⚡ Strong acids
Every acid molecule splits into ions. Hydrochloric acid, nitric acid and sulfuric acid are strong acids.
HCl → H+ + Cl-
Reactions of Acids » Strong and Weak Acids
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.4.2.6 (Higher tier only)
- The difference between a strong acid and a weak acid
- The difference between dilute and concentrated
- Why strength and concentration are not the same thing
- How a change of one pH unit changes the hydrogen ion concentration
Higher tier only
This whole lesson is for students sitting the Higher tier paper. Foundation tier students do not need it.
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Unlock This CourseStrong acids and weak acids
Vinegar's ethanoic acid is a weak acid - only a small fraction of its molecules ionise in water
When an acid dissolves in water, its molecules can split up into ions. This is called ionisation. The ions that make a solution acidic are hydrogen ions, H+. Acids do not all ionise by the same amount, and that is what "strong" and "weak" describe.
Key terms:
- Ionised: split up into ions when dissolved in water.
- Strong acid: an acid that is completely ionised in aqueous solution.
- Weak acid: an acid that is only partially ionised in aqueous solution.
🔋 Weak acids
Only some of the acid molecules split into ions. Ethanoic acid, citric acid and carbonic acid are weak acids.
Most molecules stay whole. Only a few make H+ ions.
In ethanoic acid, most of the molecules stay as whole molecules. Only a small number turn into ions.
Imagine 100 acid molecules in water. In a strong acid, all 100 split into ions. In a weak acid, perhaps only a few of them do. The weak acid has far fewer hydrogen ions.
Strength and pH
More hydrogen ions means a lower pH. So for solutions of the same concentration, the stronger the acid, the lower the pH.
For example, take 0.1 mol/dm3 hydrochloric acid and 0.1 mol/dm3 ethanoic acid. Both have the same amount of acid dissolved. But hydrochloric acid has pH 1, while ethanoic acid has a pH of about 3. The strong acid has many more H+ ions.
Dilute and concentrated
Concentrated just means lots of acid in the volume - it says nothing about how strong the acid is
"Strong" and "weak" are about how much the acid ionises. "Dilute" and "concentrated" are about how much acid is dissolved. These are two different ideas.
- Concentrated: a large amount of acid in a given volume of solution.
- Dilute: a small amount of acid in a given volume of solution.
Here "amount" means amount of substance, measured in moles. A concentrated solution has lots of moles of acid in each dm3.
🔬 Strength
Describes the degree of ionisation. It is a property of the type of acid.
🧪 Concentration
Describes the amount of acid in a given volume. You change it by adding more acid or more water.
This means you can have all four combinations:
- Concentrated strong acid: lots of acid, all ionised.
- Dilute strong acid: a little acid, all ionised.
- Concentrated weak acid: lots of acid, only partly ionised.
- Dilute weak acid: a little acid, only partly ionised.
Adding water to an acid makes it more dilute. It does not make a strong acid weak. The acid is still completely ionised.
Common mistakes
Do not say a weak acid is a "dilute" acid, or that a strong acid is a "concentrated" one. A very dilute solution of hydrochloric acid is still a strong acid. A concentrated solution of ethanoic acid is still a weak acid. Always use "strong" and "weak" for ionisation, and "concentrated" and "dilute" for the amount dissolved.
The pH and hydrogen ion concentration
The pH number tells you the hydrogen ion concentration. The link is the key idea in this lesson:
The rule
As the pH decreases by one unit, the hydrogen ion concentration increases by a factor of 10.
So a solution with pH 3 has ten times more H+ ions than a solution with pH 4. The scale goes up in powers of ten, not in steps of one.
- pH 7 is neutral.
- pH 6 has 10 times the hydrogen ion concentration of pH 7.
- pH 5 has 100 times the concentration of pH 7.
- pH 4 has 1000 times the concentration of pH 7.
When the pH goes up by one unit, the hydrogen ion concentration goes down by a factor of 10. These rules work for whole number pH values.
Worked example 1
Solution A has pH 2. Solution B has pH 5. How many times greater is the hydrogen ion concentration in A?
Step 1: the difference in pH is 5 − 2 = 3 units.
Step 2: each unit is a factor of 10, so 10 × 10 × 10 = 1000.
Answer: A has 1000 times the hydrogen ion concentration of B.
Worked example 2
A strong acid has pH 1. It is diluted by adding water until its hydrogen ion concentration is 100 times smaller. What is the new pH?
A factor of 100 is 10 × 10, so the pH goes up by 2 units.
Answer: the new pH is 3.
Putting it together
Strength and concentration both affect the pH. Diluting a strong acid ten times raises its pH by 1, because the hydrogen ion concentration falls by a factor of 10. A weak acid has a higher pH than a strong acid of the same concentration, because fewer of its molecules ionise.
You can investigate this by measuring the pH of different acids at different concentrations with a pH probe or universal indicator. Start with the most dilute solution and rinse the probe between solutions.
Common mistakes
Saying that pH 3 has "one more" hydrogen ion than pH 4. It has ten times the concentration. Also, do not say a weak acid "has no H+ ions". It has some, just fewer than a strong acid of the same concentration.
Exam-style question
Hydrochloric acid and citric acid solutions have the same concentration. The hydrochloric acid has pH 1 and the citric acid has pH 3.
(a) Explain, in terms of ionisation, why the pH values are different. (2 marks)
(b) How many times greater is the hydrogen ion concentration in the hydrochloric acid? (1 mark)
Model answer
(a) Hydrochloric acid is a strong acid, so it is completely ionised in solution. Citric acid is a weak acid, so it is only partially ionised. The hydrochloric acid therefore has a higher concentration of hydrogen ions, so it has a lower pH.
(b) The pH differs by 2 units, so the factor is 10 × 10 = 100 times greater.
Exam tip
If a question asks you to explain strength, always use the word "ionised" and say "completely" or "partially". Never use "concentrated" or "dilute" for strength.