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Chemistry Internal Assessment Hesss Law

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Introduction

Chemistry Internal Assessment DCP and CE “Hess’s Law” 2014 Raw data table: Observations: * The solution with MgSO4 a white precipitate formed * In the experiment using MgSO4 the thermos got warm * The solution with MgSO4.7H2O turned a cloudy white * In the experiment using MgSO4.7H2O the thermos got cooler Measurement Mass (g) ±0.001 Volume –measuring cylinder (mL) ±1 Mass of MgSO4 3.801 Mass of MgSO4.7H2O 7.958 Volume of H2O 50 Full Equations of Reactions 1) MgSO4(s) → Mg2+(aq) + SO42-(aq) 2) MgSO4.7H2O(s) → Mg2+(aq) + SO42-(aq) + 7H2O(l) Literature Values Constant Value Source Specific Heat Capacity of Water 4.18 J g-1K-1 http://www.sphsgator.net/sphsteachers/domanskil1/databooknew.pdf A Raw Data Table Showing the Relationship between Time Elapsed and the Temperature of the Reaction of Hydrous and Anhydrous Magnesium Sulphate using a logger pro temperature sensor. Time (seconds) ± 0.1 seconds Anhydrous MgSO4 Temperature (°C) ±0.1°C Hydrated MgSO4 Temperature (°C) ±0.1°C 0 20.7 17.3 10 20.7 17.3 20 21.8 16.8 30 23.3 15.9 40 24.4 15.7 50 25.3 15.6 60 26.1 15.6 70 26.6 15.7 80 27.1 15.7 90 27.4 15.7 100 27.7 15.8 120 28.0 15.8 130 28.0 15.9 140 28.1 15.9 150 28.1 15.9 160 28.1 16.0 170 28.1 16.0 180 28.1 16.0 190 28.1 16.0 200 28.0 16.0 210 28.0 16.1 220 28.0 16.1 230 28.0 16.1 240 28.0 16.1 250 28.0 16.1 260 27.9 16.1 270 27.9 16.1 280 27.8 16.1 290 27.7 16.2 300 27.7 16.2 310 27.7 16.2 320 27.6 16.2 330 27.6 16.2 340 ...read more.

Middle

+ 7H2O(l) ? MgSO4.7H2O(s) 37% Conclusion Theoretical or Accepted Result Experimental or Actual Result Percentage Error 37% Calculated random error Our percentage error or deviation from the expected value was 37%, which is much larger than the calculated random error of 2.46%. This implies that the experiment was significantly affected by systematic errors. Evaluation General, as the percentage error for our two experiments was relatively similar at 38% and 31.3% I can assume that the systematic errors effect both experiments reasonably evenly. Limitation Explanation Effect on results Significance of Limitation Improvements Heat Loss The thermos warmed up slightly when we added MgSO4 to the water. This suggests that heat energy, released as a result of this experiment was dissipating into the surroundings: the thermos, the temperature probe, and the air. This would have lowered the value of ?T and therefore lowered the value of ?H. Although we extrapolated our line to minimize the effect of heat loss this still would have been very significant, and is the most probably cause of our percentage error. This experiment could be improved, if the experiment was carried out in a more insulated container, which would allow heat to escape at a much slower pace. An airtight lid would also not allow heat to be lost to air. Heat Gain The thermos cooled down slightly when we added the MgSO4.7H2O to the water, which suggests that heat was gained from the surroundings. ...read more.

Conclusion

Though this wouldn?t have affected the accuracy of the results. It would have decreased the precision of our results. This may have had a significant effect as more repeats may have decreased our percentage error. If I was to repeat the experiment I would ensure that I did at least three repeats with each chemical. The surface area of anhydrous and hydrated crystals is different. The anhydrated MgSO4 was a powder while the hydrated MgSO4 was crystalline. This meant that the surface area of the chemicals was different. The larger surface area of the hydrated MgSO4 would have meant that it took longer time to react, as there would be less collision. Thus more heat could have been gained during the experiment causing ?T to decrease and therefore lowered the value of ?H This would not have been very significant. If I was to repeat this experiment I would powder the hydrated MgSO4 with a pestle and mortar prior to using it. Ensuring a more consistent surface area. Incomplete Reaction The MgSO4 may have not completely reacted in the water. As the right amount of bonds would not have been formed/broken, more heat could have been gained/lost during the experiment causing ?T to decrease and therefore lowered the value of ?H This may have been very significant but we have no evidence that an incomplete reaction occurred. We could stir the mixture faster by using a magnetic stirrer and stir bar, which was kept at 700rpm. ________________ [1] Experiments in Seventh Form Chemistry- a Laboratory Manual. Dept of Education New Zealand, 1978 ...read more.

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