1.7.11 Strength of 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 acid & base 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

The diagram shows a digital pH meter that 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

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

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 diagram shows 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 solution

The diagram shows the reaction of 0.1 mol dm^-3 of a weak acid (CH₃COOH) with Mg. The reaction produces less bubbles and hydrogen gas due to the lower concentration of H⁺ present in solution

• 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

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

• The spectator ions which are not involved in the formation of water, form the salt

The diagram shows a neutralisation reaction of HCl and NaOH and the two individual reactions that take place to form the water and salt

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

Acid reacted & salt table