Thermochemistry: Enthalpy and Hess's Law

Learning Objectives

Define enthalpy and distinguish between exothermic and endothermic reactions.

Understand the concept of standard enthalpy of formation (ΔH°f).

State Hess's Law.

Use Hess's Law to calculate the enthalpy change for a reaction from the enthalpies of formation of reactants and products.

The Heat of Reaction

Thermochemistry is the study of the heat energy associated with chemical reactions. This heat is often described in terms of enthalpy (H), which is a measure of the total heat content of a system. We are most interested in the change in enthalpy (ΔH) during a reaction.

Exothermic and Endothermic Reactions

Exothermic Reaction: A reaction that releases heat into the surroundings. The products are at a lower enthalpy than the reactants.

ΔH is negative.

Example: Combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O, ΔH = -890 kJ/mol.

Endothermic Reaction: A reaction that absorbs heat from the surroundings. The products are at a higher enthalpy than the reactants.

ΔH is positive.

Example: Decomposition of water: 2H₂O → 2H₂ + O₂, ΔH = +572 kJ/mol.

Standard Enthalpy of Formation

The standard enthalpy of formation (ΔH°f) of a compound is the change in enthalpy during the formation of 1 mole of the substance from its constituent elements in their standard state.

The standard state is the most stable form of an element at 1 atm and 25°C.

By definition, the ΔH°f of any element in its standard state is zero. (e.g., ΔH°f for O₂(g) is 0).

Values for ΔH°f for many compounds are tabulated in reference books.

Hess's Law

Enthalpy is a state function, which means the change in enthalpy for a reaction depends only on the initial and final states, not on the path taken. This leads to a very powerful principle called Hess's Law:

If a reaction is carried out in a series of steps, the ΔH for the overall reaction will be equal to the sum of the enthalpy changes for the individual steps.

This allows us to calculate the enthalpy change for a reaction without ever having to run the experiment. The most common application of Hess's Law is to use standard enthalpies of formation to calculate the overall enthalpy change for a reaction:

ΔH°_reaction = Σ ΔH°f(products) - Σ ΔH°f(reactants)

In words: The enthalpy of reaction is the sum of the enthalpies of formation of the products minus the sum of the enthalpies of formation of the reactants (making sure to multiply each by its stoichiometric coefficient).

Key Terms

Enthalpy (H): A thermodynamic quantity equivalent to the total heat content of a system.
Exothermic Reaction: A reaction that releases energy from the system to its surroundings, usually in the form of heat (ΔH is negative).
Endothermic Reaction: A reaction that absorbs energy from its surroundings, usually in the form of heat (ΔH is positive).
Standard Enthalpy of Formation (ΔH°f): The change of enthalpy from the formation of 1 mole of a compound from its constituent elements, with all substances in their standard states.
Hess's Law: A law of physical chemistry which states that the total enthalpy change during the complete course of a chemical reaction is the same whether the reaction is made in one step or in several steps.