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# Determining the Enthalpy Change of A Reaction.

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Introduction

Practical Three - Determining the Enthalpy Change of A Reaction. If Calcium Carbonate, CaCO3, is heated it decomposes into Calcium Oxide, CaO and Carbon Dioxide, CO2. CaCO3 CaO + CO2 The aim of this experiment is to determine the enthalpy change of this reaction. To do this I will react both Calcium Carbonate and Calcium Oxide, separately, with 2mol dm-3 Hydrochloric Acid, HCl. By recording the temperature changes in each reaction, and using Hess's Cycle, I will be able to work out the enthalpy change. My results were as follows: Mass of CaCO3 + weighing bottle 2.55g Mass of empty weighing bottle .30g Mass of CaCO3 used 2.25g Temperature of acid initially 20oc Temperature of solution after mixing 24oc Temperature change during reaction 4oc Mass of CaO + weighing bottle 1.5g Mass of empty weighing bottle .30g Mass of CaO used 1.20g Temperature of acid initially 21oc Temperature of solution after mixing 30oc Temperature change during reaction 9oc With these values I calculated the enthalpy change for the reaction: CaCO3 CaO + CO2 CaCl2 ?H1 - ?H2 = ?H3 Energy= mass of x Specific Heat x Temperature Solution Capacity of Water Change Mass of solution is everything that will get heated...so... Mass of HCl= vol. X density = 50cm x 1 (Density of HCl (aq) ...read more.

Middle

To find the weight of the calcium carbonate used I took the empty bottle and took that away from the initial figure. It would have been more accurate to have placed the weighing bottle on the scales and then zeroed them, as there would have been less chance of an error due to the weight of the bottle. The method we were given told us to weight out the calcium carbonate and the weighing bottle to a combined weight of between 2.4 and 2.6g. and not a specific amount. This means that had I have conducted out the experiment three times to find an average, it could have been different each time. To ensure the highest accuracy the best thing to would have been to be given a specific amount, or to do as I did and go in the middle, as to little calcium carbonate and the heat change may have not been enough. I made sure the scales were clean and were zeroed before placing the bottle on them, as any added debris would have affected the weight and my final calculation. I also used the same bottle for each part, and ensured they were both clean from any particles that could have affect the final weight. The next part of the experiment was to pour 250 cm2 of HCl in to a measuring cylinder. ...read more.

Conclusion

A smaller beaker may have also helped, as the heat would have been more concentrated. To improve this part of the experiment further it would have been better to have a conical flask with a bung in. the bung should have two holes, one for the thermometer and the other to let the gas escape. There was also another point while the reaction was taking place, and that was with the thermometer. We had to record the temperature once the reaction had stopped. I took this as the point at which it stopped bubbling, but it could have been that the thermometer could have still been adjusting to the temperature in the beaker. This is due to the amount of time it takes the mercury to react. The way to improve this would have been to use an electric thermometer, which does not need time for a liquid to expand, as the values recorded electronically. We then had to repeat this with the calcium oxide, and that method saw the same pit falls. So with both values not as accurate as they should be the final amount will also be out. If I were to do this again I would do each three times, work out the enthalpy change three times and then find the average, to give a more accurate answer. Rebecca Worley Chemistry ...read more.

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