Molar Heat of Combustion of Alkanols (Hsc Chem) Essay

Molar Heat of Combustion Aim: To find the molar heat of combustion for four different alkanols: 1. Methanol 2. Ethanol 3. 1-Propanol 4. 1-Butanol – And to compare the experimental value with the theoretical. Background: The Molar Heat of Combustion of a substance is the heat liberated when 1 mole of the substance undergoes complete combustion with oxygen at standard atmospheric pressure, with the final products being carbon dioxide gas and liquid water. (Ref. “Conquering Chemistry, Roland Smith, 2005”) The Heat Capacity of a substance is the amount of heat energy it must consume in order to raise its temperature by 1 Kelvin or 1° Celsius.

The heat capacity of 1 mol of a pure substance is known as its molar heat capacity, which can be expressed in J K-1 g-1. The heat capacity of 1 gram of a substance is known as its specific heat, which can also be expressed in J g-1 K-1. Specific Heat Capacities of: Water – 4. 18 J g-1 K-1 Copper – 0. 385 J g-1 K-1 The equation below relates the specific heat of a substance, the temperature change, and the mass of the substance and how much energy was put into the system. q = mC? T Where: q = quantity of heat (joules) m = Mass (grams) C = specific heat capacity J g-1 K-1 ?T = change in temperature (final – initial) (K or °C) Hypothesis:

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It is hypothesised that as the number of carbon chains in each alkanol increases, the energy required to raise the temperature of the water will also increase. Variables: •Independent oType of alkanol used; Methanol, Ethanol, 1-Propanol, 1-Butanol. •Dependent oMass of spirit burner. oMaximum Temperature of water reached. (This is the maximum temperature in which the water reaches after it has risen 20°C and the spirit burner put out). oEnergy released per mole of the fuel burnt. •Controlled oChange in temperature of water; 20°C rise in temperature. Apparatus: •Goggles •Bench mat •Retort stand •Boss head x 2 •Clamp x 2 •Thermometer Measuring cylinder •Copper can •Electronic scale/balance •Methanol Spirit Burner •Ethanol Spirit Burner •1-Propanol Spirit Burner •1-Butanol Spirit Burner •Water •Matches Method: 1. Placed on a pair of safety goggles. 2. Drew up 4 different tables, one for each alkanol, for results to be recorded. Included in each table of results were: three trials of the experiment, with each trial recording -The mass of the can, -Mass of can plus water, -Mass of spirit burner at the start, the end and the difference between, -The temperature of the water at the start, the end and the difference between. 3. Recorded mass of empty copper can. 4.

Set up experiment as shown below: 5. Using a measuring cylinder, added 100ml of water to copper can, weighed the can with water using an electronic scale, and then recorded the mass in table of results. 6. Hung the copper can from the clamp so that there was no more than 3cm gap between the can and the wick of spirit burner. 7. Recorded the mass of spirit burner (methanol), with the cap on. 8. Recorded temperature of water. (The bulb of the thermometer was positioned in the centre of the volume of water, and held in place with a clamp. ) 9. Lit the spirit burner (methanol), keeping the flame in the centre of the base of the copper can. 0. Allowed the temperature of the water to rise 20°C before extinguishing the flame with the cap. 11. The maximum temperature reached was then recorded (after flame was extinguished), as well as the difference recorded. 12. Re-weighed the spirit burner (methanol) with the cap on, recorded the mass and difference in initial and final mass, which is the amount of fuel combusted in grams. 13. The copper can was then rinsed with water and dried with paper towel (to cool back down and to keep the empty mass of the can at a constant. ) 14. Steps 5 – 13 were repeated (using fresh water each time), with 3 trials for each alkanol. 15.

