Page 57 - Instant notes

Enthalpy 43

sign of ∆H indicates the direction of heat flow and should always be explicitly stated, e.g.
−1
∆H=+2.4 kJ mol .
Table 1. Exothermic and endothermic processes

Heat change in system Process Value of ∆H

Heat loss (heat lost to the surroundings) Exothermic Negative
Heat gain (heat gained from the surroundings) Endothermic Positive

For a system experiencing a temperature change at constant pressure, but not undergoing
a chemical change, the definition of the constant temperature heat capacity is used in the
form C p=(∂q/∂T) p. Since ∂q equals ∂H at constant pressure, the temperature and enthalpy
changes are related through the relationship:

where ∆H T2−T1 is the enthalpy difference between temperatures T1 and T2.
Over smaller temperature ranges, within which the value of C p may be regarded as
invariant, this expression simplifies to ∆H=C p ∆T at constant pressure.
For chemical reactions, the most basic relationship which is encountered follows
directly from the fact that enthalpy is a state function. The enthalpy change which
accompanies a chemical reaction is equal to the difference between the enthalpy of the
products and that of the reactants:
∆H Reaction=ΣH (Products)−ΣH (Reactants)

This form of equation is common to all state functions, and appears frequently within
thermodynamics. Similar expressions are found for entropy (see Topics B4 and B5) and
free energy (see Topic B6).

Kirchhoff’s law

Because the enthalpy of each reaction component varies with temperature, the value of
∆H for a chemical reaction is also temperature dependent. The relationship between ∆H
and temperature is given by Kirchhoff’s law which may be written as

If the change in C p with temperature is negligible, this expression may be simplified to:
∆H T2−∆H T1=∆C p∆T