Doc Brown's Advanced A Level Chemistry

Advanced A Level Chemistry

Kinetics-Rates Part 7

Selected Case Studies of a variety of chemical reactions and their rate expressions

7.7 Acid – thiosulfate reaction e.g.

Na2S2O3(aq) + 2HCl(aq) ==> 2NaCl(aq) + SO2(aq) + S(s) + H2O(l)


Advanced A Level Kinetics Index


Case study 4.7 The acid catalysed decomposition of sodium thiosulfate

  • The reaction between e.g. dilute hydrochloric acid and sodium thiosulfate is a redox reaction catalyzed by hydrogen ions. I do not know of any other catalysts?

  • Its a typical 'rates' reaction at GCSE level to illustrate temperature and concentration factors or used as a coursework investigation. Its followed by the time it takes to form enough sulfur to obscure a black X marked on white paper. The method described in more detail on the How can we measure the rate of a chemical reaction? page.

  • It is possible to follow the reaction with a colorimeter due to the light scattering effect of the colloidal sulfur particles but the absorbance does not follow Beers Law and processing results is apparently difficult!

  • The reaction is ...

  • Na2S2O3(aq) + 2HCl(aq) ==> 2NaCl(aq) + SO2(aq) + S(s) + H2O(l) 

  • which for advanced level is much more appropriately written in the ionic form ...

  • S2O32–(aq) + 2H+(aq) ==>  SO2(aq) + S(s) + H2O(l) 

  • The thiosulfate ion on face value has an S=S bond, one S=O and two S–O bonds, but all four are 'merged' in the same delocalised pi bonding system.

    •  You could say that one sulfur is in the zero oxidation state and one in the +4 ox. state,

    • OR, perhaps 'safer' to argue, that on average each sulfur is in the +2 state (oxygen is –2, overall charge on ion 2–).

  • In the products, the oxidation states of sulfur are much clearer to define, +4 in SO2(aq) and 0 in S(s).

    • On the basis that two S's start off in the +2 state, you could argue that this is a disproportionation reaction, in which an element in an ion/compound/compound is simultaneously oxidised (+2 to +4) and reduced (+2 to 0).

  • You would expect that the rate might be controlled by the interaction of the negative thiosulfate ion and a positive hydrogen ion. You would expect the interaction of oppositely charged ions to have a relatively low activation energy, so in the rate expression:

  • rate = k[S2O32–(aq)]t[H+(aq]h, you might expect the order t and h to be 1, t=1 is quoted on the web.

  • but its likely to be at least a two step mechanism, so whether h is 1 or 2 or ?, I don't know? Whatever, the orders t and h can only be found by experiment.

    • S2O32–(aq) + H+(aq) ==> intermediate

    • intermediate ==> SO2(aq) + S(s) + H2O(l)

      • or intermediate + H+(aq) ==>  SO2(aq) + S(s) + H2O(l) 

      • or intermediate + H+(aq) ==>  intermediate 2 ==> SO2(aq) + S(s) + H2O(l) 

      • I don't know the details but you would expect the negative thiosulfate ion to combine with the 1–2? protons to form some intermediate that breaks down in one or more steps to give sulfur dioxide, sulfur and water.


Advanced A Level Kinetics Index


Revision notes for GCE Advanced Subsidiary Level AS Advanced Level A2 IB Revise AQA GCE Chemistry OCR GCE Chemistry Edexcel GCE Chemistry Salters Chemistry CIE Chemistry, WJEC GCE AS A2 Chemistry, CCEA/CEA GCE AS A2 Chemistry revising courses for pre–university students (equal to US grade 11 and grade 12 and AP Honours/honors level courses) case studies in kinetics: orders of reaction and rate expressions for the hydrochloric acid – thiosulfate reaction, acid catalysed decomposition

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Advanced A Level Kinetics Index

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