Question

Hess’s law of constant heat summation is based on
(A)$E = m{c^2}$
(B)conservation of mass
(C)first law of thermodynamics
(D)$E = h\upsilon $

Answer 1

Hint: According to Hess’s law of constant heat summation, irrespective of the multiple steps involved in a reaction, the total enthalpy change for the reaction is the sum of all the changes. This law is also the reason why enthalpy is a state function.

Complete answer:
Hess’s law states that the increase in enthalpy in a given chemical reaction is independent of the processes between the initial and final state. This law is used to find the heats of reactions which do not take place directly. It is also used to find out the heats of very slow reactions, heat of formation, neutralization, etc. Hess’s law of constant heat summation is based on the principle of conservation of energy. It can be expressed in the first law of thermodynamics. This shows that enthalpy of given chemical species is a state function. Enthalpy of a system cannot be determined but the changes in enthalpy of the system can be found.

Therefore, option (C) is the correct answer.

Note:
Hess’s law is directly derived from the law of conservation of energy and its expression in the first law of thermodynamics. By this law, we can say that the net change in the enthalpy of the overall reaction is equal to the sum of the changes in the enthalpy of each intermediate transformation involved. Hess’s law allows us to combine chemical equations to generate new chemical reactions whose enthalpy changes can be calculated, instead of directly measuring them.