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 value of a strong & weak acid table

• 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

The diagram shows a digital pH meter measures the pH of a solution using a pH electrode

• 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

The diagram shows the change in colour of the universal indicator paper when dipped in a strong (HCl) and weak (CH₃COOH) acid. The colour chart is used to read off the corresponding pH values which are between 1-2 for HCl and 3-4 for CH₃COOH

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

The diagram shows a digital conductivity meter that measures the electrical conductivity of a solution using an electrode

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 produces a lot of bubbles and hydrogen gas due to the high concentration of H⁺ present in the solution

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

The reaction produces fewer bubbles and hydrogen gas due to the lower concentration of H⁺ present in the solution

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

Table of salts produced from certain acids