There are various groups of hydrocarbons. A family of organic compounds is called a homologous series. The simplest series of hydrocarbons is the alkanes. The general formula of the alkanes is CnH2n+2. They are saturated compounds. Crude oil and natural gas are the main sources of alkanes. Natural gas is mostly methane (CH4), with small amounts of ethane (C2H6). Propane (C3H8) and butane (C4H10) are the main constituents of ‘Calor gas’ and GAZ respectively. Alkanes with between 5 and 17 carbon atoms are liquids at room temperature. For example, octane (C8H18) is one of the main constituents of petrol. Alkanes with 18 or more carbon atoms are solids at room temperature.
The table shows the first four members of the alkanes:
The alkanes become less volatile and change from gases through liquids to solids as their molecular size increases. Alkanes are typical simple molecular compounds. They are:
- Gases, volatile liquids or soft solids
- Non-conductors of electricity
- Insoluble in water.
They are also fairly unreactive. Petrol, for example, will not react with sodium or with concentrated sulphuric acid (H2SO4). The most important reaction of alkanes is combustion. Complete combustion of alcohols is safe and produces CO2 and H2O. However, incomplete is very dangerous and produces poisonous gases such as carbon monoxide. These reactions of alkanes are very exothermic.
After alkanes the most important group of hydrocarbons is the alkenes. The general formula of the alkenes is CnH2n. They are unsaturated compounds. Alkenes are made from alkanes in crude oil. They are used in the manufacture of plastics and other polymers. In alkanes, all the bonds between carbon atoms are single bonds (C---C). Alkenes are different from them because they contain a double bond between two of their carbon atoms (C == C). The simplest alkene is ethane (C2H4). The next alkene in the series is propene (C3H6).
Ethene Propene
C2H4 C3H6
H H H CH3
C==C C==C
H H H H
The ethene made by cracking is invaluable because of its ability to combine with other chemicals:
- It will react with water to form ethanol, which is an important solvent
- It combines with benzene to make the chemical needed for the manufacture of the plastic, polystyrene
- It reacts with chlorine to form the intermediate used to make the plastic, pvc
- Its molecules can be polymerized to make polythene.
The most stable arrangement for the four bonds to a carbon atom is a tetrahedral structural one in alkanes. Because of this structure in which all their carbon atoms have four single bonds, alkanes are described as saturated hydrocarbons. Alkenes such as ethane, however, contain a double covalent bond between two of their carbon atoms so they are described as unsaturated hydrocarbons. This makes alkenes much more reactive than alkanes. Other atoms can add across the double bond to make two single bonds. So alkenes readily undergo addition reactions.
The following table summarizes three important addition reactions of ethane.
During the reaction with ethane it becomes colourless. Therefore, the product, dibromoethane, is colourless. Other alkenes react with bromine water in a similar way so this reaction is used as a test for alkenes. The reaction of ethane with steam is important in the manufacture of alcohol and methylated spirits. The reaction of ethane with itself produces polythene.
Exothermic reactions are reactions that lose heat. Combustion is an exothermic reaction. Incomplete combustion is dangerous because it produces carbon monoxide. Endothermic reactions are reactions that gain heat. Such reactions are those like cracking. A mole is the amount of a substance, which contains the same number of particles, as there are atoms in 12.000g of Carbon – 12.
Both cracking and polymerisation are used to increase the yield of petrol from crude oil. Catalytic cracking is used to break up the heavy fractions to produce molecules in the C9 – C10 range. Polymerisation produces similar molecules by linking up small molecules from hydrocarbon gases.
Hypothesis
I can calculate the energy released per mole of combustion of each alcohol by using my secondary source of data, which I obtained from ‘Chemistry’.
C – C = 346 KJ/mol
C – H = 435 KJ/mol
C – O = 336 KJ/mol
O – H = 464 KJ/mol
C = O = 803 KJ/mol
O = O = 497 KJ/mol
With the above data I can now predict the values of energy required for each alcohol.
