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9701-51-OctNov-09

General comment/s:

The first part of this paper was a bit of a BEAST. Unless you were familiar with this particular version of a "clock reaction" you would easily get confused and spend lots of time trying to figure it out.

There's a Cu2+/Cu+ I2/I- S2O32-/S4O62- reaction that can be similarly confusing. Probably the best way to avoid this is to familiarize yourselves with these more difficult reactions. The syllabus only contains a limited amount of information, the chance of similar questions being asked previously on different exam papers will be quite high. This is one great reasons to really go through past years papers as much as possible, so that you see these cases and can handle them better.

It also asks about "independent" and "dependent" variables. Your syllabus (2015) has these words in page 55 about half way down, as well as in other places on pages 57,58 and 59. It is NOT mentioned in the usual list of learning requirements on page 17 - 45.

If you haven't read the syllabus fully, you really should do it now. There is much more to the syllabus than just a list of things to be learned. Read chapter 7 - about laboratory work. It pretty much tells you exactly what is expected of you and should remove (or reduce) any "shock" you might get when doing a practical paper. The same applies to the 'Learner Guide' which contains many tips as well as specific learning points.

In this experiment, what's happens here is you are doing a version of an initial rates experiment. You may remember from your theory that the initial rate is given by the gradient of the tangent on a concentration vs. time curve. The tangent to the curve is drawn at time = zero seconds and the gradient of the tangent is most steep at t=0 for obvious reasons --> it's when the concentrations are at their highest so reaction will be at its fastest then it just begins. Rate = k [A]x[B]y ...etc

It would be beneficial to recall that these curves can be obtained in a manner of ways, including (most efficiently) 'real-time' ( / dynamic) methods, using equipment that constantly measures some property of the reaction mixture, examples of equipment being a spectrophotometer, pH probe, conductivity probe, frictionless syringe (for volume changes) etc. But other methods, such as sampling - usually leading to a titration, can de done to obtain data points for the curve.

In this case however the requirement set out in the question will not involve us producing a [ ] vs. time curve on which we'd draw a tangent to it at t=0 seconds. Instead we will measure a single rate value over a time period for a different set of starting concentrations. Values of rates change over the course of a reaction (the gradient of the tangent at any particular point in time over which the experiment, will change changes depending on which point of time we use, i.e. where on the cure we draw the tangent. So if we measure rate over a period of time, then what we are actually measuring is the average rate over that time period. This information is still useful and can be used to achieve orders of reaction similar to what can be done with initial rates data. Also, if the period of time isn't too great from when the reactants are brought into contact with each other, then the average rate over this time period will, in many cases, will not be so different from the actual initial rate value (we would still be dealing with the initial steep part of the [ ] vs. time curve).

Rate = -dc / dt, i.e. the rate = - change in concentration / change in time. = k [A]x[B]y...etc.

The concentrations we deal with in the rate equation are reactant concentrations. And it's convention for delta (i.e. small change) values to be calculated as (final state) - (initial state). For reactants, final values are less than initial values, so dc values are always negative values, but it's convention to report rates as positive numbers, so we simply multiple the dc/dt term (which would ne negative) by -1 which will make the value of the rate a positive number :)

Regarding our time interval, the time at which we start timing is obvious - the instant all the reactants are mixed with each other. We need a point to stop timing to get our interval. In this case, we will stop timing when the blue-black coloured starch-iodine complex appears.

What about the dc term? We will be following the K2S2O8 species for this dc value. The initial number of moles used can be calculated as we know the starting concentration of the stock solution and how much volume to use in the reaction. We can calculate how much of the K2S2O8 will remain behind when the blue-black colour appears as its dependent on the amount of potassium thiosulfate, K2S2O3 present (which we can calculate), and the total reaction solution volume. Evaluating the dc term is settled.

