To find a relationship between the molecular size of different alcohols and the associated energy change involved in the reaction.

Steven John 11c

Introduction

The aim of the investigation was to find a relationship between the molecular size of different alcohols and the associated energy change involved in the reaction.

A preliminary experiment had been carried out already and errors were noted accordingly and were adjusted in this experiment where possible.

This investigation looked into the energy changes when different alcohols were combusted, this is the reaction of the alcohol with oxygen in the surrounding air. The alcohols are a homologous series of compounds that contain –OH as the functional group (a group of atoms in a structure that determines the characteristic reactions of a compound). For this experiment, only methanol, ethanol, propanol, butanol and pentanol will be used.

The structures of alcohols

Here’s where the alcohol structures go, sorry I couldn’t send them!

In general,

alcohol + oxygen → carbon dioxide + water

E.g. ethanol + oxygen → carbon dioxide + water

C2H5OH(l) + 3O2(g) → 2CO2(g) +3H2O(l)

Hypothesis

The larger the molecular size, i.e. the more carbon atoms it has, the larger the amount of energy given out.

Some chemical reactions are capable of releasing a lot of energy; usually it is through heat. The combustion of alcohols is a good example, but it is known that they do give off different amounts of heat.

When an alcohol is combusted, it reacts with the oxygen in the air and the products are carbon dioxide and water:

alcohol + oxygen →carbon dioxide + water

During this reaction, as with all others, bonds are first broken and then new bonds are made. In alcohol, hydrogen atoms are covalently bonded to hydrogen atoms. In oxygen gas, the atoms are held together in diatomic molecules. During this reaction, all these bonds must be broken. Chemical bonds are forces of attraction between atoms or ions, and to break these bonds or forces, energy is required; and so energy must be taken in to pull atoms apart.

Breaking chemical bonds takes in energy from the surroundings, and is an endothermic process.

When the new bonds are made between carbon and oxygen to make carbon dioxide and between hydrogen and oxygen to form water, it gives out energy.

Making chemical bonds gives out energy to the surroundings, and is an exothermic process.

When the alcohols react with oxygen, the total energy given out is greater that that taken in to break the bonds, and so overall, this reaction gives out energy and is an exothermic reaction. This energy is released as heat. The energy change involved in the combustion of alcohols is called the heat of combustion, and is what we will be finding in this investigation.

The overall change in energy for this exothermic reaction can be shown in an energy level diagram:

In this reaction, energy is given out because the bonds in the products (CO2 and H2O) are stronger than those in the reactants (CH4 and O2). This shows that the products are more stable than the reactants.

Some bonds are stronger than others and so require more energy to break them, but they also give out more energy when they are formed. Because it takes more energy to break some bonds, it shows that these are more stable.

For example,

The bond energies can be used to define the strength of the bond. Using the above information we can work out the predicted energy released per mole of the alcohol.

Methanol: 3 (C – H) + C – O + O – H

3(435) + 358 + 464

= 2127 kJ/mol

Ethanol: 5 (C – H) + C – C + C – O + O – H

5(435) + 347 + 358 + 464

= 3344 kJ/mol

Propanol: 7 (C – H) + 2 (C – C) + C – O + O – H

7(435) + 2(347) + 358 + 464

= 4561 kJ/mol

Butanol: 9 (C – H) + 3 (C – C) + C – O + O – H

9(435) + 3(347) + 358 + 464

= 5778 kJ/mol

Pentanol: 11 (C – H) + 4 (C – C) + C – O + O – H

11(435) + 4(347) + 358 + 464

= 6995 kJ/mol

My hypothesis was that the more carbon atoms an alcohol had then the more energy per mole would be released. This is because it has more bonds and so has more energy, so when these bonds are made, then heat energy will be released due to the rule of exothermic reactions, and the more bonds that are made then the more energy will be released. Because all the alcohols are part of a homologous series (i.e. each homologue is related to each other and have the same general formula, in this case it is CnH2n+1OH), the amount of energy they release should have a relationship with the molecular size. For ease of purpose, we can substitute the phrase “molecular size” with “the number of carbon atoms present”; this is because with each proceeding alcohol, the number of carbon atoms increases by one.

