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Reaction Order from Concentration-Time Data

I'm having problems with a couple of homework questions my teacher set. One of them is:

The reaction of hydrocyanic acid with water is described bythe equation
HCN(aq) + 2H2O(l) = NH4+ (aq)+ HCO2- (aq)

The concentrations of HCN at various times follow (see chart).

Make appropriate plots (see graph) of these data to:

(a) determine the order of the reaction with respect to HCN
(b) determine the value of the rate constant

I think (a) is first order, but I'm not sure of how to go about (b). I was given a chart with concentrations of HCN at different times to plot the graph.
(edited 8 years ago)
graph.png19.4KB
chart.png4.8KB
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Hey, just to check, what are the labels for your axes?
Original post by enaayrah
Hey, just to check, what are the labels for your axes?



Time for the x-axis and concentration for the y-axis.
Original post by enaayrah
Hey, just to check, what are the labels for your axes?


I've attached the chart that was given.
chart.png4.8KB
Original post by etherealinsanity
Time for the x-axis and concentration for the y-axis.


Original post by etherealinsanity
I've attached the chart that was given.


Thanks, was there any information regarding H2O?

Your part a seems about right, did you work it out by finding whether the half lives were constant throughout the reaction?

If there's no info on H2O, you can just assume that rate=k[HCN] with respect to HCN

Then rearrange to find k. Use your conc/time graph to find the rate at a certain concentration :smile:
Original post by enaayrah
Thanks, was there any information regarding H2O?

Your part a seems about right, did you work it out by finding whether the half lives were constant throughout the reaction?

If there's no info on H2O, you can just assume that rate=k[HCN] with respect to HCN

Then rearrange to find k. Use your conc/time graph to find the rate at a certain concentration :smile:


Sorry for taking so long to get back to you. No, there was no information on H2O. If I use the graph to find the rate, do I just draw a tangent to the graph? I've always had trouble with that.

Thank you for all of your help so far. You've made everything much clearer. :smile:
Is this A-Level? The only way I could go about doing this is to assume the integrated rate law [HCN](t)=[HCN]oe^-kt where [HCN]o is the initial concentration of HCN. Take logs to give ln[HCN(t)]=ln[HCN]o - kt . Plot ln[HCN(t)] against t and see if you get a straight line, if you do (within error) it is first order w.r.t HCN. From this, the magnitude of the gradient of the log graph is the rate constant.

Another way is do find instantaneous rates given the data you have, but that could be messy given the uneven data
(edited 8 years ago)
Original post by etherealinsanity
Sorry for taking so long to get back to you. No, there was no information on H2O. If I use the graph to find the rate, do I just draw a tangent to the graph? I've always had trouble with that.

Thank you for all of your help so far. You've made everything much clearer. :smile:


Yeah, just draw a tangent to the curve at any concentration, make a nice big triangle and find the change in time and change in concentration :tongue:

Then the rate is change in concentration/change in time

I'm glad this helps, I have a test on this on Monday and this should be a bit of revision :mmm:
Original post by Protoxylic
Is this A-Level? The only way I could go about doing this is to assume the integrated rate law [HCN](t)=[HCN]oe^-kt where [HCN]o is the initial concentration of HCN. Take logs to give ln[HCN(t)]=ln[HCN]o - kt . Plot ln[HCN(t)] against t and see if you get a straight line, if you do (within error) it is first order w.r.t HCN. From this, the gradient of the log graph is the rate constant.

Another way is do find instantaneous rates given the data you have, but that could be messy given the uneven data


I don't think it's A-level. There were questions about instantaneous rates and differential laws, which required worded answers earlier on on the workseet. My teacher wanted us to do the questions so that we're ready for whatever turns up on the A2 paper next year.

It's just not making a lot of sense to me.
Original post by enaayrah
Yeah, just draw a tangent to the curve at any concentration, make a nice big triangle and find the change in time and change in concentration :tongue:

Then the rate is change in concentration/change in time

I'm glad this helps, I have a test on this on Monday and this should be a bit of revision :mmm:


Okay. Thank you again and good luck on your test! I hope you do great. :smile:
Original post by etherealinsanity
I don't think it's A-level. There were questions about instantaneous rates and differential laws, which required worded answers earlier on on the workseet. My teacher wanted us to do the questions so that we're ready for whatever turns up on the A2 paper next year.

It's just not making a lot of sense to me.


Yeah this definitely seems like a differential rate law question, just being given conc and how it varies with time is a bit nasty. As Enaayrah said, you can just draw triangles and find gradients that works fine too. I just wanted to check if you had done integrated rate laws just incase it wasn't an A-Level topic :tongue:
Original post by Protoxylic
Yeah this definitely seems like a differential rate law question, just being given conc and how it varies with time is a bit nasty. As Enaayrah said, you can just draw triangles and find gradients that works fine too. I just wanted to check if you had done integrated rate laws just incase it wasn't an A-Level topic :tongue:



Thank you for your help :smile:. I think I will just draw triangles and use the gradients to find the rates, and possibly have a talk with my teacher about the other pieces of homework he's set us tomorrow.

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