Hess’s Law

In the field of both physics and chemistry the specific values related to equations are important. Germain Henry Hess in 1840,  discovered the revolutionary Hess’s law. 

A+B→C                         ΔH1 

C+D→E                          ΔH2

Therefore, if A+B+D →E             ΔH

 ΔH= ΔH1 + ΔH2

Like the above example, we can easily and accurately  solve  ΔH or the enthalpy change of the entire complex reaction by adding the intermediate enthalpy changes for each step. 

Here, we will learn about the two types of reaction – endothermic reaction and exothermic reaction. An endothermic reaction is a reaction where heat energy is absorbed during the process. In an exothermic reaction, the heat energy produced during the reaction is released into the environment. 

Importance of Hess’s Law

Because of Hess’s Law, the chemical reaction can be decomposed into several steps, and the standard enthalpy of formation can be used to determine the total energy of the chemical reaction. Standard enthalpy tables are usually edited from empirical data obtained from calorimetric measurements. You can use these tables to calculate whether more complex reactions are thermodynamically advantageous. Hess’s Law is also important for the following reasons:

1. It helps compute the enthalpies of arrangement of the compounds that cannot be defined experimentally. 
2. It helps specify the enthalpy of the transformation of compounds like coal or graphite into diamond, also termed as allotropic transformation.
3. It can help in evaluating the hydration enthalpy.

Hess’s Law depends on the first law of thermodynamics, which is:

Energy can neither be destroyed nor created, but it can be transformed into another form of energy.

According to Hess’s Law, the total enthalpy change is the same when converting a reactant A to a product B in one step, two steps, or many steps. 

Let us take a simple example. You are on the 1st floor of a 5-star hotel and want to go to the 3rd floor. This can be done in the following three ways: 

(A) Move the elevator straight from the 1st floor to the 3rd floor. 

(B) Move from the 1st floor to the 2nd floor by elevator, stop a little on the 2nd floor, and then move from the 2nd floor to the 3rd floor by elevator. 

(C) Move from the 1st floor to the 1st floor by elevator, stop a little on the 1st floor, and then move from the 1st floor to the 3rd floor by elevator. No matter which route you choose, the elevator will use the same amount of energy.

An example showing the implication of Hess’s Law:

Considering the formation of an aqueous solution of ammonium chloride from water and NH3HCL, there are two ways to achieve this reaction:

1. NH3 + HCL→ NH4CL; ΔH  =175.73 kJ

NH4CL+ water → NH4CL; ΔH  = +16.32 kJ

Adding the above equations: 

NH3+ HCL+ water → NH4CL; ΔH  = -159.41kJ

1.  NH3 + water → NH3;  ΔH  = -35.15 kJ

HCL + water →HCL; ΔH  = -72.38 kJ

NH3 + HCL  → NH4CL; ΔH  = -51.36 kJ

On adding NH3 + HCL+ water → NH4CL;  ΔH  = -158.99 kJ

Comparing the ΔH of 1 and 2, we see that they have almost the same value. This is an illustration of Hess’s Law.

Multi-Step Reaction and Multi-Different Reaction

Multi-step and multi-different reactions are the two forms of Hess’s Law. 

If a reaction occurs and the product formation needs multiple steps with the involvement of intermediate products, it is called a multi-step reaction. An example of a multi-step reaction is the formation of carbon dioxide when oxygen reacts with carbon. It is a multi-step reaction where the first intermediate product is carbon monoxide, which is again converted to carbon dioxide. 

In the case of multi-different reactions, the reactants and products of the required chemical reaction are obtained by summing many other chemical reactions. The enthalpy of the required reaction from the reactants to the product is the sum of the enthalpy changes of all those chemical reactions. An example of multi-different reactions is the combustion of sulphur, disulphide, and carbon to give enthalpy of -296.8KJ, -1075KJ, and -393.5KJ. If we see this reaction carefully in the algebraic equation, we get:

CO2  =  -393.5KJ

SO2 =  -296.8KJ

CO2 + SO2 = -1075KJ

This is an example of Hess’s Law with multi-different reactions. 

Application of Hess’s Law

Some applications of Hess’s Law are as follows:

• In the case of crystalline solid, we can find the lattice energy with the help of Hess’s Law.
• Hess’s Law is used to calculate the bond enthalpy. If we know the bond energy of the reactants, we can easily calculate the enthalpy of the product formation. 
• The heat formation, neutralisation, combustion, etc., can be calculated with the help of Hess’s Law. During the formation of hydrocarbons, heat formation can be easily calculated to find the change in heat energy. 
• It is used to determine the enthalpies of the product and the reactant. 
• Hess’s Law helps determine the enthalpy of physical change, like allotropic transformation in the case of graphite to diamond.  

Conclusion

Hess’s Law is based on the enthalpy’s state-function characteristic and the first law of thermodynamics. Therefore, the molecular enthalpy of reactants and products is constant and does not change with origin and pathway. The first law of thermodynamics states that the total energy of matter must be the same before and after any chemical or physical changes. By law, the total energy of the reactants must equal the total energy of the product. The energy difference between the reactants and the product is also fixed at a particular temperature. It does not change depending on the path that the reactant follows to form the product. You can easily find the importance of Hess’s Law and its application if you search for notes on Hess’s Law.