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

Stewart Hird

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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: 

Simple Calorimeter_1, downloadable IB Chemistry revision notes

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

Alcohol

Ethanol

Propanol

Butanol

Pentanol

Energy released per gram of alcohol burned (kJ/g)

29.8

33.5

36.3

37.6

Mass of alcohol to produce a 40 °C rise in temperature 100 cm3 of water (g)

0.56

0.51

0.45

0.42

Mass of alcohol to produce a 1 °C rise in temperature 100 cm3 of water (g)

0.0140

0.0128

0.0113

0.0105

  • 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

flammable-and-harmful-to-health

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

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Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

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