i ∑ b i ∆ f H (reactants) ( . ) where a and b represent the coefficients of the products and reactants in the balanced equation. Let us apply the above equation for decomposition of calcium carbonate. Here, coefficients ‘a’ and ‘b’ are each.
Therefore, ∆ r H = ∆ f H = [CaO(s)]+ ∆ f H [CO (g)] – ∆ f H = [CaCO (s)] = (– . kJ mol – ) + (– . kJ mol – ) – (– . kJ mol – ) = .
kJ mol – Thus, the decomposition of CaCO (s) is an endothermic process and you have to heat it for getting the desired products. (d) Thermochemical Equations A balanced chemical equation together with the value of its ∆ r H is called a thermochemical equation. We specify the physical state (alongwith allotropic state) of the substance in an equation. For example: C H OH( l ) + 3O (g) → 2CO (g) + 3H O( l ); ∆ r H = – kJ mol – The above equation describes the combustion of liquid ethanol at constant temperature and pressure.
The negative sign of enthalpy change indicates that this is an exothermic reaction. It would be necessary to remember the following conventions regarding thermo- chemical equations. . The coefficients in a balanced thermo- chemical equation refer to the number of moles (never molecules) of reactants and products involved in the reaction.
. The numerical value of ∆ r H refers to the number of moles of substances specified by an equation. Standard enthalpy change ∆ r H will have units as kJ mol – . To illustrate the concept, let us consider the calculation of heat of reaction for the following reaction : Fe O s Fe s H O l , From the Table ( .
) of standard enthalpy of formation ( ∆ f H ), we find : ∆ f H (H O, l ) = – . kJ mol – ; ∆ f H (Fe O ,s) = – . kJ mol – ; Also ∆ f H (Fe, s) = and ∆ f H (H , g) = as per convention Then, ∆ f H = (– . kJ mol – ) – (– .
kJ mol – ) = (– . + . ) kJ mol – = – . kJ mol – Note that the coefficients used in these calculations are pure numbers, which are equal to the respective stoichiometric coefficients.
The unit for ∆ r H is kJ mol – , which means per mole of reaction . Once we balance the chemical equation in a particular way, as above, this defines the mole of reaction . If we had balanced the equation differently, for example, Fe O s Fe s H O l