Core Practical: Heat of Combustion of Alcohols (Edexcel GCSE Chemistry) : Revision Note

Core Practical: Heat of Combustion of Alcohols

• Alcohols can be used as fuels as they readily undergo combustion and release heat energy

• Ethanol for example combusts in excess oxygen:

CH₃CH₂OH + 3O₂ → 2CO₂ + 3H₂O

• Some alcohols are better fuels than others i.e. they release more heat energy per mole than other alcohols

• Calorimetry studies can be performed to investigate the efficiency of alcohol fuels by measuring how much of each alcohol is needed to raise the temperature of a fixed amount of water by a set number of degrees

Heat of Combustion of Alcohols

Aim:

• To investigate the heat of combustion of four alcohols

Materials:

• Copper calorimeter can with lid, thermometer, water, spirit burner, balance

• Supply of ethanol, propanol, butanol and pentanol

Diagram: 

A simple combustion calorimeter

Method:

• Using a measuring cylinder, place 100 cm³ of water into a copper can

• Record the initial temperature of the water and the mass of the empty burner

• Fill the burner with the test alcohol and record its new mass

• Place the burner under the copper can, light the wick and place the lid on

• Stir the water constantly with the thermometer (calorimeter lids allow for this) and continue heating until the temperature rises by 25 ºC 

• Immediately extinguish the flame and measure and record the mass of the spirit burner

• Repeat procedure for other alcohols, making sure the variables are kept the same:

  • Volume of water (water should be changed each time)

  • Distance between wick and bottom of stand

• Record your results neatly in tabular format

Results:

• Different amounts of the four alcohols will be needed to achieve the same temperature rise

• Calculate the mass change for each alcohol in the spirit burner

• Use the calorimetry equation to find the heat of combustion of the alcohols in kJ per gram or kJ per mole:

Enthalpy change equation:  Q = m x c x ΔT

where:

• Q – energy transferred to water

• m – mass of water heated

• c –  the specific heat capacity – is the amount of heat needed to raise the temperature of 1 gram of a substance by 1 ^oC.

  • For water, the value is 4.18 J g^-1 C^-1 (Joules per gram per degree Celsius).

• ∆T – change in temperature

• When you have found Q you have calculated amount of heat released to burn the mass of alcohol in the experiment

• You can then work out:

  • The amount of heat released per 1 g of substance = Q/mass of substance burnt

  • The amount of heat released per 1 mole of substance = (Q/mass of substance burnt) x molar mass of substance

Sample results

Sample results for combustion of alcohols

• The results show that alcohols that are larger make better fuels as they produce the most energy per gram 

• Therefore, the order of energy density is:

  • Pentanol, C₅H₁₁OH > butanol > C₄H₉OH > propanol C₃H₇OH > ethanol C₂H₅OH

Hazards, risks and precautions

Hazard symbols to show substances that are flammable and harmful to health

• The alcohols used are flammable and often harmful to health, e.g, propan‐1‐ol, butan‐1‐ol, pentan‐1‐ol

• The alcohols should be kept away from naked flames, e.g. a Bunsen burner

• Avoid contact with the skin and breathing in the vapour

• A fume cupboard can be used for harmful alcohols