A-LevelChemistryChemical energetics9701/41 · Oct/Nov 2024 · Q214 Marks

Predict and explain the variation in enthalpy change of hydration for the ions F¯, Cl-, Br¯ and I-.

(a)Predict and explain the variation in enthalpy change of hydration for the ions F¯, Cl-, Br¯ and I-.[2]
(b)Fig. 2.1 shows an incomplete energy cycle involving calcium fluoride, CaF2. [Figure 2.1]
(b)(i)Complete line D. Include state symbols.[1]
(b)(ii)The value of the enthalpy change for process 1 can be calculated using the values of five other enthalpy changes which are not referred to in Fig. 2.1. process 1: Ca(s) + F2(g) → Ca²+(g) + 2F¯(g) Identify these five other enthalpy changes, using either names or symbols.[2]
(b)(iii)Define lattice energy, ΔΗlatt[2]
(b)(iv)Complete the expression to give the mathematical relationship between ∆Hjatt of calcium fluoride and the enthalpy changes for processes 1 and 3. ΔΗlatt =[1]
(c)Use data from Table 2.1 to calculate a value for the hydration energy, AHhyd, of fluoride ions, F¯(g). Table 2.1 value/kJ mol-1 enthalpy change of solution of calcium fluoride, CaF2(s) +13 overall enthalpy change of process 1 in Fig. 2.1 +1395 enthalpy change of formation of calcium fluoride -1214 enthalpy change of hydration of Ca2+(g) -1650 AHhyd F¯(g) = kJmol-1[2]
(d)Define entropy.[1]
(e)At 298 K, the Gibbs free energy change, AG, for the solution of compound T is +6.00 kJ mol-1. The enthalpy change of solution, ΔΗsol, of compound T is +30.0 kJ mol-1 at 298 K. Calculate the value of the entropy change, AS, for the solution of compound T at 298 K. = SV JK-1 mol-1[2]
(f)Predict whether compound T becomes more or less soluble as the water is heated from 298 K to 360 K. Explain your answer.[1]

This question uses a diagram from the original paper, which isn't shown here. Open the full paper from the past papers archive to see it.

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2024). The mark or mark type is shown in brackets.

[1] becomes less negative/less exothermic (down the group / from F to I)
[1] AND due to increase in (ionic) radius / size OR decreased attraction to water OR weaker ion-dipole force to water
[1] CaF2(aq) OR Ca2+ (aq) + 2F- (aq)
[1] atomisation energy of Ca / ∆Hat(o)m)
[1] atomisation energy of F(2) OR F-F bond energy / ∆Hat(o)m)) / BE of F-F
[1] first ionisation energy / IE₁ of Ca / ΔΗ1
[1] second ionisation energy / IE2 of Ca / ΔΗ2
[1] (first) electron affinity /EA of F / Hea
[1] energy change / energy released when one mole of an ionic compound is formed
[1] from its gaseous ion(s)
[1] (AHlatt = ) (change) 3 – (change) 1 OR (∆Hlatt = ) ∆Ηf(CaF2(s)) – ∆H(Ca²+(g)) – 2∆H(F¯(g))
[1] Correctly identifying the numbers from table 2.1 (13, 1395, 1214, 1650) and setting up the equation, including '2x' or '2ΔHhyd(F-)', e.g., 1395 – 1650 + 2x = -1214+13
[1] ΔHhyd F-(g) = -473 kJmol-1
[1] number of possible arrangements of particles / energy in a system
[1] states or clearly uses ∆G = ∆Η - TAS OR 6000 = 30 000 – (298 × AS)
[1] AS = (+)80.5(4) JK-1 mol-1
[1] becomes more soluble AND AS is positive / TAS is positive / -TAS is negative (as T inc) (so AG becomes less negative)