Background

Fossil fuels release energy on combustion as the combustion products, carbon dioxide and water, are more stable than the reactants.

Some alcohols, for example methanol and ethanol can be used as fuels.

Combustion of methanol

CH₃OH(l) + 1½O₂(g) → CO₂(aq) + 2H₂O(l)

It is difficult to measure the heat released directly. Instead the heat energy must be absorbed in water, and the energy change in the water calculated using the relationship:

q = mcΔT

Where q is the energy absorbed, m = mass of water, c = specific heat capacity of water (4.18 kJ kg^-1 K^-1), ΔT = temperature change in Celsius or Kelvin

Moles of fuel used is then calculated by dividing the mass of fuel burned by the relative mass of the fuel, and the energy per mol reacted is then given by:

Energy change per mol = q/n

Where 'n' is the number of moles, not forgetting that energy released is negative by convention.

In this experiment the energy released by several alcohol fuels is compared.

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Requirements

Chemicals

• Methanol
• Ethanol
• Propan-1-ol
• Propan-2-ol
• Butan-1-ol
• Methylpropanol

Apparatus

• Spirit burner
• Electronic balance
• Calorimeter
• Thermometer or temperature probe
• Clamp, boss and stand

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Procedure

• Weigh a sample of water into a pre-weighed calorimeter (copper can).
• Record the temperature of the water
• Weigh the spirit burner.
• Use the spirit burner to heat the water sample.
• Extinguish the spirit burner and record the highest temperature in the water.
• Weigh the water sample + calorimeter.
• Re-weigh the spirit burner.
• Record all data and repeat (each alcohol should be repeated twice, i.e. three trials)
• Determine the heat changes for all the available alcohols.

Full screen projection

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Safety

• Alcohols are highly flammable and burn with an almost invisible flame in excess air.
• Methanol is toxic
• Hot water can scald badly

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