This suggests that the unknown acid is weaker than those listed in the table, the weakest of which is HCN with an enthalpy change of – 38.2 KJ per mole. This enthalpy change is 6.1J greater than that of the unknown acid, a difference of 16%.
The unknown acid is therefore just over 4/5ths the strength of HCN.
Apparatus error: the apparatus used were not entirely accurate and each has a maximum error margin. For example, the error margin for the balance is 0.1g either side of the mass required. Collectively these errors can account for some of the error in my final value. I am going to calculate the maximum error for each piece of apparatus as a percentage of the value measured. The maximum errors for each are:
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Balance ± 0.1g = 0.1 ÷ 12.38 × 100 = 0.8 %
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250 cm³ volumetric flask ± 0.5 cm³ = 0.5 ÷ 250 × 100 = 0.2 %
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50 cm³ measuring cylinder ± 0.5 cm³ = 0.5 ÷ 50 × 100 = 1.0 %
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Thermometer ± 0.1 ºC = 0.1 ÷ 4.2 × 100 = 2.4 %
The overall apparatus error is therefore 4.4% of the final value.
Evaluation
My graph shows that, as expected, the temperature of the Sodium Hydroxide solution remains constant before the acid is added at 23.1ºC. When the acid is added at four minutes there is a rapid increase in temperature of 3.8ºC up to 26.9ºC. The temperature of the solution then falls at a slower rate to 25.6ºC at ten minutes. This is a greater rate of heat loss than I had anticipated which suggests that my methods of insulation were not as effective as I had hoped.
All my results fall on the lines of best fit and there are no anomalous results. This suggests that my results are reliable as they exhibit a strong trend, so I am able to extrapolate with confidence.
An anomalous result representing a sudden decrease in temperature could be obtained due to a change in external conditions such as a gust of wind which would increase heat loss via convection. Also, an anomalous result could show a second increase in temperature if the thermometer if the thermometer entered a region of greater concentration of acid (this would not be caused by a greater conc. of NaOH as this is already in excess). In this region more bonds will be broken releasing more energy than in the rest of the solution. This will cause an apparent warming of the whole solution.
My target value for the molar enthalpy change was – 33.8 KJ/mole so my value was 1.7 KJ less. This is an error of, 1.7 ÷ 33.8 × 100 = 5 %
This is not a significant error and 4.4% of this error could be attributed to apparatus error. However, I believe a large proportion of the error was due to heat loss during the reaction. Steps were taken to reduce heat loss; the reaction vessel was a polystyrene cup, a material that is a very good insulator, and the cup was contained in a glass beaker to act as a second heat barrier.
However, I was standing next to an open door and a constant, light breeze was blowing which would increase heat loss by means of convection. This would also explain why the subsequent heat loss which was more rapid than expected. If this were a major cause of heat loss I would also have expected the initial temperature of the sodium hydroxide to have decreased during the first three minutes. Then again if the temperature of the air was very close to that of the NaOH, this would not be the case.
If I were to repeat the experiment I would place a plastic lid on top of the cup to act as further insulation. I would also perform it in a sheltered area where there was unlikely to be a change in external atmospheric conditions.
Another, less likely, cause of error may be inadequate stirring. If the solution was not mixed enough before and during the reaction, the thermometer might have remained in a region of lower acid concentration. I could minimise the risk of this occurring by ensuring I thoroughly stir the solution. If a plastic lid were used it would reduce the loss of heat promoted by stirring.
A smaller enthalpy change could also be caused by a lower concentration of acid than intended. This could occur if the solid acid was not entirely transferred into the conical flask. This would result in fewer bonds breaking and therefore less energy being released to the surroundings.
