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Topic: the iodine clock reaction  (Read 42572 times)

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morgan

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the iodine clock reaction
« on: February 21, 2005, 03:50:05 PM »
I have done the iodine clock lab experiement (also occasionally called the primary salt effect if anyone knows it by that) and I am at a complete loss as to how to work up the data to get the rate constant k. In the lab it assumes that the concentration of iodine is constant and so we are looking at persulphate. I have the amounts of persulphate used for several solutions of differing concentrations and the time it took for the blue to appear once all the sodium thiosulphate was used up. What can I graph or what do I use to calculate k??

Demotivator

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Re:the iodine clock reaction
« Reply #1 on: February 21, 2005, 08:56:47 PM »
rate = k[persulfate]^n

first,  determine the order n in persulfate from the data. eg, if rate doubles as [persulfate] doubles, n=1. If  rate quadruples, n=2, etc.
Then, plug in the value for [persulfate] and rate for each run and solve for k. They should be similar and an average for k is calculated.

Offline Donaldson Tan

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Re:the iodine clock reaction
« Reply #2 on: February 21, 2005, 09:18:14 PM »
how about plotting a graph of log rate against log concentration?
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

Demotivator

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Re:the iodine clock reaction
« Reply #3 on: February 21, 2005, 09:59:17 PM »
how about plotting a graph of log rate against log concentration?
Ok, that would produce a slope equal to the reactant order and intercept of logk:
log(rate) = log(k[A]^n) = log(k) + nlog[A]

highchemstudent

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Re:the iodine clock reaction
« Reply #4 on: April 05, 2005, 11:00:26 PM »

 Yum i am lost on how to start a bog i think that is what this stuf is call. ok i am a home schooled High school stundent (that long) and at a local collage i am doing a chemistry lab to get a feel for collage level class. monday we will be doing experiments on clock reaction and i can not find anything in my high school level book so if any one can explain it to me some how so i can know what to expect on monday so thanks  :)

Offline Donaldson Tan

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Re:the iodine clock reaction
« Reply #5 on: April 05, 2005, 11:46:20 PM »
the idea of clock reaction has its grounds in some trivia calculus. for example, consider the reaction between thiosulphate and dilute hydrochloric acid in a glass flask. This reaction produces yellow sulphur. The bottom of the flask is labelled with a big "X". The X disappears as sulphur is produced. The "X" disappears when the concentration of sulphur exceeds a critical value (call x).

Often when the graph of reactant concentration is plotted against time, it is found that gradient is approximately constant during the short initial leg of of the reaction. Since reaction rate is the gradient, we can say that the reaction rate is thus x/t where x is the critical value whereby the reaction mixture becomes sufficiently opaque that "X" disappears. This is basis why reaction rate is said to be indirectly proportional to the time taken for the "X" to disappear.

« Last Edit: April 05, 2005, 11:47:30 PM by geodome »
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

jelly_jelly

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Iodine clock with peroxodisulfate, using thiosulhate as the clock
« Reply #6 on: April 07, 2005, 09:29:01 AM »
Hey, my aim is to determine the rate constant and activation enthalpy of the reaction, so I carried out a few experiments, e.g. vary the concentration of iodide ions; vary the concentration of peroxodisulphate and one of the experimental runs at a range of temperatures. I know that the activation enthalpy should be around 48KJ/mol but my results were far off that. Does anyone know what values of the time to change color with respect to changes in temperature and concentration??

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Re:Iodine clock with peroxodisulfate, using thiosulhate as the clock
« Reply #7 on: April 07, 2005, 07:59:01 PM »
you are trying to measure the reaction rate between iodide and perosulphate, most probably catalysed by Fe2+/Fe3+. This reaction is usually slow because it involves the collision of 2 like-charged ions.

thiosulphate removes the iodine produced by the reaction between iodide and perosulphate. when all the thiosulphate is used up, then the reaction mixture turns brown. Hence, how long it takes for the reaction mixture to change colour depends on how much thiosulphate you used. If you are measuring the initial rate, then don't add alot of thiosulphate in the mixture.
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

Clay

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Another Iodide Clock question
« Reply #8 on: April 08, 2005, 06:05:03 AM »
Hi, My class just did an experiment with Iodide Clocks whereby the concentrations of Iodide were variables but the thiosulfate, persulfate and starch amount were constant. Temperature change and the addition of a catalyst were more sections of the  experiment.

We're expected to write a prac report on the experiment identifying rate orders and the rate constant but i don't understand what information i need to put in the report. The teacher said to include in our hypothesis Quantitative Data of the change in iodine order but i don't understand what the orders are. Also once i've reached the results section of the report i cannot figure out how to calculate the rate constant from the information or whatever else is meant to be included in the results section or the discussion.

Can anybody help me with this problem and shed some light on what it is i need to include and how to find out the constants and what is needed to be graphed. (3 graphs, leading to classification of order of reaction)

Offline Donaldson Tan

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Re:the iodine clock reaction
« Reply #9 on: April 08, 2005, 06:58:08 PM »
clay: i merged all the iodine clock reaction topics together. gather your required information from the posts.
"Say you're in a [chemical] plant and there's a snake on the floor. What are you going to do? Call a consultant? Get a meeting together to talk about which color is the snake? Employees should do one thing: walk over there and you step on the friggin� snake." - Jean-Pierre Garnier, CEO of Glaxosmithkline, June 2006

Offline Twobe23

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Re: the iodine clock reaction
« Reply #10 on: January 25, 2007, 12:53:18 AM »
I'm not sure If I'm doing this right when you plot log(rate) vs log[A] how do you calculat log(rate)? when I punched in log and the rate I calculated the answer came out to be negative. I think I'm doing something wrong.

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