Question

state and explain Hess's low​

Answer 1

Hess's law states that the change of enthalpy in a chemical reaction (i.e. the heat of reaction at constant pressure) is independent of the pathway between the initial and final states.

Answer 2

Answer:

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Explanation:

Hess’ law also known as Hess’s law of constant heat summation states, “at constant temperature, heat energy changes (enthalpy – ΔHrec) accompanying a chemical reaction will remain constant, irrespective of the way the reactants react to form product”.

Hess’ law is based on the state function character of enthalpy and the first law of thermodynamics. Energy (enthalpy) of a system (molecule) is a state function. So, enthalpy of reactant and product molecules is a constant and does not change with origin and path of formation.

The first law of thermodynamics states that the total energy of the substances before and after any (physical or chemical) change should be equal. According to the law, the total energy of the reactant should be equal to the total energy of the product. Any difference in the energy between the reactants and products is also fixed at a particular temperature and will not change with the path followed by the reactants to form products. Hence, heat energy also can be considered as a reactant or product of the reaction and included in the reaction.

Hence, exothermic reactions can be written as: A + B → C + D + ΔH

Similarly, endothermic reactions become: A + B + ΔH → C + D

This allows reactions containing reactants and products to be treated as algebraic equations and carrying out mathematical operations on them. It should be remembered, that an exothermic reaction in one direction will be endothermic in the reverse direction and vice-verse.

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