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Date	November 2019	Marks available	1	Reference code	19N.2.hl.TZ0.4
Level	HL	Paper	2	Time zone	TZ0
Command term	Apply and Identify	Question number	4	Adapted from	N/A

Question

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)

Identify a conjugate acid–base pair in the equation.

[1]

a(i).

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

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

[1]

a(ii).

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

[2]

a(iii).

Calculate the standard Gibbs free energy change, $\Delta {G^\theta }$ , in kJ mol⁻¹, for the first dissociation of citric acid at 298 K, using section 1 of the data booklet.

[1]

a(iv).

Comment on the spontaneity of the reaction at 298 K.

[1]

a(v).

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

[2]

Markscheme

C₆H₈O₇ AND C₆H₇O₇⁻
OR
H₂O AND H₃O⁺ ✔

a(i).

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

a(ii).

a(iii).

« $\Delta {G^\theta }$ = −RT ln K = −8.31 J K^–1 mol^–1 × 298 K × ln(5.01 × 10^–4) ÷ 1000 =» 18.8 «kJ mol^–1» ✔

a(iv).

non-spontaneous AND $\Delta {G^\theta }$ positive ✔

a(v).

Any two 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”.

Examiners report

[N/A]

a(i).

[N/A]

a(ii).

[N/A]

a(iii).

[N/A]

a(iv).

[N/A]

a(v).

[N/A]

Syllabus sections

Core » Topic 8: Acids and bases » 8.1 Theories of acids and bases

Show 47 related questions

18M.1.sl.TZ1.19:
Which classification is correct for the reaction?

H₂PO₄⁻(aq) + H₂O(l) → HPO₄²⁻(aq) + H₃O⁺(aq)
22M.2.hl.TZ1.4a: State the relationship between NH4+ and NH3 in terms of the Brønsted–Lowry theory.
18N.2.hl.TZ0.6a.i:
State the equation for the reaction of each substance with water.
17N.2.sl.TZ0.5b.ii: State what is meant by the term conjugate base.
17N.2.sl.TZ0.5b.iii: State the conjugate base of the hydroxide ion, OH–.
17N.2.sl.TZ0.5b.i: Identify two different amphiprotic species in the above reactions.
17M.2.sl.TZ2.7a.ii:
Identify one conjugate acid-base pair in the reaction.
22M.2.sl.TZ2.3c(ii): State the meaning of a strong Brønsted–Lowry acid.
17M.1.sl.TZ1.19: Which is an acid-base conjugate pair? A. H3O+ / OH– B. H2SO4 / SO42– C. ...
17M.1.hl.TZ1.24: Which species produced by the successive dissociations of phosphoric acid, H3PO4,...
16N.1.sl.TZ0.19: Which species behave as Brønsted–Lowry bases in the following reaction? H2SO4 + HNO3...
17M.2.sl.TZ2.7a.i:
Identify the amphiprotic species.
19N.2.sl.TZ0.4a(i): Identify a conjugate acid–base pair in the equation.
18M.1.hl.TZ1.24: What describes HPO42−? A. Amphiprotic but not amphoteric B. Amphoteric but not...
18M.2.hl.TZ1.5c:
Write an equation to show ammonia, NH₃, acting as a Brønsted–Lowry base and a different equation to show it acting as a Lewis base.
22M.2.hl.TZ2.6d(ii): State the meaning of a strong Brønsted–Lowry acid.
18N.2.sl.TZ0.6a:
State the equation for the reaction of each substance with water.
21N.2.sl.TZ0.5a:
Formulate an equation for the reaction of one mole of phosphoric acid with one mole of sodium hydroxide.
21N.2.sl.TZ0.5d: Outline the reason that sodium hydroxide is considered a Brønsted–Lowry base.
17M.1.hl.TZ1.26:
Which species acts as a Lewis and Brønsted–Lowry base?

A. [Al(H₂O)₆]³⁺

B. BF₃

C. NH₄⁺

D. OH⁻
19M.2.sl.TZ2.5a(ii):
The hydrogencarbonate ion, produced in Equilibrium (2), can also act as an acid.

State the formula of its conjugate base.
17M.1.sl.TZ2.20: Which of the following is correct? A. A weak acid is a proton donor and its aqueous...
19N.3.sl.TZ0.1c(i): Justify this hypothesis.
17N.2.sl.TZ0.5c:
A student working in the laboratory classified HNO₃, H₂SO₄, H₃PO₄ and HClO₄ as acids based on their pH. He hypothesized that “all acids contain oxygen and hydrogen”.

