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My aim is to make an accurate comparison of the enthalpy changes of different alcohols and explain how and why the enthalpy change is affected by its molecular structure.

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Introduction

Chemistry Coursework Aim: My aim is to make an accurate comparison of the enthalpy changes of different alcohols and explain how and why the enthalpy change is affected by its molecular structure. Background Information: Alcohols are organic compounds in which the functional group is the -OH group, which is also called the hydroxyl group. Many belong to a homologous series which has the general formula CnH2n+1OH. In alcohols, a hydrogen atom is bonded to an oxygen atom. The oxygen atom in the hydroxyl group is in turn bonded to a carbon atom. Alcohols make good fuels because they burn easily and they release a lot of heat energy, which can be harnessed to power machines In this experiment I will be burning fuels, therefore the enthalpy change I will be measuring will be that of combustion, which s a measure of the energy transferred when 1 mole of the fuel burns completely. When an alcohol burns, it reacts with oxygen to form carbon dioxide and water. If there is a restricted supply of oxygen, it will lead to incomplete combustion, and will produce carbon monoxide instead of carbon dioxide. * alcohol + oxygen ? carbon dioxide + water complete combustion E.g. methanol + oxygen ? carbon dioxide + water CH3OH (1.5) + 1O2 (g) ? CO2 (g) + 2H2O - less oxygen is needed to ensure complete combustion of a alcohol, than a corresponding alkane because it contains an oxygen atom already * alcohol + (restricted supply of) oxygen ? carbon monoxide + water incomplete combustion This is an exothermic reaction because heat energy is given out to the surroundings. ...read more.

Middle

+ (3 x O = O) ? (4 x C = O) + (6 x O - H) STEP 3 Replaced by values: (5 x 413) + (1 x 346) + (1 x 358) + (1 x 464) + (3 x 497) ? (4 x 745) + (6 x 464) STEP 4 After doing calculations: 4742 ? 5764 STEP 5 How much heat energy is released: 5764 - 4742 = 1040 HEAT ENERGY RELEASED BY ETHANOL: 1040 kj/mol Name of alcohol: Propan-1-ol Balanced chemical equation: C3H7OH + 4.5O2 ? 3CO2 +4H2O STEP 1 This equation can be represented as: 1x + (4.5x O = O) ? (3x O = C =O) + (4x H - O -H) STEP 2 Represented in terms of bonds: (7x C - H) + (2x C - C) + (1x C - O) + (1x O - H) + (4.5x O = O) ? (6x C = O) + (8x O - H) STEP 3 Replaced by values: (7x 413) + (2x 346) + (1 x 358) + (1 x 464) + (4.5x 497) ? (6x 745) + (8x 464) STEP 4 After doing calculations: 6641.5 ? 8182 STEP 5 How much heat energy is released: 8182 - 6641.5 = 1540.5 HEAT ENERGY RELEASED BY PROPAN-1-OL= 1540.5 kj/mol Name of alcohol: Butan-1-ol Balanced chemical equation: C4H9OH + 6O2 ? 4CO2 +5H2O STEP 1 This equation can be represented as: 1x + (6x O = O) ? (4x O = C =O) + (5x H - O -H) ...read more.

Conclusion

Splints Paper Straws Solid 23.0 *C 23.0 *C 23.0 *C 23.0 *C 25.0 * C 25.0 *C 24.0 *C 26.0 *C 4 min 1 sec 1 min 50 sec 1 min 22 sec 2 min 44 sec The amount of heat transferred from the fuels to water in Kj can be found using the relationship: Energy= mass of water times temperature rise times 4,2j/g/c (In KJ) (In Kg) (In *C) * it can be assumed that at room temperature 1cm3 of water has the mass of 1g Preliminary Results: Splints= 50 cm3 + 2* C + 4.2 = 168 j/g/c Straws= 50 cm3 + 1* C + 4.2 = 84 j/g/c Paper= 50 cm3 + 2 *C + 4.2 = 168 j/g/c Solid= 50 cm3 + 3* C + 4.2 = 252 j/g/c Changes: For the actual experiment I am going to use 100 cm3 of water because 50cm3 mass of water was too large a rise as this caused too much heat to be lost to the environment, and 150cm3 was too small. Therefore the 100cm3 value was used, as this temperature was right for the enthalpy calculations. Also the amount of wick was investigated, and I found out that if it were too large, there was more heat to be lost in the environment, and if it were too small, most heat given out is lost in the can, so 5cm is thought to be the optimum length. This will be kept constant throughout, as well as the mass of water and the temperature will be kept constant, to ensure that it is a fair test. I am also going to use alcohols instead of fuels. ?? ?? ?? ?? ...read more.

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