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Clock Reactions

hi everyone im doing the bromine clock reaction, but im a bit confused to why sulfuric acid is used in this experiment.

Also does methyl orange turn pink because of the acidic solution

im also confused to how colorimetry works. i really need some help with this
Yes I believe that methyl orange is opink in acidic solution.
H2SO4 acts as a source of H+ for the redox reaction. have you done half equations yet? BrO3- requires H+ in the reaction if you take a look at the eqution about 3 posts up.
(edited 13 years ago)
Reply 2
chemicalguy
Yes I believe that methyl orange is opink in acidic solution.
H2SO4 acts as a source of H+ for the redox reaction. have you done half equations yet? BrO3- requires H+ in the reaction if you take a look at the eqution about 3 posts up.



if i increase the amount of h2so4 does this cause the solution to become colourless quicker or slower?

yes ive done half equations but i dont understand how the half equation for this reaction works
Reply 3
For the iodine clock reaction there is a method of doing a titration - that I used - I cant remember what it was called unfortunately and it I no longer have a copy. I think it was done on the basis of performing the reaction and then stopping it by adding a certain chemical, although you must make sure it is quenched properly, after a set period of time, i.e 10 secs, 20 secs etc and then titrating this with another chemical to see how much of the iodine has reacted. This is however very time consuming - as you have to do the experiment at least three times for each length of time for each concentration/other variable and then you have to titrate each one.

Also word of advice - do your calculations all on excel in a nice table then copy and past into word - I didnt and payed for it dearly as one small mistake at the begining means you have to change everything! It may seem obvious now - but it wasnt at the time.
In the bromine clock reaction you reduce bromate ions to bromine, and oxidise bromide ions to bromine. The protons 'mop' up the former ions oxygens.

5Br-(aq) + BrO3-(aq) + 6H+(aq) ---> 3Br2(aq) + 3H2O(l)

The point of the bromine clock is to determine the rate equation right? So you should see for yourself if increasing proton concentration increases rate.

The methyl orange is pink due to pH yes. You add phenol to mop up the bromine molecules you produce to form tribromophenol... once all the phenol is used up the bromine produced will bleach the methyl orange and the solution will decolourize.
Reply 5
Hi, I am also doing the bromine clock but I am so confused. Like I don't actually understand what it is I am supposed to be doing or why? :frown: Can someone please simply explain to me what it is I have to do and why? Thanks.
NSoni
Hi, I am also doing the bromine clock but I am so confused. Like I don't actually understand what it is I am supposed to be doing or why? :frown: Can someone please simply explain to me what it is I have to do and why? Thanks.


You have to measure the rate of reaction by seeing how long it takes for the colour to change.

The time is inversely proportional to the average rate.

You can then vary one of the concentrations of the reactants (keeping all others equal) to find the effect this has on the rate.

You can express the rate as:

Rate = k[A]x


Where 'A' is the component whose concentration you have varied. The idea is to work out 'x', which is called the order of reaction with respect to 'A'.

Once you have done this for one component you can do it again for the others (one at a time)

Once you have determined the orders with respect to all of the components you can work out the rate constant 'k'.

If you have enough time you can then start all over again at a different temperature, find the new 'k' and from this, using the Arrhenius equation, determine the activation energy of the reaction.
Reply 7
do you know where I can find the order with respect to phenol in this reaction?

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