DP Chemistry (first assessment 2025)

Question 19N.2.SL.TZ0.4

Date	November 2019	Marks available	[Maximum mark: 5]	Reference code	19N.2.SL.TZ0.4
Level	SL	Paper	2	Time zone	TZ0
Command term	Apply, Deduce, Identify, Justify, Outline, State	Question number	4	Adapted from	N/A

[Maximum mark: 5]

19N.2.SL.TZ0.4

A molecule of citric acid, C₆H₈O₇, is shown.

The equation for the first dissociation of citric acid in water is

C₆H₈O₇ (aq) + H₂O (l) $\rightleftharpoons$ C₆H₇O₇⁻ (aq) + H₃O⁺ (aq)

(a(i))

Identify a conjugate acid–base pair in the equation.

[1]

Markscheme

C₆H₈O₇ AND C₆H₇O₇⁻
OR
H₂O AND H₃O⁺ ✔

(a(ii))

The value of the equilibrium constant for the first dissociation at 298 K is 5.01 × 10⁻⁴.

State, giving a reason, the strength of citric acid.

[1]

Markscheme

weak acid AND partially dissociated
OR
weak acid AND equilibrium lies to left
OR
weak acid AND K_c/K_a<1 ✔

(a(iii))

The dissociation of citric acid is an endothermic process. State the effect on the hydrogen ion concentration, [H⁺], and on the equilibrium constant, of increasing the temperature.

[2]

Markscheme

(b)

Outline one laboratory methods of distinguishing between solutions of citric acid and hydrochloric acid of equal concentration, stating the expected observations.

[1]

Markscheme

Any one of:
«electrical» conductivity AND HCl greater ✔
pH AND citric acid higher ✔
titrate with strong base AND pH at equivalence higher for citric acid ✔
add reactive metal/carbonate/hydrogen carbonate AND stronger effervescence/faster reaction with HCl ✔
titration AND volume of alkali for complete neutralisation greater for citric acid ✔
titrate with strong base AND more than one equivalence point for complete neutralisation of citric acid ✔
titrate with strong base AND buffer zone with citric acid ✔

NOTE: Accept “add universal indicator AND HCl more red/pink” for M2.

Accept any acid reaction AND HCl greater rise in temperature.

Accept specific examples throughout.

Do not accept “smell” or “taste”.

Syllabus sections

Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.1—Proton transfer reactions » Reactivity 3.1.2—A pair of species differing by a single proton is called a conjugate acid–base pair. Deduce the formula of the conjugate acid or base of any Br.nsted–Lowry base or acid.

Reactivity 3. What are the mechanisms of chemical change? » Reactivity 3.1—Proton transfer reactions » Reactivity 3.1.6—Strong and weak acids and bases differ in the extent of ionization. Recognize that acid–base equilibria lie in the direction of the weaker conjugate. and are strong acids, and group 1 hydroxides are strong bases.

Reactivity 2. How much, how fast and how far? » Reactivity 2.3—How far? The extent of chemical change » Reactivity 2.3.2—The equilibrium law describes how the equilibrium constant, K, can be determined from the stoichiometry of a reaction. Deduce the equilibrium constant expression from an equation for a homogeneous reaction.

Reactivity 2. How much, how fast and how far? » Reactivity 2.3—How far? The extent of chemical change » Reactivity 2.3.4—Le Châtelier’s principle enables the prediction of the qualitative effects of changes in concentration, temperature and pressure to a system at equilibrium. Apply Le Ch.telier’s principle to predict and explain responses to changes of systems at equilibrium.

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Question 19N.2.SL.TZ0.4

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