A Data Booklet may be used.
CommonWhich of the following reactions has a negative ΔS?
Source: HCI 2018 Promo, via EJC 2022 Chemical Energetics Topical Revision, Q16
Which of the following processes is spontaneous under all conditions?
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 | + | + | + |
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?
Source: A Level 2009 P1 Q10, via EJC 2022 JC1 Chemical Energetics Tutorial, A8
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
| substance | (CH3)2N2H2(l) | N2O4(l) | CO2(g) | H2O(g) |
|---|---|---|---|---|
| ΔHf⦵ / kJ mol−1 | +48.9 | −19.6 | −393.5 | −241.8 |
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 |
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
Source: EJC 2022 JC1 Chemical Energetics Tutorial, B11(b)
CommonGive explanations as to whether the following reactions are feasible.
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) |
|---|---|---|---|
| ΔGf⦵ / kJ mol−1 | −1012 | −569 | −394 |
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 (ΔSsys), why the reaction is spontaneous under standard conditions.
Source: RJC H2 Chemistry Tutorial 5b (Chemical Energetics II), Q10(a) and (b)
CRusting of an iron bar to form iron(III) oxide.[1]
In the process of rusting, O2(g) reacts with solid iron to form solid iron oxide. Hence there is a decrease in the number of gaseous particles and ΔS is negative.
AN2O(g) → N2(g) + ½O2(g) ΔH = −82 kJ mol−1[1]
ΔG = ΔH − TΔS. For the reaction to be always spontaneous, ΔH needs to be negative (options C and D are incorrect as they have positive ΔH values) and ΔS needs to be positive.
ΔS is positive in option A because there is an increase in the number of moles of gas.
CΔH +, ΔS +, ΔG −[1]
"Cold packs" indicates that the dissolution of NH4NO3 in water is an endothermic process, thus ΔH > 0.
Ions in solution are more disordered than in the regular ionic lattice, thus ΔS > 0.
Since the process occurs readily (spontaneously), ΔG < 0.
A−66 J K−1 mol−1[1]
When water freezes at 0 °C, the equilibrium H2O(l) ⇌ H2O(s) exists. Hence ΔG = 0.
ΔS = ΔH − ΔGT = −6.0 − 0273 = −0.0220 kJ K−1 mol−1
Amount of water = 54 ÷ 18 = 3 mol. Required entropy change = −0.0220 × 3 = −0.066 kJ K−1 mol−1 = −66 J K−1 mol−1
ΔH⦵ = ΣΔHf⦵(products) − ΣΔHf⦵(reactants)
= [2(−393.5) + 0 + 4(−241.8)] − [(+48.9) + 2(−19.6)] = −1763.9 kJ mol−1[1]
ΔG⦵ = ΔH⦵ − TΔS⦵ = −1763.9 − [298(+1141.2 × 10−3)][1]
= −2.10 × 103 kJ mol−1[1]
Energy cycle: Ca(s) → Ca2+(aq) + 2e− directly (+268), or Ca(s) → Ca(g) (+178), then Ca(g) → Ca2+(g) + 2e− (1st IE + 2nd IE = +1740), then Ca2+(g) → Ca2+(aq) (x).[1]
By Hess' Law, ΔHr = +178 + 1740 + x = +268
x = −1650 kJ mol−1[1]
ΔG = ΔH − TΔS. During melting and boiling, ΔG = 0, so ΔS = ΔH ÷ T
ΔSmelting = +2.3 × 1000 ÷ 234 = +9.83 J mol−1 K−1[1]
Sum of ΔGf⦵ of reactants = −1012 kJ mol−1. Sum of ΔGf⦵ of products = −569 + (−394) = −963 kJ mol−1
So the free energy change, ΔG⦵ = −963 − (−1012) = +49 kJ mol−1[1]
The reaction will not take place spontaneously because ΔG⦵ > 0.[1]
For the reaction to be spontaneous, ΔG = ΔH − TΔS = 101 − T(0.159) < 0, so 101 < T(0.159)[1]
A temperature above 635 K (362 °C) is needed for the reaction to take place.[1]
NoteHere, we assume that ΔH and ΔS do not change with temperature.
2NI3(s) → N2(g) + 3I2(g) ΔH < 0, ΔS > 0[1]
As 4 moles of gases are produced, there is a very large increase in the entropy of the system. This reaction is also exothermic (formation of the N≡N bond). Hence, highly explosive.[1]
NH4NO3(s) → NH4+(aq) + NO3−(aq) ΔH > 0, ΔS > 0[1]
Aqueous ions are formed from ions held fixed in an orderly crystalline lattice, resulting in a large increase in the entropy of the system. Even though the reaction is endothermic, the increase in entropy compensates for the endothermic ΔH, so that ΔG = ΔH − TΔS < 0 even under standard conditions, and NH4NO3 dissolves readily.[1]