Strength of Acids & Bases (Cambridge (CIE) AS Chemistry): Revision Note

Strong & Weak Acids & Bases

• Strong and weak acids can be distinguished from each other by their:

  • pH value (using a pH meter or universal indicator)

  • Electrical conductivity

  • Reactivity

pH

• An acid dissociates into H⁺ in solution according to:

HA → H⁺ + A^-

• The stronger the acid, the greater the concentration of H⁺ and therefore the lower the pH

pH values of a strong & weak acids

• pH of 0.1 mol dm^-3 solution:

  • HCl (strong); pH 1

  • CH₃COOH (weak); pH 2.0

• The most accurate way to determine the pH is by reading it off a pH meter

• The pH meter is connected to the pH electrode which shows the pH value of the solution

Using a digital pH meter

• A less accurate method is to measure the pH using universal indicator paper

• The universal indicator paper is dipped into a solution of acid, upon which the paper changes colour

• The colour is then compared to those on a chart which shows the colours corresponding to different pH values

How to use universal indicator paper

Electrical conductivity

• Since a stronger acid has a higher concentration of H⁺ it conducts electricity better

• Stronger acids therefore have a greater electrical conductivity

• The electrical conductivity can be determined by using a conductivity meter

• Like the pH meter, the conductivity meter is connected to an electrode

• The conductivity of the solution can be read off the meter

Using a digital conductivity meter

Reactivity

• Strong and weak acids of the same concentrations react differently with reactive metals

• This is because the concentration of H⁺ is greater in strong acids compared to weak acids

• The greater H⁺ concentration means that more H₂ gas is produced

The reaction of 0.1 mol dm^-3 of a strong acid, HCl, with Mg

The reaction of 0.1 mol dm^-3 of a weak acid, CH₃COOH, with Mg

Neutralisation Reactions

• A neutralisation reaction is one in which an acid (pH <7) and a base/alkali (pH >7) react together to form water (pH = 7) and a salt:

acid + base (alkali) → salt + water

• The proton of the acid reacts with the hydroxide of the base to form water:

H⁺ (aq) + OH^– (aq) → H₂O (l)

• The spectator ions which are not involved in the formation of water are Na⁺ (aq) + Cl^– (aq)

  • These react to form the salt:

Na⁺ (aq) + Cl^– (aq) → NaCl (aq)

• The name of the salt produced can be predicted from the acid that has reacted

Salts produced from certain acids

• Hydrochloric acid forms chloride salts

• Sulfuric acid forms sulfate salts

• Nitric acid forms nitrate salts

• Ethanoic acid ethanoate salts