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To determine the standard enthalpy of formation of Magnesium Oxide using Hess Law.

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Introduction

Candidate Name: Candidate Number: Page | International Baccalaureate Diploma Program (IBDP) Session: May 2015 Chemistry HL Lab Report Lab Report Title: To determine the standard enthalpy of formation of Magnesium Oxide using Hess? Law. Criteria Assessed: * Data Collection and Processing (DCP) * Conclusion and Evaluation (CE) Candidate Name: Candidate Number: International School, Singapore AIM: To determine the standard enthalpy of formation of Magnesium Oxide using Hess?s law. INTRODUCTION: The objective of this experiment was to determine the change in enthalpy when one mole of Magnesium (Mg) reacts with half a mole of Oxygen (O2) to give one mole of Magnesium Oxide (MgO). The balanced chemical equation is as follows: Mg (s) + O2 (g) ? MgO (s) ---- ?HMgO f The reaction between Magnesium and Oxygen to form Magnesium Oxide is essentially the combustion of Magnesium and since every combustion reaction is an exothermic reaction, this reaction too is an exothermic reaction, i.e. it too will produce heat to the surroundings. In fact, the combustion of Magnesium is highly exothermic as it produces flames whose temperatures reach almost 2500oC (http://physics.stackexchange.com). At such high temperatures, a very bright white light is produced and if directly looked upon for long periods of time, the high content of ultra-violet radiation has the potential to damage unprotected eyes. Moreover, such high temperatures cannot be measured using a common thermocouple (K-type) so they need much more sophisticated setup of Ir-Rh thermocouples in an inert atmosphere. All these factors together make it extremely difficult to calculate the enthalpy of formation of MgO directly. Swiss-born-Russian scientist, Germain Henri Hess had come up with the idea of calculating the enthalpy of reaction of a certain reaction using an alternate set of stepwise reactions which would add together in such a way that it would give the same reaction. The enthalpy of reaction for each of these reactions can then be added together to give the enthalpy of reaction of the primary reaction (formation of MgO, in this case). ...read more.

Middle

As the temperature was rising, the bubbling was getting more and more vigorous. This indicates that increase in temperature leads to an increase in the reaction rate. Part X, Trial 2: Time elapsed (s) Temperature of solution (oC) ±0.25oC Time elapsed (s) Temperature of solution (oC) ±0.25oC 0 32.2 65 39.4 5 32.8 70 39.9 10 33.4 75 40.5 15 33.9 80 39.7 20 34.3 85 38.9 25 34.9 90 38.2 30 35.5 95 37.1 35 36.0 100 36.5 40 36.7 105 35.8 45 37.3 110 34.9 50 38.0 115 34.0 55 38.5 120 32.9 60 39.2 Max Recorded 40.5 Table 5: Data Collection: Part X, Trial 2 Qualitative Observations At the reaction site (calorimeter), the solution was bubbling. This indicated the production of Hydrogen gas. The origin of the bubbles was the surface of the Magnesium strip. As the temperature was rising, the bubbling was getting more and more vigorous. This indicates that increase in temperature leads to an increase in the reaction rate. Part Y, Trial 1: Time elapsed (s) Temperature of solution (oC) ±0.25oC Time elapsed (s) Temperature of solution (oC) ±0.25oC 0 32.5 65 34.2 5 32.6 70 34.4 10 32.8 75 34.5 15 32.9 80 34.1 20 32.9 85 33.8 25 33.2 90 33.7 30 33.5 95 33.4 35 33.6 100 33.2 40 33.6 105 33.0 45 33.7 110 32.9 50 33.9 115 32.7 55 34.1 120 32.7 60 34.1 Max Recorded 34.4 Table 6: Data Collection: Part Y, Trial 1 Qualitative Observations At the reaction site (calorimeter), the solution was not bubbling. This indicated that in this reaction, Hydrogen gas was not produced. In fact, only water and no gas was produced. As time went by, the solution was turning milky white. This was due to the production of MgO which is white in color and since it is insoluble in water, it turns water milky. Part Y, Trial 2: Time elapsed (s) ...read more.

Conclusion

Analytical balances have an absolute uncertainty of ±0.001g and that would result in a percentage uncertainty ten times lesser in magnitude. * The burette too could be replaced by a 15cm3 pipette which has an absolute uncertainty of much smaller magnitude. However, since the burette is not giving a very large percentage uncertainty, this improvement to the apparatus can safely be given the least preference. Only after investments in a better calorimeter and an analytical balance have been made, should one think to invest in a 15cm3 pipette. * Lastly, a great improvement to the procedure would be if a data logger software and a thermometer probe would be used to record temperature over time. By using this method, the need for graphical analysis is completely eliminated and with that the need to estimate the maximum temperature is also eliminated. A temperature logger uses a software to log temperature over time so the maximum temperature can be determined more precisely. A temperature logger would also eliminate human errors in the following ways: * Since temperature reading was required every 5 seconds, it was very difficult to accurately read the thermometer so quickly avoiding all possible parallax errors. In the total of 4 trials, 92 thermometer readings have been taken. It is likely that a few of them were taken in a haste where assuming a trend, a reading was estimated. Often times, the reading was taken up to a second later. * Stopwatch readings also had the element of human reaction time and for all 92 readings, this reaction time would differ slightly (on an average, decreasing slightly after each reading because of practice). Using a data logger, the need of a student to keep track of time would be eliminated. This way more groups could be made and rather than spending time on keeping track of time, time could be spent on gaining skills such as making a calorimeter or learning to use a burette etc. In conclusion, although the error was large, the results obtained demonstrate the applicability of Hess’ law in real life. ...read more.

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