GCSE Chemistry Coursework

Investigation into energy changes in the combustion of alcohols

Procedure

In this investigation our aim was to determine the amount of energy released through the combustion of different alcohols. Our method involved heating a can of water by a certain temperature to harness the energy given off by the burning alcohol underneath. The time is measured so that the rate of energy released can be measured. The mass of alcohol loss is also noted to see how much was used in the process. The experiment was kept as fair as possible with the ...

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GCSE Chemistry Coursework

Investigation into energy changes in the combustion of alcohols

Procedure

Preliminary Result

Through our preliminary experiment, the following theoretical errors were noted which caused results to be inaccurate:

Heat loss to environment – convection currents from windows, other experiments etc.
Heat stored in actual aluminium/steel of can.
Difference in flame sizes
Incomplete combustion of fuel, i.e. alcohol + oxygen → carbon monoxide + water.
Uneven heating of the water in the can.

Hence we adjusted the method according to the above and changes made are as follows:

Use of a draft excluder around the apparatus.
Mass and material of can noted along with its specific heat capacity and so allowing results to be adjusted accurately.
Bottom of can and tip of flame measured and kept constant throughout.
Distance between can and flame enlarged and allowing a good oxygen supply to flame.
Can constantly stirred throughout the experiment while it was heating.

Variables

In order to conduct a fair experiment, variables will be kept constant. We will use the same procedure throughout and only change the fuel, i.e. the alcohol each time.

The factors that affect the amount of energy released:

Fuel used: it is known that different fuels release different amounts of energy, so we will change the fuel we are using each time – this includes methanol, ethanol, propanol, butanol and pentanol.

Period of time: to keep this constant we will only heat the water by 20°C, according to the initial starting temperature and time how long it takes.

Amount of fuel combusted: we will measure the amount of alcohol used to heat the water by 20°C.

Amount of water heated: the more water heated, the more dispersed the energy is and so we will use a constant amount of water each time; we will use 100ml of water.

Energy lost: energy is either lost to the environment or to the can as we found in our experiment:

Energy lost to environment: this is minimised with the use of a draft excluder around the apparatus, which was a failure in the preliminary.
Energy lost to the can: as we found in our preliminary experiment, which was a problem, as we could not measure the energy lost to the can. In this experiment, we have weighed the mass of the can and with known specific heat capacities of the material of the can (steel or aluminium) we can calculate the energy lost to the can and adjust results accordingly to enable accurate results to be produced.

Specific heat capacities: it is understood that different materials are able to store different amounts of energy in a period of time, hence we use the same can throughout the experiment.

Energy released: to allow suitable and accurate comparison between results of the five different alcohols, and to allow an accurate relationship to be produced, we will convert all taken results into a value of total energy released.

Accuracy

To produce results with the maximum accuracy suitable, we repeated the experiment twice in order that we could find an average between the results and hence allowing us to identify and eliminate the anomalous results.

We used the most accurate measuring equipment available, including digital balance to 2 decimal places and a thermometer to the nearest degree.

We also learnt from the causes of errors in the preliminary and adjusted our method accordingly; i.e. the use of draft excludes and taking into accounts the energy stored in the can.

We kept the variable as constant as possible by using the same equipment throughout, based on the concept that different materials have different specific heat capacities and so store different amounts of energy; even if energy is lost we can either account for it or know that it is a constant amount each time, hence still allowing us to make an accurate comparison between results.

We also took all measurements and calculations to 2 decimal places where possible, which we thought to be a suitable accuracy for school laboratory experiments and for finding an accurate relationship. This also was suited to the equipment we chose to use.

