Help with oxidising agents/ reducing agents

[cccc]

Hi, this question was from a past paper, and I got stuck between choosing C or D, 

I know the electrode potential (E) should be greater than 0.

But what i am stuck with is which one is better reducing agent/ oxidising agent, because it doesn't seem like a fair comparison. Usually we would be comparing between Fe3+ and Br-; and then Fe2+ and Br2. Not the mixture of them together.

How to approach these questions? 

P.S. The answer to this question is D 

Any help would be greatly appreciated 

[Msj Chem]

The standard electrode potentials are:

Fe³⁺ + e^- ==> Fe²⁺ +0.77V

2Br^- + 2e- ==> Br₂ +1.09V 

In the reaction, Fe³⁺ is being reduced to form Fe²⁺ and Br^- is oxidised to form Br₂

E_cell = E_red - E_ox

E_cell = 0.77 - 1.09 = - 0.32 V (non-spontaneous) 

Can I ask which paper you got the question from?

Edited April 16, 2016 by Msj Chem

[kw0573]

The question made a mistake; the reaction is actually non-spontaneous.The reaction is spontaneous in the reverse direction: 2Fe²⁺ + Br₂ --> 2Fe³⁺ + 2Br^-. and choice D would then be the correct choice. Choice D is an objectively true statement, but you could only get the answer if the reaction was correctly stated.

You would never compare between Fe³⁺ and Br^-, because Fe³⁺ can only be reduced, and Br^- can only be oxidized. You can't say which is better oxidizing or reduction agent that way. You have to compare between HALF REACTIONS in the reduction direction, and decide which way each reaction proceeds in. Higher E^⦵ (standard REDUCTION potential) means the half reaction proceeds as REDUCTION spontaneously, or reactant is a stronger oxidizing agent. Lower E^⦵ means the half reaction proceeds as oxidation spontaneously, and hence reactant (when written as oxidation) is a better reducing agent.That's why Fe²⁺ is a better reducing agent because it is oxidized in the actual spontaneous direction.
In summary, strong oxidizing and reducing agents react to form weak oxidizing and reducing agents. Knowing this fact would make the question very straightforward had the reaction been stated in the correct direction.

 

[cccc]

On 4/16/2016 at 3:00 AM, Msj Chem said:

The standard electrode potentials are:

Fe³⁺ + e^- ==> Fe²⁺ +0.77V

2Br^- + 2e- ==> Br₂ +1.09V 

In the reaction, Fe³⁺ is being reduced to form Fe²⁺ and Br^- is oxidised to form Br₂

E_cell = E_red - E_ox

E_cell = 0.77 - 1.09 = - 0.32 V (non-spontaneous) 

Can I ask which paper you got the question from?

@Msj Chem It is from 2012 paper. 

As it is paper 1, we won't be allowed to use the data booklet, therefore won't be able to get those values and compare them. Is there any other way? 

Thanks 

[cccc]

On 4/16/2016 at 3:20 AM, kw0573 said:

The question made a mistake; the reaction is actually non-spontaneous.The reaction is spontaneous in the reverse direction: 2Fe²⁺ + Br₂ --> 2Fe³⁺ + 2Br^-. and choice D would then be the correct choice. Choice D is an objectively true statement, but you could only get the answer if the reaction was correctly stated.

You would never compare between Fe³⁺ and Br^-, because Fe³⁺ can only be reduced, and Br^- can only be oxidized. You can't say which is better oxidizing or reduction agent that way. You have to compare between HALF REACTIONS in the reduction direction, and decide which way each reaction proceeds in. Higher E^⦵ (standard REDUCTION potential) means the half reaction proceeds as REDUCTION spontaneously, or reactant is a stronger oxidizing agent. Lower E^⦵ means the half reaction proceeds as oxidation spontaneously, and hence reactant (when written as oxidation) is a better reducing agent.That's why Fe²⁺ is a better reducing agent because it is oxidized in the actual spontaneous direction.
In summary, strong oxidizing and reducing agents react to form weak oxidizing and reducing agents. Knowing this fact would make the question very straightforward had the reaction been stated in the correct direction.

 

@kw0573 Thanks! But as this is paper 1, any alternative method to get the answer, as we can't compare electrode potential values? (as we won't be able to access the data booklet) 

[kw0573]

@cccc
Please read my response in detail. I didn't reference any ACTUAL values, but I went through the derivation that strong oxidizing and reducing agents react to form weaker reducing and oxidizing agents in a spontaneous reaction.
When the reaction does proceed spontaneously, for example
Ca + Zn²⁺ --> Zn + Ca²⁺
In the forward reaction (spontaneous direction), Zn²⁺ is reduced; in the reverse reaction, Ca²⁺ is reduced. Because in the spontaneous direction Zn²⁺ is the oxidizing agent, then Zn²⁺ is a stronger oxidizing agent than Ca²⁺ (from the concept that strong agents react to form weak). Similarly, Ca would be a stronger reducing agent than Zn. This is meant to be a straightforward recognition question as long as the stated reaction is known to be spontaneous.