Heat of combustion

The heat of combustion is the heat released when one mole of a substance is completely burnt in excess oxygen. Malay: haba pembakaran · Chinese: 燃烧热.

Combustion is exothermic, so the value is negative, and it is defined per mole of the fuel burnt, whether that fuel is a hydrocarbon or an alcohol. The phrase “in excess oxygen” matters: the fuel must burn completely to carbon dioxide and water, because incomplete combustion releases less heat and would give too small a value.

Example. In a school experiment a measured mass of ethanol in a spirit lamp heats water in a copper can. You record the temperature rise of the water, put the mass of water and the rise into Q = mcθ, and divide Q by the moles of ethanol burnt to get the heat of combustion in kJ mol⁻¹. The experimental value comes out lower than the accepted value because heat is lost to the surroundings and to the can, and some fuel may burn incompletely.

Confusion to avoid. Do not use the mass of the fuel as m in Q = mcθ. The m is the mass of water being heated; the fuel supplies the heat, it is not the substance whose temperature you measure. Then divide by the moles of the fuel, not of the water.

Comparing the heats of combustion of successive alcohols links this chapter to the homologous series idea, and SPM Chemistry often pairs the calculation with an explanation of why the experimental value falls short of the accepted one. When you explain the heat losses, name the steps that reduce them, using a draught shield, placing the flame close to the base of the can, and stirring the water, because the marking scheme rewards improvements, not just the statement that heat is lost. Keep the habit of checking your balanced equation before counting moles, since a wrong coefficient on the oxygen or the products would spoil the number of moles of fuel and therefore the final value.