Predicted values for propanol
2C3H7OH + 9O2 6CO2 + 8H2O
Propanol + Oxygen Carbon dioxide + Water
(l) (g) (g) (l)
Structural Formula:
H H H
H – C – C – C – O – H + O = O C + O
H H H O O H H
Two moles of propanol releases:
ΔH = Bond Broken – Bond Formed = 18712 – 16878 = 1834 KJ
One mole releases:
1834 2 = 917 KJ/mol
Predicted values for ethanol
C2H5OH + 3O2 2CO2 + 3H2O
Ethanol + Oxygen Carbon dioxide + Water
(l) + (g) (g) (l)
Structural Formula:
H H
H – C – C – O – H + O = O C + O
H H O O H H
Two moles of ethanol releases:
ΔH = Bond Broken – Bond Formed = 5158 – 5996 = - 838 KJ
One mole releases:
-838 2 = - 419KJ/mol
Predicted values for methanol
CH3OH + O2 CO2 + 2H2O
Methanol + Oxygen Carbon Dioxide + Water
(l) (g) (g) (l)
Structural Formula:
H
H – C – O – H + O = O C + O
H O O H H
Two moles of methanol releases:
ΔH = Bond Broken – Bond Formed = 3099 – 6924 = - 3825 KJ
One mole releases:
- 3825 2 = - 1912.5 KJ/mol
Predicted values for Pentanol
2C5H11OH + 15O2 10CO2 + 12H2O
Pentanol + Oxygen Carbon Dioxide + Water
(l) (g) (g) (l)
Structural Formula:
H H H H H
H – C – C – C – C – C – O – H + O = O C + O
H H H H H O O H H
Two moles of pentanol releases:
ΔH = Bond Broken – Bond Formed = 29540 – 27196 = 2344 KJ
One mole releases:
2344 2 = 1172 KJ/mol
By using my background information, I can predict that the energy released increases as the chain length of an alcohol increases. This prediction is based on the bond energy calculations: the energy released per mole. This means that the fuels will come in the following order:
Methanol, Ethanol, Propanol, Pentanol
The above fuels have different boiling points so will boil at different temperatures due to different chain lengths. At higher temperatures, particles are moving faster, so there are more collisions. Also, and more importantly, the collisions are more energetic. More collisions have energy greater than the activation energy.
The molar heat of combustion of a fuel is the amount released when one mole of a substance burns in the air. The graph below shows the relationship between the heat of combustion and a series of alkanes:
As shown, the increased heat of combustion seems to be associated with an increased number of atoms in the fuel. This is the reason how I made my prediction that the energy released increases as the chain length increases. This should also apply to the alcohols.
Fair Testing
To ensure that my experiment is fairly carried out I shall do the following:
-
I should not give priorities to one fuel only. By this I mean that I should put equal amounts of the fuel into the spirit lamp.
- The amount of water should stay constant throughout the experiment.
-
The apparatus should be washed with distilled water before and after use.
-
To get the same amount of the liquid a measuring cylinder should be used.
- The experiment should be carried out away from the window to prevent extra heat entering.
- The same apparatus should be used for experimenting with each fuel i.e. the same beaker size.
- Variables, such as time and temperature should be kept constant, depending on what set of results are being taken.
You cannot do some aspects of fair testing properly. For example, when measuring the same amount of fuels or water, using a measuring cylinder, there is always a tiny drop left. You cannot do anything about this. It is for this reason that no liquids can be measured accurately.
Safety
- Carry all substances with care.
- Remove loose clothing and tie back loose hair.
- Wear a lab coat, safety goggles and gloves.
- Avoid getting fuels on your skin. If you do then immediately rinse the affected area with cold water, as the fuel can be irritant.
- Do not swallow or drink any substances, as it can be very harmful.
- Do not eat or drink during any part of the experiment.
- When using matches, be sure to handle them with care.
- If a bottle is broken on the floor immediately contact the lab technician.
- After the experiment ensure to wash your hands thoroughly before touching any part of your body.
- Do not run around in the laboratory and make sure all bags are underneath the tables.
- For safety reasons, keep one window in the laboratory open.
Preliminary Work
In my preliminary work, I let a lot of heat escape. I had no insulation around the beaker, which could prevent the heat escaping. The following results show the results that I have obtained from my preliminary work:
Time Constant 5 min
Temperature Constant 10°C
Variables
The variables that could affect my results are:
-
The amount of alcohol – should be kept the same for the experiments with different fuels.
-
The condition in the lab – the experiment should be carried out away from the window to prevent extra heat.
-
The temperature – should be kept constant.
-
The amount of water – should be kept the same for the experiments with different fuels.
-
The time – should be kept constant.
Apparatus
The following is a list of the apparatus that I will be using for my experiment:
- Stopwatch
- Spirit Lamp
- Beaker
- Water
- Thermometer
- Fuels
- Balance
- Tripod
- Safety mat
- Metal gauze
- Measuring cylinder
- Matches or lighter
- Height blocks
- Aluminium foil
Method
Step 1: Set up the apparatus as shown on the following page.
Step 2: Take one of the four fuels and weigh it on the balance.
Step 3: Put the fuel on the height blocks and place it beneath the tripod.
