The standard enthalpy of formation is the enthalpy change to form one mole of a substance from its constituent elements in their standard states. For example, the standard enthalpy of formation for solid sodium oxide would be the enthalpy change associated with two moles of sodium metal combining with ½ mole of oxygen gas to produce one mole of solid sodium oxide with all compounds in their standard states. The standard state of a gas is a pressure of one atmosphere; for a liquid or solid, it means its most stable form at one atmosphere and typically 25°C; or for a solution, it means the concentration is exactly one molar. We can use the standard enthalpies of formation to calculate the standard enthalpy change for a reaction. Since change in enthalpy is a state function, we can calculate it using a final minus initial. So we can sum the enthalpy change to form all of the products and subtract the sum of the enthalpy change to form all of our reactants. Here again, standard enthalpy change means that the reactants and products are in their standard states. Let's practice calculating the standard enthalpy of reaction for the combustion of butane. We'll start with our equation: the standard change in enthalpy for the reaction is equal to the sum of the moles of products times the enthalpies of formation for our products subtract the sum of the moles of reactants times the enthalpies of formation for our reactants. We can plug in our coefficients and compounds from the balanced chemical equation and the enthalpies of formation that are given in the problem. If we complete the calculation, we'll get a standard enthalpy change for the reaction of -5271.4 kilojoules.