It is defined as total heat content of the system. it is equal to the sum of internal energy and pressure-volume work.
H = U + PV
Change in enthalpy:
Change in enthalpy is the heat absorbed or evolved by the system at constant pressure.
∆H = qp
For exothermic reaction (System loses energy to Surroundings),
∆H and qp both are -ve.
For endothermic reaction (System absorbed energy from the surrounding).
∆H and qp both are +ve.
Factors affecting enthalpy of reaction
- Physical state of reactants and products.
- Allotropic forms of elements involved.
- Chemical composition of reactants and products.
- Amount of reactants.
Relation between ∆H and ∆U
Let us consider a general reaction A → B
Let HA be the enthalpy of reactant A and HB be that of the products.
At constant pressure and temperature using ideal gas law,
An extensive property is a properly whose value depends on the quantity or size of matter present in the system.
For example: Mass, volume, enthalpy etc are known as extensive property.
Intensive properties do not depend upon the size of the matter or quantity of the matter present in the system.
For example: temperature, density, pressure etc. are called intensive properties.