Page 59 - Instant notes

Thermochemistry 45

The Born-Haber cycle is a specific example of Hess’s law which
allows indirect measurement of the lattice enthalpy for an ionic
material from ∆H f of the material and the enthalpy changes
associated with the formation of gaseous cations and anions from
the elements in their standard states.

Related topics The first law (B1) Entropy and change (B5)
Enthalpy (B2) Free energy (B6)
Entropy (B4) Statistical thermodynamics (G8)

Standard state

The enthalpy changes associated with any reaction are dependent upon the temperature
(Topic B2). They are also dependent upon the pressure, and the amounts and states of the
reactants and products. For this reason, it is convenient to specify a standard state for a
substance. The standard state for a substance 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 (see Topic D1).
The definition of a standard state allows us to define standard enthalpy change as
the enthalpy change when reactants in their standard states are converted into products in
their standard states. The enthalpy change may be the result of either a physical or a
chemical process. The standard enthalpy change for a process is denoted as with
the subscript denoting the temperature.

Biological standard state

The standard state for hydrogen ion concentration is defined as an activity of 1
corresponding to pH=0. With the exception of, for example, stomach acid, biological
systems operate at pH values which are far removed from this highly acidic standard. It is
convenient, therefore, for biochemists to define the biological standard state of a
−7
hydrogen ion solution to be equal to pH=7, corresponding to an activity of 10 . The
standard state for all other species is an activity of 1. Biological standard conditions are
denoted by a superscript , for example . Thermodynamic values for a reaction at
the biological standard state only differ from that of the conventional standard state when
a proton is lost or gained in the reaction.