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Determination of the heat of combustion of alcohols

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Introduction

Experiment 8 Determination of the heat of combustion of alcohols Objective To determine the enthalpy change of combustion of a series of alcohols. Principle In the experiment, Methanol, Ethanol and Propan-1-ol, would be used for determining the heat of combustion. They are alcohols with chemical formula CH3OH, C2H6O and CH3CH2CH2OH respectively. The Heat of combustion (?Hc) is the energy released as heat when a compound undergoes complete combustion with oxygen. The chemical reaction is typically a hydrocarbon reacting with oxygen to form carbon dioxide, water and heat.. The heat of combustion is traditionally measured with a bomb calorimeter. It may also be calculated as the difference between the heat of formation (?Hf) of the products and reactants. In order to find the heat of combustion, a measured mass of an alcohol was burnt and the heat released was transferred to a calorimeter soft drink can. The heat capacity of the calorimeter was first determined by calibration with an alcohol of known molar of combustion. From the rise in temperature of the water in the calorimeter, the heat of combustion of other alcohols can be calculated by using E= MC?T and the definition of Standard Enthalpy of Combustion. Experimental set-up Chemicals & Apparatus Methanol, Propan-1-ol, water aluminium can,datalogger with temperature sensor , alcohol lamp ,electronic balance (0.01G), clamp and stand ,250cm3 measuring cylinder ,metal tray Precaution 1. ...read more.

Middle

12. Place the bottle cap and contents on top of a wooden block, 13. After 30 seconds, light the wick with matches and immediately place the can and its contents on a tripod (do NOT add a wire gauze) to enable the burning wick to heat up the can of water directly. The wick should be positioned as to minimize heat loss to surroundings and to ensure uninterrupted burning. 14. The water was gently stirred at frequent intervals using the temperature sensor. The temperature sensor should not be touched the bottom of the can. 15. When the temperature of the water has risen by 20 C, the flame was extinguishing and was immediately replaced the cap. 16. The temperature was continued stirring and was recorded for an additional 2 minutes. File of data was saved. 17. The alcohol lamp and cap as soon as possible were re-weighed after extinguishing the lamp. 18. Steps (6)-(17) were repeated using ethanol and propan-1-ol. 19. The heat capacity of the calorimeter and hence the heat of combustion of ethanol and propan-1-ol respectively were calculated. Result The graph showing the temperature change in certain time interval Calculation Methanol ethanol Propan-1-ol Initial weight 132.41g 100.30g 163.75g Final weight 130.46g 98.43g 162.15g Fuel used 1.95g 1.87g 1.60g The change in weight of C-1 (Methanol) : 1.95g Temperature change of water : 20oC Specific heat capacity of water = 4.2 KJ Kg -1 K-1 The mass of the water = ...read more.

Conclusion

that it's specific heat capacity may be neglected. 4. The initial temperature of water should not be much higher than room temperature and the distance between bottle cap & Coke can should not be too long to reduce heat loss to surroundings. 5. In this experiment, simple calorimetry is used to calculate the energy content of alcohol per gram, no heat loss to surroundings is assumed. However, the experimental value should be less than expected since there are some source of errors in this experiment: (i) heat lost from top and sides of aluminium can (ii) incomplete combustion of alcohol may occur (iii) specific heat capacity of aluminium can is ignored Some improvements may be done to minimize the errors: (i) determine the specific heat capacity of Coke can and count it. (ii) a lid with a hole (to insert the thermometer) to cover the Coke can and a draught screen surrounds the calorimeter. Conclusion After this experiment, it is found that there are some error occurred. The experiment was not carried out under standard conditions. Also between the can and alcohol lamp, there was a large separation which may affect the heat transfer to a calorimeter soft drink can form alcohol lamp and loss to the surrounding. Percentage of experiment error: Ethanol : [(1370-1046.34)/1370](100%) = 23.62% Propan-1-ol : [(2010-1650)/2010](100%) = 17.91% Simple calorimetry can be used to calculate the energy content of alcohol per gram. These values are important to scientists. ?? ?? ?? ?? Subject: Chemistry Experiment 8 Form: 6B Name: Eric LI (26) [15/2/2007] msn: eric0927hk@hotmail.com ...read more.

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