Comparing the Enthalpy Changes of Combustion of Different Alcohols

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Comparing the Enthalpy Changes of Combustion of Different Alcohols

I am going to perform an investigation with the aim of finding the enthalpy change of combustion (?Hc) of several different alcohols. I will then compare these results and attempt to show how and why the ?Hc of an alcohol varies according to its molecular structure.

Planning

Equipment apparatus and materials:

* Copper calorimeter

* Metal draught excluder

* Thermometer

* Digital scales (capable of mass readings given to at least 2 d.p.)

* Clamp stand

* Spirit burners containing the alcohols which will be investigated:

o Pentane (CH3(CH2)3CH20H)

o Ethanol (C2H5OH)

o Propan-1-ol (C3H7OH)

o Butan-1-ol (C4H9OH)

Method

Set up apparatus as shown in the diagram

Pour 200 cm³ water into the copper calorimeter

Take initial temperature reading of water

Weigh the spirit burner with the lid on

Light spirit burner

Stir water often and extinguish the spirit burner once the water temperature has risen by around 20°C

Take final temperature of water

Weigh the spirit burner with lid

The metal draught excluder will prevent any breeze or rush of air that may occur from carrying away any of the heat energy produced by the burning of the alcohol. It will also help to prevent heat being lost by radiation, and will help channel the heat of a large flame onto the base of the calorimeter.

The length of the wick must be kept as similar in length as possible throughout to avoid some alcohols burning at different rates to others, and therefore having different amounts of heat lost. The flame must be kept fairly small, so as to avoid excess heat passing the calorimeter in large quantities and escaping. To ensure this, I shall attempt to keep each wick protruding 0.5cm from the spirit burner.

The same calorimeter must be used throughout to avoid any differences in heat conductivity that might otherwise occur. It must be left to cool for several minutes after each run and any soot that builds up will be cleaned off so that it cannot hinder the thermal conductivity of the calorimeter.

The spirit burner must be weighed with the lid on to avoid alcohol evaporating whilst the weighing is taking place. This would result in an inaccurate record of the weight difference in each run. For the same reason, the lid must be kept on at all times when the spirit burner is not lit.

The water must be stirred regularly throughout the investigation so that the temperature of the water remains constant throughout and the reading given by the thermometer accurately reflects the heat absorbed by the water.

I will attempt to keep the temperature rise of each run similar so that results can easily be compared roughly without the need for calculations. This is also important because the amount of energy absorbed by water for an increase in temperature does not remain exactly constant - it is a curve - water needs to absorb more energy to change state, so a given amount of energy will not cause 200cm³ of water at 20°C to raise in temperature by exactly the same amount as 200cm³ of water at 70°C. Whilst this will probably not effect my investigation much (as the final temperatures will always be in a relatively small range), it may be worth considering.
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Risk Assessment

Do not inhale alcohol fumes

Take care not to touch the calorimeter or draught excluder immediately after the experiment, as they will be hot.

Take care not to spill alcohol on anything, as it could ignite.

Do not stand with your face directly above the calorimeter if the flame is strong as the hot air could burn you.

Keep wick length relatively low or the flame will be very high.

Theory

It is known that 4.2J of energy are required to increase the temperature of 1g of water by ...

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