Question

Hess 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

Answer 1

Answer: (A)

According to second law of thermo-chemistry the law states that the total heat change $(\Delta\,H)$ 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, $\Delta\,H=\Delta\, H_{1}+\Delta\,H_{2}+\Delta\,H_{3}$
Enthalpy change in a reaction is always constant and independent of the path followed