P. Bahadur Test: Redox Reactions & Electrochemistry - JEE MCQ

The amounts of different substances liberated by the same quantity of electricity passing through the electrolytic solution are proportional to their chemical equivalent weights.

Charge on one electron is 1.6 x 10^-19 C. 
1 mole of electron 6.023 x 10²³ electrons.
So charge on one mole of electron = 1.6 x 10^-19 C x 6.023 x 10²³ = 96500 C mol^-1

Primary batteries cannot be recharged and reused.

Secondary cell can be recharged and reused.

• A fuel cell is an electrochemical cell that converts the chemical energy of a fuel (often hydrogen) and an oxidizing agent (often oxygen) directly into electrical energy through a pair of redox reactions. 
• Fuel cells are different from most batteries in requiring a continuous source of fuel and oxygen (usually from air) to sustain the chemical reaction, whereas in a battery the chemical energy usually comes from metals and their ions or oxides that are commonly already present in the battery, except in flow batteries.
• Fuel cells can produce electricity continuously for as long as fuel and oxygen are supplied.

Setup of a Fuel cell:

In Corrosion, a metal is oxidized by loss of electrons and results in the formation of oxides.

• Corrosion is a natural process that converts a refined metal into a more chemically stable form such as oxide, hydroxide, or sulfide.
• It is the gradual destruction of materials (usually a metal) by chemical and/or electrochemical reaction with their environment. 
• Corrosion engineering is the field dedicated to controlling and preventing corrosion.

The best way to prevent corrosion is to prevent the surface of the metallic objects to come in contact with the atmosphere.

• The rusting of iron can be prevented by painting, oiling, greasing or varnishing its surface.
• Galvanization is another method of protecting iron from rusting by coating iron with a thin layer of zinc.
• Corrosion of iron is prevented by coating iron with non-xcorrosive substance like carbon. This process is termed as alloying .

Galvanization is a process in which iron is coated with reactive metal like Zn which get corroded hence preventing iron from corrosion.

Electro-Galvanization:

 A more reactive metal displaces a less reactive metal from the salt solution (refer to reactivity series).

The order in which metals displace each other from the solution of their salts can be given with the help of their standard electrode potential. Since magnesium has the least standard electrode potential so it is the most strong reducing agent.

So the required order we get is: Mg > Al > Zn > Fe > Cu

Reactivity Series:

The standard emf of a galvanic cell involving 3 moles of electrons in its redox reaction is 0.59V.

Eº_cell​ = 0.59V
n=3
The equilibrium constant for the reaction of the cell is given by the expression: ln K = RT nFEº_cell​
ln K = 8.314 × 298 × 3 × 96500 × 0.59 = 68.9
K≈10³⁰

The equilibrium constant for the reaction of the cell is 10³⁰.

⇒E_cell ​= 0.44 + 0.089V

⇒Ecell​ = 0.53V

k (Conductivity) =0.0248 Scm⁻¹, C (molarity) = 0.20 M, T (temperature) = 298 K
Λm​ = ? (molar conductivity) 

Λm​ = (1000×k​) / C = (1000×0.0248) / 0.20 ​= 124 S cm² mol⁻¹

The molar conductivity is 124 S cm² mol⁻¹

Κ = G x cell constant and G = 1/R.

For reduction of 1 mol of Al³⁺ to Al, 3 mol of electrons are required so total charge will be 3F.

And mass of Zn and Cu deposited will be in ratio of their equivalent mass.

Equivalent conductance increases on dilution for a strong electrolyte because of increase in mobility of ions.

Specific conductance decreases with dilution because of decrease in the number of ions per unit volume.

Acidity increases on attaching electron withdrawing group because of stability of conjugate base.

Voltmeter measures the potential.

No. of electrons = change in oxidation number of active elemnt.

• The strongest oxidizing agent is one having more positive or less negative reduction potential.
• For the strongest oxidizing agent, the oxidizing potential should be the least.

Here, the oxidizing potential of Fe⁺² is less than that of [Fe(CN)₆​]⁴⁻. Therefore, Fe⁺² is a stronger oxidizing agent than [Fe(CN)₆​]⁴⁻.

• Also, the stronger oxidizing agent should easily reduce itself.

Here, Fe⁺³ is easily reduced than Fe⁺². Therefore, among all the four, Fe⁺³ is the stronger oxidizing agent.