Since the enthalpy value is negative, it means that energy is lost, probably due to heat to the surrounding, which in this case was the water. Since it gives away energy, it is an exothermic reaction.
The value of -40 kJ ± 7 kJ can be supported by the literary values.
The literary values of the enthalpy in a NaOH and H2O is -44.4 kJ/mol. �
The experimental value was -40 kJ ± 7 kJ.
It seems as the experimental value is very good since the literary values is within the range of the experimental values uncertainty.
ERRORS
Total percentage error =
Total percentage error= ((-44.4 – -40)/ -44.4) * 100% = 9.9%.
systematic error = Total Percentage error – Random Error.
Systematic error = 9.9 – 17.2 = -7.3%.
The systematic errors are smaller than the random errors in this experiment. This is probably because the apparatus were very simple and there were many conversions of units in the calculations and also because three different measured quantities were used to calculate the ΔQ. And when you add and multiply and divide the values the uncertainties stack on top of each other and grow. And there were many calculations and measurements in this experiment which is why the random errors were so big. The systematic errors are in most cases not very big since after some lab experience one does not make simple mistakes.
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EVALUATING PROCEDURE AND APPARATUS
The procedure had a lot of weaknesses and limitations. To be able to see the limitations and weaknesses easier a list would be necessary.
We used a Styrofoam cup with a hole in the lid. This has a lot of heat loss to the surroundings since heat is easily radiated to the surrounding air. So the temperature was probably greatly affected because of this. (very significant since A LOT heat is lost to the surroundings.
We had to stir the salt in the water. This generates heat. This affects the results. (not very significant since it should not generate very much heat.)
The NaOH was in crystal form and not a fine salt. This leads to that it has less surface area and so the reaction goes slower. When the reaction goes slower it takes more time to carry out the experiment and more heat can radiate to the surrounding air and the solution has to be stirred for a longer time. (quite significant since the reaction time is important to minimize the heat loss to the surroundings)
The specific heat capacity of the water might alter when the salt is added to it. Then the c-value in the formula will change (not very significant since it will only alter it very little).
When stirring the solution and trying to keep the lid on most people failed. The lid moved and at some points fell off. This lead to an even greater heat loss to the surroundings.
Trying to add the salt into the water while holding the thermometer and stirring at the same time was hard. This lead to a lot of pauses and in some cases salt was dropped on the table as well affecting the results.
Although there are some weaknesses and one is a very significant one, the experimental value compared to the literature value was not very different. Also, the weaknesses might cancel out a little since stirring the solution generates heat and the poor apparatus makes the solution loose heat to the surrounding air. So the procedure could give a result with good quality, like in this case. And the weigh showed up to 3 decimal numbers, which lead to that the percentage uncertainty was almost negligible. So the apparatus were precise and accurate up to some level. The thermometer on the other hand showed no decimal numbers (it was not a digital electronic one).
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This was the main reason to the very high random errors. This also lead to that the experiment results were not very precise since the random errors were 17.2%! This is not very accurate. But since the results were accurate there can not have been very many systematic errors or other errors since the experimental value agreed a lot with the literary value.
IMPROVEMENTS
Since it had weaknesses which were mainly due to the poor apparatus, there is room for easy improvements in most cases. A list will be easier to read.
If we had used a caleriometer the heat loss would not have been near the heat loss that was present in this experiment.
Taping the lid on top of the cup before stirring and then maybe even taping the thermometer (which we stirred the solution with) through the hole and cover the hole with tape would have helped a lot. One would still be able to stir the solution.
If we had started with an initial temperature higher than 19 Celsius degrees the salt crystals would have dissolved by itself and generating heat by stirring would not have been a problem. But in that case a box that would have a temperature the same as the solutions initial temperature would have been needed since otherwise a lot of heat from the hot water would have been lost to the surrounding.
If the individuals that carried out the experiment had worked two and two the problem with doing many things at the same time would not have been a problem.
The systematic error was not very high at all but could have been reduced if we had done many more trials. If we had been given more time then there would have been time for more trials and the mean value ought to have less systematic errors then.
If we had been in a proper and better lab room then we would have had a greater access to precise and accurate apparatus which would have lead to less random errors. If we had used another method, such as using hydrochloric acid (HCl) instead of water or maybe water with higher initial temperature then the reaction time problem would have been reduced.
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References
Cited from Chemistry handout “ENTHALPY OF SOLUTION” received from the teacher 19/01 2009
The Lanly Company. Updated 06/19 2007. The Physics Of Heat Processing. Lanly. Retrieved February 04, 2009, from http://www.lanly.com/heating.htm
Chemistry 3rd Edition by John Green and Sadru Damji. IBO 2007. ISBN 978-1-876659-08-0. Page 138.
1998-2008 Roger Walker. Information retrieved 04/01 -2009. Page last modified : 3 rd. September 2008 .From HYPERLINK “http://www.simetric.co.uk/si_water.htm”http://www.simetric.co.uk/si_water.htm
HYPERLINK “http://ostermiller.org/calc/temperature.html”http://ostermiller.org/calc/temperature.html Copyright Stephen Ostemiller 2001-2006. Page visited 04/02 2009.
HYPERLINK “http://www.mindspring.com/~drwolfe/WWWolfe_dat_enthalpies.htm”http://www.mindspring.com/~drwolfe/WWWolfe_dat_enthalpies.htm
Page visited 05/02-2009. WWWolfe Enthalpies.
