What is Hess’s Law?

Germain Henri Hess was born in Geneva, Switzerland. In 1830, Hess took up chemistry full-time, researching and teaching, and later became an adjunct professor of Chemistry at the St. Petersburg Academy of Sciences. He is known for his work in thermochemistry.  Hess’s law is also known as the Hess’s law of Constant Heat Summation.

Read further to know what is Hess’s Law

  1. Introduction:

According to Hess, “The total heat change (ΔH) accompanying a chemical reaction is the same whether the reaction takes place in one or more steps” or “Enthalpy change in a particular reaction is always constant and is independent of the path in which the reaction takes place.

Suppose, the enthalpy change for direct conversion A –> B is ΔH. B can also be prepared from A through intermediates C and  D, for which enthalpy change are ΔH1, ΔH2 and ΔH3 respectively as shown.

The above statements are called Hess’s law of constant heat summation.

Then according to Hess’s law,

ΔH = ΔH1 + ΔH2 + ΔH3, the enthalpy of reaction is independent of the number and the nature of the intermediate steps.

For example, the standard enthalpy change of the reaction,

C (graphite) + O2(g) –> CO2(g)

is equal to -393.5  kJ/mol. This value can be determined with the help of a calorimeter. However, there are some reactions for which the direct measurement of enthalpy in the laboratory is not possible. For example the standard enthalpy change of the reaction,

C (graphite) + 1/2 O2(g) –> CO(g)

cannot be determined with the help of calorimeter because the combustion of carbon is incomplete unless and excess of oxygen is used. If the excess of oxygen is used, some of the CO is oxidized to CO2. How can then we determine the enthalpy change for such reactions when direct measurement is not possible?

Since ΔH is a state function, it is not dependant on the way reactions are carried out. Let us carry out the reactions as follows –

For the formation of CO2 directly as,

C + O2(g) –> CO2(g); where ΔH = – 393.5 kJ

CO2 can be prepared as,

C + 1/2 O2(g) –> CO(g); where ΔH1 = – 110.5 kJ

CO + 1/2 O2(g) –> CO2(g); where ΔH2 = – 283.0 kJC

Thus, total heat,

ΔH = ΔH1 + ΔH2 = – 110.5 – 283.0 = – 393.5 kJ

Hence, it is clear from the above that ΔH direct and ΔH obtained indirectly are same. It clearly indicates that when a chemical equation is added, subtracted or multiplied, the heat changes can also be added, subtracted or multiplied correspondingly.

2. Applications of Hess’s Law

The applications of Hess’s Law are as follows –

For determining the:

  • enthalpies of formation of compounds, extremely slow reactions, the transformation of one allotropic form into another.
  • bond energies.
  • resonance energy.

The result of Hess’s law is that thermochemical equations can be added and subtracted just like algebraic equations to get the desired reaction. A useful practical application of this law is that we calculate enthalpy changes for the reactions which cannon be studied directly.

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