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 mixed reactants will be placed on a paper marked with an “X”
Time taken for the precipitate to form and hide the “X” will be measured and recorded.
The experiment will be repeated for accuracy

Apparatus:

2M Hydrochloric Acid
0.4 M Sodium Thiosulphate
Distilled Water
Thermometer
Stopwatch
Measuring Cylinders ( 10ml+ 50ml)
Measuring pipette
Conical Flask (100ml)
Marker
(Access to a fume cupboard)

Method:

Dilution of solutions to lower their concentrations:

When diluting a solution, it is important to know that we are not losing any fraction of the solvent by mixing it with distilled water. The mass of the solvent remains the same in the total volume or the total moles of the solvent remains constant.

Hence, initial moles = final moles where the definition of moles is molar volume ( mol/L)- (Email. H. Kasaini)

Therefore the following equation will be used to calculate the water and acid ration for dilution:

M1V1 = M2V2

To make 25cm3 of HCl- Originally ...

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Method:

Dilution of solutions to lower their concentrations:

Hence, initial moles = final moles where the definition of moles is molar volume ( mol/L)- (Email. H. Kasaini)

Therefore the following equation will be used to calculate the water and acid ration for dilution:

M1V1 = M2V2

To make 25cm3 of HCl- Originally 2M

To make 50 cm3 of Na2S2O3 – Originally 0.4M

Dilute 2M hydrochloric acid; measure 4cm3 of HCl and 21cm3 of distilled water in separate measuring cylinders (10 an 50ml).
Mix these together in a 100ml conical flask.
Measure 50ml of Na2S2O3 with a measuring pipette and place in a 250ml conical flask. (this flask should be placed above a white paper clearly marked with an “X”)
Measure the temperature of both reactants.
Pour the diluted HCl into the Na2S2O3 and start the stopwatch immediately.
Observe the formation of precipitate from above the flask (As in the sketch above). Once the “X” is no longer visible stop the stopwatch and record the time.
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

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. This may have increased the average time that was calculated using these three results.

Similarly in my 2nd trial of my 4th experiment it took 22 second for the precipitate to fully ‘cover’ the marked paper which was slightly faster as opposed to 26 and 25 seconds which was observed in the 1st and 3rd trials. This may have decreased the average time that was calculated using these three results.

Overall accuracy of graph and result

The plotting of the results is accurate to at least three decimal places. If more concentration variations where used, more points could have been plotted hence increasing the overall accuracy of the final graph.

Sources of error

Procedure

Some (one or two drops), of the reactants were lost, when using the thermometer to measure the temperature of the solutions before mixing them together.
Difficulty in juggling between starting the stopwatch with one hand while pouring HCl into the Na2S2O3. (In one experiment the stopwatch was jammed and there was a delay of at least 2-3 seconds).
At the time of the experiment, a large amount of sun light lit up the working benches and very little shady space was available to work on.
Not letting the tip of the measuring pipette touch the solution when done pouring each time.
The inaccuracy may have occurred when using the pipette filler as I experienced some difficulty in using it. It is very easy to make an inaccurate measurement when using the pipette filler as it is hard to keep the meniscus of the liquid at the 25cm3 point on the pipette.
Dilution of reactants may not have been accurate

Measurement

When diluting each solution (a 50ml measuring cylinder for the water, 10ml cylinder for HCl and 25ml measuring pipette for Na2S2O3) there may have been some inaccuracy.
Slight spillage of solution may have occurred when mixing the two acids together or while diluting individual solutions therefore slightly lowering the total volume of the solution.

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.

Bibliography:

Think Quest,2008. Kinetics: Factors Affecting Reaction Rates [Online]
Available at <http://library.thinkquest.org/C006669/data/Chem/kinetics/factors.html>
[Accessed 7March 2011]
WebChem, 2005. Factors that Affect the Rate of a Reaction [Online]
Available at < http://www.webchem.net/notes/how_far/kinetics/rate_factors.htm>
[Accessed 7 March 2011]
Kasaini, H.K., ([email protected]), 9 March 2011. Chemistry Assignment. Email to H.Kasaini ([email protected])
Avogrado, 2010. Rate Equation [Online]
Available at < http://www.avogadro.co.uk/kinetics/rate_equation.htm>
[Accessed 9 March 2011]
Chemistry GCSE from Gondar Design, 2006. Rates of Reaction [Online]
Available at <http.purchon.com/chemistry/rates.htm>
[Accessed 9 March 2011]
Pictures :

<http://fcmoodle.glam.ac.uk/file.php/18/WJEC_Zipped_files_folders/Zipped_Files/1.3_Temperature_D_Bevan/page_03.htm>

<http://www.drbateman.net/gcse2003/gcsesums/chemsums/rates/pracC5.02.htm>

Teacher Reviews

Here's what a teacher thought of this essay

Brady Smith

20th of Feb 2012

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*

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Peer Reviews

Here's what a star student thought of this essay

crystalclearmagic

26th of Mar 2012

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.

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.

Response to 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.

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