12.0 Standard Enthalpy of Reaction

  Thermodynamics and Thermochemistry

The enthalpy of reaction under standard state condition is called standard enthalpy of reaction. Factors which influences the heat of reaction

1. The physical state of reactants and products

$${H_2}\left( g \right) + \frac{1}{2}{O_2}\left( g \right) = {H_2}O\left( g \right);\quad \quad \Delta H = - 57.8{\text{ }}k.cal$$

$${H_2}\left( g \right) + \frac{1}{2}{O_2}\left( g \right) = {H_2}O\left( l \right);\quad \quad \Delta H = - 68.32{\text{ }}k.cal$$

Thus the difference in the value of $\Delta H$ if water is obtained in the gaseous or liquid state.

2. Allotropic forms of the elements: Heat energy is also involved where one allotropic form of the element is converted into another

$${C_{\left( {Diamond} \right)}} + {O_2}\left( g \right) \to C{O_2}\left( g \right)\quad \quad \quad \Delta H = - 94.3{\text{ }}kcal$$

$${C_{(amorphous)}} + {O_2}\left( g \right) \to C{O_2}\left( g \right)\quad \quad \quad \Delta H = - 97.6{\text{ }}kcal$$

(3) The way of presentation of reaction

$${N_2}\left( g \right) + 3{H_2}\left( g \right) \rightleftharpoons 2N{H_3}\left( g \right);\quad \quad \quad \Delta H$$$$\frac{1}{2}{N_2}\left( g \right) + \frac{3}{2}{H_2}\left( g \right) \rightleftharpoons N{H_{3\left( g \right)}};\quad \quad \quad \Delta {H_1} = \frac{{\Delta H}}{2}$$$$3{N_2}\left( g \right) + 9{H_2}\left( g \right) \rightleftharpoons 6N{H_3}\left( g \right);\quad \quad \quad \Delta {H_2} = 3\Delta H$$

$$2N{H_3}\left( g \right) \rightleftharpoons {N_2}\left( g \right) + 3{H_2}\left( g \right);\quad \quad \quad \Delta {H_3} = - \Delta H$$

(4) Temperature: Heat of reaction or enthalpy of reaction also depend on the temperature at which the reaction is carried out. This is due to variation in the heat capacity of the system with temperature.

(5) Reaction carried out at constant pressure or constant volume

(6) If two reaction are added then their $\Delta {H_r}$ are added and when they are subtracted, there $\Delta {H_r}$ are also subtracted.