Chemistry - Chapter 14 + 15: Oxidation & Reduction // RedOx Titrations

oxidation

the loss of electrons // increase in oxidation number

• addition of oxygen

• removal of hydrogen

reduction

the gain of electrons // decrease in oxidation number

• removal of oxygen

• addition of hydrogen

oxidising agent

a substance that allows oxidation to occur by it itself being reduced (gains electrons)

reducing agent

a substance that allows reduction to occur by it itself being oxidised (losing electrons)

oxidation number

the charge an atom in a molecule would appear to have when the electrons are distributed according to certain rules (arbitrary rules)

Rules for assigning oxidation numbers

• atom in its elemental state is always 0

• ox number of simple ion is the charge on that ion

• group 1 elements lose 1e- → +1

• group 2 lose 2e- → +2

• group 6 gain 2e- → -2

• group 7 gain 1e- → -1

• complex ions - sum of ox numbers of atoms is equal to total charge on that ion

• atom in compound with highest e/n is considered to have gained the electron/s and will therefore have the oxidation number

exceptions to rules

• F will always have -1 in its compounds due to having highest e/n

• metal hydride → H has highest e/n value and will therefore “appear” to have ox number for -1

• peroxides → electrons are shared equally & therefore, oxygen appears to have ox number of -1

redox titrations

1. potassium permanganate (manganate (VII)) // ammonium iron (II) sulfate titration

2. determination of the amount of iron in an iron tablet

3. iodine/thiosulfate titration

4. determination of the percentage (w/v) of hypochlorite in bleach

potassium permanganate // KMnO4 → oxidising agent

• advantage: reacts completely & acts as an indicator

• disadvantages: impure, solution does not keep, affected by lights, must be made fresh and stored in brown bottle, easily oxidised when exposed to air

• standardised using ammonium iron (ii) sulphate

• iron sulphate must be made up with dilute H2SO4 to prevent premature oxidation from Fe2+ to Fe3+ by oxygen in the air → ensures complete reduction

• insufficient H+ will result in the incomplete reduction of Mn7+ to Mn2+ and the formation of Mn4+, a brown precipitate

• ammonium iron (ii) sulphate is a primary standard

precipitate

formation of insoluble compound

sodium thiosulfate - reducing agent

• an impure solid, hence unavailable in a pure storm → needs to be standardised using iodine