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  • Level: GCSE
  • Subject: Science
  • Document length: 3186 words

I am going to investigate how and why the enthalpy change is affected by the molecular structure of the alcohols under conditions, which I have stated under 'Plan' below.

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Introduction

Comparing the enthalpy changes of combustion of different alcohols Introduction I am going to investigate how and why the enthalpy change is affected by the molecular structure of the alcohols under conditions, which I have stated under 'Plan' below. Sources I have used the following sources in aid of devising my plan: * My lecture notes. * Salters Advanced Chemistry: Chemical Storylines * 'Enthalpy' coursework form www.essaybank.co.uk Equipment I will use the following equipment: * Copper calorimeter * Thermometer * Stand * Boss * Clamp * 200 cm³ measuring cylinder * Electronic balance * Bunsen burner * 2 Mats * 5 Splinters The above equipment are needed because the stand, boss and clamp will support and hold the metal calorimeter containing 200cm³ water, which it will be heated up by the burning alcohol underneath. The thermometer is used to measure temperature before and after the experiment and the electronic balance is used to weigh the alcohol before and after the experiment. Two mats will be used to keep heat loss, by heating the environment, to a minimum The Bunsen burner is used to light the splinter, which in turn will set alight the alcohol. Water level will be the same for each of the alcohols combusted. All of procedures are to be carried out just to make the experiment fair. Materials Only the following straight-chained alcohols will be used: * Methanol(l) CH3OH * Ethanol(l) CH3CH2OH * Propan-1-ol(l) CH3CH2CH2OH * Butan-1-ol(l) CH3(CH2)2CH2OH * Propan-2-ol(l) CH3CH(OH)CH3 These alcohols are chosen because a wide range of alcohols are needed to be compared, as an aid to decide which alcohol releases the most energy when burned. It will also provide reliable data, showing the exact enthalpy change of combustion of each of the alcohols, which will be used during the experiment. Safety The following safety aspects need to be considered, in order to conduct experiments as safely as possible: * Follow instructions exactly. ...read more.

Middle

11. While the alcohol is burning, stir the water by using the thermometer from time to time. 12. Keep on heating the water inside the copper calorimeter until the temperature rises 20°C above the recorded temperature before the test. 13. Extinguish the burning alcohol. 14. Record the temperature after the test, before it decreases. 15. Weigh the methanol after the test and record the mass. 16. Pour the water into the sink. 17. Repeat procedures 2 to 15 for the rest of the alcohols, as stated above under 'Materials'. Remember to keep the conditions for the other alcohols the same as the first alcohol, in order to make the test fair. 18. Repeat the experiment again to cancel out any anomalies. Implementing Results Combustion of different alcohols by mass Experiment Alcohol Temperature (°C) ?T (°C) Mass (g) Mass Burnt (g) Initial Final Before After 1 Methanol 22 42 20 137.657 136.365 1.292 Ethanol 22 42 20 153.423 152.423 1 Propan-1-ol 22 42 20 148.643 147.828 0.815 Propan-2-ol 22 42 20 123.424 122.606 0.818 Butan-1-ol 22 42 20 148.896 147.838 1.058 2 Methanol 17 37 20 121.475 120.264 1.211 Ethanol 17 37 20 140.541 139.606 0.935 Propan-1-ol 22 42 20 130.724 129.678 1.046 Propan-2-ol 22 42 20 147.838 147.329 0.509* Butan-1-ol 18 38 20 143.363 142.538 0.825 Table 1 *Anomaly Water level: 200cm³ ?T: 20° Analyse I have used the following formulas to calculate the values of enthalpies of combustion for all the alcohols I used during the experiment: Average enthalpy change of combustion Methanol 1. Use the Periodic Table to calculate the molecular mass of methanol. Write the mass number of each element stated in the molecular formula. If you see a number or the symbol of an element appears more than once, write the appropriate number and the mass number in brackets. Then multiply the sum in brackets first then add up the whole sum to obtain the molecular mass number, e.g.: * Mr(CH3OH) ...read more.

Conclusion

* Methanol: -430.77 kJ mol¯¹ * Ethanol: -680 kJ mol¯¹ * Propan-1-ol: -1050 kJ mol¯¹ * Propan-2-ol: -1527.28 kJ mol¯¹ * Butan-1-ol: -1292.31 kJ mol¯¹ The percentage error values suggest that my results, concerning the average enthalpy change of combustion, were unreliable. This is because when I put each alcohol on the electronic scale, the values tend to move up and down, making measurement of the mass of each alcohol difficult. There are also limitations when using a thermometer and a measuring cylinder because I had to record the mass or temperature to the nearest gram or degree. I conducted the experiment in different conditions at 20°C (293K) and 1 atm. Then the temperature has decreased to 18°C (291K) during the experiment. I also manipulated the results by rounding it up or down, so this accounts about 5% of the error percentages. The main causes of anomalous results are heat loss by heating the environment, due to poor insulation of the copper calorimeter, soot on the bottom of the copper calorimeter, incomplete combustion, tiny amounts of water has been lost when pouring from the measuring cylinder into the copper calorimeter, impurities in the water and the experiment was not conducted under standard conditions; at 25°C (298K), 1 atm and using 1 mol dm¯¹ of each alcohol. Improvements can be made to the procedure by: * Using the same burner * Keep the copper calorimeter at the same height * Pump enriched oxygen to the burner * Insulate the copper calorimeter. These procedures will ensure that the results obtained are accurate and reliable because if the same burner is used, the same amount of fuel would be burned since the length of the wick is the same. Keeping the copper calorimeter at the same height, means that the fire can reach the copper calorimeter from the burner at the same time. Pumping enriched oxygen would make combustion of the alcohols complete, so soot is not deposited on the bottom of the copper calorimeter. Finally, insulating the copper calorimeter prevents heat loss by heating the environment. Martin Yau Exam No. 6079 Centre No. 58231 1 ...read more.

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