4.12 calculate molar enthalpy change from heat energy change

The example above showed how much energy this particular mass of ethanol released.  To find out how much a mole of ethanol would release, we need to divide it by the number of moles of ethanol burned. 

Ethanol has a relative formula mass of 46, so 1 mole of ethanol is 46 grams.

Moles of ethanol           =  mass ÷ molar mass

= (145.34 - 144.82) g  ÷ 46 g/mol

= 0.0113 mol

Energy of combustion of 1 mole of ethanol         = 5.292 kJ ÷ 0.0113 mol

= 468 kJ/mol

The ENTHALPY change, ∆H, has a sign to show whether the reaction was exothermic or endothermic (see next section).  Combustion reactions are exothermic, so we must write our final answer with a negative sign, so:  ∆H for this reaction is –468 kJ/mol.