We use these values in the same we would use initial rates values. In initial rates method, we did various experiments and took initial reactant concentrations with their corresponding initial rates values in one experiment and compared them with a different experiments initial concentration and its corresponding initial rate value, while holding all the other variables constant (not an absolute essential thing to do but makes life much much easier). In this practical exercise, we will be using the measured concentration change (a (C value, rather than an actual instantaneous calculus considered dc value) over a measured time period (a (t value, rather than the actual instantaneous calculus considered dt value). Then we will compare the values between various experiments to deduce the rate equation for the K2S2O8 reactant.

The function of the S2O32- ions are to instantly 'mop up' any I2 produced) so that we get a delay in the appearance of the blue-black starch-iodine complex, which allows us to get a measurable time period, for our calculation. The amount of S2O32- can be initially set (as it is in this practical exercise) so as to get a reasonable time delay.

In general, the greater the concentration of persulfate ions, S2O82-, the faster I- will be consumed, generating 'temporary' I2 molecules more quickly {which are instantly removed by thiosulfate, S2O32-, ions}, Hence greater concentration of persulfate causes faster consumption of thiosulfate ions meaning the point at which I2 begins to build up in the mixture prodcuing the blue-black colour, happens more rapidly. Decreasing persulfate ion concentration causes the colour to appear more slowly.

The fifth line of this practical is not very well written. In my opinion, it should say... "If sodium thiosulfate and starch are present in the reaction mixture, the blue-black colour of an iodine-starch complex will appear suddenly after some period of time. This occurs when all of the S2O32- is consumed and is no longer able to remove I2 from solution, and this gives the indefinable point on the reaction"

The experiment is find how rate depends on K2S2O8.

(a) This answer uses the statement found immediately above the question, but they use 'sneaky' wording. The don't say "concentration of S2O82- is halved", but they disguise that fact by instead saying it is diluted by half. So concentration drops by half and the rate of reaction drops to half also. Hence rate is proportional to [S2O82-]1

The next part of the question, to my shame I misread. I only half-read it and expected it to say "explain your reasoning" :( and then I went into "auto-answer" mode justifying the "Rate is proportional to [S2O82-]1" statement saying "As concentration halves and rate halves" but the question isn't what I imagined it. It asks about the particles., i.e. asking what's happening on the 'particle' level. So they are asking about kinetic theory. The answer should have been as conc. halves only half the number of collisions with E ( Ea occur so rate halves.

(b)

The independent variable - this is the factor those influence on a system we are trying to find. In simple experiments, all other variables are usually fixed and only the independent variable is changed, allowing us to conclude that the change in the result purely due to the change in the independent variable.

The dependent variable - this is the factor what will change depending on the chance in the independent variable.

To give a biological example, an independent variable may be the type of snake poison. The dependent variable would be how long it takes a bitten animal (of equal species, body mass, age, general health status etc) to die after being bitten by a particular snake.

1 (c). In my opinion, I'm not keen on the answer in the mark scheme. It's true the first reaction consumes I- and that if the amount of I- is small, it would run out, but it's easy to ensure this wouldn't happen simply by ensuring a large excess of I- ions are present in the beginning. The I- doesn't have to be recycled2, even though it is of course environmentally better to do so, but how enough I- ions are secured is of no importance.

I originally answered this question with all my emphasis on the I2 species. Saying something like "If I2 didn't react with S2O32-, the blue-black complex would immediately begin to form, no time interval could be recoded, but if I2 is consumed, a sharp blue-black complex colour change can be observed after some time period which can be recorded"

However, I don't see any marks to be awarded from my answer even though I think it is far better than the answer the author of the question wanted.

The examiners report says " Only a very few candidates were able to identify the need for the iodine concentration to remain constant." - so it turns out I am not one of them, :( , but I still cannot see the need to keep the I- constant (surely the I- could vary as long as there was enough still to react with S2O82- , and the most common trick in chemistry to ensure a concentration is constant is to use a large excess. So at the moment I cannot explain the awarding of this particular mark. Boo hoo.

