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Endothermic Reactions.

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Introduction

Endothermic Reactions I am going to investigate the temperature drop induced by the reaction: NH4Cl + 100 H20 = NH4Cl (aq 100 H20) Calculating the amount of solid required to react with a certain amount of water NH4Cl + 100 H20 = NH4Cl (aq 100 H20) 1 mole NH4Cl = 14 + 4 + 35.5 = 53.5g 1 mole H20 = 2 + 16 = 18g/cm3 (assuming that 1g is the same as 1cm3) It can therefore be assumed that one mole of water occupies 18cm3 of space. Ratio: NH4Cl : H20 1 : 100 53.5 : 18*100 53.5/18 : 1800/18 2.97 : 100 Plan I will take 100cm3 of water and 2.97g of NH4Cl and combine them in a polystyrene beaker whilst measuring the change in the temperature with an accurate thermometer (accurate to 0.1cm3). I will surround my beaker with cotton wool wadding and a sheet of aluminium foil shiny side in to reduce heat loss. I will also make a cardboard lid surrounded in a similar way to further reduce heat loss to the surrounding atmosphere. When I add the NH4Cl to the water, I will first take the temperature of the distilled water (so no impurities exist in the water) ...read more.

Middle

In this way I hope that heat loss to the surrounding atmosphere will be reduced to an absolute minimum. In order to keep the test fair, I will also use exactly the same amounts of NH4Cl and water to do the experiment with, the same polystyrene cup, the same wadding and aluminium foil and the same thermometer. I will also measure the temperature after the reaction has taken place. Results Initial Temperature Temperature after Reaction Change in Temperature 22.8 19.1 3.7 22.4 17.5 4.9 22.5 18.2 4.3 22.4 18.6 3.8 22.9 17.7 5.2 Average change in temperature = 4.38 �C Calculations to find DH DH = mcDT DH = 100*4.18*4.38 DH = 1831 J DH = 1.831 kJ In order to find the number of Joules per mole given off, I must use the number of moles of solid that I used in my experiment to multiply the value produced by substituting values into the above equation. I used 1/18 of a mole of solid in my experiment; I must therefore multiply by 18 in order to find the number of Joules per mole. DH = 1.831*18 DH = 32.96 kJ/mol Evaluation The value that my teacher has given me for DH is 16.4 kJ/mol. ...read more.

Conclusion

I would also suggest that perhaps the distilled water that I used was not 100% pure and perhaps this could have had an effect on the reaction, the cup was also washed after each experiment with tap water that contains impurities and therefore could also affect my results by a small margin. I believe that a combination of these errors led to the large error in my value for DH. If I was to repeat this experiment I would be more careful. I would make sure that each container used was cleaned by distilled water as opposed to tap water before use and that I used a fresh container for each new set of results. I would ensure that my measurements were as accurate as I could make them. I think I should also take slightly more ammonium chloride and water to reduce any affect that a slight error in weighing of the substance made. In conclusion, my experiment has shown me simply that the reaction between ammonium chloride and water is endothermic and a reasonable drop in temperature takes place when the ammonium is dissolved. It has also taught me that I need to be more careful in my experimental technique if I wish to obtain good results in my experiments. ...read more.

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