Notice the differences between this equation and eqn 2.37. First, we do not write Δq,
because q is not a state function and the energy supplied as heat cannot be expressed
as qf − qi. Secondly, we must specify the path of integration because q depends on the
path selected (for example, an adiabatic path has q = 0, whereas a non-adiabatic path
between the same two states would have q ≠ 0). This path-dependence is expressed by
saying that dq is an ‘inexact differential’. In general, an inexact differential is an
infinitesimal quantity that, when integrated, gives a result that depends on the path
between the initial and final states. Often dq is written pq to emphasize that it is inexact
and requires the specification of a path.