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Enthalpy Change of the Formation of Calcium Carbonate

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Introduction

Date: 11/11/08 Title: Enthalpy Change of the Formation of Calcium Carbonate Objective: To determine the enthalpy change of the formation of calcium carbonate Theory: The enthalpy change of formation is the enthalpy change of the reaction when one mole of a substance is formed from its constituent elements is their standard states under standard conditions. The equation for the formation of calcium carbonate from its elements under standard conditions is Ca(s) + C(s) + 3/2O2(g) --> CaCO3(s) . It is defined as elements or compounds in their normal physical states, a pressure of 1 atm (101325 Nm-2), and a temperature of 25 oC (298 K).Assuming the solution in the polystyrene cup has the same specific heat capacity as water. The enthalpy change of formation of Calcium Carbonate can be represented as follows by Hess's law: Apparatus: measuring cylinder x2, polystyrene cup x2, thermometer x1, 100ml beakerx1 Chemicals: Calcium metal, Calcium Carbonate, 1.0M Hydrochloric acid Procedures: A. Reaction of calcium with dilute hydrochloric acid (1)

Middle

(6) The experiment is repeated and the average temperature rise is determined if there is time. Result: Part A Part B Polystyrene cup Calcium Polystyrene cup Calcium carbonate Mass (g) 2.18 1.01 2.18 3 Final temperature (?) 26.5 55 26.5 28 Initial temperature (?) 26.5 26.5 26.5 26.5 Temperature rise(?) 0 28.5 0 1.5 Calculations: The enthalpy change of formation of water: -286 kJ mol-1 The enthalpy change of formation of carbon dioxide: -393 kJ mol-1 Part A The ionic equation for the reaction taken place Ca(s) + 2H+(aq.) --> Ca2+(aq.) + H2(g) Heat evolved by one mole of calcium. Heat change = (100/1000)(4.2)(28.5) = 1197 = 11.97kJ Number of moles of calcium = 1.01/40.1 = 0.025187 mol Heat evolved by one mole of calcium =11.97/0.25187 = 475 kJ mol-1 Part B The ionic equation for the reaction taken place CO32-(aq.) + 2 H+(aq.) --> CO2(g) + H2O(l) Heat evolved by one mole of calcium carbonate. Heat change = (100/1000)(4.2)(1.5) = 0.63kJ Number of mole of calcium carbonate = 3/(40.1+12+16*3)

Conclusion

In order to make the experiment more accurate, we can have the following improvement, like uses the vacuum flask calorimeter instead of the polystyrene foam cup or uses the cotton wool to cover the outer surface of the cup for reduce heat loss to surroundings, use a more accurate reading thermometer. Find out the specific heat capacity of the solution and the thermometer. Use the method of "weighing by difference" to find out the amount of the samples used. The law which is used is conservation of energy and Hess's Law, Hess's law of constant heat summation states that the total enthalpy change accompanying a chemical reaction is independent of the route by which the chemical reaction takes place, The law of conservation of energy states that energy can neither be created nor destroyed but can be changed from one form to another. The importance of Hess's law lies in the fact that enthalpy changes can be calculated for reactions that cannot be carried out in the laboratory, or reactions that are too slow or involve the formation of side products. So, I think it is useful. End of report ?? ?? ?? ?? 2

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