AQA Chemistry 5.4.2.5 · Higher only

Strong and weak acids

Strong means completely ionised; concentrated means a lot of acid per dm³. Two different axes. And each pH unit is a factor of ten in hydrogen ion concentration.

Combined Science · Higher tier

Calculates

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Two spills.

Vinegar is an acid. So is the acid inside a car battery.

If you had to guess, which would you rather spill on your hand?

Degree of ionisation

All of it, or only some of it.

A strong acid is completely ionised in aqueous solution: hydrochloric, nitric and sulfuric acids. A weak acid is only partially ionised: ethanoic, citric and carbonic acids. The weak acid’s ionisation is reversible, so its equation has a reversible arrow and most of it stays as molecules.

HCl H⁺ + Cl⁻ strong: complete

CH₃COOH CH₃COO⁻ + H⁺ weak: partial, reversible

For a given concentration, which acid has the lower pH?

Pick one.

Two axes

Strong is not concentrated.

Concentrated and dilute describe the amount of substance per unit volume. Strong and weak describe how completely the acid ionises. Any combination is possible. Place each beaker.

0.001 mol/dm³ hydrochloric acid

Place it.

5 mol/dm³ ethanoic acid

Place it.

10 mol/dm³ sulfuric acid

Place it.

0.01 mol/dm³ citric acid

Place it.

Reacting with excess magnesium: equal volumes of the two acids at the same concentration give the same total volume of hydrogen, because they contain the same amount of acid and the weak acid keeps ionising as its hydrogen ions are used up. The weak acid just fizzes more slowly.

Equation · pH and hydrogen ion concentration

As the pH decreases by one unit, the hydrogen ion concentration of the solution increases by a factor of 10. A difference of n whole units is therefore a factor of 10ⁿ. This is a power, not a product, so there is no triangle: one line.

factor = 10^{(difference in pH)}

The lower pH has the higher concentration. Three units is 10 × 10 × 10 = 1000, not 30.

Think again · dilute and weak

“Add water to hydrochloric acid and it becomes a weak acid.”

Your call

Choose

Hydrochloric acid is diluted with a lot of water. What is it now?

Sort

Which word?

Place each description.

Tap a card, then tap the box it belongs in. Tap a placed card to take it back.

Command words in this lesson

Explain: Why a strong acid has a lower pH than a weak acid of the same concentration.
Describe: The difference between strong and concentrated.
Calculate: How many times the hydrogen ion concentration changes between two pH values.

Key fact

Strong acids ionise completely; weak acids only partially. Same concentration: the strong acid has the lower pH. Each pH unit down is ten times the hydrogen ion concentration.

Key note · AQA 5.4.2.5 · Higher only

Strong and weak acids

A strong acid is completely ionised in aqueous solution (hydrochloric, nitric, sulfuric). A weak acid is only partially ionised (ethanoic, citric, carbonic), written with a reversible arrow.
For a given concentration of aqueous solutions, the stronger an acid, the lower the pH: 0.1 mol/dm³ HCl is about pH 1; 0.1 mol/dm³ CH₃COOH about pH 3.
Concentrated and dilute describe the amount of substance per unit volume. Strong and weak describe ionisation. A dilute strong acid and a concentrated weak acid both exist.
As the pH decreases by one unit, the hydrogen ion concentration increases by a factor of 10. A difference of n units is a factor of 10ⁿ: pH 1 against pH 3 is 100 times.
With excess magnesium, equal volumes of the two acids at the same concentration give the same total hydrogen; the weak acid reacts more slowly.

Photograph this card. Tomorrow, cover it and say each line first.

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Mixing up strong with concentrated?

pH values for 0.1 mol/dm³ acids are approximate: hydrochloric about 1, ethanoic about 3.