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If I was using this method (http://www.rsc.org/learn-chemistry/wiki/Expt:The_effect_of_concentration_on_reaction_rate) however used pre-prepared concentrations of sodium thiosulfate solution, which intervals would be more suitable, 0.1M, 0.2M, 0.3M, 0.4M or 0.5M, 1M, 1.5M and 2M? Also, what concentration of Hydrochloric acid should be used (I've seen a variety of suggestions elsewhere). This is for a secondary school practical. Thank you.

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There's a nice demonstration of the experiment performed by Annis Hapkiewicz.

She uses a 0.15 M $\ce{Na2S2O3}$ stock solution. She prepares 5 beakers with 50 40, 30, 20 and 10 mL of the stock solution filled up to a total volume of 50 mL with destilled water.

This corresponds to thiosulfate concentrations of 0.15, 0.12, 0.09, 0.06, and 0.03 M.

I couldn't figure out the concentration of the hydrochloric acid used, but the times for the appearance of the sulfur cloud (disapperance of the cross) were given.

The range is between 20 s and 5 min.

I'd probably go with the 2 M HCl for safety reasons and perform the experiment with the thiosulfate concentrations given above. Check the times and adjust the concentrations if necessary.

You don't want it to to fast to be measured by the kids, but you don't want the highest concentration to take half an hour.

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