Test: Batteries fuel cells and corrosion(13 Oct) - JEE MCQ

(b) Electrons flow from anode to cathode-correct.

In lead-acid storage battery, specific gravity of H₂SO₄ changes and water is to be added periodically. In maintenance free battery calcium-containing lead alloy that prevents the electrolytic decomposition of H₂O. These batteries are sealed and no maintenance is required

Pipes are made of Iron. Iron can be oxidised toFe²⁺.
Fe → Fe²⁺ + 2e^-
but when connected to Mg, which is more electropositive that Fe, Mg is oxidised to Mg²⁺ and in turn reduces Fe²⁺ (rust) to Fe

It is called cathodic protection but at the cost of Mg (sacrified anode).

I. Temperature affects corrosion. However, solubility of dissolved gases decreases with rise in temperature.
II. In acidic medium, pH < 7 (acid rain), enhances corrosion

In basic medium, pH > 7
Fe²⁺ changes to Fe (OH₂) and then to Fe(OH₃)(s) (Fe₂O₃ - rust)
III. Impure metals set up their own electrochemical cells and thus, corrosion is enhanced.
IV. Conc. HNO₃ forms an oxide protective layer on the surface of a metal. When metal (as Al) reacts with cone. HNO₃, there is reaction but it ceases due to formation of an oxide (Al₂O₃) layer. Thus, HNO₃ can be stored in Al vessel.

(a) In oxidised state, shine of the metal is furnished, thus, corrosion is the oxidative deterioration of a metal. (b) A mini-cell is set up at the surface at which corrosion takes place.

Reaction is spontaneous-correct.
(c) Impurities also react with active metals by making own cells. Thus, correct.

In the anodic reaction, the metal that is more easily oxidised loses electrons to produce metal ions. In case (a), zinc (more electropositive than Fe) is oxidised. In case (b), iron (more electropositive than Cu is oxidised).
In the cathodic reaction, oxygen gas which is dissolved in a thin film of water on the metal is reduced to OH^-.
Thus, rusting of iron does not take place in (a) but it is in (b).
Fe²⁺ and OH^- ions from the half-reactions initiate these further reactions

Fe can be protected by making a layer of zinc on it. It is called galvanisation. At the cost of Zn (which is oxidised), Fe is protected.

Thus, Fe which is cathode is protected.
It is called cathodic protection and active metal is thus, sacrified anode.

To make cell reaction spontaneous, E° _cell > 0

Thus, battery emf is independent on [OH^-]
 ΔG° = - nF E°_cell 
= - 2 x 96500x 1.27
= -245100 J 
= -245.1 kJ
Thus, energy obtained per mole of Ni₂O₃ consumed i s -245.1 kJ.

Al metal is used to remove Ag₂S.

NaHCO₃ in hot water is used as electrolyte and dissolves displaced S^2-.