Molar Heat combustion chemistry - investigate the effect of molar mass on the molar heat of combustion of adjacent members of a homologous alcohol series.

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To investigate the effect of molar mass on the molar heat of combustion of adjacent members of a homologous alcohol series.


Chemists refer to the energy stored in a substance as the heat content or enthalpy of the substance. The heat of reaction is determined by the difference in the enthalpy between the reactants and products.

The molar heat of combustion of a substance is the quantity of heat liberated when one mole of that substance is burnt completely in air. In the case of a hydrocarbon, the products are carbon dioxide and water.

In this experiment you will determine the molar heat of combustion of methanol, ethanol, 1propanol, 1butanol; and 1pentanol.

Experimental Procedure


  • Electronic Balance ±0.01g
  • Measuring Cylinders ±1 mL
  • Aluminium Can
  • Cotton Wool
  • Theromometer ±0.5°C
  • Retort Stand and Clamp
  • Spirit Burner containing alcohol sample
  • Alcohols (methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol)


The alcohols are highly flammable. Always place the alcohol lamp on a tray to contain any accidental spillage of alcohol. Carefully use the cap of the alcohol lamp to smother its flame. Avoid skin contact with the chemicals. Lab coats and safety glasses must be worn at all times during the practical to ensure personal protection.


  1. Design a data table to record the results of the experiment – this must be done prior to the experiment by all individuals.
  2. Measure 100g of water into an aluminium can and clamp it in position above the spirit burner.
  3. Insert the thermometer and cotton wool plug into opening of the can (ensure that the thermometer does not touch the bottom of the can).
  4. Record the initial temperature of the water.
  5. Select an alcohol burner and ensure that there is at least 20 cm3 of the alcohol in the burner.
  6. Weigh the burner and its contents.  
  7. Place the burner under the centre of the flask (shield from drafts) and light the burner.  
  8. Gently stir the water with the thermometer and when the temperature has risen by 20oC, extinguish the burner by carefully replacing the cap.
  9. Continue stirring for 10 seconds, then read and record the final water temperature.
  10. Re-weigh the burner and record its final mass.
  11. Repeat your measurements 2 more times for each alcohol.
  12. Perform the experiment using ethanol, methanol, 1-propanol, 1-butanol and 1pentanol.

Raw Data:

Table 1. Quantitative Raw Data

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Table 2. Qualitative Data

Processing Raw Data:

Sample calculation: 1. Finding the  in mass

2. Finding the  in temperature

3. Finding the heat of combustion

4. Finding molar mass

5. Finding the number of moles

6. Finding the molar heat of combustion

7. Finding the average molar heat of combustion for each fuel

8. Finding the uncertainties of the average

Sample calculation 1. Finding the  mass:

To find the  mass, use the following equation:

E.g. (values from methanol trial 1):

Calculating ...

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