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Enthalpy change

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Introduction

Investigation 19 MEASURING ENTHALPY CHANGES Method: Part A 1) Put 100 cm3 of water at about 60 �C into a polystyrene cup by mixing boiling water from the kettle with cold water from the tap. 2) Dry about 25 g of ice with a piece of paper towel and weight them both. 3) Rapidly record the temperature of the water and transfer the ice to it, then record the mass of the paper towel and the water it has absorbed. 4) Stir the water ice mixture and record the temperature of the water as soon as all the ice has just melted. Part B 1) Take the polystyrene cup and use measuring cylinder to put 50 cm3 of 1 M aqueous copper sulphate in it. 2) Weigh out accurately about 5 g of powdered zinc into a weighing bottle. 3) Stir and record the temperature of the copper sulphate at half minute intervals for 2 minutes and then add the powdered zinc. 4) Record the temperature at half minute intervals until the temperature has been falling for ten consecutive recordings. Part A: the heat required for a change of state Firstly, I needed to boil the water in the kettle, so that it can be used in the experiment. ...read more.

Middle

Also, I calculated how many moles of zinc I am going to add to the solution. MZn=65g 1 mole - 65 g x- 5 g x=0.077 moles of Zn Since I don't know, I decided to calculate how many moles of zinc are actually going to react with copper sulphate. The equation for the reaction looks as follow: CuSO4 (aq) + Zn (s) � ZnSO4 (aq) + Cu (s) or Cu2+(aq) + Zn (s) � Zn2+ (aq) + Cu(s) 1 mole - 250 g x- 12.5 g x=0.05 moles of CuSO4 As the proportion between Zn and CuSO4 is 1:1, 1 mole CuSO4 - 1 mole Zn 0.05 moles - x x=0.05 moles of Zn will actually react Which means that zinc is in excess. Next step was to record-firstly-the temperature of the solution, then add zinc. Table 2: The measurements of time for the second reaction. Time [�0.50 s] Temperature [�0.05�C] 0.00 19.00 30.00 18.90 60.00 18.80 90.00 18.80 120.00 18.80 150.00 42.70 180.00 64.40 210.00 67.60 240.00 66.40 270.00 64.80 300.00 63.80 330.00 63.10 360.00 62.20 390.00 61.40 420.00 60.40 450.00 59.50 480.00 58.70 510.00 58.00 540.00 57.20 570.00 56.40 600.00 55.60 630.00 54.90 660.00 54.20 690.00 53.40 720.00 52.80 Rounded to 2 decimal place First two minutes of the temperature was measured for the solution of CuSO4 only. ...read more.

Conclusion

error = error= error=1.38% This lead me to conclusion that the method of collecting data and the way I processed it was accurate and very satisfying. Extrapolations on the graph allowed me to use precise values of measured datas and therefore to calculate the enthalpy changes. Errors occured during the experiment and might have affected the final results: * wrong or error in calculations to create the solution * balance with only 2 decimal places * the uncertainity of cylinders * human reaction during measurements [0.15s] * adding powder in not exact time * polystyrene cup might not have been isolated all the time, so loss of energy could occur * some zinc powder stayed on the Vernier coach thermometer * the value of heat capacity differs; I assumed it was 4.18 g-1 K-1 To improve the experiment, I might make some changes: * make more readings to reduce the uncertainity * more stirring during the experiment, to make sure all substances dissolved * use very icy ice (taken straight from the fridge) so it wouldn't melt too soon * be more accurate while measuring- add all substances at the exact time * use more accurate balance and weigh all substances twice * use thermometer and Vernier coach to make sure that temperature recorded is the same as measured * have more time to do more measurements, prelong the time of measurements ?? ?? ?? ?? Gracja Kowalska kl. 2 IB 1 ...read more.

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