give reason ----a yellow ppt is observe on addition dil.hcl to sodium thiosulfate
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Answer:
The reaction between sodium thiosulphate(Na2S2O3) and hydrochloric acid (HCl) is usually the reaction chosen by most teachers to explain to their students the concept of kinetics and reaction rate. In the beginning, both compounds are clear, but when mixed together they form a yellow precipitate – solid sulphur. The rate at which the solution turns cloudy demonstrates the speed at which the two compounds are reacting with each other.
What happens when sodium thiosulphate and hydrochloric acid mix?
The reaction between sodium thiosulphate and hydrochloric acid is as follows:
Reaction 1: Na2S2O3 + 2HCl -> 2NaCl + SO2 + S + H2O
Typically, rate of reaction is measured by simply measuring the time it takes from adding the two compounds to when the precipitate is formed. The easiest way to visualise this point is by marking the bottom of the beaker and placing it over a piece of white paper. This marking is visible for a while, but as the reaction progresses, the opaque solution makes it difficult to see. The clock stops counting when the marking is no longer visible. It is interesting to see that changing the concentration of either compound or the temperature has an impact on how long it takes for the precipitate to form (see below).
However, it’s not as easy as it seems…
Like most things in chemistry, a simple reaction hides how exactly the two compounds mix together. This example is no exception and the reaction between sodium thiosulphate and hydrochloric acid is not as easy as it seems…
In fact, the decomposition of sodium thiosulphate in the presence of hydrochloric acid is significantly more complex that suggested by Reaction 1. The reaction is acid-catalysed, which means that the acid concentration is an important factor influencing when the reaction equilibrium is achieved. The reaction undergoes several intermediate steps, producing HSnO3-, which in turn will continue to react with further S2O32- to generate polymeric ions containing multiple sulphur atoms. When the sulphur chain is long enough – at S8 – it closes on itself to form a ring of elemental sulphur, which then precipitates.
Reaction 1: Na2S2O3 + 2HCl -> 2NaCl + SO2 + S + H2O
Step 1: Na2S2O3 -> 2Na+ + S2O32-
Step 2: HCl -> H+ + Cl-
Step 3: S2O32- + H+ -> HS2O3-
Step 4: H-S-SO3- + nS2O32- -> H-S-(S)n-SO3- + nSO32-
Step 5: H-S-Sn-SO3- -> H+ + S--Sn-SO3-
Step 6: S--S7-SO3- -> S8 + SO32-
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