Enthalpies of reaction
The overall change in the enthalpy between products and reactants in a reaction is called
the reaction enthalpy. The reaction enthalpy can be either positive or negative. As an
example, consider the reaction:
The change enthalpy when 1.000 mol of CO reacts completely with 0.500 mol of O at
25 C can be measured by calorimetry and is observed to be
The negative sign means that heat is given off by the system, rather than absorbed by it.
When heat is given off, the reaction is said to be exothermic, and
will be negative. If
heat is absorbed, the reaction is said to be endothermic. The reverse of the above reaction
would be endothermic:
Because enthalpy is an extensive quantity, if we multiply the reaction by a constant, the
enthalpy of the reaction is multiplied by the same constant. For example, if we consider
the decomposition of 2.000 mol of CO to give 2.000 mol CO and 1.000 mol of O
according to
the enthalpy of the reaction would be
Example:
Consider the reaction of solid phosphorous with liquid bromine:
Suppose 2.63 g of phosphorous reacts with an excess of bromine. Calculate the enthalpy
change.
Solution:
Noting that the specified
of -243 refers to a reaction of 2.000 mol of solid
phosphorous, we calculate the number of moles that correspond to 2.63 g:
Thus, the enthalpy change when 0.0849 mol of phosphorous react is
Hess's Law
Suppose a reaction is carried out in several steps. We have already seen that the overall
reaction is the algebraic sum of the individual steps. Because energy/enthalpy is an
additive quantity,
the overall enthalpy change for the overall reaction, obtained by adding the
individual chemical equations, will be the sum of the enthalpy changes associated
with each of the individual steps/chemical equations.
The above statement is known as Hess's Law.
Hess's Law can be used to calculate the enthalpy of reactions that cannot be carried out
straightforwardly in the lab. As an example, consider the reaction of solid carbon, in the
form of graphite, with oxygen gas to produce carbon monoxide:
This reaction cannot be carried out directly experimentally. Rather, a two-step process is
used, in which solid carbon is first converted to carbon dioxide via the reaction
whose enthalpy can be easily measured at 25 C. Then the decomposition of CO to give
oxygen gas and carbon monoxide can be added to this reaction according to: