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Determine the enthalpy change of the decomposition of sodium hydrogen carbonate by thermochemical measurement and the application of Hess' Law.

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Introduction

W.J.E.C. AS-Level Chemistry CH3c Assessed Coursework - Experiment 2 Aim:- to determine the enthalpy change of the decomposition of sodium hydrogen carbonate by thermochemical measurement and the application of Hess' Law. Risk Assessment 2.0mol.dm-3 Hydrochloric Acid - low hazard; may still cause harm if it comes into contact with eyes or broken skin. If the hydrochloric acid comes into contact with skin, notify supervisor and wash affected area with water. Should spillage occur, again notify supervisor and dilute with water before mopping up. Sodium Carbonate - is an alkaline metal and poses a hazard. Should it come into contact with skin wash immediately. Throughout the experiment safety goggles and a lab coat should be worn. Your eyes are a highly sensitive region of your body and care should be taken to protect them. Apparatus Two plastic cups Two large beakers Two thermometers - graduated in 0.5c divisions Biurette NB. ...read more.

Middle

A biurette is the most accurate way of measuring this volume and, in order to keep the experiment 'fair' all variables need to remain constant. * Place the acid into a plastic cup, which should be supported by a large beaker. The large beaker also adds insulation. * Take the temperature of the acid, using the thermometer, every thirty seconds for three minutes. Record these readings. * At 3.5 minutes add the sodium hydrogen carbonate and stir, using the thermometer, to make sure that it all reacts. * Record the first temperature of the mixture at four minutes. * Continue recording the temperature of the mixture every thirty seconds for a further six minutes. * We are now ready to test the second sample of sodium carbonate. * Measure accurately, approximately 2.0g of the sodium carbonate in the same way as we measured the sodium hydrogen carbonate. ...read more.

Conclusion

We then draw a line of best fit along for the points from four minutes onwards. Where this line meets the line of mixing (a vertical line drawn from 3.5 minutes) we find the maximum/minimum temperature. This temperature is used in determining the maximum temperature. Calculations To find maximum temperature change: i) for the exothermic reaction: maximum (final) temperature - initial temperature ii) endothermic reaction: initial temperature - minimum (final) temperature We then calculate the heat change using the following equation: Heat Change = mass.c.temperature change Where: mass = mass of solution (1ml = 1g) c = heat capacity of solution (4.20J.g-1K-1) heat change = heat change determined using the etrapolated line of the graph To find out the heat change for one mole of sample we multipy the heat change by one over the amount of moles present. i.e. Energy Change x (1/n) where n = number of moles To convert into Kj we multiply the energy change for one mole by 1/1000 ...read more.

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