Homework Help: Chemical kinetics - reaction rate

1. Dec 5, 2011

chizuru

1. The problem statement, all variables and given/known data
2HCl + Na2S2O3 -> 2NaCl +S + SO2 + H2O
Find the order of reaction with respect to S2O32- and H+

volume of 3.0M HCl was held constant at 2ml

2. Relevant equations
S2O32- + 2H+ -> S + SO2 + H2O
rate = k[Na2S2O3]m[HCl]n (see number 3.)

3. The attempt at a solution
i just started taking calculus so I'm having a hard time understanding most of the equations I find in the net and most of them skip the part of solving so I can't find enough examples.
I tried to graph the S2O32- part first

Concentration of reaction against time(left)
Rate of Reaction against Concentration of a Reactant(right)

it "looks" like the graph of a second order. could someone please tell me if it is correct and another way of solving it aside from graphing and some calculus? thanks in advance
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Dec 5, 2011

chizuru

and additional question, may i know the effect of having different volumes of water?

3. Dec 5, 2011

Staff: Mentor

Please explain what you plotted, to be honest it doesn't look like anything to me.

Edit: adding water just dilutes solutions.

4. Dec 5, 2011

epenguin

And "time for reaction to finish" doesn't really mean anything. It never finishes. Though I can make a guess, operationally what was it?

5. Dec 6, 2011

chizuru

Sorry for being unclear. The "time for reaction to finish" should be the "time elapsed for the solution to be completely cloudy".
For the graph at the left: the x axis is the time elapsed for the solution to be completely cloudy and the y axis is the number of moles/L.
For the graph at the right: the x axis is the number of moles/L and the y axis is the number of moles/(L * time)

6. Dec 6, 2011

epenguin

It is almost embarrassing. I am inclined to advise don’t waste clean thought on such a dirty experiment. Any calculus would certainly be overkill.

But if you really do have to write up something:
Do you have any idea of the concentration of product S at which the “solution becomes completely cloudy”? If that is just a small fraction of the Na2S2O3 and you know what it is, call it Ccrit, you could just about say that Ccrit/(time to reach Ccrit) is a velocity in moles.L-1s-1. And if you don’t know what Ccrit is then 1/(time to reach Ccrit) is proportional to velocity. This is something like your RHS fig turned sideways and reflected. Which looks roughly linear.
With the proviso that that is the vaguest roughest experimental endpoint imaginable.
And that if Ccrit is not comparatively small as said, forget trying to do anything at all.

And that you would have to clean up your presentation.
I don’t know why your table has 3 points and your graph 5.
Your vertical axis on left table can’t possibly be moles/L
You couldn’t find the order in HCl as you haven’t varied it.

I don't know if I got across I have rarely seen such a bad experiment, sorry.