Comparing the enthalpy changes of combustion of different alcohols

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Comparing the enthalpy changes of combustion of different alcohols

Plan

Aim:

  To compare the enthalpy changes of combustion of different alcohols.

Apparatus and Quantities:

  • 4 Spirit Burners containing – methanol, ethanol, propan-1-ol, butan-1-ol
  • Small copper can to act as a calorimeter
  • 0-110°C thermometer to measure temperature
  • 100 cm3 measuring cylinder
  • Aluminium shield to reduce heat loss to surroundings
  • Digital Balance
  • Retort Stand, Boss and clamp (x2)
  • Water (800 cm3)

Method:

  1. Using a 100 cm3-measuring cylinder, measure 100 cm3 of water and this water in the copper can. 100 cm3 of water should be used, as this is an appropriate amount of water as this fills ¾ of the can, so it is just enough water to give a sufficient temperature rise. The copper can is used as copper is a good conductor and therefore it will distribute the heat equally.
  2. Take the first spirit burner with methanol and weigh this using the digital balance as it is more accurate. Record this as the initial mass. This will then be used to calculate the amount of mass used during the combustion.
  3. Place the spirit burner on the table. Then, using a retort stand with a boss and clamp securely tightened on the stand, attach the copper can with 100 cm3 water on the clamp and place it approximately 7 cm away from the spirit burner so that the flame can effectively heat the water.
  4. Using a 0-110°C thermometer, measure the temperature of the cold water and record this as the initial temperature. This will then be used to calculate the temperature rise.
  5. Then using the second retort stand (with boss and clamp) attach the aluminium shield to the clamp and place this over the copper can to reduce the heat lost to the surroundings. The apparatus should be set up as shown below:

  1. By adjusting the clamp, lower the aluminium shield so that the wick can be carefully lighted. Carefully, using a splint of flame, light the wick of the spirit burner so that the water could get heated.
  2. Immediately, adjusting the clamp again, take the aluminium shield up to sufficiently cover the copper can and tighten the clamp.
  3. Allow the spirit burner to heat up the water by about 15°C. Use a thermometer to measure the temperature. This is an appropriate temperature as a lower temperature could be harder to achieve as the fuel might heat up very quickly, resulting in a quick rise of temperature which would limit the time needed to heat the water. Also a high temperature would take longer to achieve, limiting experimental time.
  4. Therefore, stir the water until the temperature has risen by 15°C. Then, adjust the clamp holding the aluminium shield and lower the shield in order to extinguish the flame. Then extinguish the flame by placing the lid over the spirit burner. Note the highest temperature reached using a thermometer and record this temperature as the final temperature. This can then be used to calculate the temperature rise.
  5. Finally take the spirit burner and weigh it using a digital balance. Record the mass as the final mass. This can be used to calculate the mass used, as the initial mass subtracted by the final mass will result in the mass used.
  6. After recording these measurements, repeat the same experiment for ethanol, propan-1-ol and butan-1-ol and record the results.
  7. Finally repeat the experiment for the 4 fuels again to find an average result as this will produce more accurate measurements to calculate the enthalpy changes for combustion.
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Calculating results:

  Once measurements of the mass of water, the temperature rise of water, and the mass of fuel used are recorded, use these formulae to work out the enthalpy changes of combustion.

First, work out the energy transferred by using this formula:

Energy transferred to water = (4.2 x mass of water x temperature rise) J

by burning fuel

                                            = cmΔT

(C= 4.2 Joules which is the specific heating capacity of water and ...

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