AQA · A-Level · Chemistry
Course version: 7405 · Version from Sept 2016
Weak acids and weak bases dissociate only slightly in aqueous solution.
Full specification wording
Weak acids and bases, Ka, for weak acids (A-level only): Weak acids and weak bases dissociate only slightly in aqueous solution.
A weak acid ionises only slightly, so the equilibrium CH₃COOH ⇌ H⁺ + CH₃COO⁻ lies well to the left and typically fewer than two molecules in a hundred have donated a proton at any moment.
Checked against the AQA 7405 specification (opens in a new tab) on .
- Point 1 of 2
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
A weak acid ionises only slightly, so the equilibrium CH₃COOH ⇌ H⁺ + CH₃COO⁻ lies well to the left and typically fewer than two molecules in a hundred have donated a proton at any moment. That is why 0.100 mol dm⁻³ ethanoic acid has a pH near 2.9 while 0.100 mol dm⁻³ hydrochloric acid, fully dissociated, has a pH of 1.00. A weak base does the same in reverse: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ generates only a small [OH⁻]. Strength and concentration are separate ideas, so a concentrated weak acid can have a lower pH than a dilute strong one. Because the total amount of acid is unchanged, a weak acid still neutralises the same volume of alkali as a strong acid of equal concentration; only the starting pH and the reaction rate differ.
What earns the marks
- Stating that only a small proportion of the molecules dissociate, so the equilibrium lies to the left.
- Writing the dissociation with an equilibrium sign, such as CH₃COOH ⇌ H⁺ + CH₃COO⁻.
- Comparing two acids by the initial rate of gas evolution with a carbonate, the stronger acid giving the steeper curve.
- The same final volume or mass loss, because the amount of acid rather than its strength fixes the yield.
- Explaining a difference in strength by the effect of a group on the polarity of the O–H bond.
Common mistakes and how to put them right
Confusing weak with dilute, so 5 mol dm⁻³ ethanoic acid is described as a strong acid.
Using a single arrow in the dissociation equation of a weak acid.
Predicting a smaller final volume of carbon dioxide from the weaker acid, when only the rate differs.
Assuming a weak acid needs a different volume of alkali to neutralise it than a strong acid of the same concentration.
Exam tips
- On a mass-loss or gas-volume graph, strength changes the gradient and amount changes the plateau; check which one the question is testing.
- Always draw the equilibrium arrow for a weak acid; the single arrow is a common and avoidable loss.
Try the lesson
The start of the Weak acids and bases, Ka, for weak acids (A-level only): Weak acids and weak bases dissociate only slightly in aqueous solution. lesson, made from checked teaching notes for this specification point.
Have a go first
Which statement describes a weak acid in aqueous solution?
Listen
A weak acid ionises only slightly, so the equilibrium CH₃COOH ⇌ H⁺ + CH₃COO⁻ lies well to the left and typically fewer than two molecules in a hundred have donated a proton at any moment. That is why 0.100 mol dm⁻³ ethanoic acid has a pH near 2.9 while 0.100 mol dm⁻³ hydrochloric acid, fully dissociated, has a pH of 1.00. A weak base does the same in reverse: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ generates only a small [OH⁻]. Strength and concentration are separate ideas, so a concentrated weak acid can have a lower pH than a dilute strong one. Because the total amount of acid is unchanged, a weak acid still neutralises the same volume of alkali as a strong acid of equal concentration; only the starting pH and the reaction rate differ. A strong answer earns marks for these points. stating that only a small proportion of the molecules dissociate, so the equilibrium lies to the left. writing the dissociation with an equilibrium sign, such as CH₃COOH ⇌ H⁺ + CH₃COO⁻. comparing two acids by the initial rate of gas evolution with a carbonate, the stronger acid giving the steeper curve. the same final volume or mass loss, because the amount of acid rather than its strength fixes the yield. Exam tip: On a mass-loss or gas-volume graph, strength changes the gradient and amount changes the plateau; check which one the question is testing.
Key words
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partial dissociation
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position of equilibrium
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strength versus concentration
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comparing rates
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See every definition
- partial dissociation
- Only a small proportion of the molecules dissociate in aqueous solution, so the equilibrium lies to the left.
- position of equilibrium
- For a weak acid the equilibrium lies well to the left, as typically fewer than two molecules in a hundred have donated a proton at any moment.
- strength versus concentration
- Strength and concentration are separate ideas, so a concentrated weak acid can have a lower pH than a dilute strong one.
- comparing rates
- The stronger acid gives the steeper curve in the initial rate of gas evolution with a carbonate, while the amount of acid fixes the same final volume or mass loss.
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