1. HC
CHEMISTRY HC
CHEMISTRY
CHEMISTRY IN SOCIETY
(C) CHEMICAL ENERGY

2. Enthalpy of CombustionHC
CHEMISTRY HC
CHEMISTRY
(C) CHEMICAL ENERGY
Enthalpy of Combustion

3. (B) CHEMISTRY IN SOCIETY Enthalpy of CombustionHC
CHEMISTRY HC
CHEMISTRY
(B) CHEMISTRY IN SOCIETY
Enthalpy of Combustion
After completing this lesson you should be able to :
The specific heat capacity, mass and temperature change can be used to calculate the enthalpy change for a reaction using Eh = cm∆T The quantities c, m and ∆T can be calculated given relevant data.
The enthalpy of combustion of a substance is the enthalpy change when one mole of the substance burns completely in oxygen.

4. Enthalpy of combustion
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Enthalpy of combustion
The enthalpy of combustion is the heat energy given out when 1 mole of fuel burns completely in oxygen.
The enthalpy of combustion of methane can be represented by the equation
CH4(g) + O2 (g) CO2(g) H2 O(l)

5. Enthalpy of combustion
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The heat energy released when alcohols burn can be measured
The enthalpy of combustion of a substance is the amount of energy given out when one mole of a substance burns in excess oxygen.

6. Specific heat capacity
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Specific heat capacity
Calculating the energy change during a chemical reaction in water.
c =specific heat capacity
m =mass in Kg
 T =temperature change
E = c. m.  T
The mass of water can be calculated by using the fact that 1 ml = 1 g.
The value for c is usually taken as 4.18 kJ kg –1 oC-1 6

7. Measuring the enthalpy of combustion of alcohols
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Measuring the enthalpy of combustion of alcohols
Weigh a filled alcohol burner
Measure 50 cm3 water into a copper calorimeter
Take temperature of the water
Light the burner and use it to heat the water to approx 60oC
Stir the water and take the highest temperature reached
Reweigh the burner and remaining fuel

8. Enthalpy of combustion
Procedure
1. Weigh the spirit burner (already containing ethanol) with its cap on and record its mass. (The cap should be kept on to cut down the loss of ethanol through evaporation)
2. Using the measuring cylinder, measure out 100 cm3 of water into the copper can.
3. Set up the apparatus as directed by your teacher/lecturer.
4. Measure and record the temperature of the water.
Remove the cap from the spirit burner and immediately light the burner.
Slowly and continuously stir the water with the thermometer. When the temperature has risen by about 10 °C, recap the spirit burner and measure and record the maximum temperature of the water.
7. Reweigh the spirit burner and record its mass.

9. Specific heat capacity
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Calculation
(a) The heat energy gained by the water (Eh) can be calculated using the formula:
Calculating the energy change during a chemical reaction in water.
c =specific heat capacity
m =mass in Kg
 T =temperature change
E = c. m.  T
The mass of water can be calculated by using the fact that 1 ml = 1 g.
The value for c is usually taken as 4.18 kJ kg –1 oC-1 9

10. The heat energy released on burning 0.25 g of ethanol = 5.225 kJ
CALCULATION
Suppose 0.25 g of ethanol had been burned and the temperature of the water had risen by 12.5 °C.
The heat energy gained by the water (Eh) is calculated using the formula:
Eh = c m DT
Eh = 4.18 x 0.10 x 12.5
= kJ
We assume that the heat energy released by the burning ethanol is gained only by the water.
The heat energy released on burning 0.25 g of ethanol = kJ
Ethanol: CH3CH2OH Mass of 1 mole = 2(12) + 6(1) = 46 g
We can now calculate the heat energy released on burning 1 mole of ethanol.
0.25g kJ
46g
0.25 x
= 961 kJ
The enthalpy of combustion of ethanol = kJ mol-1
(A negative sign is used because combustion is an exothermic reaction)

11. Measuring the enthalpy of combustion of alcohols
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Measuring the enthalpy of combustion of alcohols
The heat energy gained by the water (Eh) is calculated using the formula:
Eh = c m ∆T
Eh = x x
= kJ

12. Measuring the enthalpy of combustion of alcohols
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Measuring the enthalpy of combustion of alcohols
We assume that the heat energy released by the burning alcohol is gained only by the water.
The heat energy released on burning ……….. g of ……………anol ………….. kJ
So one mole g of anol kJ
The enthalpy of combustion of …………anol = -………….. kJ mol-1
(A negative sign is used because combustion is an exothermic reaction)

13. Sources of inaccuracy Heat loss to surroundings
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Sources of inaccuracy
Heat loss to surroundings
Ignore heat rise of calorimeter
Incomplete combustion
Possible loss of fuel by evaporation from wick

14. COPY
Calorimetry
To eliminate these inaccuracies a bomb calorimeter is used
The burning fuel (or food) is supplied with oxygen to encourage complete combustion
The combustion chamber is entirely surrounded so there is no heat loss to the surroundings

15. Commercial ‘bomb’ calorimeters
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Commercial ‘bomb’ calorimeters
The calorimeter is heated electrically. Energy required to heat the entire apparatus by 1 0C is calculated.

16. Enthalpy of combustion
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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.
–704 kJ mol-1

17. 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.
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33.3oC

18. COPY 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. Calculate the enthalpy of combustion of ethanol
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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.
g 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.
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.

19. COPY
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. Calculate the enthalpy of combustion of ethanol
-1269 kJ mol-1

20. COPY
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.
kJ mol-1

21. COPY
g 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.
26.4oC

22. COPY
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.
10.66oC