In class we conducted an experiment where we timed reactions between sodium thiosulphate and hydrochloric acid. We kept the concentration of the $\ce{HCl}$ constant, while changing the concentrations of sodium thiosulphate.
We then graphed this data to discover there is a linear relationship between the concentration of thiosulphate and 1/time, indicating the it is a first order reaction. This means in the rate determining step there will be one thiosulphate ion.
In the analysis question it asks you to come up with a theoretical mechanism. I'm having alot of trouble with this because the first order relationship found on the graph indicates there is 1 thiosulphate ion in the rate determining step, but the overall equation also has one thiosulphate ion.
Can anyone help me out, I have tried multiple methods but none seem to add up to the overall equation.
$$\ce{2 HCl (aq) + Na2S2O3 (aq) -> 2 NaCl (aq) + SO2 (g) + S (s) + H2O (l)}$$