METHANOL:
PROPANOL:
In order for the energy to be released when the fuel is burnt, the breaking and making of bonds must take place.
Here is ethanol’s equation which has its bonds broken to produce energy.
Here is the structure of ethanol’s molecules before and after they’ve been broken.
Here is ethanol’s energy level diagram to show how bonds are broken and made to finally produce the net energy out.
This tells us that there is more energy given out when bonds are made than taken in when bonds are broken.
Overall, this is an exothermic reaction meaning the heat is transferred to its surroundings.
Experiment
In my experiment, I am measuring the temperature of water when heated by burning the different alcohols mentioned earlier. I have made points to make this a fair test and have made a prediction.
Measuring energy is not easy and cannot be done directly. I can measure the energy used to heat up an object, if I know its mass, heat capacity and temperature rise. The equation I use for this is:
H = m c T
(Increase in energy = mass of water x capacity of water x temperature rise)
The mass of water is always 4.2g and the capacity of water will be kept at 20cm³, whilst the temperature rise is 20°c. This will measure the energy.
PREDICTION
“I predict that the alcohol with the most hydrogen atoms (Hexanol) will give out the most net energy because it makes more bonds. This is because it is a larger molecule.”
METHOD & EQUIPMENT
I am using a copper beaker in this experiment, as it is a good conductor of heat and will help to heat the water and keep some of the energy inside.
- Clamp stand, 2 x clamps, 2 x bosses, copper beaker, plastic cup lid, thermometer, measuring cylinder, wooden splints, scales, metal dish and alcohol burner.
- Firstly, set up a clamp stand with two clamps and bosses.
- Using a measuring cylinder, measure 20cm³ of water and pour into the copper beaker, then set the beaker in one of the clamps. The copper beaker is used to conduct heat to the water.
- Make sure the copper beaker is 1cm away from the wick of the alcohol burner by measuring accurately with a ruler and fixing the beaker in place.
- Next, set the thermometer in the other clamp, slightly higher than the first, so that the end of the thermometer is touching the top of the water (make sure the bottom of the thermometer is not touching the bottom of the copper beaker).
- Place the plastic cup lid over the beaker with the thermometer through the straw hole. This provides insulation, stopping some of the heat energy escaping from the top of the beaker.
- Measure the temperature of the water before lighting the alcohol and record on paper.
- Weigh the mass of the alcohol burner (with lid) on the scales and record its’ mass before being lit.
- Place the burner underneath the beaker and light. Wait for the temperature to rise by 30ºc and then put the lid over the burner to stop it burning.
- Weigh the mass of the alcohol (again with lid) and record it’s mass after burning.
- Repeat process (at least twice for each alcohol) until all alcohols are completed and compare results.
FAIR TEST
To make this experiment a fair test, I will do the following things.
- I will keep the same mass of water throughout my experiments. I will use 20cm³ of water measured in a measuring cylinder, which will be poured into the copper beaker for each of the alcohols I test.
- I will use a copper beaker in my experiment so as some of the heat energy will not escape from the beaker into the air. Copper is a good conductor of heat and will heat up the water quickly and efficiently but will also allow heat to escape.
- I will keep the distance between the wick of the burner and the beaker the same too (1cm) for each alcohol. This is because if I used random measurements, the fuel would burn at different rates, making it an unfair test.
- I will keep the end of the thermometer from touching the bottom of the beaker so as the heat conducted by the copper will not affect the rise in temperature. This is why I will use a clamp to hold the thermometer.
- Using a plastic cup lid to cover the top of the beaker prevents some energy from being lost to the atmosphere, keeping most of the heat energy in the beaker.
- I will make sure to measure the temperature before lighting the burner so I know how much the temperature has to rise to. All alcohols will have a temperature rise of 30ºc. This is so the temperature will be kept the same and it seems like decent temperature, not too high, not too low.
- I will then weigh the mass of the alcohol before and after the experiment, so I can tell how much alcohol has been burnt to cause the temperature rise. Because the mass of the alcohols will all be different by other people using them, I will later have to convert the results of the mass to the same. These results can be converted to the number of moles*.
- When weighing the mass of alcohol, the lid will always be replaced so as it will not affect the mass. (E.g. if I don’t replace the lid after recording results with the lid ON, then the mass will be affected by the absence of the burner’s lid).
