Analysis of the Standard Enthalpy of Combustion for Alcohols

Aim:

To investigate the standard enthalpy change of combustion for 5 consecutive alcohols in the alcohol homologous series, methanol, ethanol, propan-1-ol, butan-1-ol and pentan-1-ol, by using a calorimetric method to calculate the heat gained by the 100cm3 water in the experiment, and thus the heat lost by the alcohol lamp at standard temperature and pressure (298 K and 101.3 kPa).

Background Knowledge:

Alcohols are organic compounds containing Oxygen, Hydrogen and Carbon. The alcohols are a homologous series containing the functional –OH group.

As we move down the homologous series of alcohols, the number of Carbon atoms increase. Each alcohol molecule differs by –CH2; a single Carbon atom and two Hydrogen atoms.

Combustion is the oxidation of carbon compounds by oxygen in air to form CO2 and H2O. Combustion produces heat as well as carbon dioxide and water. The enthalpy change of combustion is the enthalpy change that occurs when 1 mole of a fuel is burned completely in oxygen.

When alcohol undergoes complete combustion it produces carbon dioxide and water as products, and energy is released.

The standard enthalpy of combustion of an alcohol (âH°comb) is the enthalpy change when one mole of an alcohol completely reacts with oxygen under standard thermodynamic conditions (temperature of 25°C and pressure of 101.3 kPa). The standard enthalpy change of combustion of alcohols varies depending on their molecular size. The greater the number of carbons, the higher the standard enthalpy of combustion, as there is the presence of more bonds. The larger the alcohol molecule, the more bonds will be broken and formed, and therefore more heat will be produced.

Using experiments, the standard enthalpy of combustion of an alcohol can be found, buy first finding the heat released during the reaction using the equation

Heat=mass of water ×specific heat capacity of water ×rise in temperature of water

Note: The specific heat capacity of water is 4.18 Jg-1°C-1.

and then finding the number of moles of alcohol burnt, and dividing the heat by this number.

Equipment:

1. 250 cm3 Conical flask

2. 100 cm3 ± 0.08 cm3 pipette

3. Loggerpro thermometer

4. 5 x different consecutive alcohol spirit burners (eg. methanol, ethanol, propanol, butanol and pentanol)

5. Stand

6. 2 x clamps

7. Scales

8. 1500 cm3 distilled water

9. Heat proof mat

10. Matches

Method:

1. Connect the temperature sensor to the datalogger. Connect the datalogger to the computer. Ensure the datalogging software is loaded and set to record the temperature of the sensor. Set the sampling rate to 1 sample per second for 210 seconds.

2. Using the pipette, pipette 100 cm3 distilled water into the conical flask.

3. Set up the stand, and clamp the conical flask 25 cm from the table. Also clamp the temperature probe 30 cm from the table, so that it is submerged in the distilled water but not in contact with the conical flask walls.

4. Weigh the alcohol lamp (including its cap) using the scales and record the mass.

5. Place alcohol lamp directly under the conical flask on a heat proof mat.

6. Click ‘collect’ on datalogger to start recording the temperature. After 30 seconds, light the alcohol lamp.

7. When the datalogger reaches 210 seconds immediately extinguish the flame by replacing the cap. ‘Store the latest run’ in loggerpro.

8. Re-weigh the alcohol lamp (including cap) as soon as possible after extinguishing the lamp.

9. Repeat steps 2 – 8 with the same alcohol to obtain trail 2, and trial 3 results.

10. Repeat steps 2 – 9 for 4 other consecutive alcohols.

11. Calculate the average change in mass of each alcohol and calculate the change in temperature of water for each trial.

12. Calculate energy absorbed by this using q=mcâT then calculate âH°comb=qn

13. Plot the graph of âH°combversus number of carbons in alcohol.

Apparatus:

temperature probe

datalogger device

5 cm

25 cm

alcohol lamp

loggerpro collector on computer

heatproof mat

100 cm3 distilled water

conical flask

clamp

clamp

Variables:

1. Independent

The alcohol used to heat water will be changed, however all alcohols will be primary.

The range of alcohols will be 5 consecutive alcohols from the homologous series; methanol, ethanol, propan-1-ol, butan-1-ol, pentan-1-ol.

1. Dependent

The change in temperature of the 100cm3 distilled water when heated by an alcohol lamp.

1. Measure the initial temperature and final temperature using loggerpro. The change in temperature can be calculated by: ΔT=T(final)-T(initial)

1. Controlled

Finding the âH using âH°comb=qn

Controlled Variables

How is it controlled?

Effect on experiment if uncontrolled

Type of liquid

Using only distilled water for all trials throughout the experiment.

Different liquids could result in a difference in the strength of attractive forces between particles, meaning a different specific heat capacity which would affect the calculation of energy gain to water using the equation q=mcâT, and thus an incorrect enthalpy change value.

Volume of liquid used

Measure 100cm3 of distilled water by using 100 cm3 ± 0.08 cm3 graduated pipette for each trial.

If the volume was not exactly 100 cm3 it would directly affect the mass of the water which will affect the q=mcâT value and thus the âH value.

Material glassware

Use the same brand and materials of a conical flask for all trials.

Different materials have different conductivity and may absorb more heat from the alcohol lamp, affecting the overall heat absorbed by the distilled water. Using the same material and brand of conical flask ensures that this is the same for each experiment.

Temperature of surroundings

For standard enthalpy of combustion, the temperature must be 25°C however in a classroom this is hard to control, so for each experiment the temperature will stay constant at 19°C.

If the surrounding temperature was to be changing, the distilled water could be losing more, or gaining more heat energy from the surroundings, directly affecting the temperature change and therefore, q=mcâT and the âH value.

Distance between the conical flask and alcohol lamp

A clamp will be set at a distance of 25 cm from the table, and this the flask will sit at the same height each trial.

If the distance changes, the heat lost to the surroundings varies and the heat that reaches the bottom of the calorimeter also varies. This will lead to a difference in rise in temperature of water (âT), and therefore an incorrect calculation for q=mcâT and âH value.

Pressure of surroundings

For standard enthalpy of combustion the pressure must be 1 atm, however in a classroom this is hard to obtain, so all experiments will be done in a room with the same pressure.

Might influence the vapour pressure point, which will affect the q=mcâT value, and thus the âH.

Duration of heating

The water will be headed for 180 seconds.

This ensures that all experiments have the same time to heat the water which directly effects the change in temperature and thus the q=mcâT calculation and the âH value.

References:

http://gandhijkt.org/blog/wp-content/uploads/2011/03/chemistry-sample-lab-report.pdf

http://www.ausetute.com.au/heatcomb.html

http://www.s-cool.co.uk/a-level/chemistry/chemical-energetics/revise-it/enthalpy-changes