1(d) There's a possible 'trap' here. A single solution of S2O82-(aq) is mentioned, and it has a fixed concentration. Consequently so some students may get 'locked into' not being able to see the concentration of this solution can be changed because distilled water is provided, hence the S2O82-(aq) can be diluted. This brings another point... the concn of the solution given is the maximum conc you can use. You cannot start making up your own higher concentrations for use in the experiments.

They are helpful in giving you a set of quantities for the first experiment. You should take note of the total volume used and try to keep this volume the same for ease of comparison between other experiments. You could vary the total volume but you'd then lose the ability to do relative comparisons and instead would almost certainly have to calculate concentrations and then do absolute comparisons in order to draw conclusions. Relative comparisons are almost always much easier to do than absolute ones. So ALWAYS give statements in which you've processed each and every one of the guide points.

So, an answer similar to mine below is required (it generally follows the guidelines given):

• A burette should be used throughout to dispense volumes, minimizing volume based errors. (guideline #3)

comments: Accuracy is always an issue. Experiments using volumes should always use accurate equipment. Either a pipette or a burette. When working with volumes between 50cm3 and 0cm3, burettes have the advantage over pipettes in that they can easily deliver different volumes, avoiding the need to have a large range of pipettes. Measuring cylinders are only used in qualitative tests, not quantitative experiments like this one, or when excess reagents are to be measured, although some opinion holds that measuring cylinders can be used whenever they will not cause any significant error. But having to evaluate whether it will cause a significant error is time consuming in tests/exams. This drain on time is avoided by using burettes.

• Conduct a series of experiments where the concentrations of K2S2O82- is lowered by reasonably spaced degrees. This is achieved by decreasing the volume of K2S2O82- used each time by 5ml in what will ultimately be an unvarying volume of reaction mixture. A suitable range in giving volumes to be used for the K2S2O82- would be 40, 35, 30, 25, 20, 15 and 10 ml to be dispensed into a beaker topped up with distilled water to a total volume of 40ml where necessary, to be used in a total volume of 90cm3 of reaction mixture. (guideline #1, #2)

comments: CIE's guideline #1 mentions concentration, so address their point specifically. If you just start mentioning differing volumes as a way to get different concentrations, you may forget to specify that a fixed volume for the reaction mixture must be used in order to achieve these different concentrations. If the total reaction mixture volume was to vary, you could, even by using different volumes of S2O82- still have the same concentration of persulfate. Forcing yourself to address the issues in the guidelines will help you pick up marks more quickly and with less risk of losing marks.

A good range of volumes, pretty much covering the range available (40 to 0 ml) are selected to give a good spread of results. It would be a poor choice to select volumes of 40, 39.8, 39.6, 39.4, 39.2 and 39 ml to use. Similarly, efforts to obtain a reasonable number of data points should be done to address experimental precision (reproducibility of results). A typical A-level kinetics should be planned to yield an approximate minimum of 5 points through the available range of values the experiment can be conducted over. The more points the better, but a minimum of 5 usually suffices.

• In a separate beaker, and for each experiment, dispense out 20ml of KI, 20 ml S2O32- solution and 10 ml of starch solution. This beaker will have the contents of the other beaker (containing S2O82- solution) mixed into it eventually (guideline #2, #4)

comment: We need to fix these quantities, as we only want to vary the persulfate, so see how its change affects the result.

• The temperature of each beaker must be the same before mixing. Allow to equilibrate to room temperature before mixing. (guideline #4, #6)

comments: Two of perhaps the most large sources of error have been addressed and steps taken to minimize them.

• Start timing the moment the one solution is poured into the other solution. Maintain this order of mixing, i.e. if the persulfate beaker was first poured into the iodide beaker then keep mixing the solutions in this way. (guideline #4, #5)

• The instant the blue-black colour appears, stop timing and record the time elapsed. This is the dependent variable. (guideline #5)

• Calculate the number of number of moles of thiosulfate used and take this value away from the moles of persulfate used. This gives the moles of persulfate.

