A2 Chemistry Help!!

if a half equation is: Zn2+ + 2e- ----> Zn and it's E is -0.76 V
would: Zn------> 2e- +Zn2+ also be -0.76V? or would it be 0.76V?

If you are talking about standard electrode potentials then they are equilibria expressed (by convention) as reductions.

Zn²⁺(aq) + 2e <==> Zn(s) ........... E = -0.76 V

The negative sign shows that there is a tendency for the equilibrium to move to the side of releasing electrons compared with the standard hydrogen electrode.

It is not a value for an equation.

If I balanced this half equation with another equation and E standard came out as positive, could I reverse the equation and e standard would be negative?


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In this case you are dealing with a difference between two Eº values and yes the positive from E(red) - E(ox) will give the direction of spontaneous change.

Just to clarify if there was a reaction between potassium dichromate ( +1.33 V) and Chlorine ( +1.36)

How would you explain why the answer would be -0.03V? I know how to do it but don't know the reasoning behind it

As I said, in terms of standard reduction potentials, the E(cell) is given by E(red) - E(ox), where E(red) is the species being reduced and E(ox) is the species being oxidised.

Your two reagents will not react as they are both oxidising agents and can only be reduced.

According to the exam question, they do react if solid potassium dichromate is used and hot concentrated sulphuric acid is added?

Not with chlorine it won't...

here's the link to the question:


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where?

http://papers.xtremepapers.com/Edexcel/Advanced%20Level/Chemistry/1996%20June/Question%20Paper%20June%201996%20Unit-6.pdf

Q20)

You must learn to read questions carefully:

An acidic solution of potassium manganate(VII) will liberate chlorine from dilute sodium
chloride solution but an acidic solution of potassium dichromate(VI) will not. Solid potassium dichromate(VI) will liberate chlorine gas from concentrated hydrochloric acid

This does not say that dichromate ions are reacting with chlorine,it says that dichromate ions react with chloride ions (from hydrochloric acid,not sulfuric acid!!).

In this case the dichromate ions are being reduced and the chloride ions oxidised.

You get the value for E(cell) and find that it is negative,

However, E values only apply to standard conditions and the conditions given are anything but ...

Hence under these conditions there is reaction.

sorry about the confusion was tired

wait may you tell me the equation? because I thought you do the most postive minus the negative one to get the E value? so you would get +0.03? or

my equation was 2cr3+ + 3Cl2 + 7H20 ------> cr207- + 6cl- + 7H+ something like that

or has you say Er- Eox= so the reducing agent is on the left of the E cell diagram?

Hmm no.
If your equation is $2Cr^{3+} + 3Cl_2 + 7H_20 ------> Cr_2O_7^- + 6Cl^- + 7H^+$


You need to consider the half reactions. From the equation you can see that
$Cr^{3+} ---> Cr_2O_7^-$ which is an oxidation
and $Cl_2 ---> 2Cl^-$ which is a reduction

So therefore you do $\Delta E^{\circ} = E_{chlorine} - E_{chromium}$

To answer your question, you can't reverse the sign on a standard reduction potential, it's fundamentally wrong

My chemistry teacher did chromium- Chlorine? so i'm confused


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