DP Chemistry (first assessment 2025)

Test builder »

Question SPM.2.HL.TZ0.4

Select a Test
Date May Specimen-2023 Marks available [Maximum mark: 14] Reference code SPM.2.HL.TZ0.4
Level HL Paper 2 Time zone TZ0
Command term Calculate, Deduce, Describe, Explain, Sketch, State and explain Question number 4 Adapted from N/A
4.
[Maximum mark: 14]
SPM.2.HL.TZ0.4

Hydrochloric acid is an important chemical reactant and industrial chemical.

(a)

A pH probe is placed in a small volume of 0.10 mol dm-3 solution of hydrochloric acid. The pH is recorded while a steady stream of distilled water is added to the acid at constant temperature.

(a.i)

On the axes, sketch the graph of pH against volume of water added.

[3]

Markscheme

start at pH = 1  ✔
curve with decreasing gradient  ✔
must finish below pH = 7  ✔

(a.ii)

The experiment is repeated using 0.10 mol dm-3 ethanoic acid at the same temperature. Calculate the initial pH of the ethanoic acid.

pKa ethanoic acid = 4.76

[2]

Markscheme

Ka =[H+][CH3COO-][CH3COOH]

OR

Ka =[H+]2[CH3COOH]

OR

pKa = 2pH - 1

OR

pH =pKa+12=  ✔

2.88  ✔

(b)

Chloride ions can form complex ions with some transition metals. The formulas and colours of three compounds of cobalt are:

Compound Colour
[Co(NH3)6]Cl3 orange–yellow
[Co(NH3)5Cl]Cl2 purple
[Co(NH3)4Cl2]Cl green

(b.i)

Deduce the oxidation state of cobalt in [Co(NH3)5Cl]Cl2 and the charge of the complex ion.

Oxidation state: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 


Charge of complex ion: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

[2]

Markscheme

Oxidation state: +3  ✔
Charge of complex ion: 2+  ✔

(b.ii)

Describe the bonding of chloride ions in [Co(NH3)4Cl2]Cl.

[2]

Markscheme

two chlorides covalently bonded/coordination bond to cobalt ion  ✔
one chloride ionically bonded to complex ion  ✔

(b.iii)

Explain why these complex ions are coloured.

[3]

Markscheme

3d sub-level split in presence of ligands  ✔
light is absorbed when electrons promoted between split-levels  ✔
colour seen is complementary to the light absorbed  ✔

(b.iv)

State and explain, in which of the complex ions, the electron transitions responsible for the colour require the highest energy. Use the colour wheel and the electromagnetic spectrum in sections 5 and 15 of the data booklet.

[2]

Markscheme

complex [Co(NH3)6] Cl3  ✔
absorbs blue-purple light/wavelength 424 nm/shortest wavelength/highest energy  ✔