82 ENERGETICS
i Enthalpy
increase ,
M+(g)+e~+X(g)M(g)+X(g) (^3 4) V M^4 (
It
M(s)+'/2X 2 (g) [I 5
AHq
v MX(s) i
- The above Born-Haber cycle represents the enthalpy changes in
the formation of an alkali metal halide MX from an alkali metal
(Li, Na, K, Rb, Cs) and a halogen (F 2 , C1 2 , Br 2 or I 2 ).
(a) Name the halogen for which the enthalpy change 2 has the
largest value.
(b) Name the alkali metal for which the enthalpy change 3 has
the largest value.
(c) Name the halogen for which the enthalpy change 4 has the
smallest value.
(d) Name the alkali metal halide for which the enthalpy change
5 has the smallest value.
(e) The following is a list of the enthalpy changes for potassium
bromide (m kJ mol ~^1 ):K(s)
K(g)
iBr 2 (g)
Br(g) +-* K+(g) -f
-> Br(g)
g- -^Br-(g)
-Br~(g)->KB(s)AH= +92- e~ AH= +418
A/f = +96
AH= -326
AH= -677
Calculate the standard enthalpy of formation. AHp of
potassium bromide.
(JMB, A)- Comment on the following:
(a) Despite the thermochemical data contained in the following
equations, sodium metal reacts vigorously and exothermically
with chlorine gas.Na(s) -> Na+(g); AH = 144kcal.
£Cl 2 (g)->Cr(g); AH= -61.8 kcal.