There are two types of organic compounds used in this experiment: alkanes and alcohols. Alkanes contain only carbon and hydrogen atoms and all C-C and C-H bonds are single bonds (intermolecular forces). The two alkanes used in this experiment are n-pentane and hexane. (The letter "n" means that all the carbon atoms are attached in a chain. n- Pentane is called "normal" pentane.) Alcohols contain the -OH functional group in addition to carbon and hydrogen atoms. That excludes any different arrangements, which are simply due to the molecule rotating as a whole, or rotating about particular bonds. Where the atoms making up the various isomers are joined up in a different order, this is known as structural isomerism. Structural isomerism is not a form of stereoisomerisms. Not all of the carbon atoms in the molecule form a continuous chain of carbons. Some of the carbons are branched off of the continuous chain. These branched hydrocarbons may have the same molecular formula as a normal alkane but the ways that the atoms are connected to one another differ.
These isomers arise because of the possibility of branching in carbon chains. For example, there are two isomers of butane, C4H10. In one of them, the carbon atoms lie in a "straight chain" whereas in the other the chain is branched. During the propagation of polymer chains, branching can occur. In radical polymerisation, this is when a chain curls back and bonds to an earlier part of the chain. When this curl breaks, it leaves small chains sprouting from the main carbon backbone. Branched carbon chains cannot line up as close to each other as un-branched chains can. This causes less contact between atoms of different chains, and less opportunities for induced or permanent dipoles to operate. A low density results from the chains being further apart. Also lower melting points and tensile strengths are evident, because the intermolecular bonds are weaker and require less energy to break.
Prediction:
I predict that the more carbon bonds in an alcohol, the higher the boiling point would be. This is because the carbon bonds need high temperatures to be broken apart and for the liquid the boil. This changes it all. The carbon bonds are very strong. The isomers are hard. The carbon's intermolecular forces are what would make them hard to boil. Isomerism will affect the results.
Apparatus:
Method 1:
- Boiling tube
- Paraffin
- Beaker
- Methanol
- Ethanol
- Propanol
- Propan-2-Ol
- Bunsen Burner
- Gauze
- Hexanol
- 2Methyl-Propan-2-Ol
- Bunsen Burner
- Gauze
- Heat Proof Mat
- Thermometer
- Heat Proof Mat
- Thermometer
Method 2:
- Heating Mantle
- stand
- Connecting tube
- flask
- Methanol
- Ethanol
- Propanol
- Propan-2-Ol
- Butanol
- Pentanol
- Hexanol
- 2Methyl-Propan-2-Ol
- Butanol
- Pentanol
Diagram:
Method 1:
- Collect the apparatus and wear goggles
- Light the Bunsen burner and fill a beaker with 100ml of paraffin
- Fill a boiling tube with 50ml of an alcohol and place a thermometer in it
- Put the beaker on the gauze and put the boiling tube in the beaker
- Let the paraffin boil, checking the thermometer frequently
- Record the temperature when the thermometer stops rising.
- Carefully take the boiling tube out of the beaker using tongs and let it cool
- Repeat step 3 & 4 with a different alcohol
- Carefully clear up the equipment
Method 2:
- Collect apparatus and set it up
- Fill the beaker with an alcohol and switch on the heating mantle
- Attach the beaker to the clamp, being careful not to burn yourself
- Insert a thermometer into the beaker, being careful not to burn yourself
- Record the temperature when alcohol begins to flow through the tube consistently
- Make sure the heating mantle doesn’t overheat and smoke
- Repeat this with other alcohols
- Repeat for any anomalous results
- Pack away the equipment carefully
Results:
A table to show my results compared to my prediction:
A table to show the real boiling points of certain alcohols:
Conclusion:
In conclusion I found that the more carbons in a structure of an alcohol the higher the boiling point would be. This supports my prediction. I found that the intermolecular forces are very hard to break down. All but one of my predictions were close to the result, by 5°. This was because my scientific knowledge, allowed me to make the predictions. My scientific knowledge helped me use the theory that the more carbons in the chain, the lower the boiling point of an alcohol will be. This is due to branching and the forces between the molecules. The smaller chains had low boiling points due to their being fewer forces to break up.
My prediction linked with the results well. All of my results supported my results apart from one, which was an anomalous result.
Evaluation:
The anomalous results are scattered around. The first one is for Propan-2-Ol. This result cannot be explained at all. It was a lot lower then it was meant to have been. The 2nd and 3rd anomalous result is from the preliminary work. The preliminary method wasn’t as accurate, and is not suitable for this investigation at all. This is due to the thermometer only being able to warm up to the heat of the water.
The only reason that may explain why there was an anomalous result, is the fact that the same flask was used for all the alcohols. This means it was impure and some left over of the last alcohols may not have been washed out. The previous alcohols may have been heated up instead and affected the result. Also the thermometer may have been at fault, and only a little bit of water may have been pouring through, at not such a steady rate.
A way of improving the whole thing would be to use a stand and use it to hold the boiling tube over the Bunsen burner. Here a different alcohol in a boiling tube each could be placed over to boil. This way, we would be able get the actual boiling point of the alcohols. It would be connected to the connecting tube and to a beaker so we could be able to see when the alcohol has boiled or not. This helps mix method 1 and method 2 together. It also helps make sure everything works well, and the heating mantle cannot affect the result. Also, different boiling tubes could be used, so the impurities could be ironed out.