Practical investigations
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I am analysing my results concerning the effects on the rate of reaction of varying the concentration of hydrochloric acid added to sodium thiosulphate. I added 1 to 6 cm3 of 2M HCl to 25 cm3 of 0.4 M sodium thiosulphate, with the total volume of all reactants being kept at 60 cm3 using distilled water. The graph of rate against volume of HCl for my results levels off, when I was expecting HCl to be of first order in the rate equation. I am completely at a loss as to explaining why this occurred. Is it an experimental error on my part, or is something happening chemically? Thoughts of a possible limiting factor have crossed my mind, though not quite sure what could be acting as the limiting factor.
Ulex replies What a very interesting situation! I think I can see what the problem might be.
For each run of the experiment, the initial concentration of sodium thiosulphate is 0.4 x 25/60 mol dm-3, i.e. 0.17 mol dm-3. If you are to find the order with respect to the HCl, it is important that the concentration of the sodium thiosulphate should be in excess and should remain constant during the run of the experiment. When you used 1 cm3 of 2M HCl, the initial HCl concentration was 2 x 1/60 mol dm-3, i.e. 0.033 mol dm-3. This means that the sodium thiosulphate was around 5 times the concentration of the acid – not really enough, but not too terrible. However, when you used 6 cm3 of the HCl, the two concentrations were almost the same and the sodium thiosulphate was no longer in excess.
With such dilute solutions, the concentrations vary during the precipitation of the sulphur. In the first run, only the acid varies much in concentration, but in the last one both the acid and the sodium thiosulphate vary. Although you are using more acid, which increases the rate, you are effectively using less thiosulphate, which decreases the rate – hence the levelling off. That’s my theory and I’m sticking to it!
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updated: 25 January 2006
