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Date	May 2017	Marks available	3	Reference code	17M.2.hl.TZ1.4
Level	HL	Paper	2	Time zone	TZ1
Command term	Determine	Question number	4	Adapted from	N/A

Question

Millerite, a nickel sulfide mineral, is an important source of nickel. The first step in extracting nickel is to roast the ore in air.

The reaction for the formation of liquid tetracarbonylnickel is shown below:

${\text{Ni(s)}} + 4{\text{CO(g)}} \to {\text{Ni(CO}}{{\text{)}}_4}{\text{(l)}}$

Formulate an equation for the oxidation of nickel(II) sulfide to nickel(II) oxide.

[1]

The nickel obtained from another ore, nickeliferous limonite, is contaminated with iron. Both nickel and iron react with carbon monoxide gas to form gaseous complexes, tetracarbonylnickel, ${\text{Ni(CO}}{{\text{)}}_{\text{4}}}{\text{(g)}}$ , and pentacarbonyliron, ${\text{Fe(CO}}{{\text{)}}_{\text{5}}}{\text{(g)}}$ . Suggest why the nickel can be separated from the iron successfully using carbon monoxide.

[1]

Calculate the standard entropy change, $\Delta {S^\theta }$ , of the reaction, in ${\text{J}}\,{{\text{K}}^{ - 1}}$ , using the values given.

[2]

c.i.

Calculate a value for $\Delta {H^\theta }$ in kJ.

[1]

c.ii.

Use your answers to (c)(i) and (c)(ii), to determine the temperature, in °C, at which the decomposition of liquid tetracarbonylnickel to nickel and carbon monoxide becomes favourable.

(If you did not get answers to (c)(i) and (c)(ii), use $- 500{\text{ J}}\,{{\text{K}}^{ - 1}}$ and $- 200{\text{ kJ}}$ respectively but these are not the correct answers.)

[3]

c.iii.

Suggest why experiments involving tetracarbonylnickel are very hazardous.

[1]

Markscheme

${\text{2NiS(s)}} + {\text{3}}{{\text{O}}_{\text{2}}}{\text{(g)}} \to {\text{2NiO(s)}} + {\text{2S}}{{\text{O}}_{\text{2}}}{\text{(g)}}$

[1 mark]

formation of «gaseous» pentacarbonyliron is slower
OR
«gaseous» complexes form at different rates
OR
gases have different rates of diffusion «due to difference in masses»
OR
difference in thermal stability of «gaseous» complexes
OR
difference in boiling points of «gaseous» complexes
OR
difference in solubility of «gaseous» complexes
OR
difference in surface affinity «onto solid absorbent»
OR
difference in chemical properties of «gaseous» complexes

Accept any other valid answer.

[1 mark]

$\sum {S_{{\text{RHS}}}^\theta = 313.4{\text{ }}\ll {\text{J}}\,{{\text{K}}^{ - 1}}\gg }$
AND
$\sum {S_{{\text{LHS}}}^\theta = \ll (4 \times 197.6) + 29.9{\text{ J}}\,{{\text{K}}^{ - 1}} = \gg {\text{ }}820.3{\text{ }}\ll {\text{J}}\,{{\text{K}}^{ - 1}}\gg }$

$\Delta {S^\theta }\ll = \sum {S_{{\text{RHS}}}^\theta - \sum {S_{{\text{LHS}}}^\theta = } {\text{ }}313.4 - 820.3\gg = - 506.9{\text{ }}\ll {\text{J}}\,{{\text{K}}^{ - 1}}\gg }$

Award [2] for correct final answer.

[2 marks]

c.i.

$\Delta {H^\theta }\ll = - 633.0 - 4 \times ( - 110.5)\gg = - 191{\text{ }}\ll kJ\gg$

[1 mark]

c.ii.

«when» $\Delta G = 0$ «forward and backward reactions are equally favourable»

«when $\Delta G = 0$ , ${\text{T}} = \frac{{\Delta H}}{{\Delta S}}$ », ${\text{T}} = \ll \frac{{191{\text{ kJ}}}}{{0.5069{\text{ kJ}}\,{{\text{K}}^{ - 1}}}} = \gg {\text{ }}377{\text{ }}\ll {\text{K}}\gg$

«temperature =» 104 «°C»

Award [3] for correct final answer. Use of –500 J K^–1 and –200 kJ gives 127 °C.

Award [2 max] for T < 104 «°C».

Accept ΔG < 0 and T > 104 «°C».

[3 marks]

c.iii.

CO is toxic/poisonous
OR
Ni(CO)₄ decomposition deposits nickel in the lungs
OR
tetracarbonylnickel is toxic/poisonous
OR
tetracarbonylnickel is highly flammable «auto-ignition temperature of 60 °C»

[1 mark]