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Question on Iodine Clock Reaction

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Sophie Amelia

Tue Jan 10 2012, 01:21PM

Registered Member #4201 Joined: Wed Nov 09 2011, 07:42AM
Location:
Posts: 20

Hey,

I have some chemistry questions on Iodine Clock Reaction.

Two colorless solutions are mixed. After 10 seconds, the colorless mixture suddenly turns blue.

â€¢ Solution A
o 0.6 grams Starch
o 30 mLs of Acetic Acid
o 4.1 grams of Sodium Acetate
o 50 grams of Potassium Iodide
o 4.7 grams of Sodium Thiosulfate
Allow mixture to cool and dilute to 1 liter with distilled water

â€¢ Solution B
o 500 mLs of 3% Hydrogen Peroxide
o 500 mLs of distilled water

Chemical Equation:

3 I-(aq) + H202 + 2 H+ (aq) = I3- (aq) + 2 H20 (l)
I3- (aq) + 2 S2O3 2- (aq) = 3 I- (aq) + S4O6 2- (aq)
2 I3- (aq) + starch = starch-I5- complex + I- (aq)

I have some burning questions:

1. why is there a need for both acetic acid and sodium acetate?
2. How did they know how much of each chemical is needed to react with one another? Are there any mathematical calculations involved?
3. If i were to replace acetic acid with sulfuric acid, do i still need sodium acetate? What is the use of the sodium acetate?

I hope someone could bring light to my questions.

Thanks!! :)

Fulmen

Wed Jan 11 2012, 11:24AM

Registered Member #3883 Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87

I don't have any experience with this reaction, but I can take a guess:
Acetic acid is a pretty weak acid. Sodium acetate is formed by reacting acetic acid with sodium hydroxide, a strong base. The result is not neutral like with a strong acid and a strong base but rather a base.
The mixture of acetic acid and sodium acetate will produce a buffer solution, a solution that has a moderate pH that is quite stable even when "disturbed" by the addition of additional acids or base. I'm guessing the reaction of the clock is sensitive to pH, by making a buffer solution you can easily get the right pH range without fine-tuning it with a pH meter. The acetic acid could also have a catalytic effect.

I doubt you will get the same reaction with sulfuric acid as it will not produce the same buffering effect.

Sophie Amelia

Wed Jan 11 2012, 11:39PM

Registered Member #4201 Joined: Wed Nov 09 2011, 07:42AM
Location:
Posts: 20

Fulmen wrote ...

I don't have any experience with this reaction, but I can take a guess:
Acetic acid is a pretty weak acid. Sodium acetate is formed by reacting acetic acid with sodium hydroxide, a strong base. The result is not neutral like with a strong acid and a strong base but rather a base.
The mixture of acetic acid and sodium acetate will produce a buffer solution, a solution that has a moderate pH that is quite stable even when "disturbed" by the addition of additional acids or base. I'm guessing the reaction of the clock is sensitive to pH, by making a buffer solution you can easily get the right pH range without fine-tuning it with a pH meter. The acetic acid could also have a catalytic effect.

I doubt you will get the same reaction with sulfuric acid as it will not produce the same buffering effect.

Hmmmmm... acetic acid and sodium acetate yields H+ ions as a buffer solution. Other than acting as a buffer, i think the H+ somehow enhance the chemical reaction. Could you help explain how it enhances the reaction? In terms of molecular level. like... maybe it increases mobility of other ions etc.??

OOh, and one more question:

This is the chemical equation of reacting hydrogen peroxide with potassium iodide:

H2O2 + 2KI + 2H -->2K + I2 + 2H2O ------1

However, if you were to look at the first equation of the iodine clock reaction:
3 I-(aq) + H202 + 2 H+ (aq) = I3- (aq) + 2 H20 -------2

... why does equation 1 produces I2 whereas equation 2 produces I3-??

Thanks

Wastrel

Fri Jan 13 2012, 01:10PM

Registered Member #4095 Joined: Thu Sept 15 2011, 03:19PM
Location: England.
Posts: 122

With care, extra sodium acetate and sulphuric acid would substitute for acetic acid. You would need to know if sodium hydrogen sulphate is a stronger or weaker acid than acetic to know if you should calculate on the basis of one or two H+ per H2SO4.

Alternatively the final concentration of acid is within the range of vinegar and you may find a cheap white vinigar can be used instead of the acid (and most of the distilled water in A with the method you are using). Some other methods have the acid in B with the peroxide to prevent it decomposing the thiosulphate, producing sulphur dioxide gas. So long as you don't wait too long before mixing it probably makes no difference.

Iodine is not very soluable in water but it is soluable in a solution of iodide ions, I2 + I- <==> I3- and you may consider these equivalent.

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