71For the question 3, I think it is decided by -
1. Electrochemical Series/Preferential discharge theory
2. Concentrations of the ions In case it is very high or very low.
3. Electrodes.
Read this carefully as it would relate all these three factors:
Preferential discharge theory : According to this theory “If more than one type of ion is attracted towards a particular electrode, then the ion is discharged one which requires least energy or ions with lower discharge potential or which occur low in the electrochemical seriesâ€.
The potential at which the ion is discharged or deposited on the appropriate electrode is termed the discharge or deposition potential, (D.P.). The values of discharge potential are different for different ions.
The decreasing order of discharge potential or the increasing order of deposition of some of the ions is given below,
For cations : Li+, K+, Na+, Ca2+, Mg2+, Al3+, Zn2+, Fe2+, Ni2+, H+, Cu2+, Hg2+, Ag2+, Au3+
For anions : SO42–, NO3–, OH–, Cl–, Br–, I–.
(This should be enough)
For question 2:
The things that undergo corrosion aren't pure. They are usually impure forms of the metal or alloys and this probably serves a source for supply of EMF externally.
This difference causes one metal to corrode preferentially over the other when both metals are in electrical contact and immersed in an electrolyte.
In an alloy/impure object (which has undergone corrosion) one metal will be either more noble or more active than the next, based on how strongly its ions are bound to the surface. Two metals in electrical contact share the same electrons, so that the tug-of-war at each surface is translated into a competition for free electrons between the two materials. The noble metal will tend to take electrons from the active one, while the electrolyte hosts a flow of ions in the same direction.
In this way EMF is generated.
eg A common example is the carbon-zinc cell where the zinc corrodes preferentially to produce a current.
