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# K and the order of reaction watch

1. I'm doing my chemistry coursework, for which I'm finding the orders of reaction with repect to several reactants and then finding k.

The reaction is the bromide/bromate/hydrogen ions one. For bromate, my rate vs concentration graph was (roughly) first order, and I then drew a ln(concn of bro3-) vs time graph, which is meant to be a straight line with a gradient of -k, but its turned out as a slope, as i used experimental data to plot it. How am I meant to get the value of k from the slope of a curve?

Another graph I drew was that for H+ ions (2nd order). It was a graph of 1/(concn H+) vs time, for which k was 0.04 dm3mol-1sec-1. Can I use this value of k to help me get the value of k from my bromate curve, seeing as the two values of k should be the same?

Thanks if you can help
2. (Original post by *Georgine*)
I'm doing my chemistry coursework, for which I'm finding the orders of reaction with repect to several reactants and then finding k.

The reaction is the bromide/bromate/hydrogen ions one. For bromate, my rate vs concentration graph was (roughly) first order, and I then drew a ln(concn of bro3-) vs time graph, which is meant to be a straight line with a gradient of -k, but its turned out as a slope, as i used experimental data to plot it. How am I meant to get the value of k from the slope of a curve?

Another graph I drew was that for H+ ions (2nd order). It was a graph of 1/(concn H+) vs time, for which k was 0.04 dm3mol-1sec-1. Can I use this value of k to help me get the value of k from my bromate curve, seeing as the two values of k should be the same?

Thanks if you can help
Let say you have a general reaction, A + B ----> C
1) You change either one of concentration of A of B, then keep the other one constant, do at few different concentration, you can get the order of reaction wrt to that reactant(A or B)

2) rate eqn, rate = k [A]^a [B]^b

a and b are the respective orders.

3) To simplify my point is that, you can't get the rate constant without first knowing the orders of reaction wrt to both A AND B. Then it is just using your data plug in values into the rate eqn, then find k, it should remain constant at the same temperature. Though if it isn't it is up to you discuss this in your writeup.
3. (Original post by shengoc)
Let say you have a general reaction, A + B ----> C
1) You change either one of concentration of A of B, then keep the other one constant, do at few different concentration, you can get the order of reaction wrt to that reactant(A or B)

2) rate eqn, rate = k [A]^a [B]^b

a and b are the respective orders.

3) To simplify my point is that, you can't get the rate constant without first knowing the orders of reaction wrt to both A AND B. Then it is just using your data plug in values into the rate eqn, then find k, it should remain constant at the same temperature. Though if it isn't it is up to you discuss this in your writeup.
Even if I'm using loads of experimental data to draw graphs, instead of doing a few textbook questions? I drew the two graphs according to the table here: http://www.chm.davidson.edu/vce/Kine...dRateLaws.html
If I calculated k your way, I'd only be using the data from 1 run, which wouldn't make good use of all the data I have.
4. (Original post by *Georgine*)
Even if I'm using loads of experimental data to draw graphs, instead of doing a few textbook questions? I drew the two graphs according to the table here: http://www.chm.davidson.edu/vce/Kine...dRateLaws.html
If I calculated k your way, I'd only be using the data from 1 run, which wouldn't make good use of all the data I have.
True, but i think the one they have on their website make use of A ---> B

Their ways and eqns are of course valid for their reacting system,
but if you A + B ----> C

you can't apply these for your exp, as you mentioned that
"I'm finding the orders of reaction with repect to several reactants and then finding k.", these become different system, you'd have to find the orders, and from your set of data, you can easily work out a lot of k values from each set of exp conditions. From these, you can compare and contrast hopefully how similar they will be at the same temperature.
5. (Original post by shengoc)
True, but i think the one they have on their website make use of A ---> B

Their ways and eqns are of course valid for their reacting system,
but if you A + B ----> C

you can't apply these for your exp, as you mentioned that
"I'm finding the orders of reaction with repect to several reactants and then finding k.", these become different system, you'd have to find the orders, and from your set of data, you can easily work out a lot of k values from each set of exp conditions. From these, you can compare and contrast hopefully how similar they will be at the same temperature.
Ah, I hate it when I draw irrelevant graphs. Thanks for your help

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