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Date November 2017 Marks available 1 Reference code 17N.2.hl.TZ0.6
Level HL Paper 2 Time zone TZ0
Command term Calculate Question number 6 Adapted from N/A

Question

Many reactions are in a state of equilibrium.

The following reaction was allowed to reach equilibrium at 761 K.

H2 (g) + I2 (g) 2HI (g)               ΔHθ < 0

The pH of 0.010 mol dm–3 carbonic acid, H2CO3 (aq), is 4.17 at 25 °C.

H2CO3 (aq) + H2O (l) HCO3 (aq) + H3O+ (aq).

State the equilibrium constant expression, Kc , for this reaction.

[1]
a.i.

The following equilibrium concentrations in mol dm–3 were obtained at 761 K.

Calculate the value of the equilibrium constant at 761 K.

[1]
a.ii.

Determine the value of ΔGθ, in kJ, for the above reaction at 761 K using section 1 of the data booklet.

[1]
a.iii.

Calculate [H3O+] in the solution and the dissociation constant, Ka , of the acid at 25 °C.

[3]
c.i.

Calculate Kb for HCO3 acting as a base.

[1]
c.ii.

Markscheme

Kc [HI] 2 [ H 2 ][ I 2 ]

a.i.

45.6

a.ii.

ΔGθ = «– RT ln K = – (0.00831 kJ K−1 mol−1 x 761 K x ln 45.6) =» – 24.2 «kJ»

a.iii.

[H3O+] = 6.76 x 10–5 «mol dm–3»

Ka ( 6.76 × 10 5 ) 2 ( 0.010 6.76 × 10 5 ) / ( 6.76 × 10 5 ) 2 0.010

4.6 x 10–7

Accept 4.57 x 10–7

Award [3] for correct final answer.

c.i.

« 1.00 × 10 14 4.6 × 10 7 =» 2.17 x 10–8

OR

« 1.00 × 10 14 4.57 × 10 7 =» 2.19 x 10–8

c.ii.

Examiners report

[N/A]
a.i.
[N/A]
a.ii.
[N/A]
a.iii.
[N/A]
c.i.
[N/A]
c.ii.

Syllabus sections

Additional higher level (AHL) » Topic 17: Equilibrium » 17.1 The equilibrium law
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Additional higher level (AHL) » Topic 17: Equilibrium
Additional higher level (AHL)

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