Page 157 - Chemical equilibria Volume 4

Determination of the Values Associated with Reactions – Equilibrium Calculations 133
Using the following relation, which is accurate at constant pressure:
dH
C = [4.43]
P
dT
we deduce the coefficients for relation [4.41].
4.4.1.2. Non-isothermal measurements by differential scanning
calorimetry
The sample is stabilized at a temperature T 1 in a low-inertia thermal
fluxmeter (differential scanning calorimeter). Linear programming of the
temperature with the constant slope dT /dt = β is then imposed in the kiln of
the instrument, up to the temperature of T 2. The thermal flux signal is then
recorded between times t 1 and t 2 (Figure 4.4), which are sufficiently far apart
for the flux signal to be stable. The experimental temperature T is taken as
the mean of the two temperatures T 1 and T 2.

c P and C P respectively denote the specific mass heat of the sample and its
molar specific heat capacity at constant pressure; m is its mass and M the
molar mass of the substance in question. We can write the following for the
flux:

dH dT m
Φ = = mc = C β [4.44]
dT P dt M P

Flux Temperature

Temperature T 2
ramp
T

T 1

0
Heat flux

Φ
time
t 1 t 2

Figure 4.4. Differential scanning calorimetry