Practical investigations
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This is about the dreaded thiosulphate experiment. The demonstration was to mix 50 cm3 of Na2S2O3 (0.4 mol dm-3) with 5 cm3 of HCl (2 mol dm-3) diluted with 20 cm3 of water. You have to justify concentrations and volumes for the plan. I've spent a long time trying to figure it out but I can't get it. The concentration of the Na2S203 is 0.4 whilst the HCl is 0.4 too (as 5/25 x 2). Does that mean anything? Or does the demonstration imply the total solution for Na2S2O3 should be 50 cm3, whilst for HCl, it should be 25 cm3 and use those volumes in the experiment?
Ulex replies
I’m a bit uncertain what is being sought here by way of justification. I will assume that you are given 0.4M thio and 2M hydrochloric acid and asked to produce a suitable mixture for giving an appreciable initial sulphur precipitate in a reasonable time. Several thoughts occur to me.
• It is normal practice to have one of the reactants (usually the acid) in considerable excess over the other. You can then vary the concentration of the thio without affecting the performance of the experiment in any other respect. If the acid and thio have the same concentration, the experiment becomes very slow when the concentrations reach low values. In this case, however, 0.4M seems quite a high concentration so perhaps it is not necessary to stick to the usual convention.
• When you mix two solutions together, the effective concentration of both of them changes. If they are mixed in equal volumes, it is easy to calculate the new concentration. If they are mixed in unequal volumes, the new concentration is a little trickier to determine.
• My recollection is (though I have not checked it recently) that mixing thio and acid both at 0.4M concentration would result in inconveniently quick precipitation.
• The volumes you quote seem to me to be inconveniently large so that you would use up a massive volume of both solutions if you were to do a run of several experiments with different concentrations.
It would be enormously helpful for us to see the wording of the original question!
You are to plan an experimental procedure leading to a graphical method to determine how the concentration of Na2S2O3 and HCl affects the rate of reaction. Explain how you would use suitable graphs to determine the rate equation for the reaction between Na2S2O3 and HCl. You are supplied with
1) 0.4 mol dm-3 of Na2S2O3
2) 2 mol dm-3 of HCl
Observe what happens when your teacher mixes together 50 cm3 of Na2S2O3 (0.4 mol dm-3) with 5 cm3 HCl (2 mol dm-3) diluted with 20 cm3 of H2O.
You will need to use this demonstration as a basis to justify the choice of
concentrations/volumes of each reagent used in your subsequent plan.
Effectively you are going to adapt Experiment 11.4 (p 256 in Nuffield Chemistry Students’ Book) except that you are going to use the basic method to study changes in concentration rather than temperature. Read the basic idea, including the bit about rate being inversely proportional to time.
As you’ve already worked out, after diluting the HCl it will be 0.4M.
You will need two sets of experiments:
1. Keep the concentration of the acid constant at 0.4M and vary the thio concentration by progressively diluting it. In the first experiment of the set you use 10 cm3 of each of the two 0.4M solutions. In the second experiment you use 10 cm3 of the 0.4M acid and 10 cm3 of a solution of thio made by taking 9 cm3 of the 0,4M solution and making it up to 10 cm3 with water. (What is its concentration now?) In the third and subsequent experiments you continue to dilute the thio in this way.
2. Keep the concentration of the thio constant at 0.4M and vary the acid concentration in the same way.
Work out the concentrations and rates of reaction. Plot graphs of rate against concentration for the two sets of experiments.
Carry out a risk assessment before starting any practical work, and check with your teacher.
You should acknowledge this help in any account which you write. Also give a reference to Experiment 11.4 in the Nuffield Chemistry Students’ Book if you refer to it.
Risk assessment
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updated: 29 April 2004
