AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Strong and weak acids (HT only): For a given concentration of aqueous solutions, the stronger an acid, the lower the pH.
Acids release hydrogen ions, H⁺, in aqueous solution.
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Acids release hydrogen ions, H⁺, in aqueous solution. A strong acid, such as hydrochloric acid, is completely ionised, so a 0.1 mol/dm³ solution contains 0.1 mol/dm³ of H⁺ ions. A weak acid, such as ethanoic acid, is only partially ionised, so the same total concentration yields far fewer H⁺ ions. Because pH depends inversely on the concentration of H⁺ ions, the strong acid has the lower pH. For example, 0.1 mol/dm³ hydrochloric acid has a pH of 1, while 0.1 mol/dm³ ethanoic acid has a pH of about 3. The key comparison must be made at equal concentration: a stronger acid means a greater extent of ionisation, more H⁺ ions, and consequently a lower pH value.
What earns the marks
- Acids produce hydrogen ions, H⁺, in aqueous solution.
- A strong acid is completely ionised, whereas a weak acid is only partially ionised.
- At the same total acid concentration, a greater degree of ionisation results in a higher concentration of H⁺ ions.
- As the concentration of H⁺ ions increases, the pH decreases, meaning the stronger acid has the lower pH.
- A valid comparison requires both acids to be at the same concentration, for example comparing 0.1 mol/dm³ hydrochloric acid with 0.1 mol/dm³ ethanoic acid.
Common mistakes and how to put them right
Thinking that a strong acid is simply more concentrated.
Put it right: Strength describes the extent of ionisation, while concentration describes moles of acid per unit volume.
Assuming a weak acid always has a high pH regardless of concentration.
Put it right: The comparison only holds for a given concentration; a concentrated weak acid can still have a fairly low pH.
Stating that a strong acid has a higher pH because it is stronger.
Put it right: A higher concentration of H⁺ ions results in a lower pH value.
Exam tips
- Always state that the two acids are being compared at the same concentration before drawing a conclusion about their pH.
- If asked to explain the difference in pH, write a linked sequence: greater ionisation → higher concentration of H⁺ ions → lower pH.
Try the lesson
The start of the Strong and weak acids (HT only): For a given concentration of aqueous solutions, the stronger an acid, the lower the pH. lesson, made from checked teaching notes for this specification point.
Have a go first
Two aqueous solutions have the same concentration. Solution A contains a strong acid and solution B contains a weak acid. Which statement about the pH of the two solutions is correct?
Listen
Acids release hydrogen ions, H⁺, in aqueous solution. A strong acid, such as hydrochloric acid, is completely ionised, so a 0.1 mol/dm³ solution contains 0.1 mol/dm³ of H⁺ ions. A weak acid, such as ethanoic acid, is only partially ionised, so the same total concentration yields far fewer H⁺ ions. Because pH depends inversely on the concentration of H⁺ ions, the strong acid has the lower pH. For example, 0.1 mol/dm³ hydrochloric acid has a pH of 1, while 0.1 mol/dm³ ethanoic acid has a pH of about 3. The key comparison must be made at equal concentration: a stronger acid means a greater extent of ionisation, more H⁺ ions, and consequently a lower pH value. A strong answer earns marks for these points. Acids produce hydrogen ions, H⁺, in aqueous solution. A strong acid is completely ionised, whereas a weak acid is only partially ionised. At the same total acid concentration, a greater degree of ionisation results in a higher concentration of H⁺ ions. As the concentration of H⁺ ions increases, the pH decreases, meaning the stronger acid has the lower pH. A common slip: Thinking that a strong acid is simply more concentrated. Instead, strength describes the extent of ionisation, while concentration describes moles of acid per unit volume. Exam tip: Always state that the two acids are being compared at the same concentration before drawing a conclusion about their pH.
See it
Key words
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Acid strength and extent of ionisation
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Hydrogen ion concentration and pH
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Comparing strong and weak acids at equal concentration
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See every definition
- Acid strength and extent of ionisation
- A strong acid is completely ionised, whereas a weak acid is only partially ionised.
- Hydrogen ion concentration and pH
- As the concentration of H⁺ ions increases, the pH decreases.
- Comparing strong and weak acids at equal concentration
- At the same total acid concentration, a greater degree of ionisation results in a higher concentration of H⁺ ions, so the stronger acid has the lower pH.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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