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#### shohaib712

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##### Help with iodine clock reactions please
« on: February 05, 2019, 07:53:35 PM »
Im currently self teaching myself alevel chemistry and at the moment im very stuck on clock reactions. Please could someone help me with the following.

I dont get the markscheme for question 2:

Question:

Markscheme:

What does it mean by the second point that "rate will be proportional to 1/t". Firstly shouldnt it be the initial rate thats proportional to 1/t and I dont understand how that is relevant to answering the 2nd question. Also i dont understand how changing the volume would affect the amount of iodine being removed, shouldnt it be the concentration of thiosulphate ions that would remove the iodine from teh solution, not the volume?

Also why do we assume that the rate calculated from the color change is the initial rate? Do we just assume that the rate of the reaction is constant therefore 1/t is aproximately the initial rate. And does the longer the time period taken to identify color change of solution from colorless to blue/black results in less accurate inital rate value?

Sorry for so many questions im just confused by this topic.

#### mjc123

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##### Re: Help with iodine clock reactions please
« Reply #1 on: February 06, 2019, 04:49:46 AM »
Yes, it's initial rate, and the assumption, as you correctly suggest, is that (at least over the time before the iodine colour first appears) the rate is constant. In order that the time should be inversely proportional to the rate, the amount (number of moles) of iodine removed by the thiosulfate must be the same in each case, therefore the amount of thiosulfate must be the same. Assuming that you have a stock solution of a given concentration, that means the volume you add must be the same (moles = concentration x volume).

The quantities of reagents are chosen so that the thiosulfate runs out while the extent of the iodine-producing reaction is still relatively small; the concentrations of the reagents in this reaction haven't changed much and the rate is approximately constant over the time t. Theoretically, the inaccuracy ought to be the same for all reactions - except that for very fast reactions, the uncertainty in your measurement of t increases the inaccuracy.

#### shohaib712

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##### Re: Help with iodine clock reactions please
« Reply #2 on: February 06, 2019, 06:49:29 AM »
Thank you!