Discuss factors governing heat of reaction. How is it determined ?

Heat of reaction also known as Enthalpy of reaction. It is define as the heat energy released during chemical reaction at constant pressure. It is thermodynamic unit. It measure energy release or gain in chemical reaction.

Factors governing heat of reaction :-

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

1.       Physical State of Reactants and Products

              Enthalpy changes with change in physical state due to  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

                  

                               Enthalpy depends upon the quantity of reactants used if  the number of moles of reactants are doubled its enthalpy change also and it becomes doubled.

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.)

Determination :-

It is determined by two method :
1. Experimentally
2. Numerically

1. Experimentally determination :-

It is experimentally determined by the help of calorimeter which is isolated system at constant pressure.

2. Numerically determination :-

It is also determined numerically by

H°= ∑ L*Enthalpy of Product - ∑ k *Enthalpy of Reactant

Where L and K is stoichiometry coefficient.