Define the term 'enthalpy'.
What will happen to the internal energy if work is done by the system ?

Explanation

The enthalpy of reaction is defined as the summation of internal energy of the reactin system, plus the product of pressrue and volume. It is given by :
`H=U+PV`
The first law of thermodynamics states that:
`DeltaU=q+w` where
U is the internal energy of the system
Q is the heat that may flow into or out of the system and W is the work done on or by the system
If work is done by the system then its internal energy must decrease. Work done by system is considered negative, Thus,
`DeltaU=q-W.`