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The object of this investigation is to determine the enthalpy change for the reaction CaCO3 (s) → CaO (s) + CO2 (g) by an indirect method based on Hess' Law. Hess's law states that the enthalpy change for any chemical reaction

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Determining the Enthalpy Change of a Reaction Introduction The object of this investigation is to determine the enthalpy change for the reaction CaCO3 (s) � CaO (s) + CO2 (g) by an indirect method based on Hess' Law. Hess's law states that the enthalpy change for any chemical reaction is independent of the route taken provided that the initial and final conditions are identical. So the temperature change during these reactions below can be measured and the enthalpy changes ?H1 and ?H2 calculated. For Example: Using Hess' law with the calculated values for ?H1 and ?H2 it is possible to calculate a value for ?H3. Results Table Mass of CaCO3 + weighing bottle 3.43 g 3.46 g 3.43 g Mass of empty weighing bottle 0.92 g 0.93 g 0.93 g Mass of CaCO3 used 2.51 g 2.53 g 2.50 g Temperature change of acid initially 21 ?C 22 ?C 22 ?C Temperature of acid after mixing 24 ?C 25 ?C 24 ?C Temperature change during reaction 3 ?C 3 ?C 2 ?C Mass of CaO + weighing bottle 2.36 g 2.34 g 2.36 g Mass of empty weighing bottle 0.98 g 0.97 g 0.97 g Mass of CaO used 1.38 g 1.37 g 1.39 g Temperature change of acid initially 20 ?C 22 ?C 21 ?C Temperature of acid after mixing 35 ?C 34 ?C 34 ?C Temperature change during reaction 15 ?C 12 ?C 13 ?C The results for the temperatures are to the nearest ...read more.


It's an exothermic reaction. Using Hess' cycle I will use the values that I have calculated for ?H1 and ?H2 to work out the value for ?H3. ?H3= ?H1 ? ?H2 = (- 25.00 kJmol-1) - (- 111.43 kJmol-1) = 86.43 kJmol-1 This value is positive because heat is absorbed from the surroundings. It's an endothermic reaction. I have been told the actual value for ?H3, which is 178.00. So I will calculate the percentage by which my value is out by the actual value. 178.00 ?86.43 = 91.57 (91.57 ? 178.00) x 100 = 51% Evaluation Errors in procedure: When the CaO and CaCO3 were put into the cup there was a delay before the lid was put on. This could have caused heat to escape out of the cup and the temperature change would not have been as great compared to if there was no delay. Also when the lid was put on there was a hole made in it so that the thermometer could be put into the acid and measure the change in temperature. But the hole had a larger circumference than the circumference of the thermometer and heat could have escaped through this gap. The CaO and CaCO3 were not stirred after being emptied into the cup so not all of the reactants could have reacted with the HCl. This means that the temperature change during the reaction could have been lower than it should have been. ...read more.


Therefore I could have used another layer of polystyrene which surrounded this piece of polystyrene with an air gap in between. This would reduce the amount of heat loss during the reaction. Here is a diagram to show the improvement that I have suggested: The CaCO3 and CaO weren't stirred after it was put into the cup. So lumps of the CaCO3 and CaO gathered at the bottom of the cup. Therefore not all of the reactants reacted with the HCl and when they did the reaction would have occurred slower and the temperature change wouldn't have been as great. If I were to make an improvement here it would be to use a mechanical shaker for a fixed period of time so that all of the reactants reacted with the HCl. Improvements to the method of taking measurements: As I have already mentioned and discussed I will use a digital thermometer. I will use a measurement scale that can read beyond 2.d.p so my results will be more accurate. I will use the same measurement scales so there will be no variation in my results which would occur if I used two different scales due to them being calibrated differently. Also because bits of the CaO and CaCO3 were scattered on the top of the weighing scale and were recorded as being in the weighing bottle. I could have blown the off the scattered bits of CaO and CaCO3 on the measuring scales away and measured the weighing bottle again to see if any more CaO and CaCO3 would need to be added. ...read more.

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