SAT Subject Test Chemistry,10 edition

on the enthalpies of the initial and final states, not on the path. Thus, to find the enthalpy change of
a reaction, ∆Hrxn, one may subtract the enthalpy of the reactants from the enthalpy of the products:

∆Hrxn   =   Hproducts   –   Hreactants

BASIC CONCEPT

ΔH  negative    = exothermic

= heat  given   off

ΔH  positive    = endothermic

=heat   absorbed

A positive ∆H corresponds to an endothermic process (absorbs heat), and a negative ∆H corresponds
to an exothermic process (releases heat). The value of enthalpy change (or of enthalpies in general)
is dependent on external conditions such as temperature and pressure. As we shall see, enthalpy
plays an important role in determining the favorability of a reaction, but first we need to discuss
ways in which one can calculate or obtain values for changes in enthalpy.

Hess’s Law

Hess’s law is simply the application of the concept of path-independence to enthalpy. It states that if
a reaction can be broken down into a series of steps, the enthalpy change for the overall net
reaction is just the sum of the enthalpy changes of each step. The steps need not even correspond
to actual processes carried out in the real world or in the lab, but can be purely hypothetical. For
example, consider the reaction:

Br 2    (l) →   Br 2    (g) ∆H  =   31  kJ

The enthalpy change of the above reaction will always be 31 kJ/mol provided that the same initial
and final states Br 2 (l) and Br 2 (g) are operative. Instead of direct vaporization, Br 2 (l) could first be
decomposed to Br atoms and then recombined to form Br 2 (g); since the net reaction is the same