Step 4: Measure the water to 50 ml in the measuring cylinder and then put it
into the beaker. Put the aluminium foil around the tripod.
Step 5: Record the temperature of the water by placing a thermometer in the
beaker of water and then put it on the tripod.
Step 6: Take off the lid and light the fuel with either a candle or a
matchstick and time it for 5 min with the stopwatch.
Step 7: Put the lid back on the fuel to blow out the flame. Take out the
thermometer and record the temperature of the water. Also weigh the fuels. Take down results.
Observation
By doing the experiment I obtained the following results:
Set 1 Results
Time Constant – 5 min
Temperature Constant - 5°C
Set 2 Results
Time Constant – 5 min
Temperature Constant - 5°C
Averages
Time Constant – 5 min
Temperature Constant - 5°C
The above average results have been rounded up to the nearest whole number, just for the temperature before, after and change. Weight change has been rounded to 1 d.p.
Calculations
Specific Heat Capacity = 4.2
ΔH = Mass of water x Specific Heat Capacity x Temperature Change
Methanol
ΔH = Mass of water x 4.2 x Temperature Change
= 50 x 4.2 x 42
= 8820 J = 8.82 KJ
R.M.M = 32
2.3 g = 8.82 KJ
= 8.82 x 32
2.3
= -122.7 KJ/mol
Ethanol
ΔH = Mass of water x 4.2 x Temperature Change
= 50 x 4.2 x 46
= 9660 J = 9.66 KJ
R.M.M = 46
1.0 g = 9.66 KJ
= 9.66 x 46
1.0
= - 444.4
Propanol
ΔH = Mass of Water x 4.2 x Temperature Change
= 50 x 4.2 x 44
= 9240 J = 9.24 KJ
R.M.M = 60
1.3 g = 9.24 KJ
= 9.24 x 60
1.3
= - 426.5
Pentanol
ΔH = Mass of Water x 4.2 x Temperature Change
= 50 x 4.2 x 50
= 10500 J = 10.5 KJ
R.M.M = 88
1.7 g = 10.5 KJ
= 10.5 x 88
1.7
= -543.5
Analysis
My predicted statement was that:
The energy released increases as the chain length of an alcohol increases. This means that the fuels will come in the following order:
Methane, Ethanol, Propanol, Pentanol
As shown in my graph, my book and predicted values are generally higher than my practical values. There could be many reasons for this:
- Heat energy escaped as I used a glass beaker
- Values could have been read inaccurately
- The predicted values were based on the bond energy calculations. However, my experimental results were based on both calculations and practical results.
In my graphs, by comparatively studying them, we can see that the energy released more or less increases as the chain molecule does. However, my predicted values show a positive relationship as they increase.
In my graph there are a few anomalous results. My book value for ethanol is lower than it should be so I must have misread the value. The practical results do not support my theory. There is no relationship. This may be explained as misread values. However, the more likely reason is that because there were many other students doing the same practical the heat loss that they had from their experiment may have affected my experiment. This is because the amount of heat loss from the other practicals going on varied and inter-related with my experiment giving either more heat to it by giving a slightly higher temperature. This means that I could not keep my temperature variable constant.
Conclusion
The aim of my investigation was to compare the enthalpy of four different fuels. My book values were very high and so were my predicted values. My practical results did not show a relationship due to the conditions in the laboratory by other same practicals taking place. Therefore, their heat loss affected my results.
This means that my results are not firm enough to draw a conclusion as the conditions in the laboratory affected my results by varying them. This is shown not only in my results but also in my graph and my calculations.
Evaluation
To improve my experiment I could do quite a lot of things. Firstly, I could have used a calorimeter and an insulated lid on top to prevent heat loss. All the windows should have been closed so that the heat could not escape outside. The experiment should not have been carried out near a window as I was allowing extra heat in. I should have used the same balance, as different balances have different values.
It was good that I used height blocks to get the spirit lamp underneath the beaker. For extra prevention I used aluminium foil so that the heat reflected back to the calorimeter. This was good. I also used height blocks to minimise heat loss and tried to put the spirit lamp under the beaker. However, the spirit lamp was not directly under the beaker.
I felt that it was good that I took a range of readings by getting two sets of results then taking the average. This improved the accuracy of my results. However, I could have taken a second opinion of the readings. By asking someone else what reading they saw I could see whether I made a mistake in my reading and from some of my results I see that I have made mistakes.
Bibliography
Chemistry
JA Hunt and A Sykes
Letts Science Double Award
Website:
www.yahoo.com
The Material World
Second Edition
John Holman