Here is an example of the homologous series for the alcohols and their general formula: CnH2n+1OH
The members of a homologous series have similar chemical reactions. This is because each contains the same "functional group" - the atoms which give the homologous series its typical reactions. In the alcohols the functional group is -OH. Although the chemical reactions shown by the members of an homologous series will be similar, they will often show a trend in reactivity (i.e. a step by step change in reactivity as the group is descended) because the hydrocarbon chain has some effect. In the alcohols the first few are completely soluble in water whereas the later members with long hydrocarbon chains are almost insoluble.
Standard Conditions: In comparing enthalpy changes it is essential to ensure the conditions of the system are the same before and after the reaction because ΔH is affected by temperature, pressure and concentration of solutions. The standard conditions for temperature and pressure are 298K and 1 atmosphere respectively. Any enthalpy change measured under these conditions is described as a standard enthalpy change of reaction, ΔHΘ298. Also the substances involved are in their normal physical states. The standard enthalpy change of combustion, ΔHΘc, 298 is the enthalpy change when 1 mole of a substance is completely burnt in oxygen under standard conditions.
Preliminary Work:
When I first tried to measure the enthalpy change of combustion, I had achieved a level of very inaccurate results, which would have been no where near as accurate as the real enthalpy change, and would have a very high percentage error. This is because on my first attempt, I did not take into account the size of the flame, or how high above the flame the can was. This gave a great range of results, since for each of the trials, I did not measure the height, so there is a great chance that the height was different every time. I found that, to make the flame of the spirit burner around the same height, I had to make sure that the wick was approximately the same length, coming out of the burner. Also I had to make sure that the flame was more concentrated and less spread so to do so, we had the wicks of the spirit burners made tighter and held firmly together with some wire. Another thing I found for the preliminary work was that a lot of soot is formed, which means that a lot of the energy is also lost, since there is not complete combustion, and this would give some inaccurate results. Finally, I found that my first choice of using 150ml of water made the experiment too slow, so I found that 100ml worked well since lower that this will heat up to quickly and make it harder to take clear results.
Apparatus:
- Metal can with concave bottom
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0-50oC thermometer
- Spirit burners
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100cm3 measuring cylinder
- Mass measuring balance
- Bunsen burner
- Draught excluder
- Clamp and stand
The alcohols needed are:
- Ethanol
- Methanol
- Butanol
- Propan-1-ol
- Propan-2-ol
Method:
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Firstly, you have to set up the experiment as shown in the diagram below therefore the apparatus needed are the following: a thermometer, clamp + stand, spirit burner (ethanol, methanol, butanol, propan-1-ol, propan-2-ol), large tin case (to go around flame as protection), measuring cylinder, 100cm3 water, small can (for 100cm3 water).
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Once the apparatus is set up, you need to measure the mass of the spirit burner, with the glass lid. Then you light the wick of the spirit burner and let the temperature rise around 15oC (or another set temperature every time).
- Once you have done so, blow out the wick and quickly place the lid on the spirit burner so not much of the fumes are lost.
- Then measure the mass of the spirit burner again (don’t forget the lid) and you can see that some of the alcohol has been burnt away.
- Record the change in temperature and change in mass of the spirit burnet, then repeat the experiment 3 times for each alcohol. The results should be recorded in a table like so:
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Next thing to do are the calculations. The formula you need for this is Energy transferred = mcΔT. In this formula, m is the mass of water in grams, c is the specific heat capacity of water (4.17 Jg-1K-1) and ΔT is the change in temperature or temperature rise. Once you have this value and you have recorded it, you need to find the no. of moles which is calculated using the equation No. of Moles = mass / relative formula mass.
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Finally you need to calculate the enthalpy change of combustion (ΔHc) by using the following formula: ΔHc = Energy Transferred / No. of moles. Then the ΔHc is divided by 1000 to get it in kilojoules per mole (kJ mol-1). Once this is done, you just need to find an average of it.
Diagram:
Risk Assessment:
Alcohols are highly flammable, as is their vapour, and they will catch fire at temperatures above 13 oC. Alcohols are also toxic and if inhaled, or swallowed absorbed by skin, can cause severe consequences. If swallowed, one should wash out mouth and drink some water. Medical attention may be needed if victim shows drunken symptoms or if methanol or methylated spirit is involved. If vapour is inhaled, the victim should move to an area of fresh air to recover and try to keep warm. If liquid is splashed in eyes flood the eye with running tap water for 10 minutes and then seek medical attention. If split on skin or clothes, then remove contaminated cloths and wash affected area thoroughly with cold water. Soak contaminated clothing to reduce fire risk. If split in laboratory, shut off all sources of ignition e.g. Bunsen burners and open all windows, then apply mineral absorbent to the spill. Scoop up the remains into a bucket and add water to reduce chances of catching fire.
Results:
Here are my results of the enthalpy change of combustion of these alcohols, with the true results and the theoretical (bond enthalpy) results:
My percentage errors are as follows:
Analysis:
My graph shows a curve for the 3 enthalpy change of combustion results. I can tell from my graph that my results are not accurate but are in the correct proportions compared to the other correct results. This means that I did the experiment well and the errors were mainly caused but unavoidable instances. My results are percentage inaccuracies of the other results. My graph also shows that as the structure of the alcohols gets bigger, the enthalpy change of combustion also increases (negatively). From my results I found that as the relative formula mass (RFM) of an alcohol increases, so does the ΔHc and I feel this is because as the RFM increases, the size of the alcohol gets bigger because it has more carbon atoms in it meaning that it will therefore have more bond and therefore more bonds need be broken. In Propan-1-ol and propan-2-ol, I found that the latter had a higher enthalpy change of combustion.
Evaluation:
I found that there were many sources of error in my experiment. These were things such as the heat of the flame was not able to be kept fixed all on its target area, the can, and some of the heat was lost to the surroundings. However this was inevitable with the equipment we had at our disposal, so I had to keep this to a minimum, by putting the can closer to the flame. The next problem was that of incomplete combustion, which produced soot on the can and therefore, this was a further example of waste of some of the energy. This all meant that I was unable to measure the ΔHc to an accurate degree. Another problem I had was that I had to make sure that no fuel was wasted, but since we had to light the wick our selves, this caused problems, such as, when removing the lid, some of the fuel evaporated and also not blowing out the wick at the exact time that we removed the can from the flame, so there was a little bit of fuel lost this way. Another error was that when we stopped heating the water, the temperature on the thermometer still carried on increasing, meaning that there was still heat being given to the water, but the thermometer was not quick enough to show it. This meant that to make it a fair test, I had to stop heating the can, as soon as the thermometer showed that the temperature had risen by the chosen amount. Another source of error was controlling the height of the flame, which I had to control by finding spirit burners which each gave a flame of approximately the same height. To improve my experiment, I would use other more reliable equipment, such as a can with less area being exposed at the top, meaning that less energy would be lost through the air. I would also use insulated draught excluder so that it cannot absorb the heat like the metal one did that I used.
References:
- www.pinkmonkey.com/studyguides/subjects/chem/chap13
- www.scinet.elecuter.co.uk
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Salters Advanced Chemistry, Chemical Ideas 2nd edition 2000, Heinemann Educational, Oxford.
