ENTHALPY OF COMBUSTION

Standard enthalpy of combustion (ΔHC) is the enthalpy change when 1 mole of a substance burns (combines vigorously with oxygen) under standard state conditions; it is sometimes called “heat of combustion.” For example, the enthalpy of combustion of ethanol, −1366.8 kJ/mol, is the amount of heat produced when one mole of ethanol undergoes complete combustion at 25 °C and 1 atmosphere pressure, yielding products also at 25 °C and 1 atm.

Enthalpies of combustion for many substances have been measured; a few of these are listed in Table 2. Many readily available substances with large enthalpies of combustion are used as fuels, including hydrogen, carbon (as coal or charcoal), and hydrocarbons (compounds containing only hydrogen and carbon), such as methane, propane, and the major components of gasoline.

As Figure 3 suggests, the combustion of gasoline is a highly exothermic process. Let us determine the approximate amount of heat produced by burning 1.00 L of gasoline, assuming the enthalpy of combustion of gasoline is the same as that of isooctane, a common component of gasoline. The density of isooctane is 0.692 g/mL.

Hess’s Law states that the total enthalpy change for a reaction is independent of the route taken

Enthalpy changes of formation are useful for working out enthalpy changes you can’t find directly by doing an experiment.

You need to know the ΔH^θ_f for all the reactants and products that are compounds.

Definition

The enthalpy change when one mole of that substance is formed from the most stable allotropes of its elements in their standard states under standard conditions

The ΔH^θ_f for elements is zero – the element is being formed from the element so there’s no change in enthalpy.

Step 1: Write the balanced equation for the reaction. This will be ΔH^θ_r

Step 2: Under the equation write a list of the elements present. This must be balanced

Using Hess’ Law; Route 1 = Route 2