Energetics I (Edexcel AS Chemistry)

Exam Questions

1 hour9 questions

5 marks

Phosphorus(V) chloride, PCl₅, can be thermally decomposed to phosphorus(III) chloride, PCl₃ , and chlorine, Cl₂ . The equation for this reaction is

PCl₅(g) → PCl₃(g) + Cl₂(g)

The enthalpy change for this reaction cannot be measured directly.

Complete the Hess’s Law cycle to include the enthalpy change of formation of both phosphorus chlorides.

Include the labels of the missing enthalpy changes.

ΔH is the enthalpy change for the vaporisation of the substance from the state shown to the gaseous state.

(3)

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ii)

Calculate the enthalpy change for the thermal decomposition of PCl₅(g) to PCl₃(g) and Cl₂(g), using the data given in the table.
Include a sign and units in your answer.

(2)

	Enthalpy change / kJ mol^–1
Δ_fH [PCl₅(s)]	–443.5
Δ_fH [PCl₃(l)]	–319.7
Δ_vH [PCl₅(s)]	+64.9
Δ_vH [PCl₃(l)]	+30.5

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1b2 marks

Another source gave a different value for the enthalpy change of this reaction.

PCl₅(g) ⇌ PCl₃(g) + Cl₂(g) Δ_rH = +87.9 kJ mol^–1

Explain the effect, if any, of increasing the temperature on the position of the equilibrium at constant volume.

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5a1 mark

When solid calcium sulfate dihydrate, CaSO₄·2H₂O, is heated in a crucible, it forms solid calcium sulfate hemihydrate, CaSO₄·1⁄2H₂O.

Write an equation, including state symbols, for this reaction.

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5b1 mark

Which two terms could be used to describe this reaction?

		Enthalpy change	Type of process
☐	A	endothermic	hydration
☐	B	exothermic	hydration
☐	C	exothermic	dehydration
☐	D	endothermic	dehydration

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8 marks

When water is added to calcium sulfate hemihydrate, there is a rise in temperature.

A student decided to investigate this reaction using the following procedure:

Step 1	10 cm³ of distilled water is measured using a measuring cylinder having an uncertainty of ±0.5 cm³ , and is placed in an insulated cup with a lid.
Step 2	A thermometer with an uncertainty of ±0.5 °C is placed in the water.
Step 3	Exactly 10.00 g of calcium sulfate hemihydrate is weighed out using a balance with an uncertainty of ±0.005 g.
Step 4	The weighed quantity of calcium sulfate hemihydrate is added to the water in the insulated cup.
Step 5	The mixture in the insulated cup is stirred until no further temperature change is observed.

Results

Temperature of the water before adding the solid	= 23.5 °C
Maximum temperature of the mixture after adding the solid	= 26.3 ^°C

Other data

Molar mass of calcium sulfate hemihydrate, CaSO₄·1⁄2H₂O	= 145.2 g mol^–1
Density of water	= 1.00 g cm^-3

Calculate the minimum volume of water needed to convert 10.00g of CaSO₄·1⁄2H₂O into CaSO₄·2H₂O.

(2)

ii)

Calculate the enthalpy change, in kJmol^–1, for this reaction.

Include a sign in your answer and give your answer to an appropriate number of significant figures.

Assume that the liquid has a mass of 10.00 g and a specific heat capacity of 4.18 J g^–1 °C^–1.

(4)

iii)

Deduce which measurement has the greatest uncertainty in this experiment.
Justify your answer by calculating the percentage uncertainty of this piece of apparatus.

(2)

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2 marks

Hess’s law can be used to determine enthalpy changes for reactions which cannot be obtained directly.
An example is the reaction of anhydrous copper(II) sulfate with water to form hydrated copper(II) sulfate, CuSO₄.5H₂O.

The following outline procedure was carried out.

Step 1	42.75 g of deionised water was weighed out in a polystyrene cup and the temperature measured.
Step 2	0.0250 mol of hydrated copper(II) sulfate was added to the water in the polystyrene cup with stirring, making a total of 45.00 g of water.
Step 3	The temperature change was recorded.
Step 4	Steps 1 to 3 were repeated using 45.00 g of deionised water and 0.0250 mol of anhydrous copper(II) sulfate.

Calculate the mass of 0.0250 mol of hydrated copper(II) sulfate, CuSO₄.5H₂O.

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3 marks

The reaction of hydrated copper(II) sulfate with water is shown.

CuSO₄.5H₂O (s) + aq → CuSO₄(aq) ∆H₁ = +18.2 kJ mol⁻¹

Calculate the temperature change that would have given this enthalpy change for the stated experimental procedure.
Give your answer to a measurable number of significant figures and state whether the temperature increases or decreases.

[Specific heat capacity of the solution = 4.18 J g⁻¹ ^oC⁻¹]

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4 marks

The reaction of anhydrous copper(II) sulfate with water is shown.

CuSO₄(s) + aq → CuSO₄(aq) ∆H₂ = −84.5 kJ mol⁻¹

Draw to scale, on the graph paper, a labelled enthalpy level diagram which shows the enthalpy changes for the reactions of water with hydrated copper(II) sulfate (∆H₁) and anhydrous copper(II) sulfate (∆H₂).

(3)

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ii)

Use your enthalpy level diagram in (c)(i) to determine the enthalpy change, ∆_rH, for the reaction

CuSO₄(s) + 5H₂O (l) → CuSO₄.5H₂O (s)

You must show your working on the diagram.

(1)

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1d1 mark

State why the enthalpy change for the reaction of one mole of anhydrous copper(II) sulfate with five moles of water to form hydrated copper(II) sulfate, CuSO₄.5H₂O, cannot be measured directly.

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Substance	Δ_c $H^{⦵}$ / kJ mol⁻¹
C(s)	−394
H₂(g)	−286
CH₄(g)	−890

		Equipment	Measurement uncertainty for each reading
☐	A	burette	±0.05 cm³
☐	B	50 cm³ measuring cylinder	±1 cm³
☐	C	25 cm³pipette	±0.06 cm³
☐	D	50 cm³pipette	±0.08 cm³

Substance	Standard enthalpy change of combustion/ kJ mol⁻¹
CO	-283
H₂	-286

Bond	Mean bond enthalpy / kJ mol⁻¹
C–C	347
C–H	413
C–O	358
O–H	464
O=O	498
C=O	805

Bond	Mean bond enthalpy / kJ mol^–1
$N - N$	158
$N = N$	410
$N \equiv N$	945
$N - H$	391
$H - H$	436

☐	A	orange to green
☐	B	green to orange
☐	C	purple to colourless
☐	D	no change

☐	A	$K_{C} = \frac{[N_{2}] [3 H_{2}]}{[2 {NH}_{3}]}$
☐	B	$K_{C} = \frac{[2 {NH}_{3}]}{[N_{2}] [3 H_{2}]}$
☐	C	$K_{C} = \frac{{[{NH}_{3}]}^{2}}{[N_{2}] {[H_{2}]}^{3}}$
☐	D	$K_{C} = \frac{[N_{2}] {[H_{2}]}^{3}}{{[{NH}_{3}]}^{2}}$

☐	A	17.6 %
☐	B	50.0 %
☐	C	82.4 %
☐	D	100 %