Page 396 - SUBSEC October 2017_Neat

02112020/CAPE/SPEC/MS/2017
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CHEMISTRY
UNIT 01 — PAPER 02
MARK SCHEME
KC UK XS

(c) (i) Hess’s law of constant heat summation states that for any
extensive property (enthalpy), the ∆H in going from 1
reactants to products is a constant regardless of the path
taken, as long as the reaction goes from the same initial
state to the same final state. [1 mark]

(ii) Standard enthalpy of formation is the enthalpy change when
one mole of a compound is formed from the elements in their
standard states. [1 mark] 1

(iii) Stage I: ∆H A [1 mark]
θ
Stage III: ∆H D [1 mark] 3
θ
Stage IV: ∆H E [1 mark]
θ

(iv) Lattice energy is the enthalpy of formation for one mole
of the ionic compound from gaseous ions under standard 1
conditions. [1 mark]

(v) Lattice energy of MX(s)= ∆H F –(∆H A + ∆H I + ∆H D + ∆H E)
θ
θ
θ
θ
θ

[1 mark]
2
= -431 – (+86 + 408 + 122 - 372) [1 mark]
-1
= -675 kJ mol
[1 mark]

(d) The difference in lattice energies

st
nd
 For Mg, one has to take into consideration the 1 and 2
ionization energies of Mg. (+736, 1450). [2 marks]
4
 For Cl, the two electrons that are released are given one
each to 2 chlorine atoms. Adding one electron as a time
– you need to multiply E.A by 2. [2 marks]

(e) The same overall reaction for each since a strong acid and a
strong base are reacting.

+
H + OH - H2O 1
+
2H + OH - 2H2O

Since one is dealing with molar enthalpy change the values of 1
∆H will be the same. [2 marks]

Total 30 marks 10 15 5