From this, heat energy is lost/given to the air illustrating that this is an exothermic reaction (the energy difference between the reactants and the products determines the heat given out when the products are formed). Meanwhile, the air molecules around the reaction receive this excess heat energy, which causes them to vibrate faster and faster and makes them hotter and hotter. The energy is passed from these particles to other ones (by ‘bumping’ into each other) and conducted along others until it eventually reaches the can of water. To raise the temperature of the water, energy must be given to it’s molecules to make them vibrate faster and produce heat in the same manner as air. So the heat energy is conducted through the metal atoms of the can and to the water molecules. After enough heat energy has been conveyed to the water so as to satisfy its energy needs, then the temperature will increase by a certain degree. The many water molecules vibrating at a fast rate cause this.
Independent Variable:
The independent variable is the different alcohols that we are using.
Dependent variable:
The dependent variable is the amount of energy released when the alcohol is involved in combustion.
The other important variables (the ones I must control) are:
- Distance between the spirit burner and the can
- Environmental factors such as wind and air temperature
- Volume of water in the can
- Other systems absorbing heat/ material of the can
- Heat being lost by evaporation
- Incomplete combustion occurring
These variables are important in my investigation for the following scientific reasons:
The distance between the spirit burner and the can is important as the greater the distance from the spirit burner, the more heat that will need to be produced in order for enough to reach the water. This implies that a larger mass of alcohol will be burnt. Heat travels through the air when they ‘bump’ into each other and this heat is generated by the combustion of the alcohol. If the distance between the two objects is increased, then the conduction of heat from the reaction to the can will take longer meaning:
- Some particle heat energy will be lost to the surroundings along the way.
- The water will get less heat energy at the end of it all, indicating that it will take longer to raise its temperature.
- The longer the water takes to heat, the more the mass of the alcohol is lost through burning.
So the alcohol will be involved in more combustion reactions (burning away its mass) if the distance is increased, in order to provide enough heat energy for the water to raise its temperature.
Environmental factors may affect the alcohol’s distribution of heat energy by causing some to be lost to the surroundings. Of these, wind is a prime factor, as it would blow the flame from side to side and away from the can. Excess heat energy from the combustion reaction will be wasted on the air around the system and other objects – hands, table etc. Consequently, the system water will not receive much heat energy and the alcohol will needlessly burn away, releasing heat energy to the wrong place. To sustain a constant flame, heatproof mats (substituting for wind breakers) will be placed at strategic points to ensure that as little disturbance as possible is caused to the system. The air temperature is also an important factor although it is very difficult to control. This is because the hotter the air is, the easier it will be to conduct heat and the less time it will take for the heat to reach the water (do not need to heat up the air to a high temperature but only to. Not to mention that less heat will be lost and the alcohol will not have to burn so much more of itself.
The volume of water in the can is vital in the experiment and must be controlled to ensure the specific heat capacity is kept the same for all experiments. Different volumes of water will have varied heat contents and will absorb more/less energy to raise their temperatures to a certain extent. Larger volumes of water (500g) will have a large amount of water molecules, each of which have the same individual heat capacities and these will form a large total specific heat capacity for the water. The 500g of water will then need to obtain a big quantity of heat to change it’s temperature meaning that alcohol mass is lost to produce this heat, whereas less mass would be lost and less energy would be required if a 200g water volume was heated (since it has smaller total specific heat capacity.)
Another factor that is also related to specific heat capacity is that other equipment in the experiment may absorb some of the heat energy lost from the combustion of alcohol. These would be the can and thermometer since they are an integral part of the system. When heat energy is released, it travels through the air and to the can, which has to take in the energy and then pass it to the water. But it does not transfer all of this heat energy to the water because the can itself takes some of it to suit it’s specific heat capacity. The remainder of the heat energy that enters the water may also go to the thermometer if it is touching the can. To prevent more heat being taken from the water, we have to hold the thermometer such that it is not in contact with any part of the can. Another reason for this precaution is because we are only measuring the temperature of the water and if it is touching the can, then we are measuring the temperature of the can.
Heat energy is lost by the water through the top of the can due to evaporation. As the water gets hotter (receiving heat energy), its molecules start to move around faster and faster, which causes them to break their weak intermolecular bonds. Only the faster particles can achieve this and so they ‘break free’ of the liquid and evaporate near the surface. However, this evaporation lowers the average energy of the water molecules left and so the water becomes cooler. Subsequently, the heat energy produced by the combustion reaction does not really heat up the water (heat is drifting out of the top) and the alcohol burns for longer; decreasing in mass. To counteract this, a lid can be placed on top of the can so that the heat energy is contained within and little evaporation occurs.
Incomplete combustion is essentially not a serious factor but when there is a small quantity of oxygen around the reaction – then it will not form the proper products of complete combustion and the excess energy given out may be different. There is little we can do to prevent this although there should be enough oxygen in the atmosphere to support complete combustion anyway.
Prediction:
I predict that as the alcohol’s number of carbon atoms grow, the energy given out will increase and the mass of alcohol lost will decrease. The alcohol ethanol will lose the most mass in order to fulfil the energy necessary to raise the temperature of water. So even a large mass of ethanol will not give out much exothermic heat energy meaning 1 mole of it will not as well. This is the energy chart for this:
The alcohol octanol will lose the least mass when producing the energy needed to raise the temperature of water. So this means that a small mass of octanol will dissipate a huge amount of exothermic heat energy as will one mole of it. This is the energy chart for octonal’s combustion:
So to repeat my prediction: I think that as the alcohols get more and more carbon atoms and different bonds, the energy they give out will increase and their mass loss will decrease when doing this.
