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Enthalpy Change - Analysis and Results

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Introduction

RESULTS ALCOHOL AVERAGE MASS BURNT (G) METHANOL 1.54 ETHANOL 1.02 PROPAN-1-OL 0.77 BUTAN-1-OL 0.69 PENTAN-1-OL 0.55 HEXAN-1-OL 0.52 Following the gathering of results, the enthalpy change of combustion of the alcohols was calculated using the equation from page 2: Energy Transferred = Mass of water x Temperature rise x specific heat capacity (4.17) Now to calculate the enthalpy change of combustion, the following steps are taken: 1.) Energy per gram = Energy Transferred � Average mass change of alcohol 2.) Energy per mole = Energy per gram x Molecular mass The Energy Transferred from each alcohol is constant as the variables, volume of water (200ml) and temperature rise (15oC) were kept constant. Therefore the calculation of energy transferred from each alcohol is: Energy Transferred = cm?T = 4.17 x 200 x 15 = 12510 J To convert this into KJ, we divide it by 1000 = 12510 � 1000 = 12.510 KJ. Hence 12.510 KJ of energy is transferred by each alcohol to raise 200ml of water by 15oC. The calculations of the enthalpy change of combustion for each alcohol can be seen on the following page. 1.) Methanol - CH3OH : molecular mass = 32g Energy per gram = Energy Released � Average mass change = 12.510 � 1.54 = 8.10 KJ Energy per mole = Energy per gram x molecular mass = 8.10 x 32 = 259 KJ mol -1 2.) ...read more.

Middle

which suggests that a lot less mass is needed to combust an alcohol as the alcohol increases in size. The reason behind this is the reactivity of the alcohols depend on there carbon atoms, where the more carbon atoms, the more reactive the alcohol would be, therefore it would use less fuel to heat the water. For example by looking at graph 1 you can see that ethanol which has two carbon atoms uses 1.02g of fuel to burn where as hean-1-ol has six carbon atoms and only uses 0.52g of fuel to heat the water up by 15oC. By observing graph 3 you can clearly see that energy density is proportionate to the increase in alcohol size, where methanol produces the least amount of energy per gram (8.1KJ) and hexan-1-ol produces the most amount of energy per gram (24.1g). Observing graph 2 we can see that two sets of results are plotted: 1. Enthalpy change of combustion of my own results 2. Enthalpy change of combustion of average bond enthalpies The graph shows a strong positive correlation between the molecular mass of each alcohol and the enthalpy change of combustion. Looking at calculations made in my prediction, more energy is needed to break the bonds as the molecular mass increases. As each alcohol increases an extra carbon and two hydrogen atoms are added on and therefore each alcohol's molecular mass rises by 14. ...read more.

Conclusion

However, I did find an anomalous result in methanol where the point on graph 1 did not match that of the best fit line and therefore was seen as an anomaly. The reasons behind why such a result was obtained could have being to the fact that methanol was the first alcohol used and therefore the surrounding conditions could have affected the outcome, in which a lot of mass was needed to raise the temperature by 15oC. There were a number of limitations which could have affected the final outcome which are listed below with explanation. I. Whilst the alcohols were combusting there was a significant amount of heat being lost to the surroundings and therefore not heating the water efficiently. This would have affected the outcome of the results as if no heat was to be lost, the water could have heated by 15oC far quicker and therefore only a small amount of the alcohol would have being used. II. Human error also affected the final outcome in which factors such as weighing, measuring and reading off measurements. The measuring of exactly 200ml of water could have being read off incorrectly on the measuring cylinder, in which the amount of energy released would have being affected. III. Even though I stirred the water through out the experiment, I did not stir each trial equally with the same amount of effort and therefore heat being spread over the copper calorimeter would have being different for each trial. ?? ?? ?? ?? CHEMISTRY COURSEWORK ...read more.

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