What is Hess's Law?

Hess’s Law states that 'the total enthalpy change in a chemical reaction is independent of the route by which the chemical reaction takes place as long as the initial and final conditions are the same'. This law is also known as the law of conservation of energy.

In other words, the enthalpy change of a reaction remains the same as long as the products and reactants are the same, regardless of how the reaction happens.

To illustrate Hess’s Law, consider reactants A and B combining directly to form product C, which is the direct route. Alternatively, A and B could react to form substances F and G (intermediates) which then combine to form C. This is an indirect route. 

According to Hess' Law, if we add up the enthalpy changes of the indirect route, it will be the same as the direct route. 

Steps in applying Hess' Law
  1. Write the balanced equation at the top.
  2. Draw the cycle with elements at the bottom.
  3. Draw in all arrows, making sure they go in the correct directions.
  4. Apply Hess’s law, taking into account the number of moles of each reactant and product.

Hess' Law in enthalpy of formation

  • We must find the enthalpy change of the reaction, ΔHr.
  • The enthalpy change in the blue loop (2(-950.8) + ΔHr) must be the same as the enthalpy change in the red loop (-1130.7 + -393.5 + -285.8).
  • 2(-950.8) + ΔHr = -1130.7 + -393.5 + -285.8
  • Thus, we can calculate ΔHr, which is +91.6 kJ mol−1 
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Hess' Law in enthalpy of combustion

  • We must find the enthalpy change of formation of C2H6, ΔHf.
  • The enthalpy change in the blue loop (-1559.7 + 3(0) + ΔHf) must be the same as the enthalpy change in the red loop (2(−393.5) + 3(−285.8)).
  • -1559.7 + ΔHf = 2(−393.5) + 3(−285.8)
  • Thus, we can calculate ΔHf, which is −84.7 kJ mol-1
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