which is the strongest reducing agent Help!!!

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uprine100

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18. Which of the following is the STRONGEST reducing agent?

A. MnO(s)
B. Mg2+(aq)
C. Cl-(aq)
D. Zn(s)

explanation
The best reducing agent is the compound that is most favorably oxidized. According to the table, Mg2+(aq) is an oxidizing agent, because it has no electrons to lose and can gain two electrons to form the neutral metal. This eliminates choice B. According to the chart, it is unfavorable to oxidize Cl-(aq) or MnO(s) (both have negative voltages for oxidation), so choices A and C are eliminated. The oxidation potential for Zn(s) is +0.76 V, making it the strongest reducing agent of the four answer choices.

i understand the concept but i don't get how magnesium is no the answer. is there an easy way to do problems like this?

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If it was a regular Mg (with 0 charge), then I too would pick Mg. Generally there is a trend toward light alkalis being best reducing agents with Li being the strongest red. agent in the periodic table. Here though Mg2+ already shed 2 of its el-ons. Losing any more would be very unfavourable considering it is in noble gas configuration.
I wouldn't worry about exact oxidation potentials to solve problems like these and take advantage of trends and electron configuration stability concepts.
Strongest reducing agent donates el-ons the easiest, which one of these would put up the least fight and go from there.

This of course applies only to single atom elements. I am not sure exactly how I would eliminate MnO. I guess you could say it is an ionic compound and el-ons are locked in a tight lattice and thus are unlikely to be ripped out very easily. Def not as easy as Zn with a sea of valence shell el-ons.
 
Ok wait, this is a picture supplied with the question? Because since you didn't post it in the OP I used trends only. Either way, even if this info is provided, we still don't have all the equations we need to compare everything numerically.

Reversing 3 of those equations gets us emfs of oxidation for all but 1 of the answer choices.
Answer choice is Mg2+ not Mg. So we need an equation like this:
Mg3+ + e -> Mg2+ or Mg4+ + 2e -> Mg2+ or something like that. The product has to be the answer choice. Then you take the negative of the value and compare.

Using these equations (if they were provided) only allows to eliminate 2 answer choices and narrow it down to Zn and Mg2+. Now we have to rely on trend knowledge and general concept that atoms in noble gas configs are very stable and don't like to lose el-ons.

"zn and mg both got reduced and have (-) E value so appling what you said i could also say Zn has already gained electrons and wont lose... "
Why wouldn't it lose some? Reverse the equation and it tells how favourable (from emf perspective) the oxidation is.
 
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looking at the picture rom the question above it seems zn and mg both got reduced and have (-) E value so appling what you said i could also say Zn has already gained electrons and wont lose...

Ok wait, this is a picture supplied with the question? Because since you didn't post it in the OP I used trends only. Either way, even if this info is provided, we still don't have all the equations we need to compare everything numerically.

Reversing 3 of those equations gets us emfs of oxidation for all but 1 of the answer choices.
Answer choice is Mg2+ not Mg. So we need an equation like this:
Mg3+ + e -> Mg2+ or Mg4+ + 2e -> Mg2+ or something like that. The product has to be the answer choice. Then you take the negative of the value and compare.

Using these equations (if they were provided) only allows to eliminate 2 answer choices and narrow it down to Zn and Mg2+. Now we have to rely on trend knowledge and general concept that atoms in noble gas configs are very stable and don't like to lose el-ons.

"zn and mg both got reduced and have (-) E value so appling what you said i could also say Zn has already gained electrons and wont lose... "
Why wouldn't it lose some? Reverse the equation and it tells how favourable (from emf perspective) the oxidation is.

thanks both for your help.

"reduction potential" as i get it, states how likely it is that the product will be reduced. therefore, the greater the emf value, the more favorably it will get reduced.

then, shouldn't i get how favorably it will be oxidized if i just reverse the eqn? with that logic, i can see that MnO and Cl are oxidizing agents (therefore eliminated from answer choices) and choose between Zn and Mg. I chose Mg following my logic: Mg 2.37V > Zn .76 V

Please tell me where I'm flawed with my logic.

Thanks!
 
narrow it down to Zn and Mg2+. Now we have to rely on trend knowledge and general concept that atoms in noble gas configs are very stable and don't like to lose el-ons.

This, Mg2+ is already stripped of two electrons, no way in hell he's going to lose more over something so easily oxidized as Zn. We use Zn for cathodic protection because it loves to give up electrons.
 
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This, Mg2+ is already stripped of two electrons, no way in hell he's going to lose more over something so easily oxidized as Zn. We use Zn for cathodic protection because it loves to give up electrons.

i've always felt simple explanations are the best explanations.

thanks!
 
i've always felt simple explanations are the best explanations.

thanks!

Sure and as far as where your logic broke down it was here....

Mg2+ + 2e- = Mg (-2.37) therefore the inverse, Mg losing two electrons, is 2.37V, which is really favorable, you are correct there. However losing TWO MORE electrons to go to Mg4+ is incredibly unfavorable compared to Zn losing it's first two, which is decently favorable at .76V.

Hope that makes sense.
 
This, Mg2+ is already stripped of two electrons, no way in hell he's going to lose more over something so easily oxidized as Zn. We use Zn for cathodic protection because it loves to give up electrons.
just went through this exact problem, and this answer is what made me understand the question - thank you!
 
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