(i) a Brønsted–Lowry acid as a species that donates a proton and a Brønsted–Lowry base as a species that accepts a proton (see also 2.1.4 Acids)

(ii) use of the term conjugate acid–base pairs

(iii) monobasic, dibasic and tribasic acids

Learners should be able to identify acid–base pairs in equations for acid–base equilibria.

Theory of acids and bases

the role of H⁺ in the reactions of acids with metals and bases (including carbonates, metal oxides and alkalis), using ionic equations (see also 2.1.4c, 2.1.5e)

(i) the acid dissociation constant, K_a, for the extent of acid dissociation (see also 2.1.4b)

(ii) the relationship between K_a and pK_a

Strong and weak acids

use of the expression for pH as:

pH = –log[H⁺]

[H⁺] = 10^–pH

pH

use of the expression for the ionic product of water, K_w

Ionic product of water

calculations of pH, or related quantities, for:

(i) strong monobasic acids
(ii) strong bases, using K_w

Strong and weak acids

calculations of pH, K_a or related quantities, for a weak monobasic acid using approximations

Approximations for weak acid calculations:

[HA]_equilibrium ≈ [HA]_{undissociated}
i.e. [HA] >> [H⁺]

[H⁺]_equilibrium ≈ [A^–]_equilibrium
i.e. negligible dissociation of H₂O.

Learners will not be required to solve quadratic equations.

Strong and weak acids

limitations of using approximations to K_a related calculations for ‘stronger’ weak acids

Including reasons why
[HA]_equilibrium ≈ [HA]_{undissociated}
may no longer be valid.

Strong and weak acids

a buffer solution as a system that minimises pH changes on addition of small amounts of an acid or a base

Buffer solutions

formation of a buffer solution from:

(i) a weak acid and a salt of the weak acid, e.g. CH₃COOH/CH₃COO^–Na⁺
(ii) excess of a weak acid and a strong alkali, e.g. excess CH₃COOH/NaOH

Buffer solutions

explanation of the role of the conjugate acid–base pair in an acid buffer solution, e.g. CH₃COOH/CH₃COO^–, in the control of pH

Buffer solutions

calculation of the pH of a buffer solution, from the K_a value of a weak acid and the equilibrium concentrations of the conjugate acid–base pair; calculations of related quantities

Buffer solutions

explanation of the control of blood pH by the carbonic acid–hydrogencarbonate buffer system

The H₂CO₃/HCO₃^– buffer is present in blood plasma, maintaining a pH between 7.35 and 7.45.

Buffer solutions

pH titration curves for combinations of strong and weak acids with strong and weak bases, including:

(i) sketch and interpretation of their shapes
(ii) explanation of the choice of suitable indicators, given the pH range of the indicator
(iii) explanation of indicator colour changes in terms of equilibrium shift between the HA and A^– forms of the indicator

No indicator is suitable for a weak acid/weak base titration.
The indicator should be considered as a weak acid, HA.

pH titration curves Acid-base indicators

the techniques and procedures used when measuring pH with a pH meter.

PAG11