[vJames]

Hi all, I'm a high school student and I've just made my account here because I need a little bit of help. Chemistry is my favourite subject and I really want/need to know about analyzing my data.

As the topic is on reaction rate, I picked the reaction between sodium thiosulphate and hydrochloric acid and the effects of concentration on reaction rate. It was a very common experiment among the students this year and the past year and was a reaction that provided results that gave a lot of information to talk about in the EEI (as I've heard) to show the teacher my investigation of the reaction.

The basic method involved drawing a cross on a piece of paper and putting a beaker above it. Next, two known concentrations of sodium thiosulphate and HCl were poured into a beaker and the time was measured for the sulfur precipitate to form to a level where you could no longer see the cross on the paper.

Now I understand that the use of the cross and paper is to just ensure that when the concentration is changed in later tests, that you have a consistent point where you know the reaction is at. I understand that the sulfur colloid does not mean that the reaction is necessarily completed when you cannot see the cross, its just an indicator for reference. We conducted the experiment with 5 different concentrations of both HCl and sodium thio, these were 0.2, 0.4, 0.6, 0.8 and 1.0M. So we had 25 tests at different moralities of each and conducted 3 trials at each concentration combination, giving us 75 total trials.

I'd read online and in textbooks about the reaction order of experiments and the rate equation. My teacher had told us to plot our concentrations against time and then with ln(concentration) vs. time and finally 1/concentration vs. time to determine our reaction order with respect to the concentration of HCl and sodium thiosulphate.

I read online to figure out why we used the natural log (ln) and the inverse of the concentration and stumbled across the differential rate laws and their integrated forms. Ultimately, I wanted to determine the reaction order and show some worthwhile graphs. We found HCl to fit the ln conc. vs. time graph with the greatest accuracy and the 1/concentration vs. time graph for the sodium thio. If I'm correct, these signify a first order and second order reaction with respect to each reagent.

Jumping ahead I realised that the gradient would be equal to -k for zero/first order reactions and k for second order reactions. I understood the differential and integrated rate laws and then planned to make a few extra graphs of reaction rate vs. (concentration)² to support my findings. I then attempted to determine the reaction rate of each concentration test when I realised that we can't actually calculate reaction rate...

Sorry for writing up so much text and I know it's stupid that I did not see this earlier, I just hope that my position is understood.

With the data I have, are the only graphs I can make; concentration vs. time, ln(concentration) vs. time and 1/concentration vs. time?
Does this mean that the rate constant, k, cannot be calculated, neither can the reaction rate since I do not know the final concentration of the products/reactants?
Finally, is there any high-school level way of still calculating reaction order?

I really need some guidance on what I can do with my data.

Thanks

[Borek]

Short version - you did experiments on kinetics of HCl and Na₂S₂O₃ reaction measuring time required for the cross to disappear. You think the data you have allow determination of the reaction order, but not the real reaction rate, and you ask if your assessment of the situation is correct.

Yes.

[vJames]

Thanks, is there anything else that I can graph besides those three I mentioned and do their gradients have any significance (equal to the rate constant, k, or such)?

[mjc123]

First, learn the difference between molarity and morality - though hopefully you can use both at the same time!
Now, as I understand it, you did 75 experiments, and for each experiment you know the initial concentrations of HCl and thiosulfate, and the time taken to reach a certain extent of reaction (when the sulfur obscured the cross). You do not know the concentrations at this endpoint, or at any other time after the beginning of the reaction. So you cannot plot a concentration-time graph (or log conc or 1/conc or anything else) and the integrated rate equations are no use to you.
Now we have to assume (I don't know it for certain, but it is a necessary assumption for you to get any results out of your data, and for this to be a worthwhile teaching lab) that the cross disappears at an early enough stage of the reaction that, to a first approximation, the reactant concentrations have not significantly changed, and the reaction rate is equal to the initial rate over this period. Then the time to reach this point, t_e, is inversely proportional to the initial rate. Now suppose you have a rate law
Rate = k[A]^α[B ]^β...
Then 1/t_e = const.[A]₀^α[B ]₀^β... (We don't know what const is because we don't know how the absolute concentrations are varying).
So to find the order with respect to A you plot ln(t_e) vs ln([A]₀) for a set of experiments where [B ]₀ is held constant, and the slope is -α.
You cannot determine k from the data available to you.

[vJames]

Haha, sorry for that mistake.

mjc123, I've read over your reply a number of times and I think I understand what you are saying.

I used this website below as a reference when completing my experiment and I was wondering do the graphs they produce have any significance? You are talking about graphing ln(t_e) vs ln([A]₀) whereas they just graph 1/t_e vs conc.

https://www.flinnsci.com/media/622120/91860.pdf

The report talks about reaction order frequently but can the reaction order be calculated from what they have graphed or only from the graph you propose?

[mjc123]

If the reaction is first order in [A]₀, then the graph of 1/t_e vs. [A]₀ will be a straight line. Observation of a good straight line through the origin is diagnostic of first order. I don't think the t_e vs. [A]₀ graph is very useful - it's not obvious to the eye whether the curve is 1/x, or 1/x², or whatever. You could do a curve fitting, but it's better to do it with a linear graph. The advantage of the log-log graph is that it's linear whatever the order, and you can just get the order from the slope.

[vJames]

Thanks for the help, I'm definitely in a better position on this now.