Acids, bases and buffers flashcards

Acids, bases and buffers is a central topic in A-Level Chemistry that builds from basic definitions to quantitative calculations. Students must understand the Bronsted-Lowry theory of acids and bases as proton donors and acceptors, and be able to identify conjugate acid-base pairs. The pH scale is logarithmic, so students need confidence using pH = -log[H+] and its reverse. Strong acids and bases dissociate fully, while weak acids reach a partial equilibrium described by Ka and pKa. Buffer solutions resist pH change and are made from a weak acid and its conjugate base; students must be able to explain how they work and calculate their pH. Neutralisation, titration curves and indicators are also key, as is understanding how to calculate the pH at equivalence points and how to choose an appropriate indicator for a given titration.

1. Card 1

   What is a Bronsted-Lowry acid?

   A proton donor. It releases an H+ ion to another species during a reaction.

2. Card 2

   What is a conjugate acid-base pair?

   Two species that differ by a single proton (H+). When an acid donates H+, the species left behind is its conjugate base.

3. Card 3

   How is pH defined mathematically?

   pH = -log[H+], where [H+] is the hydrogen ion concentration in mol dm-3. A lower pH means a higher [H+].

4. Card 4

   What is the ionic product of water, Kw?

   Kw = [H+][OH-] = 1.0 x 10-14 mol2 dm-6 at 25 degrees C. It allows pH calculations for strong bases.

5. Card 5

   How do you calculate the pH of a strong acid?

   Because a strong acid fully dissociates, [H+] equals the acid concentration. Then apply pH = -log[H+].

6. Card 6

   What is Ka and what does a larger Ka value indicate?

   Ka is the acid dissociation constant. A larger Ka means greater dissociation and therefore a stronger weak acid.

7. Card 7

   Write the expression for Ka for a weak acid HA.

   Ka = [H+][A-] / [HA]. The concentration of water is omitted as it is effectively constant.

8. Card 8

   What two assumptions are made when calculating the pH of a weak acid?

   That dissociation is so small that equilibrium [HA] equals initial [HA], and that [H+] from water is negligible.

9. Card 9

   What is a buffer solution and what does it do?

   A buffer solution resists large changes in pH when small amounts of acid or alkali are added to it.

10. Card 10

    How does an acidic buffer resist a rise in pH when alkali is added?

    The added OH- reacts with the weak acid component, which donates H+ to neutralise it, keeping [H+] roughly constant.

11. Card 11

    How does an acidic buffer resist a fall in pH when acid is added?

    The added H+ is removed by the conjugate base (A-), which accepts the proton, preventing a significant drop in pH.

12. Card 12

    How do you choose a suitable indicator for a titration?

    The indicator's pKin must lie within the steep vertical section of the titration curve around the equivalence point.

• Many students confuse a strong acid with a concentrated acid, but strength refers to the degree of dissociation while concentration refers to the amount of solute per unit volume.
• When calculating the pH of a weak acid, students often forget to take the square root of Ka multiplied by the concentration, instead using Ka directly as [H+].
• A common error in buffer questions is stating that a buffer keeps pH constant rather than explaining that it resists large changes in pH, which loses marks in extended answer questions.

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