Question

How do you draw a Hess’s law diagram?

Answer 1

The Hess’s law is major in physical chemistry which states that the total enthalpy change for a reaction is independent of the route by which the chemical change takes place. Formation of heat is defined as the enthalpy change which accompanies the formation of substance in a standard state from its elements also taken in standard state.

Complete step by step solution:
As we know that Hess law takes the independent route for the reaction. In thermodynamics, we need the initial and final states. So, we need to construct a Hess Cycle using the given information;

While we observe the diagram, in terms of energy, the dashed long route is equal to the black arrow route as the arrows start and finish in the same place. This diagram represents and follows the Hess law. So, we can write as; $(4\times 157.3)+\Delta H=292$
Therefore, $\Delta H=-337.2kJ$
We have found $\Delta H$ for below equation; $4Cu+2{{O}_{2}}\to 2C{{u}_{2}}O+{{O}_{2}}$
This means the same ; $4Cu+{{O}^{2}}\to 2C{{u}_{2}}O\text{ }$
This means the formation of two moles of copper(I) oxide. So, we need to find the change in enthalpy for the formation of copper(I) oxide. Therefore,
$\Delta {{H}_{f}}\left[ C{{u}_{2}}O \right]\text{=}\dfrac{\Delta H}{2}=-\dfrac{337.2}{2}=-168.6\text{ }kJ/mol\text{ }$
So, for the formation of one mole of copper(I) oxide we need $-168.6\text{ }kJ/mol\text{ }$ enthalpy.

Note: We must know that Hess’s law is a version of the fine law of thermodynamics. So, it means the energy is always conserved. Often Hess’s law cycles are used to measure the change in enthalpy for a reaction that can’t be measured directly by experiments. Instead, the alternative method/ reactions are carried out that can be measured experimentally.