Chemical changes flashcards

Topic 4: Chemical changes

Oxidation and reduction in terms of oxygen

Oxidation: gain of oxygen. Reduction: loss of oxygen.

Oxidation and reduction in terms of electronsHigher

Oxidation is loss of electrons; reduction is gain of electrons.

The reactivity series in the specification, most reactive first (with carbon and hydrogen)

Potassium, sodium, lithium, calcium, magnesium, [carbon], zinc, iron, [hydrogen], copper.

How is a metal extracted if it is below carbon, and if it is above carbon?

Below carbon: reduce the oxide by heating with carbon (the oxide is reduced, carbon is oxidised). Above carbon: electrolysis of the molten compound.

Ionic equation for iron displacing copper from copper sulfate. Which species is oxidised?Higher

Fe + Cu²⁺ → Fe²⁺ + Cu (SO₄²⁻ is a spectator ion). Iron loses electrons, so it is oxidised; copper ions are reduced.

Products of acid + metal carbonate, and of acid + alkali

Acid + metal carbonate → salt + water + carbon dioxide. Acid + alkali → salt + water.

Required practical 1: making a soluble salt from an insoluble oxide or carbonate

Warm the acid, add the solid until some is left unreacted (excess), filter off the excess, evaporate some water from the filtrate, leave to crystallise, filter and dry. Excess makes sure all the acid reacts.

Ionic equation for neutralisation

H⁺(aq) + OH⁻(aq) → H₂O(l)

Strong acid vs weak acid, with examplesHigher

Strong: completely ionised in aqueous solution (hydrochloric, nitric, sulfuric). Weak: only partially ionised (ethanoic, citric, carbonic). Strength is not the same as concentration.

A solution changes from pH 5 to pH 3. By what factor does [H⁺] change?Higher

Each fall of 1 pH unit multiplies [H⁺] by 10, so two units → × 100.

Electrolysis of an aqueous solution: rules for the products

Cathode: hydrogen if the metal is more reactive than hydrogen, otherwise the metal. Anode: the halogen if a halide ion is present, otherwise oxygen.

Extracting aluminium: why cryolite, and why the anodes are replaced

Aluminium oxide is dissolved in molten cryolite to lower the melting temperature, so less energy is needed. Oxygen made at the carbon anode reacts with it to form carbon dioxide, so the anode wears away.