HIGHER GRADE CHEMISTRY CALCULATIONS

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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 proportion to calculate the amount of heat
given out when 1 mole, 32g, of methanol burns.
0.19 g ? -4.18 kJ
So 32 g ? 32/0.19 x 4.18 -704 kJ
Enthalpy of combustion of methanol is 704 kJ
mol-1.
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• Calculation using enthalpy of combustion

Calculations for you to try. 1. 0.25g 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 -4.18 x 0.5 x 3.3
- 6.897kJ
Use proportion to calculate the enthalpy change
when 1 mole, 46g, of ethanol burns.
0.25 g ? -6.897 kJ
So 46g ? 46/0.25 x -6.897
-1269 kJ mol-1.
2. 0.1 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 -4.18 x 0.2 x 10.6
- 8.86 kJ
Use proportion to calculate the enthalpy change
when 1 mole of methane burns.
0.1 mol ? -8.86 kJ
Higher Grade Chemistry
So 1mol ? 1/0..1 x -8.86 -88.6
kJ mol-1.
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• 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 -2220 kJ
By proportion burning 0.22 g of propane DH
0.22/44 x 2220 - 11.1kJ
Higher Grade Chemistry
Final water temperature 20 13.3 33.3oC
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• Calculation using enthalpy of combustion

Calculations for you to try. 1. 0.1g of
methanol, CH3OH, was burned and the heat given
out used to raise the temperature of 100 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
0..1/32 x 727 - 2.27 kJ
Final water temperature 21 5.4 26.4oC
Higher Grade Chemistry
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• Calculation using enthalpy of combustion

Calculations for you to try. 2. 0.2g 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
0..2/16 x 891 - 11.14 kJ
Higher Grade Chemistry