Page 135 - Chemical equilibria Volume 4

Determination of the Values Associated with Reactions – Equilibrium Calculations 111
4.2.1. Experimental determination of the reaction enthalpies by
calorimetry
The development of knowledge of reaction enthalpies is closely linked to
that of reaction heats, and therefore to calorimetry and thermochemistry.
Experimental measurement of a reaction heat at constant pressure at a
temperature determined by calorimetry requires the measurement to respect
a set of essential precautions:

– the first condition is that the tester must control the triggering of the
reaction so that it does not start before the measurements begin to be taken;

– the reaction must be sufficiently quick as to enable us to measure the
heat involved over the course of the whole reaction without the losses of heat
from the calorimeter due to radiation interfering with the measurement;
– the reaction must not be accompanied by secondary reactions for which
the reactions’ heats are not known; and
– the products used must be sufficiently pure.

These conditions mean that direct measurement of the heat of a large
number of reactions is still tricky, and therefore a significant amount of data
still needs to be improved.

4.2.2. Calculation of the standard enthalpy at another temperature

If we know the enthalpy at a temperature T 0, it is easy to use a relation
such as [4.7] to calculate that enthalpy at any temperature T, provided we
know the variations of the molar specific heat capacities of the reaction
components between temperatures T 0 and T and the enthalpies of state
change of those components within that temperature range.

4.2.3. Influence of the pressure on the reaction enthalpies

Consider the general relation giving the variations of enthalpy with
pressure:

∂ H = PV [4.11]
∂ P