Hess’s Law of Constant Heat Summation

What Is Hess’s Law?

Hess Law of heat summation states if the reaction happens in a single step or a series of steps, the enthalpy will always remain the same. Hence, the Hess Law of constant heat summation states that regardless of the reaction’s path, its enthalpy will always stay the same.

In other words, we can say that the total enthalpy change is independent of the steps taken by the reaction.

State and Explain Hess’s Law of Constant Heat Summation

Russian chemist and Doctor Germain Hess was the person who formulated the Hess Law. He observed that the heat change of a specific reaction is equal to the sum of the heat change of a series of reactions, which is equal to the sum of the overall reaction.

In simple words, the heat change for a specific reaction in a single step is equal to the heat change of the same chemical reaction that happens in multiple stages.

To sum up, the total enthalpy change in a chemical reaction is equal to the sum of all the changes, no matter the number of steps it takes to complete.

Thus heat change is not affected by the step taken by the chemical reaction to the process. It remains the same for a specific reaction irrespective of the chemical reaction’s path.

Therefore, the definition of Hess law of constant heat summation is:

“The enthalpy change in a chemical reaction is the same and independent of the path or steps the reaction takes.”

Explain Hess’s Law with Example

Let us take an example to understand the Hess law of constant heat summation.

Example 1

In preparing methylene chloride, the chemical reaction can take two courses. Let us observe both the ways and understand the Hess Law of constant heat summation.

Chemical reaction path I:

CH4​(g) + 2Cl2​(g)→ CH2​Cl2​(g) +2HCl(g), ΔH10​=−202.3kJ

Chemical reaction path II:

CH4​(g) + Cl2​(g)→ CH3​Cl(g)+ HCl(g), ΔH20​=−98.3kJ

CH3​Cl(g) + Cl2​(g) → CH2​Cl2​(g) + HCl(g), ΔH30​=−104.0kJ

Total heat change for the chemical reaction path II can be obtained by adding the steps:

CH4​(g) + 2Cl2​(g)→ CH2​Cl2​(g )+ 2HCl(g) ΔH0 = (-98.3-104) = −202.3kJ

Thus you can see that for the preparation of methylene chloride, whether you go by path 1 or path 2 the total enthalpy change for both paths remains the same.

ΔH10​=ΔH20​+ΔH30​=−202.3kJ

Example 2

Now consider the formation of carbon dioxide. The two ways CO2 can be formed are:

Path 1: By burning carbon in the presence of an excess of oxygen.

C(s)+O2→CO2(g)

ΔrH= -393.5 k J

Path 2: By burning carbon in a controlled supply of oxygen that forms CO. This CO is then converted to CO2.

C(s)+1/2O2(g)→CO(g);  ΔrH  = -110.54 kJ

CO(g)+1/2O2(g)→CO2(g);  ΔrH  = -283.02 kJ

The total heat change in the above reaction can be calculated by adding the heat change of both steps.

C(s)+O2(g)→CO2(g);  ΔrH  = -393.5 kJ

As you can see, ΔrH remains the same in path 1 and path 2. Hence this proves the Hess law of constant heat summation.

Example 3

Let us consider the formation of an aqueous solution of ammonium chloride NH3HCl and water. The two ways the chemical reaction can take place are:

Path 1:

 NH3(g) + HCl(g) → NH4Cl(s); ΔrH  =175.73 kJ

NH4Cl(s)+aq → NH4Cl(aq); ΔrH  = +16.32 kJ

On adding both the steps

NH3(g)+HCl(g)+aq → NH4Cl(aq); ΔrH  = -159.41kJ

Path 2:

NH3(g) +aq → NH3(aq);  ΔrH  = -35.15 kJ

HCl (g)+aq →HCl(aq); ΔrH  = -72.38 kJ

NH3(aq)+HCl(aq)  → NH4Cl(aq); ΔrH  = -51.36 kJ

On adding all the steps for total heat change

NH3(g) + HCl(g)+aq → NH4Cl(aq)  ΔrH  = -158.99 kJ

Therefore, as you can see, the enthalpy change for a specific chemical reaction of formation of ammonium chloride remains the same irrespective of the paths. (However, the slight difference in 0.42kJ may be due to experimental error.)

What are the applications of the Hess Law of Constant heat summation?

The application of Hess law of constant heat summation is as follows:

1.     The Law helps to calculate the heat change of chemical reactions and the formation of compounds that are not practically possible to conduct.
2.     It helps in calculating the heat change of allotropic transformations.
3.     It is used in the determination of the enthalpy of hydration.
4.     It is applicable in calculating the enthalpy change of extremely slow chemical reactions.
5.     The Law is applicable in determining bond energies.
6.     It is used in knowing the lattice energy.

Conclusion

The Hess Law of constant heat summation is one of the most important laws in thermodynamics. It is based on the First Law of thermodynamics which deals with heat change. Hess is significant because it proves that enthalpy is a state function.

Hess’ law is based on the initial and final condition of the reactants and not on the steps taken by the chemical reaction.