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# Comparing the enthalpy changes of combustion of different alcohols

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Introduction

Comparing the enthalpy changes of combustion of different alcohols Planning Introduction In this assessment I will be comparing the enthalpy change of combustion of different alcohols, to investigate if enthalpy change is affected by the molecular structure of an alcohol. The standard enthalpy of combustion is the enthalpy change when one mole of a substance burns completely with oxygen under standard state. I will use different alcohols as a means of comparing the molecular structure. The different alcohols I will be using are: Methanol, Ethanol, Propan-1-ol, Propan-2-ol and Butan-1-ol. I chose these alcohols as they each change in structure by one extra carbon atom, (except Propan-2-ol, which is similar to Propan-1-ol, but has its hydroxyl group attached carbon 2 instead of carbon 1) allowing me to have a variety of different examples to compare. I will be measuring the amount of energy transferred from the fuel to the water. I know that it takes 4.2 Joules of energy to raise 1 gram of water by 1 C. This can be shown in the equation 'energy transferred = mass of water x temperature change x 4.2'. Once I have the energy transferred, I will use the formula 'moles = mass/molecular mass' to find out the energy transferred per mole. The independent variable for my experiment is: * The type of alcohol used (Methanol, Ethanol, Propan-1-ol, Propan-2-ol and Butan-1-ol). The dependant variables for my experiment are: * The specific heating capacity of water. * Mass of water in grams. * Change in temperature of water. The control variables for my experiment are: * The change in temperature of water (15�C, keeping temperature rises within 0.2�C of each other). * The mass of water used. Changes would mean false results, as different amounts of energy are needed to heat different volumes of water, due to its specific heat capacity. * The height of the wick. A higher wick would mean different amounts of energy being released each time, creating an unfair test. ...read more.

Middle

water heated x 4.2 x temperature rise 100g x 4.2J x 15.2�C =6,384.00J Now I will get the amount of Ethanol burned in terms of moles. Moles = Mass of Fuel burned / Mr of fuel 1.6g / 46.07 = 0.035 Now I will find the amount of energy transferred to the water per mole of Ethanol Enthalpy per mole = Energy transferred / moles 6,384J / 0.035 = 182,400 J/mol-1 To convert the answer into KJ, I divide this figure by 1000. 182,400 J/mol-1 / 1000 = 182.40 kJmol-1 Average (177.33 kJmol-1 + 182.40 kJmol-1)/2 = -179.87 kJmol-1 Propan-1-ol Calculations 1st Recording Firstly, I will need to find the amount of fuel burned during the combustion of Propan-1-ol Change in Weight = Initial Weight - Final Weight 173.53g - 172.08 g = 1.45 g Now, I will need to find the change in temperature of the water from the combustion of Propan-1-ol Temperature Difference = Final Temperature - Initial Temperature 35�C -19.9�C = 15.1�C Now I will find the energy transferred from the fuel to the water by using the equation Energy Transferred = Mass of water heated x 4.2 x temperature rise 100g x 4.2 x 15.1�C =6,342J Now I will get the amount of Propan-1-ol burned in terms of moles. Moles = Mass of Fuel burned / Mr of fuel 1.45g / 60.1 = 0.024 Now I will find the amount of energy transferred to the water per mole of Propan-1-ol Enthalpy per mole = Energy transferred / moles 6,342J / 0.024 = 264250 J/mol-1 To convert the answer into KJ, I divide this figure by 1000. 264250 J/mol-1 / 1000 = 264.25 kJmol-1 2nd Recording Firstly, I will need to find the amount of fuel burned during the combustion of Propan-1-ol Change in Weight = Initial Weight - Final Weight 155.88g - 154.48g = 1.4g Now, I will need to find the change in temperature of the water from the combustion of Propan-1-ol Temperature Difference = Final Temperature - Initial ...read more.

Conclusion

This means that the enthalpy changes of combustion which I have worked out are significantly below the actual amount, but equally proportionate to each other. As this diagram shows, a significant amount of heat was lost to the surroundings of my experiment. Heat was continually being lost. Heat was being radiated in all directions. Some heat was absorbed by the draught shield. Some heat was absorbed by the copper calorimeter. Only a small amount of the heat was actually transferred to the sample of water. In this experiment, I was limited to the equipment and supplies of the laboratory. Had this not been an issue, I would have improved my experiment to increase accuracy. To begin with, I would have used a draught shield with a diameter exactly matching that of my copper calorimeter. This would reduce heat loss significantly, as less heat would be able to escape between the copper calorimeter and draught shield. Other places in which I could improve my experiment are with using more advanced apparatus. In my experiment I could use an electronic thermometer, which displays temperature to greater accuracy than to which I could read. This would tell me exactly when to extinguish the flame, thus improving the overall accuracy of my results. In this practical procedure, there were several particularly important aspects of ensuring that the data I collected was precise and reliable. One of which was ensuring that each recording of data that I included in my results had been carefully and competently measured. This was due to the many measurements I had to take for my experiment, and if one of the measurements I took was wrong, it would affect the rest of my results. Another aspect which was important to ensuring the data I collected was accurate was ensuring that the apparatus I used each time was the same. This is to avoid variation in the materials and thickness of some of the equipment, which has the potential to alter my results. ...read more.

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