Quick question on how Ecell is presented on the MCAT

[DrDreams]

Guys,

I have a quick question, I just took a FL and one of the questions I got wrong was an Ecell calculation question.

I know we cannot discuss them out of the FL thread so I just want to know this general trend.

On the MCAT, the half reaction value they give you, and the sign that goes with it, is it based on how they present the half reaction to you, or would they write a oxidation reaction and give you a reduction potential and let you figure it out? (WHEN the two half reactions are given separately)

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[justadream]

@DrDreams

There's no hard and fast rule with regard to this. You need to figure it out.

I doubt they're always going to give you the reduction potential for the species being reduced and the oxidation potential for the species being oxidized explicitly (that'd be too easy).

 

[DrDreams]

@DrDreams

There's no hard and fast rule with regard to this. You need to figure it out.

I doubt they're always going to give you the reduction potential for the species being reduced and the oxidation potential for the species being oxidized explicitly (that'd be too easy).

Thats what I thought, and I got it wrong. But intuitively it now makes sense thought right? If they give you two different half reactions and E cell of each half reaction, followed by a combined reaction. You have to assume that the half reaction e cell values given are for as the half reactions are written right? I didn't think so So i got it wrong.

 

[justadream]

@DrDreams

What do you mean?

If the reaction involves zinc and copper, obviously the half-reaction E cell values will pertain to Zinc or copper.

However, you can't simply add up the values as given in the table. You need to figure out which is being reduced (and for this, take the reduction potential) and what is being oxidized (and for this, take the oxidation potential).

Feel free to PM me your question if this is from any AAMC source except AAMC 10/11 (so any other AAMC FL, SA, Official guide).

 

[Shirafune]

Oxidation or Reduction potentials are given as values for the direction of the half-cell displayed. If you are given two Ecell values for two reduction reactions, you will need to figure out which is being oxidized and which is being reduced so that the overall Ecell is positive.