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Topic: redox equation writing: H2So4, NaBr, KMnO4...  (Read 17675 times)

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redox equation writing: H2So4, NaBr, KMnO4...
« on: June 04, 2006, 01:31:49 AM »
hi, a new member here:)
we did a bunch of redox labs at school, and now we need to write the equations and balance them...
however, i have no idea how we're supposed to know the products in a redox reaction, because the teacher never covered that... she just showed us how to balance already written equations.
so here it goes:

stock solution: KMnO4 + H2SO4 -> ? (doesn't look like double displacement..?)
to NaBr, add water and stock solution, then mineral oil. shake well. record the color of layers.

well, what i got is that the purple stock solution turned yellow when added to NaBr. there seemed to be only 2 layers, though; oil and aqueous solution. but the oil was not involved in the reaction.
and the lab says that Mno4 produces Mn2+ and Br is a brown liquid... but it doesn't help, since i still cannot figure out what the products are!

so please help... if i learn how to do this one, i think i'll be okay with the 9 problems that are left.
thanks in advance:D

Offline Dan

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Re: redox equation writing: H2So4, NaBr, KMnO4...
« Reply #1 on: June 04, 2006, 07:17:33 AM »
Ok, so you know KMnO4 in acid reacts with bromide ions to produce Mn2+ and bromine (Br2)

So, the ionic equation is:

MnO4- + H+ + Br-  -----> Mn2+ + Br2 + __________

Work out the missing product (hint: what happens to the O), and balance.
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Offline Will

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Re: redox equation writing: H2So4, NaBr, KMnO4...
« Reply #2 on: June 04, 2006, 10:51:03 AM »
It is also a good idea to learn this half equation:

MnO4- + 8H+ + 5e- --> Mn2+ + 4H2O

The half equation for the bromide to bromine is 2Br- --> Br2 + 2e-.

Now all you have to do is multiply each equation by a coefficient to make the electrons on each side equal (so you can cancel them out and put the equations together) (ie multiply manganate one by 2 and bromide one by 5); sorry if that gives too much away, Dan!

Borek's site ChemBuddy explains it well.

As a separate thing: Doesn't KMnO4 react with conc. H2SO4 to give green-looking Mn2O7? (See bottom of this page from woelen's website.)


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Re: redox equation writing: H2So4, NaBr, KMnO4...
« Reply #3 on: June 06, 2006, 10:59:57 PM »
thank you guys so much!! you've been a lot of help:D

to will17: that's what i read from other posts, but they all agree that the product is highly unstable and dangerous... hmm i wonder if that applies only for solid KMnO4?

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