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Enthalpy Change

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Introduction

Skill P - Planning * The aim of this practical experiment is to determine the enthalpy change for this reaction by an indirect method based on Hess's Law. Both Calcium Oxide and Calcium Carbonate react readily with 2 mol dm Hydrochloric Acid solutions. The temperature changes during these reactions can be measured and the enthalpy changes calculated. * Chemicals and apparatus: * 250 cm measuring cylinder * 2 mol dm Hydrochloric Acid * 250 cm beaker * 0 - 10 0� C thermometer (graduations to 1� C ) * 2.4 - 2.6 g of Calcium Carbonate * 1.3 - 1.5 g of Calcium Oxide * Procedure 1. Weigh out a weighing bottle containing between 2.4 and 2.6 g of Calcium Carbonate 2. Weigh out a weighing bottle containing between 1.3 - 1.5 g of Calcium Oxide 3. Using the measuring cylinder provided place 50 cm3 of 2 mol dm-3 hydrochloric acid (an excess) into a 250 cm3 glass beaker. 4. Measure the temperature of the acid using the thermometer provided. 5. Add the calcium carbonate/ calcium oxide to the acid. 6. Take the temperature again when the reaction is complete. ...read more.

Middle

4.2 7 H= 1514.1 We have to calculate the molar heat. Therefore, 1 mole of CaO = 40 + 16 = 56 KJ mole CaCO CaCl We can calculate based on Hess's Law. Therefore, Hence, = CaO = -18.19 - (- 56.52) = 38.33 KJ mole (Endothermic reaction) Skill E - Evaluating Evidence A balance was used fact that leads to limited accuracy, because it is open to the air and is susceptible to outside forces. The procedure was good and the mix of reactants was intimate, so we can say that the experiment were suitable. The first reaction (CaCO) was a very slow reaction. Therefore, when reading the temperature we had to check it constantly and pick up the biggest value. The second one (CaO) was a very fast reaction, so when reading the temperature, we had to choose the first biggest value, because after that that point the temperature will decrease and the results will be anomalous. 2.42 g of CaCO and 1.50 g of CaO was taken with a tendency to show variation of 0.01g. This will lead to 0.004% error for CaCO and 0.006% for CaO. ...read more.

Conclusion

A greater proportion of molecules exceed the activation energy at higher temperature. The reaction takes place quicker if the reactant, hydrochloric acid, is more concentrated, because there are more molecules per volume and they will exceed the activation energy. This thing applies to the first reaction which involved CaCO, and which was very slowly because it was in lumps form. So, the first experiment could be considerable improved by turning the CaCOin powdered form. In order to find out the enthalpy change we used Hess's Law which shows that whatever the route from given reactants to products, the overall energy change must be the same. Therefore, each of the two enthalpies calculated from the two experiments count half per cent on the final conclusion. Hence, CaCO 2.42 0.01g 0.4 % error on final conclusion + the temperature change CaO 1.50 g 0.01g 0.6 % error on final conclusion + the temperature change A considerably effect on the validity of the final conclusion have the signs of the heat evolved (whether the reactions are endothermic or exothermic). The first two experiments are exothermic reactions and as the final enthalpy change has a positive value, the reaction is an endothermic one. ...read more.

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