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
- 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
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