Comparison of Enthalpy Changes of Combustion of Different Alcohols.

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Emma Powditch 12RP

Chemistry Coursework

Comparison of Enthalpy Changes of Combustion of Different Alcohols

Aim: To investigate how and why the enthalpy of combustion of varying alcohols is affected by molecular structure.

The enthalpy change of combustion refers to the amount of energy transferred when one mole of fuel burns completely.  This value was obtained by using various fuels to heat water, knowing 4.2J of energy are required to raise the temp of 1g of water by 1oC, using the following method.  The enthalpy change of one mole of alcohol can be calculated from the amount of energy the water has absorbed.  

Apparatus

Fuel burners and the following alcohols

Methanol (CH3OH)

Ethanol (C2H5OH)

Propan-1-ol (C3H7OH)

Butan-1-ol (C4H9OH)

Octan-1-ol (C8H17OH)

Copper Calorimeter

Thermometer

250cm3 measuring cylinder

Goggles

Draft excluder

Electronic scales (to 3dp for accuracy purposes)

Stirring rod

Diagram

Method

  1. Pour 200cm3 of water into a copper calorimeter and record the temperature.
  2. Using a clamp stand support the calorimeter over your chosen spirit burner. (At this point it is useful to erect some kind of draft excluder, for increased accuracy, in my case I used a tin can.)  Any disturbances could affect the infra red radiation being produced by the burning alcohol and help direct it towards the calorimeter.
  3. Weigh the burner and alcohol including lid, otherwise the alcohol will evaporate and disrupt the results, record the result in a suitable table.
  4. Replace the burner and light wick.
  5. Heat the water until it becomes 15 oC warmer and extinguish the burner, replace lid, whenever your burner is not in use.
  6. Reweigh your burner and lid and record the result.
  7. Repeat the experiment with the other alcohol burners keeping each variable the same, record your results.  Use the same calorimeter but let it cool down between each experiment; clean off any soot deposits, which could affect the conduction of heat.

I kept the increment of temperature change as constant as possible to prevent the need for unnecessary calculations in arriving at my final enthalpy changes.  Due to the extra energy water needs to change sate the relationship between the increase in temperature isn’t always the same creating an unpredictable curve.  An amount of energy will not cause the 200cm3 of water at an initial temperature of 70oC to the rise as that of 30oC initially.

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Risk Assessment

  • Wear eye protection
  • Be sensible around naked flames, alcohol burners often have flames that are not clearly visible in sunlight.
  • Experimental equipment may be hot during and for some time after the experiment, so refrain from touching it before it cools sufficiently.
  • Alcohols are flammable, avoid spillages whenever possible.
  • Alcohols fumes can be harmful.
  • Remain at a safe observing distance whilst carrying out the experiment.
  • Long wicks encourage large flames so keep the wick to a minimum.

Accuracy

In an effort to make the experiment a ‘fair test’ the only ...

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