I think it will happen for the following scientific reasons:
As the size of the carbon chain grows, more bonds are added to the structure of the alcohol. This means that each time, more energy from the surroundings must be extracted in order to break these starting bonds (endothermic stage). Yet, the more energy that is taken to break the bonds, the more energy is used to form the product’s bonds and this makes the energy of the products greater than that of the reactants. The alcohols start off with methanol, which has 0 carbon – carbon bonds, 3 carbon – hydrogen bonds, 1 carbon – oxygen bond and 1 oxygen – hydrogen bond. When the products are formed, excess energy is released which is mainly due to the amount of C = O bonds being made. The products of methanol have 2 C = O bonds and 4 O – H bonds. Since a C = O bond has a high energy value of 805, many of them will ensure that the energy of the products is greater than that of the reactants. Here are the theoretical values for the input, output and exothermic heat energy of methanol along with a diagram and the bond energy values:
Now let’s take ethanol as an example. The bonds are 1 C – C bond, 5 C – H bonds, 1 C – O bond and 1 O – H bond. The number of C – H bonds has risen by 2 and the carbon bond has gone up by one. For the products, there are 4 C = O bonds and 6 O – H bonds which is two more bonds than methanol. Here is the input and output calculation for ethanol:
The theoretical input energy for the ethanol is greater than that of methanol as is the output energy and the exothermic heat energy difference. The same is shown with proponal, butanol, pentanol, heptanol and octanol.
The pattern that we see is that when the alcohols gain a carbon, they have to break an extra 2 C – H bonds (and C – C bonds if it is connected to another carbon) and this makes it need more energy from the surroundings which comes out as more energy when the products are formed. The theoretical values show that the more bonds in the reactants, the greater the bonds in the products and the more the theoretical energy difference. Also, since there is a regular change in the structure of the alcohols, then there is probably a regular change in the energies too.
There is a regular change in the exothermic energy given out. Here is a graph of my theoretical values in order to show the relationship between the energy given out and the number of carbons:
The number and range of results I will need, to obtain reliable evidence are:
I will need to have at least 20 results; twice for each different alcohol, perhaps thrice if possible. If each experiment is done at least once, then averages of mass differences can be obtained and will make the ‘energy given out by one mole of alcohol readings’ of each, much more accurate. The range of these results will be from the alcohol ‘ethanol’ to the alcohol ’octanol’ which is 7 alcohols in all as we are excluding methanol and heptanol. This will enable me to make bond energy charts for each alcohol to easily identify and illustrate the relative heat energies released.
I will require the apparatus for my investigation:
Spirit burners of each alcohol, 5 heatproof mats, metal stand and clamp, thermometer, can, ruler, measuring cylinder, splint and a cardboard can lid.
The way I will use this apparatus to obtain reliable evidence is shown below:
Once the apparatus has been assembled as shown, fill the measuring cylinder with a specific volume of water and pour into the can. The starting temperature of the water should be recorded and the temperature it will go up to should be calculated by adding the original temperature to a temperature rise (e.g. 20°C + 7°C =27°C). Check that the distance between the alcohol and spirit burner and that the temperature rise is the same for every experiment. Weigh the alcohol before the experiment and then place in the shelter of the windbreakers, so it is directly underneath the can. Light the alcohol using a lighted splint and close it in with another heatproof mat. Safety glasses must be worn as the alcohols are flammable and if some gets on your hands, they must be washed immediately in case any flame touches your hand. Also, it would be safer and less of an equipment hazard if one Bunsen burner were used. Put the cardboard lid on top of the can and the thermometer through it (punch hole in lid and slide thermometer through). After the water temperature goes up to the one decided, put out the alcohol and weigh its finishing mass. This should be recorded. To prepare for a repeat – empty out the can and refill and change the alcohol with another of the same type. Repeat the procedure for other alcohols and take average.
I have used the following to help me plan my investigation:
I have used the following books to help me plan my investigation:
- ‘Physics for you’ by Keith Johnson. On page 37 to 39, I found out information concerning measuring heat energy, its values, specific heat capacity and a table displaying specific heat capacities of different substances.
- ‘Revision guide for GCSE Double Science – Physics’ where on page 67 to 68, there is information about heat transfer, evaporation, the conduction of heat and vibrating particles.
- ‘Chemistry for you’ by Lawrie Ryan (Revised National Curriculum Edition). On page 178 to 179, there is information regarding organic molecules such as alcohol; their structures, physical properties and homologous series. On page 182, there is information on the combustion of alcohols, a brief explanation of the combustion experiment and combustion formulas. On page 186, which is about energy transfer, there is more detailed coverage of incomplete/complete combustion and fuels. On page 190 to 195, there are things about exothermic/endothermic reactions, what happens to the temperature during these reactions, energy level diagrams, making and breaking bonds and finally bond energy calculations.
- ‘Nuffield Book of Data’ sheet has exothermic heat energies of all the alcohols combustion reactions and all the varied bond energy values.
I have done the following experiments to help plan my investigation:
- Experiment on page 191 to see what substances have endothermic or exothermic reactions.
- Previous practise experiment of the alcohol investigation, which helped me to find corrections for certain procedures, various precautions, tips that would produce less inaccuracy, discover key factors and how to set up the apparatus.
Here is the set up of my results table as an example of what I learnt from the practise experiment:
