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Molar Heat combustion chemistry - investigate the effect of molar mass on the molar heat of combustion of adjacent members of a homologous alcohol series.

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Introduction

Aim To investigate the effect of molar mass on the molar heat of combustion of adjacent members of a homologous alcohol series. Introduction Chemists refer to the energy stored in a substance as the heat content or enthalpy of the substance. The heat of reaction is determined by the difference in the enthalpy between the reactants and products. The molar heat of combustion of a substance is the quantity of heat liberated when one mole of that substance is burnt completely in air. In the case of a hydrocarbon, the products are carbon dioxide and water. In this experiment you will determine the molar heat of combustion of methanol, ethanol, 1-propanol, 1-butanol; and 1-pentanol. Experimental Procedure Equipment * Electronic Balance �0.01g * Measuring Cylinders �1 mL * Aluminium Can * Cotton Wool * Theromometer �0.5?C * Retort Stand and Clamp * Spirit Burner containing alcohol sample * Alcohols (methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol) Safety The alcohols are highly flammable. Always place the alcohol lamp on a tray to contain any accidental spillage of alcohol. Carefully use the cap of the alcohol lamp to smother its flame. Avoid skin contact with the chemicals. Lab coats and safety glasses must be worn at all times during the practical to ensure personal protection. Method 1. Design a data table to record the results of the experiment - this must be done prior to the experiment by all individuals. ...read more.

Middle

10032 �400 4 9196 �400 5 9614 �400 1-Pentanol 1 8778 �400 2 8778 �400 3 8360 �400 4 9196 �400 5 9614 �400 Sample Calculations 4. Finding molar mass Table 6. Molar mass Alcohol Molecular formula Molar Mass (g mol-1) Methanol CH4O 32.05 Ethanol C2H6O 46.08 1-Propanol C3H8O 60.11 1-Butanol C4H10O 74.14 1-Pentanol C5H12O 88.17 Sample Calculations 5. Finding the number of moles E.g. (values from methanol trial 1): Table 7. Number of moles: Alcohol Trial Number Number of moles Uncertainty of number of moles Methanol 1 0.034 0.0006 2 0.041 0.0006 3 0.025 0.0006 4 0.025 0.0006 5 0.027 0.0006 Ethanol 1 0.018 0.0004 2 0.023 0.0004 3 0.015 0.0004 4 0.018 0.0004 5 0.020 0.0004 1-Propanol 1 0.0092 0.0003 2 0.0083 0.0003 3 0.011 0.0003 4 0.0058 0.0003 5 0.0070 0.0003 1-Butanol 1 0.0082 0.0003 2 0.0066 0.0003 3 0.0082 0.0003 4 0.0082 0.0003 5 0.0085 0.0003 1-Pentanol 1 0.0084 0.0002 2 0.010 0.0002 3 0.0077 0.0002 4 0.0059 0.0002 5 0.0090 0.0002 Sample Calculations 6. Finding the molar heat of combustion E.g. (from methanol trial 1) Table 8. Molar heat of combustion: Alcohol Trial Number Molar Heat of Combustion (kJ mol-1) Uncertainty of Molar Heat of combustion (kJ mol-1) Methanol 1 245.88 20 2 209.00 10 3 384.56 30 4 384.56 30 5 340.59 20 Ethanol 1 487.67 30 2 381.65 20 3 557.33 40 4 487.67 30 5 418.00 30 1-Propanol 1 954.13 70 2 1007.23 80 3 722.00 60 4 1369.31 100 5 ...read more.

Conclusion

Length of the wick The length of the wick controls the size of the flame and the size of the wick disintegrates from the flame. The smaller the wick, the smaller the flame, the slower it would take to heat the water to 20oC meaning more heat would be lost to the atmosphere and surroundings as the process is longer. By measuring the length of the wick before every trial and having a fixed length as the length of the wick is adjustable in a spirit burner. Positioning of the flame The positioning of the flame was not always at the same place under the aluminium can. Sometimes it was in the middle or more to side and etc. This varies the energy transfer to the water. By using a gauze mat to spread out the heat equally or to place a mark on the aluminium can so that the flame will always be in the same position. Parallax and random errors There were different groups who conducted different sections of this experiment to save time. Different experimenters may have different methods to reading the thermometers which causes parallax error. Having different people conducting the same experiment also increases random error. For example, more movement around the flames could occur more near one apparatus than another. To reduce random errors, more trials should be conducted and it should be the same group/person conducting the experiment. Having the same person or group conducting the experiment would most likely reduce both parallax and random errors or keep them consistent. ?? ?? ?? ?? ...read more.

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