Calculated the number of moles of fuel used, (for each trial, then the average was calculated). 16. Used the formula ‘q = mC? T’ to calculate the heat/energy produced, (for each trial, then the average was found). 17. Calculated Molar heat of combustion, (using the two averages previously calculated) and comparing this value with the theoretical value. 18. Steps 15 – 17 were repeated for each alkanol. Results: All Measurements have an uncertainty of ‘+/- 0. 05’ for both the mass (in grams) and the temperature (in degrees Celsius). Methanol TrialMass of can (g)Mass: can + water (g)Mass – Spirit Burner (g) (+/- 0. 05g)Temperature (°C) (+/- 0. 5°C) StartEndDifferenceStartEndDifference 1. 78. 2178. 3208. 4207. 41. 01740. 523. 5 2. 78. 2178. 4207. 4206. 11. 3173922 3. 78. 2178. 3206. 1205. 11. 01739. 522. 5 Ethanol TrialMass of can (g)Mass: can + water (g)Mass – Spirit Burner (+/- 0. 05g)Temperature (°C) (+/- 0. 05°C) StartEndDifferenceStartEndDifference 1. 78. 2178. 3245. 6244. 80. 81840. 522. 5 2. 78. 2178. 4244. 8244. 10. 717. 540. 523 3. 78. 2178. 2244. 1243. 40. 7184123 1-Propanol TrialMass of can (g)Mass: can + water (g)Mass – Spirit Burner (+/- 0. 05g)Temperature (°C) (+/- 0. 05°C) StartEndDifferenceStartEndDifference 1. 78. 2178. 2217216. 30. 71839. 521. 5 2. 78. 2178. 216. 3215. 70. 618. 54022 3. 78. 2178. 4215. 7215. 10. 618. 54122. 5 1-Butanol TrialMass of can (g)Mass: can + water (g)Mass – Spirit Burner (+/- 0. 05g)Temperature (°C) (+/- 0. 05°C) StartEndDifferenceStartEndDifference 1. 78. 2178. 2260259. 40. 618. 54122. 5 2. 78. 2178. 4259. 4258. 80. 618. 54122. 5 3. 78. 2178. 4258. 4258. 30. 51941. 522. 5 Data Analysis: Combined Specific Heat Capacity of water and copper = 4. 18 + 0. 385 = 4. 565 J g-1 K-1 Methanol (molar mass = 32. 04 g mol-1) oNumber of moles of fuel used = Mass of methanol burnt ? Molar mass of fuel (i)n = 1. 0 ? 32. 04 (ii) n = 1. ? 32. 04 = 0. 0312 mol = 0. 0405 mol (ii)n = 1. 0 ? 32. 04 = 0. 0312 mol Average = 2(0. 0312) + 0. 0405 = 0. 1029 ? 3 = 0. 0343 mol oCalculate energy required to change temperature of water using: ?H = -mC? t (i)? H = -178. 3 x 4. 565 x 23. 5 = -19127. 58 J = -19. 1276 KJ (ii)? H = -178. 4 x 4. 565 x 22 = -17916. 72 J = -17. 1967 KJ (iii)? H = -178. 3 x 4. 565 x 22. 5 = -18313. 64 J = -18. 3136 KJ Average = -19. 1276 + -17. 1967 + -18. 3136 = -55. 3579 ? 3 = -18. 4526 KJ oMolar heat of combustion (M. H. C) M. H. C = -18. 4526 ? 0. 0343 = -537. 97 KJ mol-1 Ethanol (molar mass = 46. 68 g mol-1) oNumber of moles of fuel used = Mass of methanol burnt ? Molar mass of fuel (i)n = 0. 8 ? 46. 068 = 0. 0174 mol (ii)n = 0. 7 ? 46. 068 = 0. 0152 mol (iii)n = 0. 7 ? 46. 068 Average = 0. 0174 + 2(0. 0152) = 0. 0152 mol = 0. 0478 ? 3 = 0. 0159 mol oCalculate energy required to change temperature of water using: ?H = -mC? t (i)? H = -178. 3 x 4. 565 x 22. 5 = -18313. 64 J = -18. 3136 KJ (ii)? H = – 178. 4 x 4. 565 x 23 = – 18731. 11 J = -18. 7311 KJ (iii)? H = -178. 2 x 4. 565 x 23 -18710. 11 J = -18. 7101 KJ Average = -18. 3136 + -18. 7311 + -18. 7101 = -55. 7548 ? 3 = -18. 5849 KJ oMolar heat of combustion (M. H. C) M. H. C = -18. 5849 ? 0. 0159 = -1168. 86 KJ mol-1 1-Propanol (molar mass = 60. 095 g mol-1) oNumber of moles of fuel used = Mass of methanol burnt ? Molar mass of fuel (i)n = 0. 7 ? 60. 095 = 0. 0116 mol (ii)n = 0. 6 ? 60. 095 = 0. 0099 mol (iii)n = 0. 6 ? 60. 095 = 0. 0099 mol Average = 0. 0116 + 2(0. 0099) = 0. 0314 ? 3 = 0. 0105 oCalculate energy required to change temperature of water using: ?H = -mC? t (i)? H = -178. 2 x 4. 565 x 21. 5 = -17489. 88 J = -17. 4899 KJ (ii)? H = -178. 3 x 4. 65 x 22 = -17906. 67 J = -17. 9067 KJ (iii) ? H = -178. 4 x 4. 565 x 22. 5 = -18323. 91 J = -18. 3239 KJ Average = -17. 4899 + -17. 9067 + -18. 3239 = -53. 7205 ? 3 = – 17. 9068 KJ oMolar heat of combustion (M. H. C) M. H. C = -17. 9068 ? 0. 0105 = -1705. 41 KJ mol-1 1-Butanol (molar mass = 74. 122 g mol-1) o Number of moles of fuel used = Mass of methanol burnt ? Molar mass of fuel (i)n = 0. 6 ? 74. 122 = 0. 008 mol (ii)n = 0. 6 ? 74. 122 = 0. 008 mol (iii)n = 0. 5 ? 74. 122 = 0. 007 mol Average = 2(0. 008) + 0. 007 = 0. 023 ? 3 = 0. 0077 mol oCalculate energy required to change temperature of water using: ?H = -mC? t (i)?

