TIP
This paragraph gives the definition of the molar heat of formation.Notice that the ΔH for an endothermic reaction is positive, while that for an
exothermic reaction is negative. It should be noted also that changes in enthalpy
are always independent of the path taken to change a system from the initial state
to the final state.
Because the quantity of heat absorbed or liberated during a reaction varies
with the temperature, scientists have adopted 25°C and 1 atmosphere pressure as
the standard state condition for reporting heat data. A superscript zero on ΔH
(i.e., ΔH^0 ) indicates that the corresponding process was carried out under
standard conditions. The standard enthalpy of formation (ΔH^0 f) of a compound
is defined as the change in enthalpy that accompanies the formation of 1 mole of a
compound from its elements with all substances in their standard states at 25°C.
This value is called the molar heat of formation.
To calculate the enthalpy of a reaction, it is necessary to write an equation
for the reaction. The standard enthalpy change, ΔH, for a given reaction is usually
expressed in kilocalories and depends on how the equation is written. For
example, the following equations express the reaction of hydrogen with oxygen in
two ways:
Experimentally, ΔH^0 f for the formation of 1 mole of H 2 O(g) is −241.8 kJ.