7.7.2 Energetics
QUESTIONS
Section A
1 The Born-Haber cycle for sodium chloride
is represented below:
Na+(g)
+ Cl(g)
z Na+(g) + Cl-(g)
y Na+(g) + ½
Cl2(g)
x Na(g) + ½ Cl2(g) u
w Na(s)
+ ½ Cl2(g)
v NaCl(s)
Which one of the following statements
is correct?
A
The bond dissociation energy of chlorine is y.
B The electron affinity
of chlorine is z.
C
The first ionisation energy of sodium is w.
D
The lattice energy of sodium chloride is v.
2
The
reactions represented by the following equations are part of the Born-Haber
cycle for sodium bromide. Which one of the reactions is exothermic?
A
Na (s) Na
(g)
B Na (g) Na+
(g) +
e-
C
Br (g) + e- Br- (g)
D Br2(g) 2Br(g)
Section B
1. Lithium chloride is formed from lithium (atomic number
3) and chlorine (atomic number 17). Its
lattice energy may be calculated from a Born-Haber cycle using the following
experimental data.
DH/kJ mol-1
First
ionisation energy of lithium =
+520
Heat
of atomisation of lithium =
+159
Heat of formation of
lithium chloride =
-409
Heat of atomisation of chlorine per mole
of chlorine atoms = +122
Electron affinity of
chlorine atoms =
-349
(a) Using the usual chemical symbols, the state symbols (s),
(1), (g) and the symbol for an electron, e-, write an equation in
each case to define the following terms.
(i)
The first ionisation energy of lithium
(ii)
The second ionisation energy of lithium.
(iii)
The heat of formation of lithium chloride.
(iv)
The electron affinity of chlorine.
(v)
The lattice energy of lithium chloride.
[5]
(b)
(i)
Construct a
labelled Born-Haber cycle for the formation of lithium chloride.
[6]
(ii)
Using the
constructed Born-Haber cycle, or any other method, calculate the lattice energy
of lithium chloride.
kJ mol-1
[2]
(c) The lattice enthalpies for sodium chloride, potassium chloride and potassium bromide are + 780, +710 and +680 kJ mol-1 respectively. Comment on these results and that for lithium chloride.
[4]
NI98S