[OhMyGod]

Hi there,

Got a question here on the topic of Electrochemistry.

Supposed that we have a Voltaic Cell (Electromchemical Cell) made up of Mg2+/Mg Half cell and the Fe3+/Fe2+ halfcell. Which of the below statement are true?
1. Addition of H20 to Fe3+/Fe2+ halfcell has no effect on the cell emf.
2.Addition of aq. NaOH to Mg2+/Mg halfcell decreases the cell emf.
Answer is that only 1 is true.

Worked out this overall equation by myself by looking at the Eo values.

2Fe3+ + Mg --> Mg2+ + 2Fe2+ Eo(cell) = +3.143V

Cracked my mind on this, still no idea. why is #1 true and why shld be the result in the case of #2(adding aq NaOH) if the result mentioned in #2 is false.

Would some kind soul help me out with this one?

[DevaDevil]

the potential of an electrochemical cell is given by the Nernst-equation:
E = E⁰ - RT/nF * ln a_Redu / a_Ox in aqueous ions that means E = E⁰ - RT/nF * ln [Redu] / [Ox]; activity of a solid is set at 1

For your total equation the Nernst equation would be E⁰ - RT/2F ln [Mg²⁺][Fe³⁺]²/[Fe²⁺]² this is just adding the halfcell Nernst equations.

for answer 1 adding water to the couple causes [Fe²⁺] and [Fe³⁺] to change equally, so the ratio [Fe²⁺] / [Fe³⁺] will be the same, hence the cell potential will stay the same.

for answer 2: assuming NaOH will not participate in the reaction, you will in effect add water and reduce the concentration Mg²⁺.
look at the equation: a decrease in Mg²⁺ concentration will lead to a decrease in the concentration (activity) portion of the equation - aka a decrease in value for the natural logarithm part, and since that is subtracted from the standard potential of the cell, it leads to an increase in cell potential.

[OhMyGod]

Firstly, thanks for the long reply mate 

The Nernst Equation that you have used is currently not in my syllabus and Ive never seen it before.

But from what u have explained, its rather comprehensive and I could understand it.

After some thoughts, I remembered that my teacher had used Le Chatelier's Principle(LCP) to explain this sort of question to me before.

Therefore, am I right to explain the answer in this way below:

1 is correct because, additional of water to Fe2+/Fe3+ cell would just make the concentration of [Fe2+] and [Fe3+] change by the same ratio and hence, by LCP, there is no shifting of equilibrium and E⁰ stays the same.

2 is wrong because addition of NaOH to Mg2+ cell would result in some form of precipitation of MgOH and that decreases concentration of Mg2+ in water. Or if no precipitation, there will nonetheless be dilution due to the Aqueous state of NaOH. Hence, according to LCP, this decrease in [Mg2+] would then result in the equilibrium shifting more to the right to produce more of the Mg2+ so that to increase [Mg2+] once again. Shifting of the equilibrum to the right would result the Forward reaction to be more feasible, and hence, cell potential increases.

So are my explanation valid?

Thanks once again.

[DevaDevil]

when using only LCP your explanations are correct.