Apparatus

Below is a lost op apparatus used in the experiment:

Measuring cylinder – to measure out amount of water to be heated 100ml.
Fuels (methanol, ethanol, propanol, butanol and pentanol) – used to heat water in can. Stored in spirit burners and weighed with the lid.
Beverage can – used to store water, either made of aluminium or steel (allows conduction of heat to the water; has only a small hole to prevent heat loss).
Draft excluder – to prevent heat loss from naked flame to environment.
Thermometer – to check temperature of the water.
Heatproof mat – to conduct experiment upon.
Clamp and retort stand – used to hold can above flame.
Ruler – used to measure distance between bottom of can and tip of flame.
Electronic balance – used to measure mass change of fuel, i.e. amount combusted.

Method

This is where the apparatus diagram goes.

Put on safety goggles.
Set up apparatus as shown above.
Measure out 100ml (100g) of water from tap with measuring cylinder so that bottom of meniscus is on the 100ml mark.
Weigh the mass of can and note the material it is made of.
Pour all 100ml of water into the can, and put thermometer in and note down the temperature of the water.
Clamp the filled can to the retort stand.
Take one of the five alcohols and weigh it on electronic balance with the lid.
Place directly under can on a heatproof mat.
Take lid off spirit burner and light the wick, immediately set up the draft excluder around apparatus.

Set the height of tip of flame to the bottom of the can, i.e. height x to desired height and keep constant throughout with all fuels. If the height is too short, it was found that incomplete combustion occurred and there was an inefficient energy transfer, if height is too high, then a lot of energy is lost

and there is an inefficient energy transfer. It was found that around 5cm was a suitable height.
Use thermometer to stir the water in the can constantly throughout the procedure, not allowing it to touch base of can.
When temperature of water has risen by 20°C, e.g. from 21°C to 41°C, replace lid on spirit burner to extinguish the flame.
Weigh new mass of spirit burner and calculate change in mass.
Repeat experiment again so an average can be found between results.
Repeat entire procedure with remaining fuels.

GCSE Chemistry Coursework

Investigation into energy changes in the combustion of alcohols

Results

Below is a table of our results, all measurements are correct to 2 decimal places:

GCSE Chemistry Coursework

Investigation into energy changes in the combustion of alcohols

Calculations

Using the results obtained, we can calculate the amount of energy released by the candle by calculating the amount transferred to the can and the water using the specific heat capacity theory.

Energy released = specific heat capacity x mass x change in temperature

For the can:

Mass of can = 15g

Material = aluminium

Specific heat capacity of aluminium = 8.9 J g-1 °C-1

Energy released = (9.8 x 15 x 20) ÷ 1000

= 2.67 kJ

For the water:

Mass of water = 100g

Specific heat capacity of water = 4.18 J g-1 °C-1

Energy released = (4.18 x 100 x 20) ÷ 1000

= 8.36 kJ

Total energy released:

= 2.67 + 8.36

= 11.03 kJ

The total energy released by all the alcohols is 11030 J because they all heated the water and can by the same amount. To compare between the results, we would need to express the energy released in terms of per gram of alcohol combusted:

Energy released per gram = total energy ÷ mass of alcohol used

Then we need to calculate the amount of energy released per mole, so that we can compare them with our predicted results made earlier to see what the difference was:

Energy released per mole = energy released per gram x RMM

All values to 2 decimal places

GCSE Chemistry Coursework

Investigation into energy changes in the combustion of alcohols

Conclusion

I found that as the molecular size of the alcohol increased so did the amount of energy released. From the graph, it is possible to see that the number of carbons in each alcohol is proportional to the amount of energy released. The actual results were much smaller that the predicted results and also at a shallower gradient.

This shows that as the number of carbons in an alcohol increases so does the amount of energy it releases on combustion. This is because with larger molecules of each alcohol, i.e. with more carbon atoms, there are more bonds between atoms in the molecules and because each bond has its own bond energy, then the more bonds that are made then the more energy is released. Though breaking the bonds in the molecule requires energy and larger molecules require more amounts, it means that it forms more carbon dioxide and more water and so more bonds are made which means more energy released.

