Page 58 - Instant notes

B3
THERMOCHEMISTRY

Key Notes
The standard state for a material is defined as being the pure
substance at 1 atmosphere pressure, and at a specified
temperature. The temperature does not form part of the definition
of the standard state, but for historical reasons data are generally
quoted for 298 K (25°C). For solutions, the definition of the
standard state of a substance is an activity of 1. The standard
enthalpy change for a process is denoted as with the
subscript denoting the temperature.
The definition of the biological standard state is identical to the
standard state, with the exception of the standard state of
−7
hydrogen ion activity, which is defined as equal to 10 or pH=7.
Biological standard conditions are denoted by a superscript ,
for example . Thermodynamic values for a reaction under
standard biochemical conditions only differ from that of the
conventional standard state when a proton is lost or gained in that
reaction.
For the purposes of concise discussion, the enthalpy changes
associated with a number of common generic processes are given
specific names, although in thermodynamic terms, these
processes are treated identically.

Hess’s law of constant heat summation is primarily a restatement
of the first law of thermodynamics. It may be summarized as
‘The overall enthalpy change for a reaction is equal to the sum of
the enthalpy changes for the individual steps in the reaction
measured at the same temperature’ Hess’s law is particularly
useful in calculating enthalpy changes which cannot be easily
measured.
Tabulated values of the enthalpy of formation of materials may
be used to calculate the enthalpy change associated with a
reaction using the following, derived from Hess’s law:
∆H reaction =Σ∆H f (products)−Σ∆H f (reactants)
The enthalpy of combustion of reactant and product materials
may be used to calculate the enthalpy change associated with a
reaction in a similar manner to that of the enthalpy of formation:
∆H reaction =Σ∆H c (reactants)−Σ∆H c (products)
The ease with which ∆H c values may be obtained is offset by the
more limited scope of the expression.