Page 156 - Chemical equilibria Volume 4

132 Chemical Equilibria
Certain coefficients may be equal to zero.
In order to establish such a function, we can either take measurements at
different temperatures, each being taken at constant temperature, or take
measurements at a temperature which varies in a linear fashion over time.
4.4.1.1. Isothermal measurements with a drop calorimeter

In the isothermal method of determining the specific heat capacities, we
use a drop calorimeter.

Flux Φ
Sample
T 0

T
Fluxmeter time
a b

Figure 4.3. Drop calorimeter

We drop the sample (Figure 4.3(a)), taken to temperature T 0 (usually
25°C), into a thermal fluxmeter previously stabilized at the measuring
temperature T, which is usually higher than T 0. The heat flux is recorded
(Figure 4.3(b)) as a function of time, until the signal finally returns to its
initial value. The area beneath the curve gives the energy which was needed
to raise the temperature of the sample from temperature T 0 to the temperature
T. We work from this value of the exchanged energy to find its value per
mole by multiplying it by the ratio m/M of the mass of the sample to the
molar mass of the substance of which it is composed. The experiment is
restarted at different temperatures T. From these measurements, we establish
the coefficients of a law in the form:
b c d
3
H = aT + T + T − [4.42]
2
2 3 T