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Determination of the Enthalpy Change of a Reaction

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Introduction

Determination of the Enthalpy Change of a Reaction Determine the enthalpy change of the thermal decomposition of calcium carbonate by an indirect method based on Hess' law. Using the proposed method of obtaining results, these values were gathered: Reaction 1: CaCO3(s) + 2HCl(aq) ?Cl2(aq) + CO2(g) + H2O(l) Experiment Number Mass of CaCO3 (g) Temperature Change (?) 1 2.50 2 2 2.55 2 1/6 3 2.50 2 1/4 4 2.53 2 1/6 5 2.47 2 � 2.51 2.12 Reaction 2: CaO(s) + 2HCl(aq) ?Cl2(aq) +H2O(l) Experiment Number Mass of CaO (g) Temperature Change (oK) 1 1.30 9 1/2 2 1.36 10 1/3 3 1.46 11 4 1.35 10 1/6 5 1.40 10 1/2 � 1.37 10.3 � in both cases represents the mean of the data. Using the equation for enthalpy change: ?H = mc?T Where: m = Mass of liquid to which heat is transferred to (g) c = Specific heat capacity of aqueous solution (taken as water = 4.18 J.g-1.K-1) ?T = Temperature change (oK) We can thus determine the enthalpy changes of reaction 1 and reaction 2 using the mean (�) of the data obtained. Reaction 1: ?H = 50 x 4.18 x -2.12 ?H = -443.08 This value is for 2.51g of calcium carbonate, not 100.1g which is its molecular weight. Therefore: ?H = -443.08 x (100.1 / 2.51) = -17670.2 J.mol-1. ?H = -17.67 kJ.mol-1. Reaction 2: ?H = 50 x 4.18 x -10.3 ?H = -2152.7 This value is for 1.37g of calcium oxide, not 56.1g which is its relative molecular mass. Therefore: ?H = -2152.7 x (56.1 / 1.37) = -88150.7 J.mol-1. ?H = -88.15 kJ.mol-1. Hess' law states that: 1"The total enthalpy change for a chemical reaction is independent of the route by which the reaction takes place, provided initial and final conditions are the same." This means that therefore the enthalpy change of a reaction can be measured by the calculation of 2 other reactions which relate directly to the reactants used in the first reaction and provided the same reaction conditions are used, the results will not be affected. ...read more.

Middle

These are that the specific heat capacity of all solutions to which a temperature change has occurred is taken as the value for water. This is relatively high due to its high stability from its hydrogen bonding. Therefore with solutions with a lower specific heat capacity, less thermal energy is required to raise the temperature of the solution and the enthalpy change is not as great as suspected. Also the density of the solution is judged as water which has a density of 1. That is, for every 1 cm3 of liquid, it has a mass of 1 gram. This is because Density = Mass / Volume. However, no solutions have a density of 1 unless of course they are pure water. This means that there could, in fact be a larger amount of molecules to which the thermal energy is transferred to. Therefore with solutions with greater density, more thermal energy is required to raise the temperature of the solution and the enthalpy change is greater than suspected. Taking into account all of these potential areas of limitation of the method, if the experiment were to be repeated, it would have to include as many methods in which to prevent these sources of error to occur. This would include: conducting the experiment in a vacuum flask, using a digital thermometer to display an exact change in temperature of the solution, the flask to contain a method of introducing the solid reagent without loss of heat, provide standard conditions under which to conduct the experiment and use the true specific heat capacity and density of hydrochloric acid when calculating enthalpy changes. Results: For CaCO3: T1 = 17 T2 = 19 DT= 02 using 2.57g of CaCO3 For CaO: T1 = 18 T2 = 27 DT= 09 using 1.39g of CaO Analysis: In order to determine the enthalpy change for the thermal decomposition of calcium carbonate, we must work out the enthalpy changes for both the reactions of calcium carbonate and calcium oxide with hydrochloric acid. ...read more.

Conclusion

There were also a number of other problems with the experiment, but these were of lower significance than those mentioned above. The most easily noticed one of these is the measuring cylinder - we were instructed to use a 250cm3 measuring cylinder to measure out 50cm3 of 2 mol dm-3 HCl. This could have produced an error, as it is using a large cylinder to measure out a relatively small amount of acid. However, whilst meaning that this variable was not entirely under control during the experiment, as the acid was in excess, it is not especially crucial that exactly 50cm3 was used - it does not affect the readings taken. Another variable was the concentration of the acid. This could have affected the results, by the concentration not being accurate. If this were the case, however, then the error would have been systematic, and so will have appeared in both sets of results, as the acid was drawn from the same bottle. Another one of these small errors is the accuracy of the balances - the readings given may not have been entirely accurate. However, this slight inaccuracy (?0.004g) pales in the face of the other experimental problems, so it can be safely disregarded. These problems were not the only ones with the experiment - the major factor being that it was only performed once, with no repeats - these values were taken to be correct, with no comparisons made. This could easily be rectified by performing a suitable number of repetitions - for example, 4 repetitions could be made, and an average taken. This would vastly improve the reliability of the end results, as the average would more accurately reflect the true temperature change. Overall, there were a large number of problems with this experiment, and correspondingly, there are a large number of things that I would like to change if I were to be able to repeat this experiment. The experiment was successful I that results were obtained, but I suspect that these results are vastly different to the actual values. ...read more.

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