Determining order of reaction please help

[123aleksandra]

HL chemistry... why why i took this course...

well anyway i need to determine the order of reaction with respect to H2O2 for the reaction with KI in acidic solution

And i have completely no idea how to calculate this.

My teacher asked for help explained me that though I had 6 for the first semester now I became completely brainless and she is not going to waste her time on such student.

Encouraged by the teacher I tried a few times took three different tutors and still nobody can explain me how to do this.

I begin you please help me

[sweetnsimple786]

You don't know how to calculate given a data table or you don't know how to set up an experiment to find the order?

When you have H₂O₂ in water with KI, you'll get H₂O₂ + 2I +2H⁺ --> 2H₂O + I₂

So you'd set up a chart with trials in one column, the concentration of H₂O₂ in the next column and concentration of I in the next column and the reaction rate in the last column. If you have nice numbers, you can easily figure out what the order is. That is, if the concentration of one reactant doubles and the concentration of the other reactant is constant in the one trial [as compared to the concentrations in another trial], then looking at the change in rate will tell you the order of the reactant whose concentration changed. If the rate doubles, you've got a first order. If the rate doesn't change, the order is 0. If the rate squares, you've got 2nd order. If it is 9 times the original rate, that's third order, and so one.

If the numbers aren't so nice, you say rate = k[ A]^xy [x and y are the orders & k is some constant & [ ] is the concentration of the reactant]

So you compare two trials.

rate1/rate2 = [A1]^x[b1]^y/[A2]^x[b2]^y = [A1/A2]^x[b1/B2]^y

You want to choose trials where one of the concentrations is held constant. If you don't have a trial like that, then you need to compare two other rates and have a system of equations deal. Also, you know that if you have a 1st order, then you have to plot ln [ A] on the y axis and time on the x to get a straight line. If you have a 2nd order, you have to plot 1/[ A] on the y and time on the x to get a straight line. For zero order, I think you just plot [ A] versus time. Does that help?

[123aleksandra]

It might help if i know anything about order of reaction, but as for right now I have an experiment involving beaker with distilled water, H2SO4 0.1 mol, starch KI 0.1 mol, H2O2 and the time in which the blue color of the solutions reappeared after adding 2cm3 of 0.05 mol Na2S2O3 until the amount of added Na2S2O3 was equal 20cm3. I obtained in this manner 10 readings of time and have completely no idea what to do with them.

The second part of this investigation was: determine the exact concentration of H2O2 as follows. Pipette 10 cm3 of approximately 0.1 M H2O2 into erlenmeyer flask, add 25 cm of 0.1 H2SO4, 2 cm of starch and 15 cm of 0.1 mol KI and titrate the mixture against the 0.05 mol Na2S2O3 using starch as indicator. repeat this twice.

I know that at this stage I should be able to do this but really I have no idea how and there is no other place I can turn for help so please could you explain me what should I calculate and how?

[sweetnsimple786]

I haven't experimented like this. However, I think this might help you: http://www.wpi.edu/Academics/Depts/Chemistry/Courses/General/kinperoxide2.html

And when you say mol, you probably mean molar. molar expresses concentration while mol expresses quantity.

[Vahl]

As sweetnsimple786 explained you will have to make a table of the different results in the first lab:

Initial conc. KI------------Initial conc. H2O2-------------------Rate

x --------------------------y -----------------------------------something#1 mol dm-3 s-1

2x--------------------------y------------------------------------something#2 that is doubled something#1 if the order with respect to KI is 1, or quadrupled if it is 2 etc.

x---------------------------2y-----------------------------------something#3 same as with KI

This works easily if you have nice round numbers. I am basically repeating sweetnsimple, try and read it in your textbook once again ... Your description of the second experiment is a little fuzzy, try to write down the equation of the reaction taking place to see what you are titrating to measure.

If they are equimolar the amount of moles that you titrated into the solution is the amount of moles of H2O2 present.

Edited March 14, 2010 by Vahl