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The Determination of a Rate Equation.

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A2- Assessed Practical 12 The Determination of a Rate Equation Jared Ching Planning: Background: When adding 50cm3 of Na2S2O3 to 5cm3 of HCl, there is a gradual colour change from transparent to a thick cloudy yellow colour. The yellow colour is a result of sulphur (S) forming: Na2S2O3 + 2HCL S + SO2 + 2NaCl + H20 The investigation is about the effect of changing the concentration of the respective solutions and find out the effect this has on the rate of the reaction. When hydrochloric acid is in excess to the sodium thiosulphate, the reaction may be different to when sodium thiosulphate is in excess. This will show the effect of concentration on the rate of reaction. We know that the reaction changes colour and eventually becomes so cloudy that it is like a solid, where no light can pass through it, this will be due to sulphur molecules becoming abundant in the reaction. Therefore this will be helpful in knowing when to know when the reaction is over. A commonly used method is to place a black "X" on a piece of paper underneath the container used for the reaction, when the "X" disappears, does not become visible, that is the point where all repeats of the experiment will be stopped. Several factors affect the rate of reaction, such as temperature and concentration. These factors are governed by the number of collisions between molecules occur, the collision theory. ...read more.


This is also shows that the sodium thiosulphate is always in excess, as the starting concentration for HCl is 0.2M and reduces down the table; in comparison the sodium thiosulphate is set at 0.4M. Safety Precautions: 1. Wear lab coat and safety goggles at all times 2. Do not ingest, get on your hands or put into eyes reagents. 3. Do not run in the lab 4. When attaching pipette to pipette fillers, ensure pressure is only applied at the end near the filler so the pipette is less likely to cause any injury if the glass were to break. Fair Test: * The experiment should be done on the same day if possible to ensure variable factors such as temperature is constant * Repeat each experiment twice to reduce human error * The same person must take readings, in particular when the "X" disappears, everyone has different vision so the "X" will disappear at different times for different people * Same equipment should be used to ensure greatest accuracy Results: When sodium HCl is in excess: Sodium Thiosulphate (cm3) Water (cm3) Hydrochloric Attempt 1 (sec) Attempt 2 (sec) Attempt 3 (sec) Average Conc. (cm3) (sec) (M) 35 0 10 17 17 16 16.67 0.4 30 5 10 17 18 18 17.67 0.34 25 10 10 22 21 22 21.67 0.29 20 15 10 25 24 26 25.00 0.23 15 20 10 32 31 32 31.67 0.17 10 25 10 47 47 51 48.33 0.11 5 30 10 121 116 118 118.33 0.057 Conc. ...read more.


* More repeats for the second method could have been done to ensure an accurate set of results. * A colorimeter could have been used to measure the change in colour, this would be far more accurate and improve results. * The same apparatus could have been used. * The equipment could be washed out and allowed to dry fully, the traces of distilled water may affect concentrations and cumulatively build up. For example, I noticed that sometimes after leaving a conical flask to stand for a while, there was about 3cm3 of water at the base of the flask. The results are quite reliable, but when considering all the faults and inaccuracies there is a fairly considerable margin of error within these results. Referring to the graphs, there is not a precise correlation, although clearly a linear graph, there are still points that look more like anomalies than slightly inaccurate results. However it could be argued that this investigation is based on what the individual sees as the "end" of the reaction, which determines the amount of time it takes for the reaction to complete. Therefore if the individual is consistent the results may still show a true representation of the real outcome, even if not completely accurate. I think the results show good relationship, especially when HCl is in excess and the concentration of sodium thiosulphate is varied, all points are very close together. On the whole the results prove the order of reaction with respect to the reagent, and even if results were made to be more accurate, the same outcome would arise. This makes the results accurate enough to justify the conclusions made. ...read more.

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