Hess's Law (DP IB Chemistry: SL): Revision Note

Hess's Law

• In 1840, the Russian chemist Germain Hess formulated a law which went on to be known as Hess’s Law

• This went on to form the basis of one of the laws of thermodynamics. The first law of thermodynamics relates to the Law of Conservation of Energy

• It is sometimes expressed in the following form:

Energy cannot be created or destroyed, it can only change form

• This means that in an isolated system, the total amount of energy present is always constant

• Hess’s law can be used to calculate the standard enthalpy change of a reaction from known standard enthalpy changes

• 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 means that whether the reaction takes place in one or two steps, the total enthalpy change of the reaction will still be the same

Diagram to show Hess's Law

• Hess’ Law is used to calculate enthalpy changes which can’t be found experimentally using calorimetry, eg:

3C (s) + 4H₂ (g) → C₃H₈(g)

• The enthalpy change for this reaction can't be found experimentally, because carbon and hydrogen don't react directly under standard conditions