“In endothermic reactions, energy is taken in from the surroundings. Examples are photosynthesis and thermal decomposition.” (Key science Chemistry by Eileen Ramsden)
In terms of chemical bonds an exothermic reaction is “when the energy given out is greater than the energy taken in to break old bonds.” (Key Science Chemistry by Eileen Ramsden) Therefore an endothermic reaction is “when, the energy taken in to break the old bonds is greater than the energy given out when new bonds form.” (Key science Chemistry by Eileen Ramsden)
The enthalpy depends on breaking and making bonds.
Bond energy Calculations
We can use bond energies to calculate a value for the heat of reaction.
Methanol + O
Each bond contains certain levels of energy.
The overall energy change for methanol = -535 KJ mol
The overall energy change for Ethanol = -1031 KJ mol
The overall energy change for Propanol = -1527 KJ mol
The overall energy change for Butanol = - 2023 KJ mol
However the Data Book values for the combustions are -715 KJ mol (Methanol), -1371KJ mol (Ethanol), -2010 KJ mol (Propanol), -2673 KJ mol (Butanol).
In both cases, the pattern is similar and there is a numerical trend. The overall energy changes increases proportionally to the amount of the carbon and hydrogen atoms present.
Apparatus
- Scales
- Thermometer, 0 to 100
- Measuring cylinder 100 cm
- Retort Stand and clamp
- Glass beaker
- Stop Watch
- Spirit burner for ethanol, Ethanol, Propanol, Butanol
- Card box
- Heat proof mat
- Matches
Diagram
Preliminary Work
I did some preliminary work, in order to find out the optimum conditions to do the investigations in. I found from the results that the amount of time the spirit burner should be burnt for was 1 minute. This is because I found that if I burnt it any longer, water began to evaporate. I also found that 50cm was the best volume of water that I could use. I tried using more volume of water but it took a longer time to heat. I also tried using less volume of water but some of it evaporated. My results of this preliminary experiment can be seen below.
Therefore the optimum condition for the experiment is 50cm of water and to burn the spirit burner for 1 minute.
Table of Results from the Preliminary Work
Method
The apparatus is set up as shown above. The apparatus will be placed in a card box over a heat proof mat. This is to minimise the amount of heat lost to the environment.
Firstly, I will fill a beaker of water containing 50cm, measuring it out using a measuring cylinder. Then, I will measure the temperature of the water. This will give me the start temperature. After that I will weigh a spirit burner containing an alcohol with its lid. This will give me the starting mass of the spirit burner. Having done this, I will clamp the beaker so the bottom is about 5 cm above the wick of the spirit burner. Then I will light the spirit burner and the stir the water with the thermometer. I will allow the spirit burner to burn for 1 minute. After that, I will reweigh the spirit burner with the lid, which will allow me to work out the change in mass. I will also note the maximum temperature of the water.
Having got all the readings, I will wash the base of the beaker just in case if soot forms due to in combustion. I will also replace the water and I will do 3 repeats of each alcohol so as to improve the reliability of the experiment.
Fair Test + Control
To ensure this investigation is carried out fairly and in a controlled manner,
- I will try to keep the same burner,
- Keep the same distance of the spirit burner and the beaker ( 5 cm),
- I will use the same scales
- Use the same thermometer
- Use different water and clean any soot produced underneath the beaker.
With reference to my preliminary work, the variables that must be controlled are:
- Volume of water – If there is more water, it will take more energy to heat in comparison to if there is less water. Therefore, the amount of water has to be kept constant. In my experiment, it will be 50cm
- Exclusion of draft – By placing all the apparatus in a card box the heat lost was significantly reduced to make this experiment more accurate. It makes the experiment more efficient as less heat is lost to the environment and it is mainly directed towards the beaker.
- Type of alcohol – This is only variable being varied.
- Height of can above flame and stirring the water - Stirring the water means that there is uniform temperature in the can, and monitoring the temperature rise to ensure uniform heating. The height of the can above the flame is also a factor, so this needs too be kept constant. If the beaker is too low then soot is produced. The distance between the wick and the bottom of the beaker will be 5cm.
- Type of beaker – If the beaker was made of copper the water will heat up quicker as more heat will be transferred because copper is a good conductor. We didn’t have any copper cans so we have to use a glass beaker. Less heat will be transferred because glass isn’t a very good conductor. Therefore the beaker has to be kept the same as different beakers have different conducting properties.
- Time of burning- I have decided to burn the alcohols for 1 minutes to make sure that enough energy can be transferred for a accurate experiment. This has to be kept constant as if I burn it for more than minute, more heat energy will be transferred.
Prediction
I predict that the more bonds there are holding the carbon and hydrogen atoms together, the more energy will be required to break them apart. Therefore the longer the molecule the more energy it takes to break its bonds. This is evident from some of the scientific knowledge I obtained. I can see that it takes more energy to break up the bonds of butanol (+7507 mol) in comparison to methanol (+2803 mol). This is also evident from the data book value. This is also evident from the data book values which I stated in my Bond energy calculation section. There is a pattern and a numerical trend from these data suggesting that as you go up the homologous series the more exothermic the reaction will become. This is because there are more carbons present in alcohols high in the homologous series. Therefore I predict that Butanol will be the most exothermic because it has the greatest mass per mole of the four alcohols provided; therefore it will produce the most heat in a given time. As a result I predict the alcohol with the greatest relative molecular mass will be the most exothermic and Methanol with the least relative molecular mass will be the least exothermic. I think that the order of the alcohols (highest heat produced to lowest heat produced) will be: butanol, propanol, ethanol and methanol.
Safety
I will also need to ensure that I conduct the experiments safely. As alcohols are very dangerous and highly flammable I will wear my safety goggles at all times. I will need to keep all lose items off. The lids on the alcohol must be kept on at all times to prevent evaporation or any spillage.
