Reactions of acids with metals

Acids react with some metals to produce salts and hydrogen.

Students should be able to:
- explain in terms of gain or loss of electrons, that these are redox reactions
- identify which species are oxidised and which are reduced in given chemical equations.

Knowledge of reactions limited to those of magnesium, zinc and iron with hydrochloric and sulfuric acids.

Neutralisation of acids and salt production

Acids are neutralised by alkalis (eg soluble metal hydroxides) and bases (eg insoluble metal hydroxides and metal oxides) to produce salts and water, and by metal carbonates to produce salts, water and carbon dioxide.

The particular salt produced in any reaction between an acid and a base or alkali depends on:
- the acid used (hydrochloric acid produces chlorides, nitric acid produces nitrates, sulfuric acid produces sulfates)
- the positive ions in the base, alkali or carbonate.

Students should be able to:
- predict products from given reactants
- use the formulae of common ions to deduce the formulae of salts.

Soluble salts

Soluble salts can be made from acids by reacting them with solid insoluble substances, such as metals, metal oxides, hydroxides or carbonates. The solid is added to the acid until no more reacts and the excess solid is filtered off to produce a solution of the salt.

Salt solutions can be crystallised to produce solid salts.

Students should be able to describe how to make pure, dry samples of named soluble salts from information provided.

The pH scale and neutralisation

Acids produce hydrogen ions (H⁺) in aqueous solutions.

Aqueous solutions of alkalis contain hydroxide ions (OH^-).

The pH scale, from 0 to 14, is a measure of the acidity or alkalinity of a solution, and can be measured using universal indicator or a pH probe.

A solution with pH 7 is neutral. Aqueous solutions of acids have pH values of less than 7 and aqueous solutions of alkalis have pH values greater than 7.

In neutralisation reactions between an acid and an alkali, hydrogen ions react with hydroxide ions to produce water.

This reaction can be represented by the equation:

\(H^+(aq) + OH^-(aq) → H_2O(l) \)

Students should be able to:
- describe the use of universal indicator or a wide range indicator to measure the approximate pH of a solution
- use the pH scale to identify acidic or alkaline solutions.

Titrations

The volumes of acid and alkali solutions that react with each other can be measured by titration using a suitable indicator.

Students should be able to:
- describe how to carry out titrations using strong acids and strong alkalis only (sulfuric, hydrochloric and nitric acids only) to find the reacting volumes accurately
- calculate the chemical quantities in titrations involving concentrations in mol/dm³ and in g/dm³.

Strong and weak acids

A strong acid is completely ionised in aqueous solution. Examples of strong acids are hydrochloric, nitric and sulfuric acids.

A weak acid is only partially ionised in aqueous solution. Examples of weak acids are ethanoic, citric and carbonic acids.

For a given concentration of aqueous solutions, the stronger an acid, the lower the pH.

As the pH decreases by one unit, the hydrogen ion concentration of the solution increases by a factor of 10.

Students should be able to:
- use and explain the terms dilute and concentrated (in terms of amount of substance), and weak and strong (in terms of the degree of ionisation) in relation to acids