Experiment: Determining the heat of combustion of alcohols

Determining the heat of combustion of alcohols is a Form 5 practical in the Thermochemistry chapter. It measures the energy released when an alcohol burns by using it to heat water, and it lets you compare the members of a homologous series. This guide sets out the aim, the apparatus, a numbered procedure, the observations to expect, and the science process skills the practical papers reward. The observations are qualitative; use your own measured masses and temperatures for any calculation.

Aim

To determine the heat of combustion of an alcohol by using the burning alcohol to heat a known mass of water, and to compare the heats of combustion of successive members of the alcohol series.

Apparatus and materials

• Copper can (calorimeter) to hold the water
• Retort stand, boss and clamp to hold the can
• Thermometer
• Measuring cylinder
• Spirit lamp or small burner containing the alcohol, with a wick
• Balance (to weigh the lamp before and after)
• A series of alcohols, for example methanol, ethanol, propan-1-ol and butan-1-ol
• Draught shield to reduce heat loss

Procedure

1. Measure a known volume of water with a measuring cylinder and pour it into the copper can; the mass of water is known from its volume.
2. Clamp the copper can above the spirit lamp and record the initial temperature of the water with the thermometer.
3. Weigh the spirit lamp containing the first alcohol and record its mass.
4. Light the lamp and let the flame heat the water, stirring the water gently with the thermometer.
5. When the temperature of the water has risen by a suitable amount, put out the flame and record the highest temperature of the water.
6. Weigh the lamp again immediately and record its new mass; the difference gives the mass of alcohol burnt.
7. Calculate the heat gained by the water and, using the mass of alcohol burnt and its molar mass, work out the heat of combustion per mole; repeat with each alcohol under the same conditions.

Expected observations

When the lamp is lit, the alcohol burns with a flame and the water in the copper can becomes warmer, so the thermometer reading rises. The mass of the spirit lamp decreases as the alcohol is used up. A layer of black soot may form on the base of the copper can, especially with the alcohols that have more carbon atoms. The two key measurements to record are the rise in temperature of the water and the mass of alcohol burnt; along the series, alcohols with more carbon atoms tend to give a larger heat of combustion per mole.

Inference and conclusion

The water becomes warmer because the combustion of the alcohol is exothermic, releasing heat that the copper can transfers to the water. The heat gained by the water is calculated with heat gained = mass of water x specific heat capacity x temperature rise (mc-theta), and dividing by the moles of alcohol burnt gives the heat of combustion per mole. As you go along the series, each alcohol has one more carbon and hydrogen unit, so more bonds are formed with oxygen on burning and the heat of combustion per mole increases. The conclusion is that the heat of combustion of an alcohol can be found from the temperature rise of the water and the mass burnt, and it increases along the homologous series.

Science process skills (Paper 3 style)

Identifying variables. The manipulated variable is the alcohol used (its number of carbon atoms), the responding variable is the heat of combustion per mole, and the controlled variables are the mass of water, the distance from the flame to the can, and the use of a draught shield.

Making an inference. From the observation that the water temperature rises, infer that combustion is exothermic and releases heat to the surroundings.

Tabulating and plotting. Tabulate each alcohol with its number of carbon atoms and its heat of combustion per mole, then plot heat of combustion against the number of carbon atoms and describe the trend.

Controlling for accuracy. Explain how a draught shield and placing the can close to the flame reduce heat loss to the surroundings, so the measured heat of combustion is closer to the true value.

Safety precautions

• Wear safety goggles, because the alcohols are flammable and burn readily.
• Keep the alcohol bottles stoppered and away from the lit lamp, so the vapour does not catch fire.
• Put out the flame with the lamp cap, not by blowing, so that burning alcohol is not scattered.
• Let the copper can and lamp cool before weighing or moving them, to avoid burns.
• Work on a heatproof mat in a ventilated area, so heat and combustion gases are handled safely.

Common errors

• Not weighing the lamp promptly. If you wait after the flame is out, more alcohol evaporates and the mass burnt is overstated; weigh the lamp immediately.
• Ignoring heat loss. Much heat escapes to the air; use a draught shield and place the can close to the flame, and note that heat loss makes the measured value lower than the true one.
• Letting soot build up. A thick layer of soot insulates the can and lowers the temperature rise; note incomplete combustion rather than ignoring it.
• Changing conditions between alcohols. A fair comparison needs the same mass of water and set-up for each alcohol; change only the alcohol.
• Inventing readings. Use the masses and temperatures you actually measure; do not copy heat values from a data book into your result.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we make sure students can carry out the full calculation, heat gained by the water with mc-theta, then heat per mole from the mass burnt, and can explain why heat loss makes the experimental value lower than the accepted one. Heat of combustion is a favourite calculation and graph question across SPM Chemistry because it links energy changes with the alcohol homologous series, and the same skills of measuring, tabulating and plotting are rewarded in the Paper 3 practical assessment (Paper 3 is a practical test assessing science process skills).