From the graph, we can also see that the actual results are much smaller than the accurate predicted ones and was due to heat loss from the reaction to the environment. Though we tried to minimise the heat loss by correcting errors from the preliminary, for example, by using a draught excluder, the energy transfer was not 100 per cent efficient as there was still air between the flame and the can which causes heat loss through convection. This means that the heat loss is the gap between the two lines at each point. It was also found that the gap increases as you proceed along the horizontal axis, i.e. as the number of carbon atoms increase, this is because with larger molecules of alcohol, energy is released as heat through a faster rate. This means that the energy transfer is even more inefficient, because it means a quicker and larger convection current in the air so more heat loss and also the can is not a perfect conductor and so can only conduct a certain amount at a time. Therefore, as the molecular size of the alcohol increases then so does the rate of energy production and so does the rate of heat loss. This means that the two lines get further and further apart as you move along the horizontal axis, i.e. the gradient of the actual results is shallower than the predicted. But in theory, the amount of energy released by the alcohol is the same amount as we predicted, only not all of it was accounted for as it was lost. Only the heat in the water and the aluminium can was taken into account.

The combustion of alcohols is an exothermic reaction, but the energy released in each case is different, and is due to the number of bonds present. Each bond has its own energy and so when made it releases a certain amount. For there to be more bonds there has to be more atoms, and more atoms means a larger molecule. This is why the larger molecules of alcohol, i.e. the ones with the more carbon atoms, produces more energy; because more bonds are made. The energy released is proportional to the molecular size of the alcohol and produces a straight line, because with each alcohol, the number of bonds increases by a fixed amount because all the alcohols are in a homologous series, and when there are no carbon atoms, there is no alcohol molecule and so no energy is released, explaining why the line passes through the origin. The graph also shows that the energy change is positive, i.e. it is an exothermic reaction, meaning more energy is given out from the formation of bonds than the energy taken in through the breaking of bonds.

The results support my prediction, though they do not match up with the predicted results perfectly, because they show the same trend. They show that as the molecular size of the alcohol increases, so does the energy released on combustion. Because the results show a proportional trend, a line of best fit can be drawn and it can be used to calculate the amount of energy produced with the same method for further alcohols, like those with six or more carbon atoms. For example, by combusting an alcohol with seven carbon atoms in the same apparatus, it would produce 1500 kJ per mole. We can also use the graph to devise a formula so that we can easily calculate the energy released in an alcohol with, say a thousand carbon atoms.

The formula for any straight line is y = mx + c [m is the gradient and c is the y-axis intercept].

Therefore the formula would be y = x + 0 [where y is the energy released and x is the number of carbon atoms in the alcohol].

The above will only calculate approximate values as the formula was devised form the graph which can cause inaccuracies.

For example, an alcohol with 15 carbon atoms would produce the following amount of energy with this apparatus:

y = x

= 15

=3214.29 kJ/mol to 2 d.p.

Hence, using the same method, we can devise a formula with the predicted accurate values. It would be:

y = 1217x + 910

The above is accurate and will calculate exactly the amount of energy produced.

For example, an alcohol with 15 carbon atoms will produce exactly this amount of energy:

y = 1217x + 910

= 1217(15) + 910

= 19165 kJ/mol

The two formulas are able to support my explanation that energy transfer is not 100 per cent efficient and that a lot of energy is always lost.

GCSE Chemistry Coursework

Investigation into energy changes in the combustion of alcohols

Evaluation

The procedure of the experiment did not allow us to obtain highly accurate results because a lot of energy was still lost to the environment, though a draft excluder was used, heat was lost through the top of the apparatus. Hence this explains why our actual results are smaller than the predicted ones – because energy is lost and so not all of it is taken into account. The procedure’s qualitative errors were a major problem, hence the large difference between the two results, though they show the same trend.

