Learners' AlleyDesigned by Khow Willard

Chemical Energetics II

Set BExam practice

Name

Time 1 hour

Total 26 marks

A Data Booklet may be used.

Multiple choice

4 marks

CommonWhich of the following reactions has a negative ΔS?

AHeating solid calcium carbonate.

BSublimation of solid carbon dioxide.

CRusting of an iron bar to form iron(III) oxide.

DReaction between aqueous sodium hydroxide and aqueous ammonium chloride.

Answer [1]

Source: HCI 2018 Promo, via EJC 2022 Chemical Energetics Topical Revision, Q16

Which of the following processes is spontaneous under all conditions?

AN₂O(g) → N₂(g) + ½O₂(g) ΔH = −82 kJ mol⁻¹

BCH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l) ΔH = −890 kJ mol⁻¹

C2CO(g) + 2NH₃(g) → C₂H₆(g) + 2NO(g) ΔH = +409 kJ mol⁻¹

DSO₃(g) → SO₂(g) + ½O₂(g) ΔH = +99.0 kJ mol⁻¹

Answer [1]

Source: CJC 2018 Promo, via EJC 2022 Chemical Energetics Topical Revision, Q18

Instant cold packs are composed of powdered ammonium nitrate and water separated by a thin plastic membrane. When the pack is squeezed, the membrane breaks and the ammonium nitrate dissolves in the water. NH₄NO₃(s) + nH₂O(l) → NH₄NO₃(aq)

Which line in the table gives the signs of ΔH, ΔS and ΔG for the overall process?

	ΔH	ΔS	ΔG
A	−	−	+
B	−	+	−
C	+	+	−
D	+	+	+

Answer [1]

Source: A Level 2008 P1 Q10, via EJC 2022 JC1 Chemical Energetics Tutorial, A7

CommonWhen water freezes, 6.0 kJ mol⁻¹ of heat is evolved. What is the entropy change when 54 g of water freezes at 0 °C?

A−66 J K⁻¹ mol⁻¹

B−22 J K⁻¹ mol⁻¹

C22 J K⁻¹ mol⁻¹

D66 J K⁻¹ mol⁻¹

Answer [1]

Source: A Level 2009 P1 Q10, via EJC 2022 JC1 Chemical Energetics Tutorial, A8

Structured questions

22 marks

The reaction of 2,2-dimethylhydrazine, (CH₃)₂N₂H₂, with dinitrogen tetroxide, N₂O₄, is an energy source for rockets.

(CH₃)₂N₂H₂(l) + 2N₂O₄(l) → 2CO₂(g) + 3N₂(g) + 4H₂O(g) ΔS^⦵ = +1141.2 J K⁻¹ mol⁻¹

(a)

Dinitrogen tetroxide, N₂O₄, may be obtained from the more commonly available NO₂. State and explain the effect on the entropy, S, of the chemical system during the conversion of NO₂(g) to N₂O₄(g).

[1]

(b)

By using the standard enthalpy change of formation values given below, calculate the standard Gibbs free energy change, ΔG^⦵, in kJ mol⁻¹, for the reaction of (CH₃)₂N₂H₂ with N₂O₄ at 298 K.

substance	(CH₃)₂N₂H₂(l)	N₂O₄(l)	CO₂(g)	H₂O(g)
ΔH_f^⦵ / kJ mol⁻¹	+48.9	−19.6	−393.5	−241.8

[3]

Source: 2025 CJC J2 Prelim P2 Q1(b)

Calcium hydroxide is produced commercially by treating lime (calcium oxide) with water. The following are some thermochemistry data.

	ΔH / kJ mol⁻¹
enthalpy change of atomisation of calcium	+178
enthalpy change of Ca(s) → Ca²⁺(aq) + 2e⁻	+268
enthalpy change of hydration of Ca²⁺ ion	x
enthalpy change of hydration of OH⁻ ion	−550

(a)

Define standard enthalpy change of hydration of calcium ion.

[1]

(b)

Using the data above, and relevant data from the Data Booklet, construct an energy cycle to determine x.

[2]

(c)

The entropy change of hydration of the calcium ion is −208 J mol⁻¹ K⁻¹. Explain why the sign is negative.

[1]

Source: TJC 2017 Promo, via EJC 2022 Chemical Energetics Topical Revision, Q42(i) to (iii)

Mercury melts at 234 K with an enthalpy change of melting of +2.3 kJ mol⁻¹. It boils at 630 K with an enthalpy change of vaporisation of +59 kJ mol⁻¹. Calculate the entropy change of the system (per mole of mercury) on

(a)

melting

[1]

(b)

boiling

[1]

Source: EJC 2022 JC1 Chemical Energetics Tutorial, B11(b)

CommonGive explanations as to whether the following reactions are feasible.

(a)

X(s) → Y(s) + Z(g) ΔH^⦵ = +250 kJ mol⁻¹

[1]

(b)

2X(g) + Y(g) → 2Z(g) ΔH^⦵ = −100 kJ mol⁻¹

[1]

(c)

X(l) → Y(l) + Z(g) ΔH^⦵ = −200 kJ mol⁻¹

[1]

(d)

2X(s) + 2Y(g) → Z(g) ΔH^⦵ = +50 kJ mol⁻¹

[1]

Source: RJC H2 Chemistry Tutorial 5b (Chemical Energetics II), Q9

The table gives standard free energy changes of formation.

substance	MgCO₃(s)	MgO(s)	CO₂(g)
ΔG_f^⦵ / kJ mol⁻¹	−1012	−569	−394

(a)

Use the data to calculate the free energy change, ΔG^⦵, for the reaction MgCO₃(s) → MgO(s) + CO₂(g). Will the reaction take place spontaneously?

[2]

(b)

Given that ΔH^⦵ of the reaction is +101 kJ mol⁻¹ and ΔS^⦵ is +159 J mol⁻¹ K⁻¹, estimate the temperature at which the reaction will take place spontaneously.

[2]

Source: RJC H2 Chemistry Tutorial 5b (Chemical Energetics II), Q6

ChallengeFor each reaction described below, write an equation and explain, by considering the changes in enthalpy (ΔH) and entropy (ΔS_sys), why the reaction is spontaneous under standard conditions.

(a)

The decomposition of solid nitrogen triiodide, NI₃, to produce N₂(g) and purple vapour is highly exothermic and explosive.

[2]

(b)

The solution cools down when ammonium nitrate crystals are dissolved in water.

[2]

Source: RJC H2 Chemistry Tutorial 5b (Chemical Energetics II), Q10(a) and (b)