Enthalpies of Chemical Changes

Enthalpy change in a chemical reaction represents the net difference between the enthalpies of the products and reactants. This change can be depicted using energy diagrams.

Notably, exothermic reactions cause a decrease in enthalpy, while endothermic reactions lead to an increase.

The standard enthalpy of formation refers to the enthalpy change when a compound forms from its constituent elements in their standard states.

Under standard conditions, the enthalpy change of a reaction is calculated by subtracting the total standard enthalpies of formation of the reactants from that of the products, each multiplied by their stoichiometric coefficients.

Hess’s law follows directly from enthalpy being a state function and states that the overall enthalpy change of a multi-step reaction equals the sum of the enthalpy changes of its individual steps, often requiring appropriate manipulation of thermochemical equations.

Multiplying a chemical reaction by a factor changes its enthalpy by the same factor, and reversing a reaction flips the sign of its enthalpy change.

When these reactions are added together, the overall reaction has an enthalpy change equal to the sum of the individual steps.