Q. Hess’s law states that
A) The standard enthalpy of an overall reaction is the sum of the enthalpy changes in individual reactions.
B) Enthalpy of formation of a compound is same as the enthalpy of decomposition of the compound into constituent elements, but with opposite sign.
C) At constant temperature the pressure of a gas is inversely proportional to its volume.
D) The mass of a gas dissolved per litre of a solvent is proportional to the pressure of the gas in equilibrium with the solution.
Explanation
(a)
According to second law of thermo chemistry the law states that the total heat change ($\mathrm{\Delta}H$) accompanying a chemical reaction is the same whether the reaction takes place in one or more steps.
It means that heat of a reaction depends only on the initial reactants and final products and not on intermediate products that may be formed.
Now, $\mathrm{\Delta}H = \mathrm{\Delta}H_{1} + \mathrm{\Delta}H_{2} + \mathrm{\Delta}H_{3}$
Enthalpy change in a reaction is always constant and independent of the path followed.
Reviewed by Dr. Abubacker
Physics Educator & NEET/JEE Specialist
Ph.D. in Physics with 19+ years of teaching experience.