H = -178. 2 x 4. 565 x 22. 5 = -18303. 37 J = -18. 3034 KJ (ii)? H = -178. 4 x 4. 565 x 22. 5 = -18323. 91 J = -18. 3239 KJ (iii)? H = -178. 4 x 4. 565 x 22. 5 = -18323. 91 J = -18. 3239 KJ Average = -18. 3034 + 2(-18. 3239) = -54. 9512 ? 3 = -18. 3171 KJ oMolar heat of combustion (M. H. C) M. H. C = -18. 3171 ? 0. 0077 = -2378. 85 KJ mol-1 AlkanolExperimental M. H. C (KJ mol-1) -Experimental M. H. C (KJ mol-1) -Theoretical M. H. C(KJ mol-1) -% Error Methanol -537. 47 -726 26% Ethanol -1168. 86 -1368 15% 1-Propanol -1705. 1 -2021 16% 1-Butanol -2378. 85 -2671 11% oGraphs showing the Experimental and Theoretical values of the molar heats of combustion for Methanol, Ethanol, 1-Propanol, 1-Butanol: Conclusion: Through investigation, it was found that the greater number of carbon chains in an alkanol, the greater the molar heat of combustion in the alkanol. The hypothesis therefore was supported, from the results obtained and the analysis of data. The greater the molar mass, the more energy required to raise the temperature of water.

Evaluation: The first obvious factor contributing to limitations is the fact that the experiment was done in a school laboratory therefore there is the presence of wind. This may interfere with the flame and therefore the heating of the water. A way to improve on this issue (although not fully solving the problem) would have been to place up a wind shield(s) to stop the wind from blowing out and interfering with the flame. Another issue is the limitations of accuracy of the instruments used. The thermometer used has an absolute error of ±0. 5 of a degree, which when calculating the data, can lead to significant changes.

Another one is the electronic scale used which has an absolute error of ±0. 05g. A way to improve on this issue would simply be to have better and more accurate instruments, for instance having a scale that had an absolute error of ±0. 005 or more. When the experimental values were compared with the theoretical values, there was not an extremely, large amount of error, with the highest being 26%. The loss of heat to the surroundings such as the air, the glass and metal parts of the spirit burner, may account for this error. These were accounted for during the experiment where there was heat felt, coming off the spirit burner, can and water.

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