A Data Booklet may be used.
Source: Learners' Alley, Chemical Energetics II slides
Source: Learners' Alley, Chemical Energetics II slides
Source: Learners' Alley, Chemical Energetics II slides
Source: Learners' Alley, Chemical Energetics II slides
CommonFor the reaction N2(g) + 3H2(g) ⇌ 2NH3(g), ΔH = −92 kJ mol−1. Which statement about the forward reaction is correct?
Source: 2024 DHS J2 Prelim P1 Q10
Use of the Data Booklet is relevant to this question. When water vaporises, 40.7 kJ mol−1 of heat is required. What is the entropy change when 72 g of water boils at 100 °C?
Source: IJC 2017 Promo, via EJC 2022 Chemical Energetics Topical Revision, Q20
Magnesium chloride is one of the most abundant salts in sea water.
| lattice energy of MgCl2(s) | −2521 kJ mol−1 |
|---|---|
| enthalpy change of formation of MgCl2(s) | −641 kJ mol−1 |
| enthalpy change of formation of Mg2+(aq) | −467 kJ mol−1 |
| enthalpy change of formation of Cl−(aq) | −167 kJ mol−1 |
Source: EJC 2022 JC1 Chemical Energetics Tutorial, A6
CommonAssume the pressure of each system remains constant throughout each process.
Source: EJC 2022 JC1 Chemical Energetics Tutorial, B8
Source: RJC H2 Chemistry Tutorial 5a (Chemical Energetics I), Q7(b) to (d)
Nitroglycerin, C3H5(NO3)3, decomposes on ignition: C3H5(NO3)3(l) → 32N2(g) + 14O2(g) + 3CO2(g) + 52H2O(g). The standard enthalpy change of this decomposition is −1423 kJ mol−1.
Source: 2019 CJC J1 Promo P2 Q4(c) and (d)
CommonDinitrogen pentoxide, N2O5, can be produced in a car engine by the following reaction sequence.
| I | N2(g) + O2(g) → 2NO(g) | ΔH⦵ = +180 kJ mol−1, ΔS⦵ = +24.8 J mol−1 K−1 |
| II | 2NO(g) + O2(g) → 2NO2(g) | ΔH⦵ = −114 kJ mol−1 |
| III | 2NO2(g) + ½O2(g) → N2O5(g) | ΔH⦵ = −110 kJ mol−1 |
Source: NJC 2018 Promo, via EJC 2022 Chemical Energetics Topical Revision, Q40(i) and (ii)
ChallengeThe following data relate to the energy changes which occur when sodium hydroxide, NaOH, and magnesium hydroxide, Mg(OH)2, dissolve in water.
| sodium | magnesium | |
|---|---|---|
| hydration energy of metal ion / kJ mol−1 | −390 | −1890 |
| lattice energy of metal hydroxide / kJ mol−1 | −896 | −2995 |
| enthalpy change of solution of metal hydroxide / kJ mol−1 | −44 | x |
| extent of hydration of metal ion (average number of attached water molecules per metal ion) | 5 | 15 |
Source: EJC 2022 JC1 Chemical Energetics Tutorial, B9
ΔHsol = +711 + (−322) + (−381) = +8 kJ mol−1[1]
Mg2+. Mg2+ has a higher charge and a smaller ionic radius than Na+, so it has a higher charge density.[1]
Hence, Mg2+ forms stronger ion–dipole interactions with water molecules.[1]
ΔG = −92 − 298(−0.199) = −92 + 59.3 = −32.7 kJ mol−1[1]
Since ΔG is negative, the reaction is spontaneous at 298 K.[1]
At 298 K, ΔG = 130 − 298(0.335) = +30.2 kJ mol−1, not spontaneous[1]
T > 1300.335 = 388 K[1]
NoteLikewise MgCO3 decomposes above 576 K but BaCO3 only above 1566 K: ΔH(MgCO3) is less positive, ΔS similar.
AIt is spontaneous only at low temperature.[1]
ΔS < 0 due to a decrease in the number of moles of gas (4 mol to 2 mol). Hence −TΔS > 0.
Since ΔH < 0 and ΔG = ΔH − TΔS, ΔG < 0 when |−TΔS| < |ΔH|. Hence the reaction is spontaneous only at low temperature.
D+436 J K−1[1]
Amount of H2O = 7218.0 = 4 mol
At the boiling point, ΔG = 0, and ΔG = ΔH − TΔS
0 = (40.7 × 103 × 4) − (100 + 273)ΔS
ΔS = +436 J K−1
Energy cycle: MgCl2(s) + aq → Mg2+(aq) + 2Cl−(aq) (ΔHsol), with both sides formed from Mg(s) + Cl2(g): −641 for MgCl2(s), and ΔHf = −467 + 2(−167) for the ions.[1]
By Hess' Law, ΔHsol = 641 + (−467) + 2(−167) = −160 kJ mol−1[1]
Since ΔHsol is highly exothermic, MgCl2 is very soluble in water.[1]
NoteThe magnitude of ΔS is often much smaller than that of ΔH. For most reactions, ΔS is less than 100 J K−1 mol−1. Hence when ΔH is very negative, ΔG is likely to be negative and the reaction is spontaneous.
ΔHsoln[LiCl(s)] = −lattice energy + ΔHhyd[Li+(g)] + ΔHhyd[Cl−(g)]
= +843 + (−883) = −40 kJ mol−1[1]
Heat absorbed = mcΔT = 50.0 × 4.20 × 3.3 = 693 J[1]
ΔHsoln[KCl(s)] = +693 ÷ 3.0074.6 = +17.2 kJ mol−1 (3 s.f.)[1]
Since |LE| is inversely proportional to the interionic distance, the lattice energy of LiCl is slightly more exothermic than that of KCl.[1]
However, since hydration energy is proportional to charge density, Li+ will have a much more exothermic hydration enthalpy than K+. Therefore, the enthalpy change of solution of LiCl is more exothermic than that of potassium chloride.[1]
ΔGdecomposition = ΔHdecomposition − TΔS
= −1423 − [298 × (+208 ÷ 1000)] = −1485 kJ mol−1[1]
Since ΔG is negative, the reaction is spontaneous at 298 K.[1]
The spontaneity is not affected by changes in temperature because ΔHdecomposition is negative, T is always positive and ΔS is positive (or −TΔS is always negative), i.e. ΔG = ΔH − TΔS = negative − [(positive) × (positive)] = negative.
Yes, the decomposition of nitroglycerin is spontaneous at all temperatures.[1]
ΔG⦵ = ΔH⦵ − TΔS⦵. ΔG⦵ (+172.6 kJ mol−1) is positive at 298 K, hence reaction I is not spontaneous at room temperature.[1]
The high temperature in the car engine causes TΔS⦵ to become more positive (or −TΔS⦵ to become more negative). This leads to a more negative ΔG⦵, and hence reaction I becomes spontaneous.[1]
The amount of gases decreases from 3 mol to 2 mol, hence there are fewer ways to arrange the gaseous particles and their energy in the system.[1]
ΔS⦵ is negative.[1]
ΔHsoln = −(lattice energy) + ΔHhyd
For NaOH: −44 = 896 + (−390) + ΔHhyd(OH−), so ΔHhyd(OH−) = −550 kJ mol−1[1]
For Mg(OH)2: x = 2995 − 1890 + 2(−550) = +5.00 kJ mol−1[1]