Enthalpies of Chemical Changes

The enthalpy change of a chemical reaction is the difference between the total enthalpy of the products and the reactants. This energy change for the molecules involved in a chemical reaction can be visualized through an energy diagram, where the lines represent the total enthalpy of the reactants and products, and the gap between them signifies the enthalpy change for the reaction. In an exothermic reaction, the system's enthalpy decreases, whereas it increases in an endothermic reaction.

The standard enthalpy of formation, ΔH^o_f, of a compound is the standard reaction enthalpy for the formation of the compound from its elements in their reference states. The standard enthalpy of formation for elements in their reference states is zero by definition, but this definition strictly applies at the standard temperature (usually 298.15 K or 25°C).

The standard enthalpy changes of a reaction happening under standard conditions are computed by subtracting the sum of the standard enthalpies of the formation of the reactants from the sum of the standard enthalpies of the formation of the products, each multiplied by its stoichiometric coefficient.

Consider the reaction of 2 moles of hydrazoic acid with 2 moles of nitric oxide to yield 1 mole of hydrogen peroxide and 4 moles of nitrogen gas under standard conditions. The enthalpy for this reaction equals the sum of the enthalpy of the formation of hydrogen peroxide and four times the enthalpy of the formation of nitrogen gas minus the sum of twice the enthalpy of the formation of hydrazoic acid and twice the enthalpy of the formation of nitric oxide gas.

According to Hess’s law, if a chemical equation can be expressed in multiple steps, the net enthalpy change for the equation can be written as the sum of the enthalpies associated with each step. Often, thermochemical reactions have to be manipulated to make the reactions sum to a specific reaction. The stoichiometric quantities and the direction of the reaction can be changed to determine a new enthalpy for each manipulated reaction.

Two crucial features of ΔH prove useful while solving problems using Hess’s law. Firstly, ΔH is directly proportional to the quantities of reactants or products. Secondly, altering the reaction (or the thermochemical equation) in well-defined ways changes the ΔH correspondingly. For instance, if you multiply or divide a chemical equation, the change in enthalpy should also be multiplied or divided by the same number.

Lastly, ΔH for a reaction in one direction is equal in magnitude but opposite in sign to ΔH for the reaction in the reverse direction. Therefore, for the reverse reaction, the enthalpy change is also reversed.