Page 140 - Physical chemistry understanding our chemical world

ENTHALPY 107

Inserting values:
−1
C V = (1 × 8.3 + 75.3) JK mol −1
−1
so C V = 83.6J K −1 mol .

The value of C V is 11 per cent higher than C p , so the water in the pressure cooker
will require 11 per cent more energy than if heated in an open pan.

Justification Box 3.3

Starting with the definition of enthalpy in Equation (3.16):

H = U + pV

The pV term can be replaced with ‘nRT’ via the ideal-gas equation (Equation (1.13)),
giving
H = U + nRT

The differential for a small change in temperature is
The values of n and R
dH = dU + nR dT are constants and do
not change.
dividing throughout by dT yields

∂H ∂U
= + nR
∂T ∂T
The first bracket equals C V and the second bracket equals C p ,so

C V = C p + nR

which is just Equation (3.24) rearranged. Dividing throughout by n yields the molar
heat capacities:
C V = C p + R (3.25)

Why does the temperature change during a reaction?

Enthalpies and standard enthalpies of reaction: H r and H r O

One of the simplest definitions of a chemical reaction is ‘changes in the bonds’. All
reactions proceed with some bonds cleaving concurrently with others forming. Each