THERMOCHEMICAL EQUATION: In Thermochemistry we deal with the energy changes associated with a chemical reaction. We learn how to apply the laws of thermodynamics on a reaction. A chemical reaction is associated with energy changes which may be represented in the form of various change in internal energy and enthalpy. The first aim is to make a chemical equation more informative that is to represent the energy changes associated with a chemical reaction with the equation and also represent the phases of various reactants and products.A chemical equation which gives us all this information is called “Thermochemical Equation”. The most important state function for a chemical reaction is the enthalpy change associated with it.
 The reason is that most of the reactions take place under constant pressure conditions and the heat exchanged between the system and surroundings at constant pressure is represented by H. Those reactions which release energy are called “exothermic reactions” and those reactions which absorb energy are called “endothermic reactions”. In exothermic reactions the enthalpy of the system decreases whereas it increases in endothermic reactions. Therefore for exothermic DH is negative and it is positive for endothermic reactions. For example, combustion of methane at constant pressure takes place with the evolution of 890 KJ energy per mol. This reaction can be represented as under:

CH4 (g) + 2O2 (g) ---------> CO2 (g) + 2H2O (g      H = – 890 KJ/mol

The above equation is a Thermochemical equation as it represents the reaction as well as the energy changes associated with it. We already know that enthalpy is the net heat content of a system. Then what does H for a reaction represent? It represents the change in enthalpy or the difference in enthalpy of products and reactants. There is a difference when we talk about enthalpy change for an ideal gas during a process and that of a chemical reaction. When we consider an ideal gas then the gas itself changes its state from some initial conditions to some final conditions but the gas remains the same chemically. In a chemical reaction the reactants change into products, some existing bonds are broken and new ones are formed, so it’s a chemical change which will be inherently associated with energy changes. That is why any change for a chemical reaction will be seen from this perspective, that is, change from reactants to products than the change in conditions.
Therefore the enthalpy change for a reaction can be represented as:
During a chemical reaction heat may flow in or out of the system. If the reaction takes place at constant volume then this heat is equal to the change in internal energy and if it takes place at constant pressure then it is equal to the change in enthalpy.
H = qP
U = qV
And the two terms can be related by the expression:
H = U + PV
H = U + (PV)
For an ideal gas we are more interested in the changes in P, V, T but for a reaction the significant parameter is the change in the number of gaseous moles due to conversion of reactants into products.
(PV) = (n) g RT
H = U + (n) g RT
Where n represents change in gaseous moles during the reaction.

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