[InD_IamginE]

Hello there! New member here, so I'm sorry if I posted in the wrong sub-forum.

Right now I'm on my 2nd year of Chemical Engineering major and will be doing a small lab experiment to determine the Arrhenius constant, energy of activation, and order of reaction for reaction of Na₂S₂O₃ (Sodium thiosulphate) and H₂O₂ (hydrogen peroxide) in (assumed to be) adiabatic batch reactor. The kinetic is assumed to follow power rule where the rate is equal to k*C_A^a*C_B^b

To determine them I will be taking notes of the solution temperature every few seconds until the reaction finishes (signified by no further increase of temperature, as all of the reactants are already consumed) and then plot the data in a logarithmic graph.

The composition I will be using, according to the module. is the stoichiometric ratio between the reactants (Na₂S₂O₃ and H₂O₂), which will be gained through another small experiment.

My question is: why should we use the stoichiometric ration for the reactants?
Mathematically, determining the equation correlating to the T-t graph while using a non-stoichiometric feed could be done rather easily.  So why should I use stoichiometric ratio for the reactant?

My professor hinted that using stoichiometric ratio could have a benefit, but he didn't tell me what :/

Or is it purely to conserve reactant?

[mjc123]

If you begin with a stoichiometric ratio of reactants, this ratio will remain the same throughout the reaction, so you can express concentrations as
C_A(t) = C_A,0(1-x) and C_B(t) = C_B,0(1-x) where x(t) is the extent of reaction.
Then dx/dt is proportional to (1-x)^(a+b), and you should be able to determine the overall order of reaction a+b from your data. As you say "order of reaction" rather than "orders", I assume this is what you are after. If you want to determine the individual orders a and b, the usual way would be to do experiments where one reactant is in large excess, and therefore its concentration remains virtually constant, to determine the order of the other.

[InD_IamginE]

Thank you for the reply!

So I guess my post wasnt clear enough.

1 I do want to know each order, as in what is a and b. Still, I could get it from my current derived equation and using trial and error method of guessing what they are and determine the result of which combination of a and b produce the most accurate result.

2 Unfortunately for this reaction, which happens very fast, it is very hard to check concentration as a function of time, thus the use of temperature to "gauge" how far the reaction have proceeded. By using energy balance and some tinkering with power law i kinetic, I gain a logarithmic equation that could be used to gain the parameters I wanted (the usual ln(y) = mx + ln(c) stuff) with y as a function of system conductivity, dT/dt,a,b,and concentration of A (concentration of B was changed to a function of concentration of A by using the stochimetric ratio to make calculation easier),m being Ea/R, x being 1/T and c as a function of heat of reaction and Arrhenius constant.

Yes, I do realize that using stochimetric ratio of reactants do make the derivation of the formula much easier(C_B=stochimeric ratio*C_A), but my professor hinted an actual, technical advantage/benefit of using stochimetric ratio instead just putting whatever ratio of reactants we want.

Note that my experiment is a very small scale with the total volume of reactor is 100 mL.

[mjc123]

Doing a reaction with stoichiometric ratio will only give you a+b. To get a and b you will have to do other experiments.
If your temperature is changing over time, how are you going to determine k as a function of T? You get k by observing how concentrations change over time at a constant T. If T is varying, how do you disentangle the temperature effect from the concentration effect. (I just tried deriving an equation, it got horrendously complicated. But I don't discount somebody having solved it in a clever way.)