Question

What is rate of corrosion (metals) in HCl, seawater and distilled water?

How does this (corrosion) affect the appearance of the metal?

Can it be explained in detail

Answer 1

Well the rate should increase in the order: `"distilled water;"` `"seawater;"` `"hydrochloric acid"`....

Explanation:

We interrogate the oxidation reaction for some unspecified metal....

`M(s) rarrM^(2+) + 2e^(-)`

And for every oxidation, electron loss, there must be a corresponding electron gain....and here the water solvent is reduced....

`H_2O(l) + e^(-) rarr 1/2H_2(g) + HO^-`

And in the acid we actually supply hydronium ions to be reduced....

`H_3O^+ +e^(-) rarr 1/2H_2(g) + H_2O(l)`

And to quantify this we draw on redox potentials ....

When `E^@` is LESS than ZERO, this means that the oxidation reaction (the reverse of the reduction reaction) is SPONTANEOUS as written....i.e. consider the alkali metals....

`Li^(+) + e^(-) rarr Li(s)` `;E^@=-3.01*V`

`Ca^(2+) + 2e^(-) rarr Ca(s)` `;E^@=-2.87*V`

And since `DeltaG^@=-nFE^@`, a NEGATIVE value of `E^@` signifies a spontaneous reaction FOR the OXIDATION, i.e. for....

`Ca(s) rarr Ca^(2+) +2e^(-)`...spontaneous as written....