Edexcel International A Level Chemistry

Topic 14: Acid-base Equilibria

Two acids at the same concentration can have very different pH. This topic puts numbers on that difference with pH, Ka and Kw, then uses titration curves and buffers to control and measure it.

The one idea

Take 0.100 mol dm⁻³ hydrochloric acid and 0.100 mol dm⁻³ ethanoic acid. Each needs exactly the same amount of sodium hydroxide to neutralise it. Yet the hydrochloric acid has pH 1.00 and the ethanoic acid has pH 2.88, so the hydrochloric acid has about 76 times as much H⁺ in it.

Concentration says how much acid is dissolved. Strength says how much of it has handed over its protons. This topic puts a number on strength, the acid dissociation constant $K_\text{a}$, and then uses it. It is the equilibrium constant from the last topic, applied to water.

Three ideas carry most of the marks:

1. The pH of a weak acid. Two assumptions turn $K_\text{a}$ into a square root, and the same assumptions run backwards to find $K_\text{a}$ from a measured pH.
2. Titration curves. The shape tells you the strengths of the acid and base, and the vertical section tells you which indicator works.
3. Buffers. A weak acid and its conjugate base resist change in pH. You can calculate their pH, make one to order, and find $K_\text{a}$ from the pH at half-neutralisation.

The three jobs

What you need first

• Dynamic equilibrium and $K_\text{c}$ from the equilibria topics. $K_\text{a}$ is a $K_\text{c}$, so you must be able to write an expression from an equation.
• Moles and concentration: n = c × V, with the volume in dm³.
• Titration technique and calculations from Unit 1.
• Logarithms on your calculator: the lg (or log) button, and the 10^x button for the reverse.

Values used in these notes

Where a question gives a value of $K_\text{a}$ or $K_\text{w}$, use the one given. Data labelled illustrative are invented to show a typical result and are not measurements.

How to use these notes

• Read the pages in order the first time. Later pages assume the earlier ones.
• Each page opens with its one idea in a sentence. If you can explain that sentence to someone else, you know the page.
• Coloured boxes mean the same thing on every page: definitions you may be asked to quote, key ideas, common mistakes, exam technique, and beyond the spec (interesting, but you will not be examined on it).
• Each page ends with questions and fully worked answers. Cover the answers, try the questions, then mark yourself.
• Use the topic on a page the night before an exam.

Pages

1. Acids, bases and conjugate pairs

   An acid gives away a proton and a base takes one, so every acid-base reaction is a proton transfer, and every reaction contains two conjugate pairs.

   12 min 14.114.2

2. pH and hydrogen ion concentration

   pH is the negative logarithm of the H⁺ concentration. Each pH unit is a factor of ten, and you convert both ways with the lg button and its inverse.

   13 min 14.314.414.5

3. Strong and weak acids

   A strong acid is completely dissociated and a weak acid only partly, so strength is not concentration. For a strong acid the pH comes straight from the concentration.

   13 min 14.614.7

4. The acid dissociation constant

   Ka is the equilibrium constant for a weak acid splitting into ions. A larger Ka means a stronger acid, and pKa turns it into a small number.

   12 min 14.814.12

5. pH of a weak acid

   Two assumptions turn the Ka expression into a square root, so you can find the pH of a weak acid without solving a quadratic.

   14 min 14.9

6. Ka from a measured pH

   Run the weak-acid calculation backwards. A known mass of acid and a measured pH give the concentration, then [H⁺], then Ka.

   13 min 14.14

7. Water and strong bases

   Water ionises very slightly, and the product of [H⁺] and [OH⁻] is the constant Kw. That constant turns the concentration of a strong base into a pH.

   14 min 14.1014.1114.12

8. Measuring pH and dilution

   Two experiments test the ideas so far. Equimolar solutions show how strength and salts change pH, and dilution by ten, a hundred and a thousand times tells a strong acid from a weak one.

   15 min 14.13(i)14.13(ii)14.6

9. Reading a titration curve

   A titration curve plots pH against the volume added. Its start, its steep section and its end all come from the amounts of acid and base, and you can calculate any point on it.

   14 min 14.15

10. Four monoprotic curves

    Strong or weak acid, strong or weak base, gives four different curves. The start pH, the equivalence pH and the size of the vertical section all depend on which partner is weak.

    15 min 14.15

11. Diprotic acids and bases

    A diprotic acid gives up two protons in two steps, so its titration has up to two equivalence points. The second is at exactly twice the volume of the first.

    14 min 14.15

12. Choosing an indicator

    An indicator changes colour over a range of pH. It gives a sharp end point only if that range lies inside the vertical section of the titration curve.

    13 min 14.16

13. What a buffer is and how it works

    A buffer holds a large reserve of a weak acid and its conjugate base. Added H⁺ is absorbed by the base, added OH⁻ by the acid, so the pH hardly moves.

    13 min 14.1714.18

14. pH of a buffer

    The pH of a buffer depends on Ka and on the ratio of the weak acid to its conjugate base. Rearrange the Ka expression, put in the amounts, and take the log.

    15 min 14.19

15. Making a buffer of a given pH

    Run the buffer calculation backwards. The target pH sets the ratio of conjugate base to acid, and the ratio sets the amounts you must mix.

    13 min 14.20

16. Buffers on the titration curve

    The flat part of a weak acid curve is a buffer. Halfway to equivalence the weak acid and its conjugate base are equal, so the pH equals the pKa and gives Ka.

    15 min 14.21(i)14.21(ii)

17. Buffers in blood, cells and food

    Enzymes and proteins only work in a narrow pH range, and microorganisms spoil food by changing its pH. Buffers keep the pH steady in both.

    10 min 14.22(i)14.22(ii)

18. Core Practical 11: finding the Ka of a weak acid

    Find the equivalence volume by titration, then add exactly half of it to fresh acid and measure the pH. At that point pH = pKa, so Ka = 10 to the power minus pH.

    15 min 14.2314.21(ii)

Also in this topic: Topic 14 on a page

Spec checklist

Every point you can be examined on in this topic, and where it is explained. Tick them off as you go.