Hess’s Law

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Define Hess Law

Hess’s law, also called Hess law of constant heat summation, is one of the important outcomes of the first law of thermodynamics. The enthalpy change in a chemical or physical process is similar whether it is carried out in one step or in several steps.

The Hess’s law of constant heat summation was derived in 1840, which is a Swiss-born Russian chemist and physician, where, Germain Hess, derived a thermochemistry relationship for calculating the standard reaction enthalpy for the multi-step reactions. In general, it exploits the state functions’ properties, where the state functions’ value does not depend on the path taken for dissociation or formation. Rather, it depends only on the state at the moment (pressure, formation volume, and more related).

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Illustration of Hess’s law

As we all know that enthalpy is a state function, and thereby, it is independent of the path taken to reach the final state from the initial state. Hess’s law says that for a multistep reaction, the standard reaction enthalpy is independent of either the pathway or the number of steps taken, rather being the sum of standard enthalpies of intermediate reactions, that are involved at a similar temperature.

The purpose of Hess’s law is to measure the neutralization enthalpies for various acid-base reactions and then use that information and Hess’s law to determine the enthalpies reaction for two salts in an aqueous solution.

Application of Hess Law

Let us discuss some practical areas where Hess’s law is applied. As an example, let us take the formation of Sulphur Trioxide gas from Sulphur, which is a multistep reaction involving in Sulphur Dioxide gas formation. Let us find the enthalpy of the standard reaction for the Sulphur Trioxide gas formation from Sulphur.

Step 1: Sulphur Dioxide gas Formation

S + O2 → SO2, where, △H1=−70.96 KCal/mol

Step 2: Conversion of Sulphur Dioxide gas into Sulphur Trioxide gas

SO2 + 12O2 → SO3, where, △H2 = −23.49KCal/mol

Standard reaction enthalpy according to Hess’s Law:

△HR = △H2 + △H1 = (-70.96) + (-23.49) = -94.95KCal/mol

Net Reaction:

S+32O2→SO3, where, △HR=−94.95KCal/mol

Therefore, in simple words, we can state as follows.

△HR = △H2 + △H1 + △H3 + △H4 + ….

Formation of Enthalpy Determination

There are various compounds including Co, C6H6, C2H6, and more, whose direct synthesis from their constituent elements cannot be possible. Their △H values are determined indirectly using Hess’s law.

Hess’s Law can be used to determine other state functions with enthalpies like free energy and entropy. The Bordwell thermodynamic cycle can be taken as an example, which takes advantage of Redox potentials and easily measured equilibriums to experimentally determine the inaccessible Gibbs free energy values.

△G(reaction) = Σ△G(product)- Σ△G(reactants)

As the entropy is measured as an absolute value, thus, in the case of entropy, there is no need to use the formation of entropy.

△S(reaction) = ΣS(product)- ΣS(reactants)

Calculating Standard Enthalpies of Reaction

From the standard enthalpies of the reactants and products’ formation, the standard enthalpy of the reaction is calculated by using Hess’s law. Generally, the cycle of Hess’s law representing the reactants and products’ formation from their respective elements in the standard state can be considered as follows.

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According to Hess’s law,

Σ△fHo(P) = Σ△fHo(R) + Σ△RHoRHo

= Σ△fHo(P) - Σ△fHo(R)

= Sum of the standard enthalpies of products’ formation − Sum of the standard enthalpies of reactants’ formation.

Uses of Hess’s Law

Hess's law of constant heat summation can be useful to determine the enthalpies of the following.

  • Heats of unstable intermediates formation such as NO(g) and CO(g).

  • The ionic substances’ lattice energies by constructing the Born-Haber cycles, if the electron affinity is known to form the anion.

  • Heat changes in allotropic transitions and phase transitions.

  • Electron affinities with a Born-Haber cycle using theoretical lattice energy.

Example of Hess’s Law

Hess’s Law, which is also called Hess’s Constant Heat Summation Law states, the overall change in enthalpy for the solution can be given by the sum of all changes independent of the various steps or phases of a reaction. This particular rule is a discovery, where enthalpy is a part of the state.


Calculate the reaction’s standard enthalpy change using the following reaction.

CO2(g) + H2(g) → CO(g) + H2O(g)

Given that, ΔrHo for CO(g), CO2(g), and H2O(g) as -110.5, -393.5, and 241.8kJ/mol respectively.


ΔrHo for the reaction can be given as 

rHo = Σ△fHo (Products) - Σ△fHo(Reactants) =[△fHo (H2O) + △fHo(CO)] - [△fHo (CO2) + △fHo (H2)]  

Substituting the values that are given, we get the result as follows.

rHo =[-241.8 - 110.5] - [-393.5 + 0],

=-352.3 + 393.5,

= 41.2 kJ.

Did You Know?

  • Hess’s law allows the enthalpy shift (even if it cannot be determined directly) to be estimated for any of the reactions. This can be achieved by carrying simple algebraic operations depending on the Hess’s law equation of the reactions by using the values, which are defined previously for the formation enthalpies.

FAQ (Frequently Asked Questions)

1. What is the basis of Hess’s law?

Hess’s law says that the increase in enthalpy in a chemical reaction, which means, the reaction heat at constant pressure is the process-independent between initial and final states.

2. What is meant by entropy?

In general, entropy refers to the idea that everything, inevitably in the universe transitions from order to chaos. It is also the measure of that transition. The term entropy has originated from the Greek term, entropy, which means a “transformation” or “a change toward.”

3. State and explain Hess's law.

in a chemical reaction, Hess’ law states that the change of enthalpy (it means, the heat of reaction under constant pressure) is independent of direction between the states of final and original. Also, this law requires the change in enthalpy (Δ H) for a reaction to be determined, even though it can not be measured directly.