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Redox reactions - Sodium? Please help :(

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slinquii

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Ohhh sh** the MCAT is coming up really soon :eek:

Can someone help me understand this problem, from my gen chem textbook:
Predict what redox reaction will take place, if any, when molecular bromine (Br2) is added to (a) a 1 M solution of NaI and (b) a 1 M solution of NaCl (Assume 25C)

Answer:
Oxidation of I- ions by Bromine.

How do you do this problem? My issue is: Why isn't sodium taken into account? It has a standard reduction potential that is the least of any, so why is it not the reducing agent?

Na+(aq) + e- --> Na(s) -2.71
Br2(l) + 2 e- --> 2Br- (aq) +1.07
Cl2(g) + 2e- --> 2Cl-(aq) +1.36
I2(s) + 2e- --> 2I-(aq) +0.53

I feel there is some rule that I am missing. Another problem, identical to the above, asks about Pb in solutions of (a) NiCl2 and (b) HCl.

How do I know whether we are using Ni, Cl, or H??
 

theonlytycrane

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Focus on this line: Br2(l) + 2 e- --> 2Br- (aq) +1.07

Br2 can get reduced (steal electrons) from things that have a lower reduction potential (lower affinity for electrons) than it.

Cl- holds onto the electrons more tightly (1.36) so Br can't steal from it.

Br can steal from I- because I has a lower reduction potential.

Na+ has no electrons anyway, so Br has nothing to steal from it.
 

slinquii

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Thanks for the reply. Can you please help a little more with this confusion..

Br2(l) + 2 e- --> 2Br- (aq) +1.07
Na+(aq) + e- --> Na(s) -2.71

I'm thinking that since Bromine has the higher reduction potential, it is reduced, and sodium has the lowest, so it would be oxidized.
This would tell me that yes, Bromine is reduced, and Na is oxidized, giving the balanced equation:
Br2(l) + 2Na(s) --> 2Br- (aq) + 2Na+(aq)

So Bromine is reduced (reduction is gain) to Br- and Sodium is oxidized (Oxidation is loss) to Na+.

So E°cell = E°cathode - E°anode
E°cell = 1.07 - (-2.71) = 3.78

This is a positive number, so it would occur.

I'm sorry, I just feel there must be something fundamental that I'm missing. Thanks
 

theonlytycrane

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Just from the problem statement:

"When molecular bromine (Br2) is added to (a) a 1 M solution of NaI and (b) a 1 M solution of NaCl (Assume 25C)"

Hang in there :)
 
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