To investigate and compare the relationships between the amounts of energy released from different alcohols in the alcohol series. BACKGROUND INFORMATION Alcohols are a series of organic homologous compounds with the general formula of C(n) H(2n+1) OH. They gain their properties, which are different to those of other compounds in the homologous series due to the OH bonded to the Carbon. The 4 simplest alcohols are Methanol, Ethanol, Propanol and Butanol. Each alcohol has one more carbon and two more hydrogens than the previous one.
When alcohols are heated to react in a copious supply of oxygen they undergo complete combustion to form carbon dioxide and water. The reaction is an exothermic one and produces a flame emitting light and thermal energy. This is because the energy put into the alcohol to break the bonds is less than the energy given out by new bonds forming. Therefore, different alcohols release different amounts of energy. The diagrams and calculations on the following sheets show the theoretical energy release of each alcohol using the bond energy values below. BOND TYPE BOND ENERGY (KJ/mol) 0-H 463 C-H 412 C-O 360 C=O.
805 (Carbon dioxide) O=O 496 C-C 348 The calculations show that: (the negative values mean energy is lost from the compounds and is therefore given out as heat or light) ALCOHOL ENERGY RELEASED (KJ/mol)) Methanol -659 Ethanol -1279 Propan-1-ol -1899 Butan-1-ol -2519 The negative values mean energy is lost from the compounds and is therefore given out as heat or light as an exothermic reaction. The graph shows that as the length of alcohol chains increases the energy released by combustion will also increase directly proportionally. This is shown by the straight line passing through the origin on the graph on page 6.
This is because the longer chains have give out more energy from the bonds formed in the products than the energy that went in to break the extra carbon and 2 hydrogen bonds. During this investigation I will be using propan-1-ol and butan-1-ol. The 1 refers to the structure of the compound, as Propanol and Butanol are capable of having more than one structure but share the same molecular formula. This is important as other isomers will release different levels of energy due to the different arrangement of bonds. PREDICTION The evidence given previously states that the longer the chain of the molecules the more energy it will release.
Therefore I expect Methanol to give of the least energy and Butan-1ol to release the most amount of energy. This is due to the enrrgy of the products in Butanol being higher than the energy in Methanol. My graph should look the same as the one on the previous page for the theoretical energy release values. However, I will expect my results to show a lower level of energy release because not all of the energy will go into heating the water. Some of the heat will be lost by being blown away and some energy will take the form of light. PRELIMINARY INVESTIGATION.
How high do I place the calorimeter above the burner? I found that a decent sized flame was 4cm and therefore I clamped the calorimeter 7cm above the top of the burner. This allowed for the flame to burn freely without the bottom of the calorimeter interfering with the combustion and making it unfair. How large should the flame be? Although the size of the flame will not effct the results diretly because the rate at which the alcohol burns is irrelevant due to the fat that the change in mass is also measured. However, the size of the flame can make the results inaccurate in other ways.
I found that the small flames flickered to much and did not produce a constant enough source of heat. However, I also found that the large flames were blown around a lot by the slightest of drafts and therefore the heat was not always being transferred into the calorimeter. I decided that 4cm was a good height to use as it was not affected by the drafts too much and did not flicker. How much water do I fill the calorimeter with? The amount of water in the calorimeter needs to be a sensible amount as too much will not heat up enough to be recorded accurately and too little will heat up so much it boils and the results are useless.
Therefore I decided to use 70cmi?? (70g) of distilled water as it was heated up enough to be recorded acuratly but did not boil. How often do I stir the calorimeter? The calorimeter needs to be stirred so that the water is moved around and uniform heating occurs. Uniform heating means that all the water is the same temperature and not just the bottom of the water is heated. Uniform heating make results more accurate. I decided to plunge the stirring wire down and up once every 5 seconds in order to mix the water. How long do I heat the calorimeter for?