Advanced A Level Chemistry Advanced A Level Chemistry - Kinetics-Rates
revision notes Part 7
7.7 The acid – thiosulfate reaction
precipitating sulfur e.g.
+ 2HCl(aq) ===> 2NaCl(aq) + SO2(aq)
+ S(s) + H2O(l)
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study 4.7 The acid catalysed decomposition of sodium
between e.g. dilute hydrochloric acid and sodium thiosulfate is a
redox reaction catalyzed by hydrogen ions. I do not know of any other
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
How can we measure the rate of a chemical reaction?
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
The reaction is
+ 2HCl(aq) ==> 2NaCl(aq) + SO2(aq)
+ S(s) + H2O(l)
which for advanced
level is much more appropriately written in the ionic form ...
+ 2H+(aq) ==> SO2(aq) + S(s)
+ H2O(l) (the correct ionic equation in
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).
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.
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:
you might expect the order t and h to be both 1.
The reaction has been shown to be a
multi-step complex mechanism, so what order h is I don't
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.
+ H+(aq) ==> '1st' intermediate
(HS2O3–, the hydrogensulfite ion
is an obvious choice)
other intermediates ==> SO32–(aq)
or SO2(aq) + S(s)
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.
Appendix 1 The
half–cell reactions for the acid–thiosulfate reaction
https://people.bu.edu/straub/courses/demomaster/colloidalsulfur.html I quote,
with some editing ...
We can balance the reduction half–reaction as
and the oxidation half–reaction as
(i) 6H+(aq) + 4e– + S2O32–(aq)
→ 2S(s) + 3H2O(l)
(here the sulfur is reduced from +2 to zero)
The overall reaction is
(ii) 3H2O(l) + S2O32–(aq)
→ 2SO32–(aq) + 6H+(aq) + 4 e–
(here the sulfur is oxidised from +2 to +4)
(i) + (ii) = (iii) S2O32–(aq) → S(s)
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
Is another complex study, rate expression
rate = k[S2O32–(aq)]3/2[H+(aq)]1/2,
two fractional orders of reaction, 1.5 and 0.5,
as opposed to rate = k[S2O32–(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 [HSnO3]–,
where n = 1 to 9, after the reversible formation of the hydrogensulfite
the sulfite ion is then formed by
reactions such as
+ [HSO3]– ==> [HS2O3]–
this continues to eventually form
which breaks down to give the
sulfur as S8 molecules
==> S8 + HSO3–
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