The results are fairly accurate to what was actually measured, they differ with the predicted results due to the main qualitative error which was heat loss. Otherwise they are fairly accurate results to what was actually transferred to the water and the can. Also we did not calculate the heat transferred to the can accurately because we assumed its temperature rise was also 20°C, which is the same as the water. This is wrong because heat is not all transferred to the water and instead to the environment, and hence the temperature of the can is actually higher than 20°C, and also explains why the actual results were smaller than the predicted. We were only measuring temperature with a thermometer to the nearest degree, this is highly inaccurate because any small error made in these measurements are magnified because we are manipulating the results to get what we want, i.e. the energy transferred. Therefore this reduced the accuracy of the results. The anomalous results that were below the line of best fit showed that the energy released was too small, this was because of extra heat loss than expected and was caused by us blowing onto the can or water to cool it and also not fully closing the draught excluder. The anomalous results that were above the line of best fit show that the energy released was too high and was due to uneven stirring of the water and so some areas of the water were hotter than the others. It was also due to the fact that the tip of the flame was too near to the bottom of the can, i.e. height x is too small, and so it was an unfair test and less heat was lost than expected.

The procedure was highly inaccurate due to the apparatus used, which caused too much heat to be lost. The apparatus was not in sealed conditions and so a lot of heat was lost to the air around it, between the flame and the can causing convection currents. If the flame was too near the bottom of the can it would mean less heat loss but also incomplete combustion and so the energy transferred would be different than expected and the carbon that forms on the bottom of the can causes inefficient heat transfer. If the flame was too far form the can then there would be a lot of heat loss and so affecting the accuracy of the results. The draught excluder proved to be of limited use as heat rises and so heat was not kept in from above where most heat energy is lost. The measurements were also not accurate enough as the results would have to manipulated. It is for these reasons that the procedure is not suitable enough to enable us to produce highly accurate results of which would be very similar to the predicted. But we must appreciate the fact that there is never a 100 per cent energy conversion and that energy is always lost.

An improve procedure, would involve the use of a thermocouple to replace this calorimeter. The thermocouple reduces heat loss greatly as it is able to create a sealed environment and so nearly all the energy released in the combustion of the alcohol is accounted for. The water is also circulated and so is heated evenly. But the calorimeter could be improved by heating the water by a larger temperature, such as 60°C. This means that the inaccuracy of the thermometer would be spread over a larger temperature and so the error factor is smaller. We could also use a digital thermometer instead which measure to 2 decimal places which would be efficient and accurate. We could also heat a larger amount of water for the same reason. The entire apparatus could be put into a sealed environment such as a large jar with vent holes at the bottom and a small hole the top for stirring the water. The oxygen needed for the reaction would be sucked into the jar through the holes at the bottom and so the heat produce would be trapped in the environment and could be measured. A more detailed trend with the results could be obtained by continuing the experiment with alcohols that had larger molecules, i.e. more carbon atoms. Also the experiment would be repeated more than twice to allow us to identify and eliminate the results even further.

The evidence is reliable in showing the sort of trend that would be produced. The anomalous results were also very small and still show the trend clearly and so the results are accurate. The difference in the actual result and the predicted results can also be fully accounted for. The actual results are also more realistic in terms of energy transfer as it takes into account the energy loss. The obtained evidence is sufficient to support a firm conclusion that as the molecular size of the alcohol increases so does the amount of energy released. This is because the results show this trend very clearly and are similar to the predicted results. The anomalies are also not far from the line of best fit and so support the trend making them reliable. Even though the actual results differ from those that were predicted, it can be explained by the fact that energy is lost to the environment.

Further work for this investigation would include testing to see the rate at which energy is produced; how long it takes for each alcohol to heat the water by a certain amount. My prediction would be that the alcohols with the larger molecules would take less time because they have more bonds and so more energy is released in a certain amount of time, and so it would heat the water faster. Additional evidence for the conclusion could also be obtained by continuing the experiment with more alcohols with more carbons and so allowing us to gain a more detailed trend in the relationship. Also by replacing the calorimeter with a thermocouple would allow us to see a more accurate trend and find other factors apart from heat loss that may cause anomalous results.