Water as a reduction and oxidation agent

[MedGrl@2022]

Hi everyone,

I was reading EK and it said that "water is both a poor oxidizing agent and a poor reducing agent." but on the Standard Reduction Potential at 25C the reduction potential was listed at -0.83. What is the value of a strong oxidizing/reducing agent? How can I determine from a reduction potential chart whether or not it is a strong oxidizing/reducing agent?

Thank you. 🙂

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[sdm33]

How can I determine from a reduction potential chart whether or not it is a strong oxidizing/reducing agent?
On the chart they all reduced so they are all oxidizing agents , the more reduced the stronger the oxidizing agent.

 

[MedGrl@2022]

How can I determine from a reduction potential chart whether or not it is a strong oxidizing/reducing agent?
On the chart they all reduced so they are all oxidizing agents , the more reduced the stronger the oxidizing agent.

But water has a negative standard reduction potential (-0.83) therefore it is a oxidizing agent? Why isn't it considered a strong oxidizing agent? What negative value is required to be a "strong" oxidizing/reducing agent?

 

[sdm33]

In terms of redox, water behaves much as it did in acid-base reactions, where we found it to be amphiprotic. In the presence of a strong electron donor (strong reducing agent), water serves as an oxidizing agent. In the presence of a strong electron acceptor (strong oxidizing agent), water serves as a reducing agent. Water is rather weak as an oxidizing or as a reducing agent, however; so there are not many substances which reduce or oxidize it. Thus it makes a good solvent for redox reactions.

 

[polyatomic]

As far as telling whether or not something is a strong oxidizing/reducing agent...I'm not sure if there's a cut off on potentials for "strong" oxidizers/reducers, but you can usually tell by the look of a compound.

Strong oxidizing agents: example = KMnO4
As you notice, there's lots of oxygens. Lots of oxygens = strong oxidizing agent. Makes sense, because "oxidize" has "ox" in it, like oxygen!
Also, like oxygen, it has atoms of high electronegativity (because oxidizing agents are reduced, so they accept electrons, and higher electronegativity means you are more willing to accept electrons)
Another tendency is that these agents have highly electronegative atoms attached to each other, like in HOOH

Strong reducing agents: example = Li
Opposite of oxidizing agents, reducing agents have low electronegativity, and so they are willing to GIVE away electrons. In the example you see Li, which makes sense because...we usually see Li as a cation (Li+) which means it likes to lose electrons, which means it's a great reducing agent!
In this case, alkali and alkaline earth metals (Li, Na, K, Ca, Ba, Mg) are good reducing agents.


But anyways, when it comes to numbers, I'm sure in questions you'll always be given a reference point. By that I mean, you'll be comparing a few compounds and asked which is the strongest of those.

 

[HardWork1]

But water has a negative standard reduction potential (-0.83) therefore it is a oxidizing agent? Why isn't it considered a strong oxidizing agent? What negative value is required to be a "strong" oxidizing/reducing agent?

1st-But water has a negative standard reduction potential (-0.83) therefore it is a oxidizing agent?

Since water has a negative standard reduction potential of -0.83. It means it will not want to reduce easily. That makes water a poor oxidizing agent. Remember oxidizing agents gets reduced, and reducing agents get oxidized. Strong oxidizing agents have positive standard reduction potentials because they will get reduced easily. Therefore since water has a negative reduction potential it will not want to get reduced making water a poor oxidizing agent.

Now, the reason why water is also a poor reducing agent, look at the other part where water appears on that table (table 7-3). It appears in the reduction of oxygen to water which is the second-half reaction. The standard reduction potential for this reaction is (1.23). Again the reducing agent gets oxidized. If water were to get oxidized, the oxidation potential would be (-1.23), It's -1.23 because you have to reverse the signs to get the oxidation potential. Since the oxidation potential is negative, water will not want to oxidize spontaneously, and therefore water is also a poor reducing agent.

Always remember, oxidizing agents get reduced, and reducing agents get oxidized.



2nd-Why isn't it considered a strong oxidizing agent?

For water to be a strong oxidizing agent here, it would have to have a positive standard reduction potential.



3rd-What negative value is required to be a "strong" oxidizing/reducing agent?

Water has both, a negative reduction potential, and a negative oxidation potential. This is why water is both a poor reducing agent, and a poor oxidizing agent.