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# Enthalpy change

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Introduction

Finding the enthalpy change that cannot be measured directly. Abstract The main purpose of the experiment was to plan and carry out experiments were the enthalpy change cannot be measured directly. And also to comment on the sources of error and assumptions made in the experiment. Introduction The standard enthalpy of formation of calcium carbonate, ?Hf is defined as the enthalpy change when one mole of calcium carbonate is formed from its elements in their standard states, under standard conditions. Equation for the reaction is: Ca (s) + C (graphite) + 1.5 O2 (g) --> CaCO3 (s) The reaction cannot be determined in the laboratory experimentally but by demining the enthalpy change when calcium carbonate reacts with hydrochloric acid, and applying the principles of Hess's law, this enthalpy change can then be measured. ...read more.

Middle

approximately 500 cm3 * Access to balance weighing � 0.1g Method Experiment 1: Calculating the enthalpy change for the reaction between calcium metal and hydrochloric acid. 1. 1.03 g of calcium was measured, number of moles of calcium used was calculated: 1.03 g � 40 = 0.02575 mol. 2. 100 cm3 of 1.0 M HCL (aq) was measured out into a polystyrene cup using a measuring cylinder. 3. The temperature of the acid was measured at 22�C. 4. After taking the temperature, calcium metal was added to the acid, and the highest temperature rise was recorded at 42.2�C. 5. Thus indicating that the reaction was exothermic and the temperature increase was: ?T= 20.2 �C. ...read more.

Conclusion

Results The enthalpy change for the reaction between calcium metal and hydrochloric acid was calculated using the equation: Enthalpy change = m X c X ?T. 100 cm3 X 4.2 kJ kg-1 K-1 X 20.2K = -8484 J. Enthalpy change in kJ mol-1 based on the number of moles of calcium metal used is: -8484 X 0.02575 = -218.463 kJ mol-1. The enthalpy change for the reaction between calcium carbonate and hydrochloric acid was calculated using the equation: Enthalpy change = m X c X ?T. 100 cm3 X 4.2 kJ kg-1 K-1 X 0.8K = -336 J. Enthalpy change in kJ mol-1 based on the number of moles of calcium carbonate used is: -336 X 0.265 = -89.04 kJ mol-1. Reaction for the reactions in the experiments are: Ca(s) + 2HCl --> CaCl2 + H2 CaCO3 +2HCl --> CaCl2 + H2O + CO2 ...read more.

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