[jnimagine]

When calculating the cell potential for a redox reaction between silver and Iron:
Ag+  +  Fe2+  -> Ag  + Fe3+
why does it give a negative value?
Shouldn't it be positive since iron is a stronger oxidizing agent than silver, so a spontaneous reaction should occur....
Also, how does concentration affect the cell potential?

[Borek]

iron is a stronger oxidizing agent than silver

Have you compared standard potentials?

Also, how does concentration affect the cell potential?

Nernst equation.

[lancenti]

Also, if you don't want to look up the Nernst Equation, you can treat it as totally an equilibrium problem i.e. using Le Chatelier's Principle.

"If I have less Fe2+, will the reduction potential be higher, or lower?"

Once you work out all such questions, just refer back to the equation

E_cell = E_red - E_ox

and you should be able to get an answer. Then use the Nernst Equation if you need to find the EXACT potential. Otherwise higher/lower can be answered by LCP.

[Borek]

Then use the Nernst Equation if you need to find the EXACT potential. Otherwise higher/lower can be answered by LCP.

Generally you are right, but difference in standard potentials in this particular case is below 30 mV, that's close enough so that Nernst equation can be necessary.

Sure, one may just assume standard conditions and ignore everything else