Question

State Hess’s law.

Answer 1

We know about the principle of conservation of energy. It states that total energy always remains the same. It cannot be destroyed or made. Hess’s law is just like that the total change in enthalpy remains the same for a chemical reaction. Enthalpy is determined as the heat of the system which undergoes change during a reaction.

Complete step-by-step answer:
Hess’s law is also known as Hess’s law of constant heat summation. According to this law, if we did a reaction in one step or more than one step, the total enthalpy change for a chemical reaction remains the same always.

During a chemical reaction, molecules dissociate into atoms, atoms form ions, and solid converts into gas. Some processes require the addition of heat whereas during some process heat releases so, the overall change in enthalpy is determined by addition and subtraction of heat value for each process.

The formula to determine the enthalpy change for a single reaction is as follows:
$\Delta {\text{H}}\,{\text{ = }}\,\Delta {{\text{H}}_{\text{f}}}{\text{(product)}} - \Delta {{\text{H}}_{\text{f}}}{\text{(reactant)}}$
Consider the example given below, the formation of ammonia from hydrogen and nitrogen. We can write the equation as follows:
${{\text{N}}_{\text{2}}}{\text{(g)}}\,{\text{ + 3}}\,{{\text{H}}_{\text{2}}}{\text{(g)}}\, \to \,{\text{2N}}{{\text{H}}_{\text{3}}}{\text{(g)}}$

Enthalpy change for this reaction is, $- 92.2\,{\text{kJ}}$.

We can form ammonia by the following way;
${{\text{H}}_{\text{2}}}{\text{(g)}}\,{\text{ + }}\,{{\text{N}}_{\text{2}}}{\text{(g)}} \to \,{\text{2N}}{{\text{H}}_{\text{3}}}{\text{(g)}}$…..$(1)$; enthalpy change is $+ 95.2\,{\text{kJ}}$
${{\text{H}}_{\text{2}}}{\text{(g)}}\,{\text{ + }}\,{{\text{N}}_{\text{2}}}{\text{(g)}} \to \,{\text{2N}}{{\text{H}}_{\text{3}}}{\text{(g)}}$…..$(2)$; enthalpy change is $- 187.4\,{\text{kJ}}$
On adding reaction $(1)$ and $(2)$ we get,
${{\text{N}}_{\text{2}}}{\text{(g)}}\,{\text{ + 3}}\,{{\text{H}}_{\text{2}}}{\text{(g)}}\, \to \,{\text{2N}}{{\text{H}}_{\text{3}}}{\text{(g)}}$
$= \, + 95.2\,{\text{kJ}}\,{\text{ + }}\,\left( { - 187.4\,{\text{kJ}}} \right)$
$= \, - 92.2\,{\text{kJ}}$
So, we can also form ammonia by the addition of reaction $(1)$ and $(2)$ which also give the same enthalpy change.

Note: The change in enthalpy is additive. If we give the heat for the occurrence of any process and we subtract it and if the heat evolves during a process we add it to determine the total change in enthalpy. The reaction in which heat evolves is known as an exothermic reaction. The reaction in which heat is utilized is known as an endothermic reaction. According to Hess’s law heat change does not depend upon the path followed by the reaction.