Hess's Law

• Hess’s Law, named after Germain Hess, states that the total enthalpy change during the complete course of a chemical reaction is the same whether the reaction is in one step or in several steps.
• It is a statement about the conservation of energy, effectively stating that energy cannot be created or destroyed. Hence, the total amount of energy in the universe remains constant.
• The principle assumes that enthalpy, defined as the heat absorbed or released in a chemical reaction, is a state function. A state function’s value is determined only by the state of the system, independent of its previous conditions or how it arrived at its current state.
• Since enthalpy is a state function, the enthalpy change from a set of reactants to a set of products is the same, regardless of the route by which the chemical reaction occurs. This concept forms the basis of Hess’s Law.
• This means that if a reaction can occur through multiple pathways or steps, the total enthalpy change for each pathway is the same, even if the individual steps have different enthalpy changes.
• Hess’s Law can be applied to calculate the enthalpy change of complex reactions which are difficult to measure experimentally. The strategy is to break down the overall reaction into a series of simpler reactions for which enthalpy changes are known (either from tables or from direct measurement).
• It’s also important to remember that if a reaction’s direction is reversed, the sign of the enthalpy change for that reaction also changes. This follows from the concept that an exothermic reaction in the forward direction becomes endothermic in the reverse direction, and vice versa.