Hess Law

Introduction

Hess law was introduced by the Russian Chemist Doctor Germain Hess in the year 1840 where he formulated many principles of thermochemistry. Hess law is an effect of the first law of thermodynamics. But it exists as a single law because of its significance. The calculation of heat of reaction is involved in this law.

The first law of thermodynamics states that the enthalpy difference in a particular reaction is equal to the quantity of energy absorbed or discharged during the reaction. Hess law also deals with the energy of a chemical reaction in which it does not depend on the path followed or steps involved instead the change in energy is always a constant.

What is Hess's law?

Hess law states that the enthalpy difference or energy alteration involved in a chemical reaction does not vary with its route or the number of steps involved. This means that the value of energy change is always constant. Whether it follows 3 steps or 4 steps, the energy change will be the same. Hess law allows for the calculation of enthalpy change accompanied by chemical reactions in which the energy changes are not known.

The value of enthalpy will rely on the number of molecules involved in a reaction since enthalpy is an extensive property. So, enthalpy change will also rely on the number of molecules involved in chemical reactions. And so, the enthalpy change will be the same even if it takes a different path or several steps. Only it is necessary to have the initial and final states.

The energy is also in the aspect of heat. So, in the exothermic process energy is liberated. While in an endothermic process energy is consumed. A reaction which is exothermic in the forward direction must be endothermic in the backward direction. In this way the value of energy in a chemical reaction is constant.

It can also be defined as the energy of any chemical process is equal to the addition of energies of all the steps, that is energy is always a constant for a particular chemical reaction. That is the overall enthalpy change, $\mathrm{\Delta\:H\:=\:\Delta\:H_{1}\:+\:\Delta\:H_{2}\:+\:\Delta\:H_{3}\:+\Delta\:H_{4}\:+\:\Delta\:H_{5}\:+\:\Delta\:H_{6}}$. It can be represented as,

Illustration of Hess law

Hess law can be illustrated by taking an example. The chemical process in Hydrogen gas produced,

$$\mathrm{C(s)\:+\:2H_{2}O(g)\:\rightarrow\:CO_{2}(g)\:+\:2H_{2}(g)\:,\:\Delta\:H\:=\:90.1\:Kj}$$

This reaction can be converted into different steps as,

$$\mathrm{C(s)\:+\:O_{2}(g)\:\rightarrow\:CO_{2}(g),\Delta\:H\:=\:-393.5Kj}$$

$$\mathrm{2H_{2}(g)\:+\:O_{2}(g)\:\rightarrow\:2H_{2}O(g)\:,\:\Delta\:H\:=\:-483.6\:Kj}$$

But the change in Enthalpy is constant, $\mathrm{\Delta\:H\:=\:90.1\:Kj}$. This can be illustrated as shown below.

Hess Law Examples

The enthalpy of the reaction which involves the formation of Sulphur trioxide gas from sulphur can be explained with the help of Hess law.

$$\mathrm{S\:+\:O_{2}\:\rightarrow\:SO_{2}\:,\:\Delta\:H_{1}\:=\:-70.96\:KCal/mol.}$$

$$\mathrm{SO_{2}\:+\:12O_{2}\:\rightarrow\:SO_{3}\:,\:\Delta\:H_{2}\:=\:-23.49KCal/mol}$$

$$\mathrm{\Delta\:H\:=\:\Delta\:H_{1}\:+\:\Delta\:H_{2}\:=\:-7096\:+\:-23.49\:=\:-94.95KCal/mol}$$

And the enthalpy change in the overall reaction is,

$\mathrm{S\:+\:32O_{2}\:\rightarrow\:SO_{3}\:,\:where\:\:,\Delta\:H\:=\:-94.95KCal/mol}$

The stepwise calculated change in enthalpy and the enthalpy change in the net reaction are the same.

Application of Hess Law

It is an important theory for the calculation of the energy of several chemical reactions. It has many applications in the chemistry field. Some of its applications are,

• Enthalpy change calculation − It can be used for the calculation of enthalpy difference in a chemical reaction where the enthalpy difference is not known. And from which we can also predict whether the reaction is exothermic or endothermic. If the value is negative, it is an exothermic reaction, while if it is positive, it is an endothermic reaction.

• Calculation of enthalpy of formation − The enthalpy of formation of a chemical process in which it is not known can be calculated with the help of Hess law. By the consequent addition and subtraction of enthalpy values of known reaction, we can easily 1. calculate the enthalpy of formation. For example, in the building of benzene from carbon and hydrogen, the enthalpy of formation cannot be calculated with normal methods. But it is possible to calculate these non- measurable properties with the help of Hess law.

• Enthalpy change associated with the physical change − Certain reactions in which the value of enthalpy change cannot be calculated can be calculated, which will be a hypothetical value. For example, in the conversion of graphite to diamond, even though the reaction is not possible, enthalpy change can be calculated.

Conclusion

Hess law is an important law in thermodynamics. It exists as a single law even if it is an outcome of the first law of thermodynamics because of its characteristic importance. According to this law, the change in enthalpy is always consistent no matter the steps taken or the path in which the reaction has been completed. And is because of the reason that enthalpy is a state function.

Hess’s law helps in the calculation of the overall energy of many reactions in which the enthalpy change is not known. The enthalpy of formation of reactions is also determined with the help of Hess law. Another important application is the calculation of enthalpy change associated with physical change.

FAQs

1. In which case is Hess's law not applicable?

Hess law cannot be used for the calculation of the entropy of a system.

2. How is Hess's law used in real life?

The food we eat is converted into energy through a series of pathways. The carbohydrates present in our body are converted into carbon dioxide and water. Hess law states that the energy changes are always constant which means that the same energy is released when we work out and is given the name burning calories. So, it is the application of Hess law.

3. What is entropy?

Entropy is the quantity of the degree of randomness or ailment in a system. Entropy will assess the quantity of thermal energy per unit temperature that is available for performing work in a system.

4. Which type of reaction is always spontaneous?

Spontaneous reactions are the reactions in which the difference in enthalpy, ∆H is negative and the difference in entropy, ∆S positive. A system which follows this condition is said to be a spontaneous reaction.

5. What is the application of Hess law in the automobile industry?

Automobile industries work based on Hess law since they focus more on the determination of different methods needed to produce engines. The most effective pathway is checked with the help of this law. For example, car companies check the amount of energy liberated by the engine when the gasoline is burnt.

6. Which law of thermodynamics is the basis for Hess's law?

The first law of thermodynamics is the foundation of Hess' law.

7. What does "ΔE" mean?

Internal energy change, also referred to as the change in heat at constant volume, is denoted by the symbol ΔE.