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Investigating the enthalpy of combustion

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Introduction

Chemistry Lab report - Enthalpy of combustion Aim: To see the trend in the combustion of the first five alkanols. Introduction: The standard enthalpy of combustion is the enthalpy change when one mole of a substance completely reacts with oxygen under standard thermodynamic conditions (although experimental values are usually obtained under different conditions and subsequently adjusted). E.g. The standard enthalpy of combustion of ethane is the energy released when one mole of ethane is completely burned in excess oxygen under standard conditions. i.e. the energy change for the reaction: C2H6 + 3.5 O2 --> 2CO2 + 3H2O By definition, combustion reactions are generally strongly exothermic and so enthalpies of combustion are generally strongly negative. It is commonly denoted as or Enthalpies of combustion are typically measured using bond calorimetry, and have units of energy (typically kJ); strictly speaking, the enthalpy change per mole of substance combusted is the standard molar enthalpy of combustion (which typically would have units of kJ mol-1). ...read more.

Middle

Dependent variable > The dependant variable is the amount of fuel burnt after the 20oC rise in temperature. This can be measured using the Digital balance. Constant > The constant variables in this experiment are the conditions in which the trials were performed. The equipment used is also constant since a change in instrument could cause a variation in the data being recorded. RESULTS MASS - Trial 1 Name of fuel Weight of fuel (g) [�0.01] START-END= Change of mass (g) START END Methanol 257.85 257.02 0.83 Ethanol 233.76 232.96 0.80 Propanol 252.24 251.70 0.54 Butanol 282.14 281.51 0.63 Pentanol 268.10 267.48 0.62 MASS - Trial 2 Name of fuel Weight of fuel (g) [�0.01] START-END= Change of mass (g) START END Methanol 253.85 253.04 0.81 Ethanol 287.72 287.12 0.60 Propanol 269.36 269.00 0.36 Butanol 272.24 271.80 0.44 Pentanol 265.16 264.71 0.45 Name of fuel Calculation for Average mass (T1 + T2)/2 (g) Average mass of fuel burnt (g) ...read more.

Conclusion

From the graph we can see a positive correlation between the length of the hydrocarbon chain and the Heat evolved per mole. Evaluation The experiment was fair and safe since all the safety measure were taken e.g. wearing goggles, handling fuels with care etc. The results were fairly accurate but showed some difference between the two trials due to the following sources of errors. Sources of Errors 1. There was a lot of heat lost to the surrounding and so we could not account for all of the ?H. 2. While measuring the gas after the experiment was over, the weight kept on decreasing the longer you kept on the weighing scale. This is because of the capillary action of the wick of the alcohol container (spirit lamp). 3. The time was not stopped on an exact rise of 200C because the temperature does not rise exactly 200C. 4. There was a slight time lapse between the time of setting up the trial, lighting the spirit lamp, and starting the stop watch ?? ?? ?? ?? Prashant Pradhan YR 11 Chemistry HL - Enthalpy of combustion ...read more.

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