Research Question

How does the number of carbon atoms in the hydrocarbon chain of primary alcohol affect its enthalpy change of combustion?

Independent Variable

The independent variable in this investigation is the number of carbon atoms in the hydrocarbon chain of primary alcohol. Five different primary alcohol will be used, namely, methanol, ethanol, propan-1-ol, butan-1-ol and pentan-1-ol.

Dependent Variable:

The dependent variable in this investigation is the enthalpy change of combustion of primary alcohol.

Enthalpy change of combustion is the energy released when one mole of fuel is completely burnt in oxygen to form carbon dioxide and water. To measure the enthalpy change of combustion, the heat given out during combustion is first absorbed by water which is calculated by q = mc T where m is the mass of water, c is the specific heat capacity of water and T is the temperature change of water. The heat given out is then divided by the number of moles of alcohol used during the investigation. As combustion is an exothermic reaction, a negative sign is added to give the final enthalpy change of combustion.

Control Variables:

The table below shows the variables that have to be controlled:

Hypothesis:

The higher the number of carbon is in the hydrocarbon chain, the greater the enthalpy change of combustion of the primary alcohol.

One of the evidence for this hypothesis comes from the IB Chemistry data booklet. The standard enthalpy change of combustion for methanol, ethanol, propan-1-ol and butan-1-ol are –726 kJmol^-1, –1367 kJmol^-1, –2021 kJmol^-1 and –2676 kJmol^-1 [1]respectively.

Another way to get this trend is to use the average bond enthalpy to estimate the enthalpy change of combustion. Though average bond enthalpies are just average values only, they do give an insight on the trend of the enthalpy change of combustion of different alcohol. The general formula for alcohol is C_nH_2n+2O. The general equation for combustion of alcohol is

C_nH_2n+2O (l) + 1.5nO₂ (g)  nCO₂(g) + (n+1)H₂O(l)

When the number of carbon increases by one, the following table shows the extra bonds to be broken and extra bonds formed:

The average bond enthalpies for the above bonds are shown in the table below: [2]

The net change in enthalpy change when one more carbon is added

= + 2 × 413 + 347 – 2 × 746 – 2 × 464

= – 1247 kJmol^-1

Thus, the enthalpy change of combustion of primary alcohol will decrease by 1247 kJmol^-1 (more negative) when one more carbon is added to the hydrocarbon chain.

Apparatus:

1. 100 cm³ measuring cylinder (±1 cm³) × 6 (one for each alcohol and the other for water)
2. (2 d.p. ) Electronic balance (±0.01 g) × 1
3. Tripod × 1
4. Copper container with lid × 15 (one for each trial)
5. Stand and clamp × 1 (use to hold the thermometer in position)
6. Mercury in glass thermometer (±0.1ï‚°C) × 2 (one is used to measure temperature of water and the other used for monitoring the temperature of surrounding)
7. Stirrer × 1
8. Aluminium heat shield × 4 (to reduce heat loss)
9. Spirit burner with wick × 15 (one for each trial)
10. Lid × 1 (for extinguishing the flame)
11. Stopwatch (±0.01 s) × 1
12. Ruler (±0.1 cm) to measure the length of wire and the position of the thermometer
13. Scissor × 1 (used to cut the wick)

Chemicals

1. 750 cm³ of distilled water (50 cm³ for each trial)
2. 100 g of each of the following primary alcohol at a concentration of 90%.

Methanol, ethanol, propan-1-ol, butan-1-ol and pentan-1-ol

Procedures

1. Prepare a spirit burner. Cut the length of its wick to approximately 3.0 cm (the portion outside the burner) with a pair of scissors and a ruler.
2. Measure 50 cm³ of methanol with a 100 cm³ measuring cylinder and pour it in a spirit burner.
3. Weight the spirit burner with an electronic balance. Record the mass of the spirit burner in the table below.
4. Measure 50 cm³ of distilled water with another 100 cm³ measuring cylinder and pour it in a copper container.
5. Place the copper container on a tripod and cover it with a lid.
6. Use stand and clamp to hold a thermometer in the beaker of water. Use a ruler to make sure that the bottom of the thermometer is 1.0 cm from the bottom of the beaker and make sure the thermometer is approximately located at the centre of the beaker. Record the initial temperature of the water and put it in the table below.
7. Place a stirrer in the water. Stir the water throughout the process of heating.
8. Surround the whole setup with heat shield made of aluminium to reduce heat loss.
9. Ignite the wick of the spirit burner with a lighter.
10. Place the spirit burner under the beaker.
11. Immediately start the stopwatch and wait for two minutes.
12. Stop the stopwatch after two minutes.
13. Use a lid to extinguish the flame.
14. Record the highest temperature reached by the thermometer and put it in the table below.
15. Allow the spirit burner to cool.
16. Weigh the final mass of the spirit burner with an electronic balance. Put the mass in the table shown below.
17. Repeat the experiment (Step 1 to 16) two more times.
18. Repeat the same experiment (Step 1 to 17) for other primary alcohols (i.e. ethanol, propan-1-ol, butan-1-ol and pentan-1-ol).

Table for recording raw data and some processed data:

Data Processing

1. The mass of fuel used (m) is calculated by subtracting the final mass of spirit burner from the initial mass of spirit burner.
2. The temperature change (T) is calculated by subtracting the final temperature by the initial temperature.
3. The heat (q) absorbed by water can be calculated by the formula

q = mc T

where

1. m is the mass of water used: 50 g (as the density of water is 1 g cm^-3)
2. c is specific heat capacity of water: 4.18 Jg^-1K^-1

4. Number of moles of fuel used can be calculated by dividing the mass of fuel used (m) by the molar mass of the fuel.

The molar mass is found by adding up the relative atomic mass of the element involved. For example, the molar mass of methanol (CH₃OH) is 12.01 + 4 × 1.01 + 16.00 = 32.05 gmol^-1.

5. The enthalpy change of combustion of alcohol is found by dividing the heat by the number of moles of fuel with a negative sign added to show that it is exothermic.
6. A graph is plotted with enthalpy change of combustion (y-axis) against the number of carbon atom is the hydrocarbon chain of the primary alcohol (x-axis).
7. Compare this graph with the graph in the hypothesis.

Safety

Alcohols are flammable. Avoid naked flame near alcohol.

[1] Chemistry data booklet (First examinations 2009). United Kingdom: International Baccalaureate, 2008. Print.

[2] Chemistry data booklet (First examinations 2009). United Kingdom: International Baccalaureate, 2008. Print.