1. Enthalpy of Combustion

2. HIGHER GRADE CHEMISTRY CALCULATIONS
Enthalpy of combustion.
The enthalpy of combustion of a substance is the amount of energy given out when one mole of a substance burns in excess oxygen.
Worked example 1.
0.19 g of methanol, CH3OH, is burned and the heat energy given out increased the temperature of 100g of water from 22oC to 32oC.
Calculate the enthalpy of combustion of methanol.
Use DH = -cmDT
DH = x x 10
DH = kJ
( c is specific heat capacity of water, kJ kg-1 oC-1)
m is mass of water in kg, 0.1 kg
DT is change in temperature in oC, 10o)
Use proportion to calculate the amount of heat given out when
1 mole, 32g, of methanol burns.
0.19 g  kJ
So g  32/0.19 x – = kJ
Enthalpy of combustion of methanol is –704 kJ mol-1.

3. Calculation using enthalpy of combustion
Calculations for you to try.
g of ethanol, C2H5OH, was burned and the heat given out raised the temperature of 500 cm3 of water from 20.1oC to 23.4oC.
Use DH = -cmDT
DH = x 0.5 x 3.3
= kJ
Use proportion to calculate the enthalpy change when 1 mole, 46g, of ethanol burns.
0.25 g  kJ
So 46g  /0.25 x = kJ mol-1.
moles of methane was burned and the energy given out
raised the temperature of 200cm3 of water from 18oC to
28.6oC. Calculate the enthalpy of combustion of methane.
Use DH = -cmDT
DH = x 0.2 x 10.6
= kJ
Use proportion to calculate the enthalpy change when 1 mole of methane burns.
0.1 mol  kJ
Higher Grade Chemistry
So 1mol  /0..1 x = kJ mol-1.

4. Calculation using enthalpy of combustion
Worked example 2.
0.22g of propane was used to heat 200cm3 of water at 20oC.
Use the enthalpy of combustion of propane in the data book to calculate the final temperature of the water.
From the data booklet burning 1 mole, 44g, of propane DH = kJ
By proportion burning 0.22 g of propane
DH = /44 x – = kJ
Rearrange DH = -cmDT to give
DT = DH
-cm
DT = = oC
-11.1
-4.18 x 0.2
Higher Grade Chemistry
Final water temperature = = 33.3oC

5. Calculation using enthalpy of combustion
Calculations for you to try.
g of methanol, CH3OH, was burned and the heat given out used to raise the temperature of 500 cm3 of water at 21oC.
Use the enthalpy of combustion of methanol in the data booklet to calculate the final temperature of the water.
From the data booklet burning 1 mole, 32g, of methanol DH = -727 kJ
By proportion burning 0.1 g of methanol
DH = /32 x – = kJ
Rearrange DH = -cmDT to give
DT = DH
-cm
DT = = oC
-2.27
-4.18 x 0.1
Final water temperature = = 26.4oC
Higher Grade Chemistry

6. Calculation using enthalpy of combustion
Calculations for you to try.
g of methane, CH4, was burned and the heat given out used to raise the temperature of 250 cm3 of water
Use the enthalpy of combustion of methane in the data booklet to calculate the temperature rise of the water.
From the data booklet burning 1 mole, 16g, of methane DH = -891 kJ
By proportion burning 0.2 g of methane.
DH = /16 x – = kJ
Rearrange DH = -cmDT to give
DT = DH
-cm
DT = = oC
-11.14
-4.18 x 0.25
Higher Grade Chemistry