Concise Physical Chemistry

c04 JWBS043-Rogers September 13, 2010 11:24 Printer Name: Yet to Come

66 THERMOCHEMISTRY

The change in
rHfor relatively small changes inTis
CP
T. We can test this

result against experimental values for the simple hydrogenation of ethene (Kisti-

akowsky et al., 1935). Using the computed result for
CP
Tover the temperature

range 355–298, the range between the temperature at which his experiments were

carried out (355 K) and standard temperature, we get

hydH^355 =− 19. (^28) [−( 355 − 298 )]= 1 .099 kJ mol−^1
for the change in
hydH^355 , from 355 K, the temperature at which the hydrogenation
was actually carried out to room temperature 298 K. This small temperature correction
gives

hydH^298 =− 137. 44 + 1. 10 =− 136 .34 kJ mol−^1
for the enthalpy of hydrogenation of ethene at 298 K. This value is to be compared to
the result of a series of experiments carried out at 298 K by a different experimental
method which led to
hydH^298 =− 136. 29 ± 0 .21 kJ mol−^1. The computed heat
capacities are probably reliable over a temperature range of±50 K or so. Furthermore,
the enthalpy of hydrogenation itself is insensitive to temperature, so we may take
experimental determinations of
hydHcarried out under normal laboratory conditions
as essentially the same as the standard state
hydH 298 ◦.
When calculations are carried out over larger temperature ranges, as they often are
in industrial applications, a polynomial approximation to the heat capacity is used.
Cp=α+βT+γT^2 +···
This enables one to determineCpfor the reactants and products of a chemical reaction
at some new temperature other than 298 K, thereby enabling one to determine the
new change in heat capacity
Cpfor the reaction:

CP=

∑

CP(products)−

∑

CP(reactants)

Variation in the
Hof physical and chemical processes with variation in pressure

can be calculated from equations of state or by acquisition of experimental data and

curve fitting. Many reactions are less sensitive to pressure change than to temperature

change over comparable ranges. Metiu (2006) has treated both temperature and

pressure variation in the industrial production of ammonia from its elements.

4.10 DIFFERENTIAL SCANNING CALORIMETRY

When moderate amounts of heat are supplied to a solution of simple salts in water,

we expect a smooth heating curve between, say 290 and 320 K, like the lower curve

in Fig. 4.3. The heat capacity of water is nearly constant over this temperature range,