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 ï„T where m is the mass of water, c is the specific heat capacity of water and ï„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:
Control variable
Reason for controlling it
Method used to control it
Time (Duration) of burning
Longer time of burning will result in higher temperature change. Though more fuel will be consumed if longer time is used, heat will be lost to surrounding at a faster rate if temperature is higher.
The time is fixed to be 2 minutes using a stopwatch for every trial
Mass of water used
More water will result in a larger surface area and greater rate of heat loss.
Use 50 cm3 of water measured by a measuring cylinder in all trials
Distance between the spirit burner and the copper container
The closer the copper container is from the spirit burner, the higher the efficiency of energy transfer and smaller the rate of heat loss.
The copper container is placed on the same tripod which makes sure the container is at the same level
Initial temperature of water
Although the temperature difference is used to find the heat, the higher the temperature means the rate of heat loss is faster. The heat given would be much underestimated.
Use a thermometer to make sure the initial temperature is approximately the same as the room temperature. Perform all the trials at approximately the same time so that the temperature can be assumed to be the same.
Position of the thermometer in the beaker
Although water is stirred, the energy distribution may not be even. One position may have more heat than the other.
Hold the thermometer 1.0 cm from the bottom of the copper container, confirmed with a ruler. Make sure the thermometer is approximately at the centre of the container.
Initial mass of alcohol in the spirit burner
More fuel in the spirit burner may provide a better contact between the alcohol and the wick, increasing the efficiency of burning and hence releasing more heat.
Use 20.00 g of alcohol for each trial, measured by an electronic balance.
Temperature of surrounding
Surrounding temperature would affect the rate of heat loss to surrounding. If the surrounding temperature is higher, the rate of heat loss would be slower. Besides, energy released during combustion can be affected by surrounding temperature.
Room temperature of about 25ï‚°C is used for all trials. It can be monitored by a thermometer. Perform all the trials in roughly the same period of time. The temperature can then be assumed to be constant. The literature values are standard values, measured in 298K and 1 atm. Temperature closer to 298 K would be prepared for easy comparison. As it requires to measure the temperature rise of water, water bath would not be an appropriate means to control the temperature.
Length of wick
If a longer wick is used in the spirit burner, the burning efficiency may be higher.
Use a new wick in every trial. Cut the wick to ensure that it has a length of approximately 3.0 cm out of the burner. The lengths are measured by a ruler.
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 CnH2n+2O. The general equation for combustion of alcohol is
CnH2n+2O (l) + 1.5nO2 (g) ïƒ 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:
Extra bonds broken
Extra bonds formed
2 C – H
2 C = O
1 C – C
2 H – O
The average bond enthalpies for the above bonds are shown in the table below: [2]
Bond
Average bond enthalpy (kJmol-1)
C – H
413
C – C
347
C = O
746
H – O
464
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:
100 cm3 measuring cylinder (±1 cm3) × 6 (one for each alcohol and the other for water)
(2 d.p. ) Electronic balance (±0.01 g) × 1
Tripod × 1
Copper container with lid × 15 (one for each trial)
Stand and clamp × 1 (use to hold the thermometer in position)
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)
Stirrer × 1
Aluminium heat shield × 4 (to reduce heat loss)
Spirit burner with wick × 15 (one for each trial)
Lid × 1 (for extinguishing the flame)
Stopwatch (±0.01 s) × 1
Ruler (±0.1 cm) to measure the length of wire and the position of the thermometer
Scissor × 1 (used to cut the wick)
Chemicals
750 cm3 of distilled water (50 cm3 for each trial)
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
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.
Measure 50 cm3 of methanol with a 100 cm3 measuring cylinder and pour it in a spirit burner.
Weight the spirit burner with an electronic balance. Record the mass of the spirit burner in the table below.
Measure 50 cm3 of distilled water with another 100 cm3 measuring cylinder and pour it in a copper container.
Place the copper container on a tripod and cover it with a lid.
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.
Place a stirrer in the water. Stir the water throughout the process of heating.
Surround the whole setup with heat shield made of aluminium to reduce heat loss.
Ignite the wick of the spirit burner with a lighter.
Place the spirit burner under the beaker.
Immediately start the stopwatch and wait for two minutes.
Stop the stopwatch after two minutes.
Use a lid to extinguish the flame.
Record the highest temperature reached by the thermometer and put it in the table below.
Allow the spirit burner to cool.
Weigh the final mass of the spirit burner with an electronic balance. Put the mass in the table shown below.
Repeat the experiment (Step 1 to 16) two more times.
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:
Type of primary alcohol
Trial
Methanol
Ethanol
Propan-1-ol
Butan-1-ol
Pentan-1-ol
1
Initial mass of spirit burner and fuel (g) (±0.01 g)
Final mass of spirit burner and fuel (g) (±0.01 g)
Mass of fuel used (g) (±0.02 g)
Initial temperature (ï‚°C)
(±0.1ï‚°C)
Final temperature (ï‚°C)
(±0.1ï‚°C)
Temperature change (ï‚°C)
(±0.2ï‚°C)
2
Initial mass of spirit burner and fuel (g) (±0.01 g)
Final mass of spirit burner and fuel (g) (±0.01 g)
Mass of fuel used (g) (±0.02 g)
Initial temperature (ï‚°C)
(±0.1ï‚°C)
Final temperature (ï‚°C)
(±0.1ï‚°C)
Temperature change (ï‚°C)
(±0.2ï‚°C)
3
Initial mass of spirit burner and fuel (g) (±0.01 g)
Final mass of spirit burner and fuel (g) (±0.01 g)
Mass of fuel used (g) (±0.02 g)
Initial temperature (ï‚°C)
(±0.1ï‚°C)
Final temperature (ï‚°C)
(±0.1ï‚°C)
Temperature change (ï‚°C)
(±0.2ï‚°C)
Data Processing
The mass of fuel used (m) is calculated by subtracting the final mass of spirit burner from the initial mass of spirit burner.
The temperature change (ï„T) is calculated by subtracting the final temperature by the initial temperature.
The heat (q) absorbed by water can be calculated by the formula
q = mc ï„T
where
m is the mass of water used: 50 g (as the density of water is 1 g cm-3)
c is specific heat capacity of water: 4.18 Jg-1K-1
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.
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.
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).
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.
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