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(1 recommendation so far) Message 1 of 2 in Discussion 
From: MSN NicknameJaimej003  (Original Message)Sent: 5/27/2008 11:08 AM
okay so i was given an experiment to do and now i have to write a lab write up. i need help with identifying the different chemistry topics in the lab(meaning like dilution, oxidation, aciids and bases, ex)

prepare 500 mL of a 3.5M H2SO4 soution. to this solution add enough potassium permanganate to make it 0.0006M with respect to potassium permanganate. (NOTE: there instructions do no direct you to make 200 mL of the acid solution and 200mL of the potassium permanganate solution and then to mix the two obtaining 400mL solution) stir the mixture well to insure that all the potasium permanganate is dissolver and that the composition of the solution is unifor throughout. put 10 mL of this solution in each of 5 test tubes. to the first test tube add one iron wire, to the second 2 iron wire, to the third 3 iron wire and so on. wait why did the solution in the test tube with 5 iron wires become decolorized more rapidly than the one with 4 wires and it in turn more rapidly with 3 wires and so on?

help please! it will be greatly appriciated!


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(1 recommendation so far) Message 2 of 2 in Discussion 
From: MSN Nickname·Steve·Sent: 5/27/2008 8:01 PM
Hello, it looks like this lab covers a lot of chemistry topics, but mainly oxidation and reduction, and reaction rate.  Here are the expected oxidation and reduction reactions that are occurring:

Oxidation        5 Fe (s)   ––�?gt;   5 Fe3+ (aq)  +  15 e�?/SUP>
Half-Rxn

Reduction       3 MnO4�?/SUP> (aq)  +  15 e�?/SUP>  +  24 H+ (aq)   ––�?gt;   3 Mn2+ (aq)  +  12 H2O (l)
Half-Rxn

Overall            5 Fe (s)  +  3 MnO4�?/SUP> (aq)  +  24 H+ (aq)   ––�?gt;   5 Fe3+ (aq)  +  3 Mn2+ (aq)  +  12 H2O (l)
Net-Ionic
Reaction

Iron metal becomes oxidized to the yellow Fe3+ ion ultimately, while the MnO4�?/SUP> ion is reduced to the nearly colorless Mn2+ ion.  But if excess iron metal is present, the situation can be a little more complicated, because the iron metal can reduce Fe3+ ion:
 
Fe (s)  +  2 Fe3+ (aq)   ––�?gt;   3 Fe2+ (aq)

Fe2+ ion is nearly colorless in dilute solution, so in the presence of excess iron wire, you may not see the yellow color of Fe3+ ion.  But if you observe a yellow color appearing in the reaction, you know that you are forming Fe3+ ion.  I'm not certain about this without observing the reaction, so for now I'll assume the reaction forming Fe3+ is correct.

We can include the sulfate spectator ions:

5 Fe (s)  +  3 KMnO4 (aq)  +  12 H2SO4 (aq)   ––�?gt;   5/2 Fe2(SO4)3 (aq)  +  3 MnSO4 (aq)  +  3/2 K2SO4 (aq)  +  12 H2O (l)

If we multiply through by 2, we will have all whole number coefficients:

10 Fe (s)  +  6 KMnO4 (aq)  +  24 H2SO4 (aq)   ––�?gt;   5 Fe2(SO4)3 (aq)  +  6 MnSO4 (aq)  +  3 K2SO4 (aq)  +  24 H2O (l)
 
 
You observed that the rate of the reaction is greater if more iron wires are in contact with the solution.  Since the rate of reaction of solid reactants depends on their surface area, I would expect this to be the reason.  Five wires have a greater iron surface area than four wires, etc.

Steve