NEET Chemistry Notes Chemical Thermodynamics-First Law of Thermodynamics
First Law of Thermodynamics
First Law of Thermodynamics
The first law states conservation of energy i.e. “Energy can neither be created nor destroyed although it may be converted from one form to another”
Sign Convention
- If W is positive — work done on the system
- If W is negative — work done by the system
- If q is positive — when heat is supplied to the system
- If q is negative — when heat is lost by the system.
is negative — energy transfer from system to surrounding
is positive — energy transfer from surrounding to system.
Enthalpy
The total heat content of a system at constant pressure is called the enthalpy of the system. Indeed, it is the sum of internal energy and the product of pressure-volume work. It is an extensive quantity and represented by the symbol H. The equation is
Enthalpy also changes, when a substance undergoes phase transition.
Enthalpy Change and Enthalpy Change of Various Processes
The difference between enthalpy of products and enthalpy of reactants is known as enthalpy change of reaction.
Standard Enthalpy Change of Reaction 
The measured enthalpy of a reaction at 25°C and 1 bar pressure is known as standard enthalpy change of reaction.
Enthalpies for Different Types of Reactions
