Do this experiment again for each of the alcohols up to pentanol, every time changing the water, and washing off any soot (carbon) on the beaker that may have been caused by incomplete combustion, which is when there is a restricted supply of oxygen, and some of the alcohol does not burn completely, giving carbon and carbon monoxide rather than carbon dioxide.
Repeat this complete experiment with all the alcohols three times for good accuracy. Now work out how much alcohol was burnt during each separate test, by taking away the amount of alcohol that you were left with from the original quantity of alcohol. Now work out the amount of energy that is going into the water by doing the following equation:
Amount of energy going = Mass of x Rise in x Specific
into the water Water Temperature Heat Capacity
KJ Kg °C 4.2
Then work out the formula mass of each of the alcohols. Then, using the formula masses, work out the number of moles of alcohol burnt by doing this equation:
Number of moles = Mass of alcohol burnt
Formula mass of alcohol
Using the answer to this equation, work out what the amount of energy produced per mole of alcohol burnt is by using this equation:
Amount of energy going into the water
Number of moles of alcohol burnt
To make this investigation fair, I will make sure that I only change one variable throughout the experiment, and keep all the other variables the same. This will make it a controlled investigation.
Variable that I will be changing
Throughout this investigation, I will be testing the effect that different alcohols have on the amount of energy produced per mole, so the only variable that I will be changing is the type of alcohol that is being used.
Variables that I will be keeping the same
The variables that I will be keeping the same throughout this investigation are:
- The rise in temperature of the water. I am keeping this the same, because if I change it, the experiments will be inaccurate because the flame will need to burn for longer if the temperature rise is higher, or if it is a lower temperature rise, the flame will need to burn for a shorter period of time. This will lower the accuracy of the results, and make it an uncontrolled test.
- The length between the bottom of the beaker and the wooden board should be kept the same, because otherwise the beaker will be nearer or further away from the flame, making the readings inaccurate.
- The mass of the water, because more water would take longer to heat up, making the investigation inaccurate.
To make this investigation safe, there are some guidelines that I will follow.
- Follow all the usual laboratory rules.
- Make sure that I do not go near any electricity with the water.
- Be careful with the alcohol burners, because they are very flammable.
- If I come into contact with the alcohol, wash it off immediately, so that it doesn’t burn my skin.
- Be sensible around the flames.
I predict that as the number of bonds, or formula mass of the alcohols increases, the energy produced per mole of alcohol burnt will also increase.
For any reaction to take place bonds must be broken and made, bond breaking requires energy while bond making releases energy. Bonds between different atoms require or release different amounts of energy when broken or made because they are different in strength.
The energy that is produced per mole is the bond energy, or enthalpy. This is the energy that is required to break one particular bond. I predicted that the energy produced per mole would increase as the number of bonds in the alcohol. This is because of the breaking of bonds between the molecules within the alcohols. The minimum energy that the molecules must possess in order for the bonds to break is called the activation energy. If they gain enough energy while they are colliding during the reaction, the process of bond breaking can start. As this process of bond breaking goes on, new bonds start to form. This is the reactant molecule reacting, and changing into the product molecules of the reaction. The activation energy in the reaction is less than the energy that is released from the new bonds. Overall, the more bonds that have to be broken within the alcohol, the more energy will be produced in the reaction.