(e) The amount of thiosulfate, S2O32-, determines when the blue-black colour appears. It has an important role in the experiment, hence it is necessary to ensure the same constant amount is dispensed so it will not have any effect on the time data obtained.

(f)

|Experiment |Beaker A |C |D |Time, t, |Rate of reaction, |

| |(Beaker B uses fixed vols as | | |taken for |-dc/dt |

| |discussed in the method) | | |colour change|Units: mol dm-3 s-1 |

| | | | |/ seconds | |

| |Vol S2O82-|Vol H2O |Total volume|Initial moles of |Amount of S2O82- (aq) used / | | |

| |(aq) / cm3|/ cm3 |/ cm3 |S2O82- (aq) / mol |mol | | |

|1 |40 |0 |40 |0.20 x (vol of |(This experiments Column C | |[This experiments D value /|

| | | | |persulfate used in |value) - 1/2 (0.01 x 90/1000) | |(90/1000) ] / this |

| | | | |this experiment/1000) | | |experiments t value |

|3 |30 |10 |40 |0.20 x (vol of |(This experiments Column C | |[This experiments D value /|

| | | | |persulfate used in |value) - 1/2 (0.01 x 90/1000) | |(90/1000) ] / this |

| | | | |this experiment/1000) | | |experiments t value |

|4 |25 |15 |40 |0.20 x (vol of |(This experiments Column C | |[This experiments D value /|

| | | | |persulfate used in |value) - 1/2 (0.01 x 90/1000) | |(90/1000) ] / this |

| | | | |this experiment/1000) | | |experiments t value |

|5 |20 |20 |40 |0.20 x (vol of |(This experiments Column C | |[This experiments D value /|

| | | | |persulfate used in |value) - 1/2 (0.01 x 90/1000) | |(90/1000) ] / this |

| | | | |this experiment/1000) | | |experiments t value |

|7 |10 |30 |40 |

|1 |0.0040 |0.00394 |0.99 |

|2 |0.0038 |0.0039 |1.0 |

|3 |0.0034 |0.00406 |1.2 |

|4 |0.0040 |0.00398 |1.0 |

|5 |0.0038 |0.0039 |1.0 |

|6 |0.0037 |0.0037 |1.0 |

|7 |0.0038 |0.0038 |1.0 |

|8 |0.0046 |0.00409 |0.89 |

|9 |0.0040 |0.00402 |1.0 |

|10 |0.0038 |0.0038 |0.99 |

|11 |0.0041 |0.00413 |1.0 |

|12 |0.0041 |0.00409 |0.99 |

Data suggests one Zn for every one I2, Hence ZnI2 formula is confirmed.

c) Anomalous results are student 3 and student 8's zinc to I2 ratio.

d) This question is worth two marks. I am not sure why. Anyway... Exclude the anomalous results and take the average moles of Zn used and the average moles of I2 used to deduce the formula..

(e) Student 3 and student 8 both put in the correct approximate masses in. So the Zn:I2 ratio is going to be anomalous due to processes done before the readings were taken (Its usual that the errors will come about due to limitations that come with the procedures used, and not human error - It's usually assumed the person doing the experiment is trained enough to avoid human errors like accidentally spilling zing on the balance etc, so human errors are the last source of errors you should think of. Try procedural errors first of all.'

One is likely to have lost some zinc on pouring off the ethanol, causing an apparent decrease in the mass of remaining excess Zn (student 3). The other is likely to have not heated the tube enough causing an apparent larger mass of Zn remaining for column D (i.e. student 8).

(f) Yes, most independently results are consistent and results are far away from any other possible whole number ratios.

(g) The decanting (pouring off of the ethanol solution and ethanol washing. Better would be to filter the mixture on a pre-weighted filter paper and washing the paper with pure ethanol allowing the filter paper to dry then recording the mass and calculating the mass of Zn remaining.

(h) Use a calibrated colourimeter to determine I2(aq) content.

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