Factors Affecting Enthalpy of Reaction

The various factors on which enthalpy of reaction depend are as follows:

1.            Physical State of Reactants and Products

                The enthalpy of reaction changes with change in physical state. This is because as the physical state changes, extent heat is evolved.

                Example:            

                                H₂ (g)  +  1/2 O₂ (g)  ------------>   H₂O (l)  ;   DH   =   – 286 KJ.

                                H₂ (g)  +  1/2 O₂ (g) ------------>    H₂O (g)  ;  DH  =  – 242 KJ.

2.            Quantity of Reactants                  

                                The change in enthalpy of reaction depends upon the quantity of reactants used. When the number of moles of reactants are doubled, the enthalpy change also becomes double.

3.            Allotropic Modification

For elements existing in different allotropic modifications, the heat of reaction is different if different allotropic form is involved in reaction.

                Example:            

C (graphite)   +    O₂ (g)  ------------>    CO₂ (g)   ;     DH  =  – 393.4 KJ

C (diamond)   +   O₂ (g)  ------------>   CO₂ (g)   ;    DH  =   – 395.4 KJ.

4.            Temperature and Pressure         

                                The enthalpy of reaction depends upon the temperature and pressure of reaction. Therefore, the values are generally expressed under standard conditions of temperature (298K) and pressure    (1 atm.)