Ex: exothermic reaction
� The heat-pack reaction with oxygen is expressed as:
4Fe + 3O2 ----------> 2Fe2O3 + 1625 kJ
� According to the equation, the reactants of this exothermic reaction lose heat,
therefore, Hproducts < Hreactants
� When Hreactants is subtracted from the smaller Hproducts, a negative value for
change in Hrxn is obtained.
� This tells that the reaction is exothermic because for exothermic reactions, the value
for change in Hrxn is always negative
� The equation for the heat pack reaction is written like this:
4Fe + 3O2 -----------> 2Fe2O3 change in Hrxn = -1625 kJ
� 1625 kJ of energy is released in the reaction
Ex: endothermic reaction
� For the cold- pack reaction the equation was:
27 kJ + NH4NO3 ---------> NH4 + NO3
� For this endothermic reaction, change in Hproducts > change in Hreactants
� Therefore, when change in Hreactants is subtracted from the larger change in
Hproducts, a positive value for change in Hrxn is obtained
� Enthalpy changes for endothermic reactions are always positive
� The equation for the cold-pack reaction and its enthalpy change is written like this:
NH4NO3 ---------> NH4 + NO3 change in Hrxn = 27 kJ
� 27 kJ is absorbed in this reaction
� Recall that q is the heat gained or lost in a chemical reaction or process
� When the reaction takes place at a constant pressure, the subscript p is sometimes
added to q
� The enthalpy change is equal to qp
� Because almost all reactions occur at a constant pressure, you can assume
that q = change in Hrxn
