Energy is given out as the temperature of the water rises. After awhile the temp of the products falls to room temp. Heat from the reaction is lost to the surroundings. The surroundings have gained energy and the reaction has lost energy. This is called a negative reaction because of the heat change and because the total energy of the products is less than the burning reactants. The heat produced is used to heat the water in the copper calorimeter. If we measure the temperature increase of the water we can work out the heat produced.
I am going to work out the heat of combustion as shown in the worked example below. To calculate this we must first identify the structures of both the reactant molecules and the product molecules. Certain bonds between atoms require a specific amount of energy to break or make them. The example I have worked out is butanol the values of each bond is shown in the table, the bonds are taken from the structural equation. Once the values for both the reactants and the products we may then calculate the heat of combustion by taking away the energy out from the amount of energy in.
Butanol
1 mole of Butanol ( N = 4 )
C4 H3 OH + 6 O2 → 4 CO2 + 5 H2O
H H H H
H C C C C O H
H H H H
Heat of Combustion = Energy in – Energy out
= 8566 – 10550
= -1984 kJ / Mole
This shows us how to work out the heat of combustion for Butanol, however the heat of combustion for the other 3 fuels can be found later in the prediction section of my investigation. The negative results for heat of combustion shows that energy is being lost to the surroundings.
Apparatus
- Copper Calorimeter
- Clamp
- Clamp stand
- Boss
- Spirit burner x4
- Heat proof mat
- 100ml measuring cylinder
- 100ml water
- Stop clock
- Digital weighing scales
- Thermometer
- Bunson burner
- Splint
Diagram
Prediction
The following table shows the heat of combustion for each of the 4 alcohols.
I predict that according to my table butanol will have the greatest weight loss as it takes the least amount of energy to heat it therefore it will be the first to begin giving out heat in the two. Ethanol will have the smallest weight change as it takes the most energy to burn which means it will release the least energy within the 2 minutes. I also predict that butanol will have the largest temperature change as it burns quicker and will heat the water up more within the 2 minutes. On the whole I believe that at this stage, theoretically butanol will turn out to be the most efficient.
Fair test
To keep my experiment fair here is a list of the constants:
- The amount of water in the calorimeter (100ml)
- This distance from the top of the spirit burner to the bottom of the calorimeter. (3cm)
- Leave the spirit burner burning from the same amount of time for each alcohol. (2min)
Here is a list of the variables:
- The alcohol in the spirit burner (methanol, Ethanol , butanol, propanol )
- The amount of alcohol is different to each spirit burner used.
Method
- Set up equipment as shown in the diagram
- Measure 100ml of water in the measuring cylinder and pour it into the copper calorimeter
- Record the temperature of the water
- Choose a spirit burner, record the name of fuel inside and the mass of the whole spirit burner (including the lid and fuel inside)
- Light the wick of the spirit burner and put it under the copper calorimeter (which will be held by the clamp)
- Stir the water gently with the thermometer
- After 2 minutes record the temperature of the water and the mass of the spirit burner (including the lid and fuel)
- Put fresh water into the copper calorimeter and repeat steps 1- 7 for each of the fuels. Test each fuel 3 times.
Results
Conclusion
The results that I obtained do not support my prediction as I predicted that Butanol will have the greatest loss. When my results clearly show that propanol had the greatest loss by a clear amount, theoretically I still believe that butanol should have had the greatest mass loss. I also predicted that ethanol would have the smallest weight change which has been proved as you can see from my results table and graph 1.1. The results table and graph 1.2 show that butanol and propanol had the greatest temperature change which in a way does support my prediction as I said that butanol would have the greatest temperature change. I also learnt from this experiment that if there are too many carbon atoms present in a fuel then the heat of combustion is decreased. If there are too few carbon atoms then the heat of combustion also decreases, according to my graph the peak where the heat of combustion is at its greatest is at 2 carbon atoms.
Evaluation
On the whole I believe that my experiment was a success as nothing went drastically wrong and my prediction was kind of right. I think that my prediction could have been proved completely right if we had repeated the results a few more times therefore increasing the range of results, I don’t believe it is down to accuracy of equipment but because of human error such as reading the thermometer wrong etc. On the first ethanol experiment I recorded the mass change as 1.5 when the other two were much lower this is the only anomalous result and I believe that it had no great effect on my findings. But it could mean that a few of my other results were like this without me realizing I believe that this is also due to human error rather than the equipment.
I could have improved my experiment by testing the available alcohols more times, which would fine tune my conclusion. Or maybe I could use more accurate equipment and made sure that the only variables were the alocohols and water. Keeping the distances exactly the same between the heat source and the water. I could also have put a sort of cone shape around the spirit burner up to the copper calorimeter which would have kept the maximum amount of heat in and would have concentrated it onto the water itself. To extend my investigation I could have firstly investigated moe fuels, which would give me a much broader range and therefore allowing me to make my conclusion more accurate. I could also have tried increasing the amount of water or decreasing it which may have produced different results but should follow the same pattern. I could also have investigated other factors that affect the heat of combustion such as length of the wick, the distance between the heat source and copper calorimeter.