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Determination of the Heat of Formation of Calcium Carbonate

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Introduction

Determination of the Heat of Formation of Calcium Carbonate Report Type: Full Date: 20th November, 2007 Grade: Objective: To determine of the heat of formation of calcium carbonate. Principle: Heat of formation of a substance is the enthalpy change when the substance is formed the substance from its constituent elements. To be more specific, under standard conditions, the enthalpy change when one mole of the substance is formed from its elements is called the standard heat of formation. An enthalpy change under standard conditions means that the reactants and products in the reaction are in standard states. The standard states are defined as the most stable form of substance at 1 atmosphere and 298 K (or 25 oC). These conditions enable comparisons to be made between sets of data. In this experiment, it is aimed to find out the heat of formation of calcium carbonate (CaCO3(s)). The equation for the formation of CaCO3(s) from its elements under standard conditions is: Ca(s) + C(s) + O2(g) CaCO3(s) Since the heat of formation cannot be measured directly, Hess's Law can be applied in calculations, with the help of values of heat of reaction from reactions of calcium and calcium carbonate with dilute hydrochloric acid. They can be calculated by using the formula E = mc?T, where m is the mass of reactants, c is the specific heat capacity of solution and ?T is the temperature change. ...read more.

Middle

+ 2H+(aq) Ca2+(aq) + H2O(l) + CO2(g) ?H2 +) H2(g) + O2(g) H2O(l) ?H3 +) C(s) + O2(g) CO2(g) ?H4 Ca(s) + C(s) + O2(g) CaCO3(s) ?Hf [CaCO3(s)] Thus ?Hf [CaCO3(s)]?=??H1 - ?H2 + ?H3 + ?H4 = (-419) - (-19.4) + (-286) + (-394) = -1080 kJ mol-1 Method 2 - Drawing Enthalpy Level Diagram: H / kJ mol-1 Ca(s) + C(s) + O2(g) + 2H+(aq) Ca2+(aq) + H2(g) + C(s) + O2(g) Ca2+(aq) + H2O(l) + C(s) + O2(g) Ca2+(aq) + H2O(l) + CO2(g) CaCO3(s) + 2H+(aq) Thus ?Hf [CaCO3(s)]?=? (-419) + (-286) + (-394) - (-19.4) = -1080 kJ mol-1 Conclusion: The heat of formation of calcium carbonate is -1080 kJ mol-1. Discussion: Reasons and Importance of Applying Hess's Law The heat of formation of calcium carbonate cannot be determined directly by calorimetric experiments as there exists several experimental difficulties listed as follow: * The extent of the reaction cannot be controlled; * Heat evolved cannot be separated into appropriate terms; * Direct combustion of calcium can be violent; and * Side reactions may arise, for example, 2Ca(s) + O2(g) 2CaO(s) C(s) + O2(g) CO2(g) Then Hess's Law is applied to calculate the heat of formation of calcium carbonate. Hess's Law states that because enthalpy is a state function, the enthalpy change of a reaction is the same regardless of what pathway is taken to achieve the products. ...read more.

Conclusion

Precautions In Experiment B, since dry powdered calcium carbonate is placed into the plastic cups directly, the cup should be kept dry in order to avoid additional mass in the cup due to the water, as this will slightly increase the experimental value. When conducting the two experiments, the lid should be loosely covered the cups to prevent pressures from building up inside the cup by the gases released. This is important as contents inside the cup should under constant atmospheric pressure throughout the processes. Since the reaction occurs under constant pressure, the heat output gives enthalpy change. Safety Precautions For calcium metal, we should never touch the metal with bare hands, as moisture on hands will react with the metal to give calcium hydroxide, which is corrosive. Use a forceps to handle it. Besides, hydrogen, which is flammable, will be released when the metal is reacting with acids. Therefore we should ensure there is no flame nearby to prevent explosions. In addition, calcium presents a hazard because of its reaction with water, so it must not be disposed into a sink. Collect calcium in a plastic or glass container for later disposal. For hydrochloric acid, although the acid used in this experiment is diluted, the solution is still mildly corrosive, so we should handle it with care. Besides, small amounts of dilute hydrochloric acid can be flushed down a sink with a large quantity of water. - End of Report - ?? ?? ?? ?? Chemistry TAS Experimental Report Determination of the Heat of Formation of Calcium Carbonate 1 ...read more.

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