- I will repeat the process twice for some alcohols to get an average of its’ results. It’s always best to double-check!
- If I do not keep the temperature rise the same, it won’t be a fair test because some alcohols will not show the correct temperature after it has risen and will affect the accuracy of my results.
I did a preliminary experiment to see how much water I would have to use, how long it would take and what length of wick to use. It was also useful to make sure the temperature rise is quick to ensure accuracy (less heat loss). My final decision is to use the method and planning written above.
PREDICTION
When water and carbon dioxide are made, energy is released. Energy is taken in to break the bonds in the fuel. The more atoms and bonds in the fuel, the more oxygen it reacts with and the more water and carbon dioxide are made. Therefore more net energy is released when more bonds are made.
Each type of bond requires a certain amount of energy to be broken or gives out that amount of energy if it is made. It is similar to a measure of strength of the bond. Textbooks give these energies in KJ/mole. Below, are the bonds needed for each alcohol and their energies. They are all in KJ/mole.
Also shown is the net energy release for 1 mole of each fuel using the difference between the bonds broken and the bonds made.
RESULTS
Here are the alcohols that I repeated the method with:
Now, to convert these results into energy amounts I need to use the formula below:
H = m c T
(Increase in energy = mass of water x capacity of water x temperature rise)
E.g./ Methanol
H = 20 x 4.2 x 30 = 2520J for 0.48g
The “20” is the 20g we used as our mass of water, the “4.2” is the heat capacity of water itself and the “30” is the temperature rise which we used for all our experiments. The total of this “2520” will be used for all of the equations below.
This then has to be has to be converted into moles in the way shown below:
E.g./ Methanol - (mass of ) = 0.48g
Here are the mass numbers for Carbon, Hydrogen and Oxygen.
CARBON = 12
HYDROGEN = 1
OXYGEN = 16
I will use these mass numbers for every alcohol’s equation.
The ‘32’ below is found by adding the mass numbers of methanol’s equation = 12 + 4 + 16= 32.
For 1 mole = 2520 x 32
0.48g = 168000J
168000J converted into KJ = 168 KJ/mole
And the amount of energy used per mole for methanol is 168KJ/mole. I have used this formula to work out all the other alcohols’ amount of energy per mole. The results are shown below:
ETHANOL – (mass of ) = 0.41g
The ‘46’ below is found by adding the mass numbers of ethanol’s equation = 24 + 6 + 16= 46.
For 1 mole = 2520 x 46
0.41g = 282731.7J (1dp)
282731.7J converted into KJ = 282.7317 KJ/mole
PROPANOL – (mass of ) = 0.42g
The ‘60’ below is found by adding the mass numbers of propanol’s equation = 36 + 8 + 16= 60.
For 1 mole= 2520 x 60
0.42g = 360000J
360000J converted into KJ = 360KJ/mole
BUTANOL – (mass of ) = 0.27g
The ‘74’ below is found by adding the mass numbers of butanol’s equation = 48 + 10 + 16= 74.
For 1 mole= 2520 x 74
0.27g = 690666.6J (1dp)
690666.6J converted into KJ = 690.6666KJ/mole
PENTANOL – (mass of ) = 0.25g
The ‘88’ below is found by adding the mass numbers of pentanol’s equation = 60 + 12 + 16= 88.
For 1 mole= 2520 x 88
0.25g = 887040J
887040J converted into KJ = 887.04KJ/mole
HEXANOL – (mass of ) = 0.26g
The ‘102’ below is found by adding the mass numbers of hexanol’s equation = 72 + 14+ 16= 102.
For 1 mole= 2520 x 102
0.26g = 988615.3J (1dp)
988615.3J converted into KJ = 988.6153KJ/mole
I have noticed from my tabulated results that the alcohols with the even number of carbon atoms have a decimal number of energy per mole. These are also the alcohols that have none of the number ‘2’ in front to balance out its’ equation.
The amount of net energy given out increases as the number of carbon and hydrogen atoms increases by the structure of each alcohol. For example, Methanol has a net energy of only 168KJ/mole (because it has a small structure) whereas Hexanol has a net energy of 988.6153KJ/mole (because it has a large structure). Therefore, the larger the structure, the more net energy given out. In this case, Hexanol gives out the most energy.