Table 3: Average of the two above results from the experiments.
I am going to calculate the amount of heat absorbed by the water in each experiment and I will use these answers to calculate a value for the enthalpy change of combustion of each alcohol. I will all my values from table 3.
I will begin with methanol, as this is the smallest molecule, and has the smallest number of bonds. I will then do ethanol, propan-1-ol, propan-2-ol and then butan-1-ol, which has the most number of carbon bonds.
Methanol
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Mass of water used: 192.75g
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Temperature rise in water: 21oC
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Mass of fuel used: 2g
- To work out the amount of heat absorbed by the water in the experiment we do the following:
(mass of water) x (temperature rise in water) x 4.2J
192.75g x 21oC x 4.2J = 17000.55J (2.d.p.)
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Molecular Formula: CH3OH
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Structural Formula: H
H C O H
H
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Mass of 1 mole of fuel: (C:12; O:16; H:1)
12+16+(4x1)=32g
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Number of moles of fuel used: 1 mole = 32g
(1/32) x (mass of fuel used)
= (1/32) x 2g = 0.0625 moles (4.d.p.)
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Energy transferred by this number of moles: (Look above at bullet point number 4)
192.75g x 21oC x 4.2J = 17000.55J
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Energy transferred by 1 mole of fuel: 0.0625 moles = 17000.55J
1 mole = (17000.55/0.0625) = 272008.5J (1.d.p.)
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Enthalpy Change of Combustion: (272008.5J/1000) = 272.0088
(1.d.p.) = -272.0Kjmol-1
Ethanol
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Mass of water used: 192.2g
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Temperature rise in water: 20.75oC
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Mass of fuel used: 1.95g
- To work out the amount of heat absorbed by the water in the experiment we do the following:
(mass of water) x (temperature rise in water) x 4.2J
192.2g x 20.75oC x 4.2J = 16750.23J (2.d.p.)
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Molecular Formula: CH3CH2OH
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Structural Formula: H H
H C C O H
H H
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Mass of 1 mole of fuel: (C:12; O:16; H:1)
(2x12)+16+(6x1)=46g
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Number of moles of fuel used: 1 mole = 46g
(1/46) x (mass of fuel used)
= (1/46) x 1.95g = 0.0424 moles (4.d.p.)
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Energy transferred by this number of moles: (Look above at bullet point number 4)
192.2g x 20.75oC x 4.2J = 16750.23J
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Energy transferred by 1 mole of fuel: 0.0424 moles = 16750.23J
1 mole = (16750.23/0.0424) = 395052.6J (1.d.p.)
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Enthalpy Change of Combustion: (395052.6J/1000) = 395.0525
(1.d.p.) = -395.1Kjmol-1
Propan-1-ol
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Mass of water used: 194.3g
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Temperature rise in water: 21oC
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Mass of fuel used: 0.85g
- To work out the amount of heat absorbed by the water in the experiment we do the following:
(mass of water) x (temperature rise in water) x 4.2J
194.3g x 21oC x 4.2J = 17137.26J (2.d.p.)
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Molecular Formula: CH3CH2CH2OH
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Structural Formula: H H H
H C C C O H
H H H
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Mass of 1 mole of fuel: (C:12; O:16; H:1)
(3x12)+16+(8x1)=60g
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Number of moles of fuel used: 1 mole = 60g
(1/60) x (mass of fuel used)
= (1/60) x 0.85g = 0.0142 moles (4.d.p.)
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Energy transferred by this number of moles: (Look above at bullet point number 4)
194.3g x 21oC x 4.2J = 17137.26J
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Energy transferred by 1 mole of fuel: 0.0142 moles = 17137.26J
1 mole = (17137.26/0.0142) = 1206849.3J (1.d.p.)
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Enthalpy Change of Combustion: (1206849.30J/1000) = 1206.849296
(1.d.p.) = -1206.8Kjmol-1
Propan-2-ol
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Mass of water used: 194.25g
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Temperature rise in water: 19.75oC
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Mass of fuel used: 0.70g
- To work out the amount of heat absorbed by the water in the experiment we do the following:
(mass of water) x (temperature rise in water) x 4.2J
194.25g x 19.75oC x 4.2J = 16113.04J (2.d.p.)
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Molecular Formula: CH3CH2OHCH2
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Structural Formula: H H H
H C C C H
H O H
H
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Mass of 1 mole of fuel: (C:12; O:16; H:1)
(3x12)+16+(8x1)= g
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Number of moles of fuel used: 1 mole = 60g
(1/60) x (mass of fuel used)
= (1/60) x 0.70g = 0.0117 moles (4.d.p.)
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Energy transferred by this number of moles: (Look above at bullet point number 4)
194.25g x 19.75oC x 4.2J = 16113.0375J
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Energy transferred by 1 mole of fuel: 0.0117 moles = 16113.0375J
1 mole = (16113.0375/0.0117) = 1381117.5J (1.d.p.)
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Enthalpy Change of Combustion: (138111.7J/1000) =
(1.d.p.) = -1381.1Kjmol-1
Butan-1-ol
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Mass of water used: 189.7g
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Temperature rise in water: 20oC
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Mass of fuel used: 1.6g
- To work out the amount of heat absorbed by the water in the experiment we do the following:
(mass of water) x (temperature rise in water) x 4.2J
189.7g x 20oC x 4.2J = 15934.80J (2.d.p.)
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Molecular Formula: CH3CH2CH2CH2OH
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Structural Formula: H H H H
H C C C C O H
H H H H
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Mass of 1 mole of fuel: (C:12; O:16; H:1)
(4x12)+16+(10x1)= 74g
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Number of moles of fuel used: 1 mole = 74g
(1/74) x (mass of fuel used)
= (1/74) x 0.70g = 0.0095 moles (4.d.p.)
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Energy transferred by this number of moles: (Look above at bullet point number 4)
194.25g x 19.75oC x 4.2J = 16113.0375J
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Energy transferred by 1 mole of fuel: 0.0095 moles = 16113.0375J
1 mole = (16113.0375/0.0095) = 1703378.3J (1.d.p.)
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Enthalpy Change of Combustion: (1703378J/1000) = 1703.37825
(1.d.p.) = -1703.4Kjmol-1
Now that I have worked out all the enthalpy changes of the alcohol's above I will put them in a table so I can easily compare them for each alcohol. This will be Table 4.
Table 4: Enthalpy Changes of Combustion of all the alcohol's tested and the amount of heat the water absorbed in each experiment.
Now I am going to use all the information above to help me investigate how and why the enthalpy changes of combustion of alcohols is effected by their molecular structure.
Here is the molecular structure of Methanol. I worked out that the enthalpy change of Combustion of Methanol is –272.0Kjmol-1.
H
H C O H
H