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Determination of a Rate Equation

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Determination of a Rate Equation Introduction: The rate of reaction is the rate of depletion of reactants or the formation of a product during a chemical reaction. It is expressed by units of concentration over the time taken for the reaction to take place. (Avogrados, 2010) Aim: To plan and carry out an experiment involving a graphical method to determine how the concentration of each component affect the rate of reaction in this reaction: 2HCl + Na2S2O3 --> 2NaCl + SO2 + S + H2O Background Information: Sodium thiosulphate and hydrochloric acid are both clear colourless solutions. They react together according to this equation: 2HCl + Na2S2O3 --> 2NaCl + SO2 + S + H2O When these solutions are mixed together, a yellow precipitate, sulphur, is produced. This causes the mixture to appear cloudy. The faster the rate of the reaction, the faster the solution appears cloudy. There are four basic factors that affect the rate of a reaction namely: Temperature, concentration/ pressure (in gases), physical state/ surface area of the reactants and the presence or absence of a catalyst [(Think Quest, 2008) and (WebChem, 2005)] Basic Idea: * Two sets of experiments will be carried out. In each case, the concentration for a single reactant will be varied * All other factors should remain constant * Conical flask containing the ...read more.


Once the "X" is no longer visible stop the stopwatch and record the time. 7. Step 1 to 6 will be repeated for varied concentrations of Na2S2O3 and HCl as in the tables above. NOTE!! This was per the plan, except that there was a shortage of Na2S2O3 and a new solution had to be made. (This new solution was used from the 3rd experiment of the second trial onwards) Analysis Experiment [HCl] mol/dm3 [Na2S2O3] mol/dm3 Time taken for precipitate to hide 'X' Avg. Time 1/time Trial 1 Trial 2 Trial 3 1 0.32 0.4 19 20 18 19 0.052 2 0.32 0.2 35 32 31 32.6 0.031 3 0.32 0.1 42 44 45 43.6 0.023 4 0.16 0.4 26 22 25 24.3 0.041 5 0.08 0.4 29 31 30 30 0.033 From these two graphs I can deduce that when varied concentrations of a single solution with respect the other is plotted against the reciprocal of time 1/t, a straight graph forms in both cases. This is an indication that both Na2S2O3 and HCl are first order reactions. Hence the rate equation for this chemical reaction will be: r= k [Na2S2O3]1 [HCl] 1 Evaluation Anomalies In my first trial of the second experiment it took 35 second for the precipitate to fully 'cover' the marked paper which was slightly slower as opposed to 31 or 32 seconds which was observed in the 2nd and 3rd trials. ...read more.


Overall accuracy and reliability of techniques Even though there is a possibility that numerous amounts of the errors listed above may have been made during the course of the experiment, I think that the overall accuracy of this method was good as not too many obvious anomalous results were obtained. Modification to minimize errors Ultraviolet rays are an influential factor of the rate of reaction. The sun light that lit up part of the working area may have influenced the rate of reaction. It would have been better for the experiment to be done with the blinds closed. The temperature of each solution could have been measured before measuring out the exact quantity required for each experiment to prevent the loss of solution. Similarly, making sure to let the measuring pipette touch the solution to ensure accuracy in the volume of solution. Significant error I think most significant error I made in this experiment was in the procedure of diluting my reactants and the apparatus I used to do this. Instead of using a 50ml and 10ml measuring cylinder and a 25ml measuring pipette to dilute my reactants, I should have used 5, 10 and 25ml measuring pipettes only in order ensure accuracy in the total volume and concentration of the final diluted solution. ...read more.

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3 star(s)

This experiment has a good evaluation, but a poorly written method and contains a very high level of possible error in the results.
This piece of work is 3*

Marked by teacher Brady Smith 20/02/2012

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Response to the question

The student shows a clear understanding of the topic and the work is clearly, logically and fluently presented. Their response is very explicit through the layout of the work and this helps examiners clearly identify the marks. It is commendable ...

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Response to the question

The student shows a clear understanding of the topic and the work is clearly, logically and fluently presented. Their response is very explicit through the layout of the work and this helps examiners clearly identify the marks. It is commendable that the student has referenced every source they have used to help them reach the answer to the set question - this is usually omitted and can consequently put the student in danger of writing plagiarized work. The diagram of apparatus supported the student's written work and likewise the graphs did too. However, it is important to remember that sometimes it might be required to reason the steps in the experiment. Whilst the student has done this very well, at times they have omitted this. For example, it is best to labour points such as repeating the experiment for reliability and then taking an average and so on. Such 'stating the obvious' sometimes makes it easier for the examiner to award marks as it is clearly showing that you understand the topic.

Level of analysis

The level of analysis (i.e. giving reasons to why the steps are done or guiding the examiner through the steps) is very good. The student presents this in a logical order which helps their structure of the work. This in turn improves their clarity and furthers the fact that they understand the topic and more importantly, the background knowledge to it. The student has reached a suitable conclusion from the graphs (and the experiment) and has documented it well.

Quality of writing

The spelling, grammar and punctuation are fine. The technical terms were used with fluency and were well integrated into the piece of work. However, I suggest perhaps a glossary of terms would add to show the examiner that the student understands the terms used. Yet, the one slip up was writing "Na2S2O3" instead of using the subscript which they had done otherwise. Such mistakes are rather damaging to the overall impression of the piece of work as it is one of the basics of chemistry. This flags up the importance of proof reading as such mistakes should have been picked up in proof reading the work. However, overall, this piece of work was completed to a high standard and I cannot give a lower level simply for the one slip up.

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