Transcript 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.
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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)
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.
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Specific heat capacity
Calculating the energy change during a chemical reaction in water.
E = c. m.  T
c
m
T
=specific heat capacity
=mass in Kg
=temperature change
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
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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
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.
5.Remove the cap from the spirit burner and immediately light the burner.
6.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.
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.
E = c. m.  T
c
m
T
=specific heat capacity
=mass in Kg
=temperature change
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
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 T
Eh = 4.18 x 0.10 x 12.5
= 5.225 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 = 5.225 kJ
Ethanol: CH3CH2OH
Mass of 1 mole = 2(12) + 6(1) + 16 = 46 g
We can now calculate the heat energy released on burning 1 mole of ethanol.
0.25g
46g
5.225kJ
46
0.25
x
5.225
= 961 kJ
The enthalpy of combustion of ethanol = - 961 kJ mol-1
(A negative sign is used because combustion is an exothermic reaction)
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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
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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
So one mole .............. g of ................anol
………….. kJ
....................kJ
The enthalpy of combustion of …………anol = -………….. kJ mol-1
(A negative sign is used because combustion is an
exothermic reaction)
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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
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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
Commercial ‘bomb’
calorimeters
The calorimeter is heated electrically.
Energy required to heat the entire
apparatus by 1 0C is calculated.
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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
Worked example 2.
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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.
33.3oC
Calculations for you to try.
1.
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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.
Calculate the enthalpy of combustion of ethanol
2. 0.01 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.
3. 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.
4. 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.
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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.
Calculate the enthalpy of combustion of ethanol
-1269 kJ mol-1
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2. 0.01 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.
-886.2 kJ mol-1
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3. 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.
26.4oC
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4. 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.
10.66oC