Which substance can act only as a reducing agent?

[merovingian12]

Homework Statement

Which substance can act only as a reducing agent?
I2
BrCl
NaBr
HIO4

Homework Equations

The Attempt at a Solution

If a substance oxidizes something, the substance itself becomes reduced (gains electrons.) Halogens have high electron affinities and accept electrons readily, so I eliminated iodine and the bromine-chlorene compound immediately.

The acid can oxidize metals (the metal would undergo a single replacement reaction with the hydrogen ion, producing hydrogen gas,) so I eliminated that one as well.

This leaves sodium bromide, which is the correct answer. However, wouldn't a metal more active than sodium, e.g. potassium, be oxidized by the acid? As far as I can tell, all four substances can act as oxidizing agents...

 

[Borek]

wouldn't a metal more active than sodium, e.g. potassium, be oxidized by the acid?

I don't see an acid in NaBr. Please try to write a reaction equation to elaborate on what you mean.

 

[merovingian12]

I'm sorry, that was a typo. I know that NaBr is a salt. What a meant was a reaction like

NaBr + K ---> KBr + Na

 

[Borek]

Technically you are right - in such reaction Na⁺ would be an oxidizer. But it would need pretty exotic conditions, I am not convinced it is possible at all.

 

[merovingian12]

Would it really need exotic conditions? If I remember correctly, as one goes down Group 1, the metals become more active. So wouldn't the reaction happen spontaneously, since potassium is higher on the activity series than sodium?

 

[Borek]

I doubt it will react easily if you will just mix both solids, and I can't think of any solvent that would be able to both dissolve salt and survive contact with metallic potassium. Perhaps liquid ammonia.

 

[merovingian12]

I'm sorry to bring back the thread, but I've been thinking about this and I still don't understand it. as far as I can tell, solid potassium metal should be oxidized by the sodium ion... so what is the problem? Is it that potassium is so reactive that it wouldn't last long enough to react with sodium, i.e. react with something else?

The only information I've been able to find in textbooks and other sources is that potassium is a "more active metal," which suggests that a single replacement reaction should occur - so why is it impossible?

 

[Borek]

solid potassium metal should be oxidized by the sodium ion... so what is the problem?

Two things. First is strictly technical. To see if they react you would need metallic potassium, mixed with NaBr, kept isolated from the air and moisture so that potassium doesn't react with other things, kept in an inert vessel able to survive temperature needed to melt NaBr (747°C), as otherwise reaction would happen only on the surface and would be very slow. That's what I meant by exotic conditions - you can't just mix them in the test glass to see what happens.

Then there is a question whether K and NaBr will really react. Reactivity is based on standard reduction potentials. Table of reduction potentials assumes every substance is in its "standard state" - and lists values based on this assumption. However, standard state means room temperature and more or less 1M solution. Conditions we need (see above paragraph) have nothing to do with the standard state - so we can't simply use the standard potential table to predict the outcome.

Let me give you an example of the counterintuitive chemistry - not directly related to the sodium/potassium thing, but showing how a simplified approach can be misleading. Gold is very inert, and doesn't react with oxygen - everyone knows that. However, gold oxidation with atmospheric oxygen is a part of a popular process used in gold mining. How come?

If you put a piece of gold in the solution of cyanides, atmospheric oxygen will oxidize gold and gold will dissolve (this is a process known as gold cyanidation). That's because complex of gold with cyanides is so stable it shifts the equilibrium of the reaction

4Au + 8NaCN + O₂ + 2H₂O <-> 4Na[Au(CN)₂] + 4NaOH

far to the right.

So you see that simply applying "reactivities" to real chemical processes can be misleading.