations to give a balanced redox equation.

What this key area is asking

The SQA wants you to describe everyday and industrial uses of oxidising agents, explain how they kill bacteria and remove colour, carry out and calculate a redox titration, and combine ion-electron half-equations into a balanced redox equation. This key area reuses the redox skills from Area 1 and applies them in titration calculations, so the half-equation and $n = CV$ methods are central.

Uses of oxidising agents

How oxidising agents work

Oxidising agents kill bacteria and other pathogens by oxidising key molecules inside the cells, which is why they make effective antiseptics. They act as bleaches by oxidising coloured compounds, breaking down the structures responsible for colour (and often smell), so stains and dyes are removed.

Redox titrations

A redox titration finds the concentration of an oxidising or reducing agent by reacting it with a solution of known concentration. The end point is often shown by a colour change of the reagent itself (for example, acidified permanganate changes from purple to colourless, and iodine with a starch indicator changes from blue-black to colourless). From the volume needed and the balanced redox equation, you calculate the unknown concentration with $n = CV$.

Combining half-equations

As in the structure key area, build the overall equation by combining the oxidation and reduction half-equations:

• multiply each half-equation so the electrons lost equal the electrons gained,
• add them and cancel the electrons,
• check the atoms and the charge balance.

This balanced equation gives the mole ratio used in the titration calculation.

Worked example: a vitamin C redox titration

Examples in context

Redox chemistry is everywhere in daily life. Hydrogen peroxide is used to bleach hair and to disinfect wounds, oxidising the pigments in hair and the molecules in microbes respectively. Chlorine-based bleaches whiten clothes and sanitise drinking water by the same electron-grabbing action. The redox titration method above is used commercially to check the vitamin C content of fruit juices and supplements, and a closely related iodine titration measures the level of dissolved oxygen in rivers and lakes. In rocketry, concentrated hydrogen peroxide and other oxidisers carry their own oxygen so a fuel can burn in the vacuum of space, the same principle that lets a sparkler burn after being lit.

Try this

Q1. State one everyday use of an oxidising agent and explain how it works. [2 marks]

• Cue. As an antiseptic, it kills bacteria by oxidising molecules in their cells.

Q2. Why does acidified permanganate make a good indicator in a redox titration? [1 mark]

• Cue. It changes colour from purple to colourless at the end point, so no separate indicator is needed.

Q3. $25.0 \text{ cm}^3$ of vitamin C reacts with $18.0 \text{ cm}^3$ of $0.0100 \text{ mol l}^{-1}$ iodine ($1 : 1$ ratio). Calculate the vitamin C concentration. [3 marks]

• Cue. $n(I_2) = 0.0100 \times 0.0180 = 1.80 \times 10^{-4}$ mol $= n(C_6H_8O_6)$; $C = 1.80 \times 10^{-4} / 0.0250 = 7.20 \times 10^{-3} \text{ mol l}^{-1}$.