Thursday, 26 February 2015

4.5.1 - Laboratory Activity : Heat of Combustion of Different Alcohol

Laboratory Activity 4.5.1:
Heat of Combustion of Different Alcohol
Aim: To compare the heat of combustion of different alcohol.
Problem statement:Do all alcohol have the same heat of combustion?
Hypothesis: Different alcohol have different heat of combustion
» Fixed variable : Water volume, thin copper container and lamp.
» Manipulated variable : Types of alcohol
» Responding variable : Heat of combustion

» Methanol
» Ethanol
» Propan-1-ol
» Distilled water

» Lamp
» Thermometer
» Measuring cylinder
» Tripod stand
» Wind screen
» Thin copper container

1. 100cm3 of water is measured with a measuring cylinder and poured into a thin copper container which is placed on top of a tripod stand. A wind screen is placed surrounding the tripod stand.
2. Record the initial temperature using thermometer.
3. A methanol lamp is weighed and placed under the metal container and lighted.
4. Stirred the water in the metal container with a thermometer.
5. When temperature of the water increases by 30°C, the flame is distinguish and the highest temperature are recorded.
6. The lamp is then weight again.
7. The experiment is repeated using other types of alcohol.

Alcohol Methanol Ethanol Propan-1-ol
Initial mass of lamp (g) M1 M4 M7
Final mass of lamp (g) M2 M5 M8
Mass of alcohol used (g) M3 M6 M9
Final temperature (°C) T1 T3 T5
Initial temperature (°C) T2 T4 T6
Change temperature (°C) θ1 θ2 θ3

Heat of combustion for methanol
Volume of water used = 100cm3 . Therefore the mass of water used = 100g
Heat change in water = mc∆θ
= (250 X 4.2 X θ1) joules
= H joules

Molar mass of methanol CH3OH = 32 + 3 + 16 + 1 = 32g
Mass of methanol burnt = M
Number of moles of methanol burnt = M32 = x moles
Therefore the burning of x moles of methanol will liberate H joules of heat.
Therefore the burning of 1 mole of methanol will liberate HxJ mol1= Heat of combustion

Heat of combustion for ethanol and propan-1-ol
The same calculation method can be used to determine the heat of combustion for ethanol and propan-1.

All alcohol contains carbon, hydrogen and oxygen. Burning of alcohol is an exothermic reaction, heat of combustion will have negative values.

The amount of heat given out will depends on the number of carbon atoms and hydrogen atoms they contain, the mass of alcohol in their molecules. The heat liberated will increase if the number of carbon atoms, hydrogen atoms and the mass of alcohol increased as shown in the following table
Alcohol Heat of combustion (kJ mol-1)
Methanol -725
Ethanol -1376
Propanol -2015

The experiment value will always be less than the theoretical value, due to the radiation heat lost to the surroundings, and energy used to heat up the apparatus(copper container and thermometer).

The energy level diagram for the combustion of alcohols are shown in the following diagram.

Wind shield is used to protect the flame from the disturbance of wind.
The flame of alcohol is placed close to the metal container without using a wire gauze .
The copper container should be as thin as possible to ensure the heat from the alcohol flame is easily transferred to the water in the container.
The alcohols used are volatile liquids as they can easily catch fire. Make sure the caps of the lamps are screwed tightly to prevent any alcohol vapour escaping from the bottles.
Use only thermometer to stir and not any other stirrer apparatus.
The weighing should be done as fast as possible to prevent lost of volatile alcohol by evaporation.

Different alcohols have different heat of combustion.

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