KINETICS 7.7 The acid catalysed thiosulfate ion decomposition reaction precipitating sulfur

[Author ©  Dr Phil Brown PhD: Doc Brown's exam revision notes suitable for students of advanced pre-university/college advanced level theoretical-physical chemistry courses: kinetics of the acid catalysed decomposition of the thiosulfate [kinetics page updated RE-EDIT]

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.

• It is 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 following GCSE level pages

• How can we measure the rate of a chemical reaction?

• Effect on rate of changing reactant concentration in a solution/gas mixture

• Effect on rate of reacting on changing the temperature of reactants

• 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!

• This is NOT a particularly good experiment for advanced level chemistry students, it is difficult to get accurate results to determine any order of reaction.

The reaction between sodium thiosulfate and hydrochloric acid

• The reaction is ...

• (1) Na₂S₂O_3(aq) + 2HCl_(aq) ==> 2NaCl_(aq) + SO_2(aq) + S_(s) + H₂O_(l) 

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

• (2) S₂O₃^2–_(aq) + 2H⁺_(aq) ==>  SO_2(aq) + S_(s) + H₂O_(l) 

  • A correct ionic equation in acid solution.

  • See Appendix 1 for the half-cell reactions.

• The redox analysis for this reaction is NOT straight forward.

• However, you can say in the reaction, the following oxidation states do NOT change:

  • Na(+1), H(+1), O(-2), Cl(-1).

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

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

    • This is not strictly speaking true, the two sulfur atoms have unique and different electronic characters, but I'm using an average sulfur oxidation state

    • In the products, the oxidation states of sulfur are much clearer to define, +4 in SO_2(aq) and 0 in S_(s), which, to further complicate matters, is actually S₈ molecules!

    • On the basis that the two sulfur atoms start off in the +2 state, you should correctly argue that this is a classic redox disproportionation reaction, in which an element in an ion/compound/compound is simultaneously oxidised.

    • Oxidation: For sulfur average of oxidation state of +2 in the thiosulfate ion to +4 to give the sulfite ion/sulfur dioxide.

    • Reduction: For sulfur average of oxidation state of +2  is reduced to 0 in the elemental sulfur formed.

    • Overall change in oxidation states:  [2S(+2)] ==> [S(0)] +  [S(+4)]

    • See alternative redox (oxidation state changes) interpretation

    • where it is equated to [S(+5) + S(-1)] ==> [S(4)] +  [S(0)]

    • [...] indicates a species of sulfur - ion, molecule or atom.

The rate equation and orders of reaction for the acid catalysed decomposition of the thiosulfate ion

• 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 for the 'bimolecular' rate expression:

• rate = k[S₂O₃^2–_(aq)]^t[H⁺_(aq]^h

  • you might expect the orders of reaction t and h to be both 1.

  • t = 1 is quoted on the web. and found to be so in most reliable experiments (but not all).

  • h is quoted from 0 to 1 from internet sources.

  • It need to said that this is a very complex reaction, but for GCSE level students, you can do good semi-quantitative experiments to see the effect of changing acid concentration or the temperature of a constant solution composition.

• The reaction has been shown to be a multi-step complex mechanism, so what order h is I don't know?

  • I've come across references that indicates the order h could be 0–1 depending on the relative concentrations of thiosulfate and acid.

  • Whatever, the orders t and h can only be found by experiment and the mechanism is likely to be complex e.g.

  • (3) S₂O₃^2–_(aq) + H⁺_(aq) ==> '1st' intermediate 

    • (HS₂O₃^–, the hydrogensulfite ion is an obvious choice)

  • (4) other intermediates ==> SO₃^2–_(aq) or SO_2(aq) + S_(s)

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

• Found this link, typically it shows the order with respect to the thiosulfate ion is 1, but no mention of order for the hydrogen ion.

• It also quotes an 'uncited' and 'uncorroborated' complex mechanism at the end.

• https://www.flinnsci.com/api/library/Download/78da6c8204aa48a294bd9a51844543ad

• See also Appendix 2 for a link to another research paper on the reaction and some of its findings.

Appendix 1 The half–cell reactions for the acid catalysed acid–thiosulfate decomposition reaction

We can balance the reduction half–reaction as

(5)  6H⁺(aq) + 4e^– + S₂O₃^2–(aq) → 2S(s) + 3H₂O(l)

(here the sulfur is reduced from +2 to zero)

and the oxidation half–reaction as

(6) 3H₂O(l) + S₂O₃^2–(aq) → 2SO₃^2–(aq) + 6H⁺(aq) + 4 e^–

(here the sulfur is oxidised from +2 to +4)

The overall reaction is

(5)  + (6)  = (7) S₂O₃^2–(aq) → S(s) + SO₃^2–(aq)

Note that equation (7) does not seem to fit in with equation (2).

It depends on whether you quote the sulfur(IV) product as aqueous sulfur(IV) oxide or the sulfate(IV) ion.

They are connected by equation (8) SO₂(aq)  + H₂O(l)  SO₃^2-(aq)  +  2H⁺(aq)

and noting the regeneration of the hydrogen ion H⁺(aq) which fits in with the notion of it being an acid catalysed reaction.

This source quotes 1st order of reaction with respect to the thiosulfate, and zero order for the hydrogen ion.

Appendix 2 A major study of the thiosulfate–acid reaction from 1958 by Robert Earl Davis

https://pubs.acs.org/doi/abs/10.1021/ja01547a018

Is another complex study, rate expression

rate = k[S₂O₃^2–(aq)]^3/2[H⁺(aq)]^1/2

with two fractional orders of reaction, 1.5 and 0.5,

as opposed to rate = k[S₂O₃^2–(aq)][H⁺(aq)]

with no mention of 1st order for thiosulfate,

and was explained using a series of nucleophilic displacement reactions at the sulfur atom.

Radiochemical studies (using radioisotopes) have shown the two sulfur atoms retain their identity throughout the reaction..

It is suggested in this paper the reaction involve ions like [HS_nO₃]^–, where n = 1 to 9, after the reversible formation of the hydrogensulfite ion,

SO₃^2– + H⁺  HSO₃^–

the sulfite ion is then formed by reactions such as 

S₂O₃^2– + [HSO₃]^–  ==> [HS₂O₃]^– + SO₃^2–

this continues to eventually form [HS₉O₃]^–

which breaks down to give the sulfur as S₈ molecules

 [HS₉O₃]^– ==> S₈ + HSO₃^–