Evaluate his hypothesis.
21N.2.hl.TZ0.5a:
Formulate an equation for the reaction of one mole of phosphoric acid with one mole of sodium hydroxide.
17M.2.sl.TZ2.7b.i:
Calculate the amount, in mol, of NaOH(aq) used.
19M.2.sl.TZ1.2a:
Draw the structure of the conjugate base of benzoic acid showing all the atoms and all the bonds.
19N.1.sl.TZ0.21: Which is an example of an amphiprotic species? A. Al2O3 B. CO32− C. P4O10 D. HPO42−
19N.1.sl.TZ0.20: What is the difference between a conjugate Brønsted–Lowry acid–base pair? A. Electron...
18N.1.sl.TZ0.19:
Which two species act as Brønsted–Lowry acids in the reaction?

H₂PO₄⁻ (aq) + OH⁻ (aq) $\rightleftharpoons$ HPO₄²⁻ (aq) + H₂O (l)

A. HPO₄²⁻ (aq) and OH⁻ (aq)

B. H₂PO₄⁻ (aq) and HPO₄²⁻ (aq)

C. HPO₄²⁻ (aq) and H₂O (l)

D. H₂PO₄⁻ (aq) and H₂O (l)
20N.2.hl.TZ0.5d:
Calculate the $K_{b}$ of the conjugate base of ethanoic acid using sections 2 and 21 of the data booklet.
19M.2.hl.TZ2.5a(ii):
The hydrogencarbonate ion, produced in Equilibrium (2), can also act as an acid.

State the formula of its conjugate base.
18M.2.sl.TZ2.2d:
Outline why H₃PO₄/HPO₄²⁻ is not a conjugate acid-base pair.
19M.2.hl.TZ2.5e:
The reaction of the hydroxide ion with carbon dioxide and with the hydrogencarbonate ion can be represented by Equations 3 and 4.

Equation (3) OH⁻ (aq) + CO₂ (g) → HCO₃⁻ (aq)
Equation (4) OH⁻ (aq) + HCO₃⁻ (aq) → H₂O (l) + CO₃²⁻ (aq)

Discuss how these equations show the difference between a Lewis base and a Brønsted–Lowry base.

Equation (3):

Equation (4):
21N.2.hl.TZ0.5d: Outline the reasons that sodium hydroxide is considered a Brønsted–Lowry and Lewis base.
21M.1.hl.TZ1.26:
Which is a Lewis acid, but not a Brønsted-Lowry acid?

A.   ${BF}_{3}$

B.   $H_{3} O^{+}$

C.   ${NH}_{3}$

D.   ${Cl}^{-}$
19M.2.hl.TZ1.2d:
Draw the structure of the conjugate base of benzoic acid showing all the atoms and all the bonds.
21M.1.sl.TZ2.19: Which cannot act as a Brønsted–Lowry base? A. HPO42− B. H2O C. CH4 D. NH3
21M.2.sl.TZ1.2b(i): State the formula of its conjugate base.
20N.1.sl.TZ0.20:
Which of these acids has the weakest conjugate base?

A.   $HCl$

B.   ${CH}_{3} COOH$

C.   ${NH}_{4} Cl$

D.   $C_{6} H_{5} COOH$
20N.2.sl.TZ0.1c(ii):
State the formula of the conjugate base of hypochlorous acid.
20N.2.hl.TZ0.1c(ii):
State the formula of the conjugate base of hypochlorous acid.
20N.1.hl.TZ0.26:
Which species is a Lewis acid but not a Brønsted–Lowry acid?

A.   ${Cu}^{2 +}$

B.   ${NH}_{4}^{+}$

C.   $Cu$

D.   ${CH}_{3} COOH$
22M.2.sl.TZ1.2e(i): State the relationship between NH4+ and NH3 in terms of the Brønsted–Lowry theory.
21N.2.hl.TZ0.5b:
Formulate two equations to show the amphiprotic nature of H₂PO₄⁻.
21N.1.sl.TZ0.21: What is the conjugate acid of HS−? A. H2S B. S2− C. H2SO3 D. H2SO4
21N.2.sl.TZ0.5b:
Formulate two equations to show the amphiprotic nature of H₂PO₄⁻.

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