The change in enthalpy of a reaction

The change in enthalpy of a reaction is the net transfer of heat of the reaction at constant pressure.

As explained in the previous article, we can use eq1 or eq2 to calculate the change in energy of a reaction. It is difficult to measure the absolute potential energies of the products and reactants, Ep and Ert. It is also hard to separately quantify the heat transferred from the surroundings to the system q1 during the bond breaking process, and the heat transferred from the system to its surroundings q2 during the bond forming process. However, qnet, the net change in energy during the reaction, is easily measured through experiments. Since many chemical reactions occur at constant pressure, the net transfer of heat qnet at constant pressure is denoted by ΔH, which is called the change in enthalpy of the system. Consequently, the potenital energy profile diagrams are replaced with enthalpy diagrams:

which can also be in the form of enthalpy curves:

where Ea is the activation energy of the reaction, i.e. the minimum amount of energy needed for the reactants to form products.

An endothermic reaction is therefore also defined as a reaction where its change of enthalpy is positive and an exothermic reaction is a reaction where its change of enthalpy is negative. An example of an endothermic reaction is the thermal decomposition of calcium carbonate where net energy is absorbed to breakdown the carbonate into the products:

CaCO_3(s)\; \begin{matrix} \rightarrow \\ heat \end{matrix}\; CaO(s)+CO_2(g)\; \; \; \; \; \; \; \; \Delta H=+ve

An example of an exothermic reaction is the combustion of methane where net energy is released in the formation of the relatively stable products of carbon dioxide and water:

CH_4(g)+2O_2(g) \rightarrow CO_2(g)+2H_2O(l)\; \; \; \; \; \; \; \; \Delta H=-ve

Theoretically, every reaction is reversible to a certain extent. As shown in the diagram below, the reverse reaction (of any reaction) has an equal magnitude of enthalpy change as the forward reaction but with an opposite sign, e.g. the reverse reaction of the combustion of methane is:

CO_2(g)+2H_2O(l)\rightarrow CH_4(g)+2O_2(g) \; \; \; \; \; \; \; \; \Delta H=+ve

Note that the activation energy for the reverse reaction is

E_a\, '=E_a+\left | \Delta H \right |


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