5.1.3 Acids, bases and buffers

This sub-topic is divided as follows;

(a,b) Brønsted–Lowry acids and bases

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

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

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

(iii) monobasic, dibasic and tribasic acids

(b) the role of H⁺ in the reactions of acids with metals and bases (including carbonates, metal oxides and alkalis), using ionic equations

(c) Acid Dissociation Constants

(c) (i) the acid dissociation constant, K_a, for the extent of acid dissociation

(ii) the relationship between K_a and pK_a

(d) pH, [H⁺], and K_w

(d) use of the expression for pH as:

pH = –log[H⁺]

[H⁺] = 10^–pH

(e,f) The Ionic Product of Water, K_w

(e) use of the expression for the ionic product of water, K_w

(f) calculations of pH, or related quantities, for:

(i) strong monobasic acids

(ii) strong bases, using K_w

(g) 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.}

(h) 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.}

Buffers: action, uses and calculations

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

(j) formation of a buffer solution from:

(i) a weak acid and a salt of the weak acid, e.g. CH₃COOH/CH₃COONa

(ii) excess of a weak acid and a strong alkali, e.g. excess CH₃COOH/NaOH

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

(l) 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

(m) 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.}

Neutralisation

(n) 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.}

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