Redox Reactions MCQs for NEET — Chemistry Questions with Answers

As stated in the NCERT text, 'decomposition of calcium carbonate is not a redox reaction.' This is because there is no change in the oxidation states of calcium (+2), carbon (+4), or oxygen (-2) in either the reactant or the products. The other options involve changes in oxidation states, making them redox reactions.

The NCERT text mentions, 'Very less active metals, which may occur in the native state such as silver (Ag), and gold (Au) do not react even with hydrochloric acid.' Metals like Zn, Mg, and Fe are listed as examples that displace hydrogen from acids (e.g., $Zn(s) + 2HCl(aq) \rightarrow ZnCl_2(aq) + H_2(g)$).

The NCERT text explicitly states, 'In section (7.2.1) we have already discussed that the metals – zinc (Zn), copper (Cu) and silver (Ag) through tendency to lose electrons show their reducing activity in the order Zn > Cu > Ag.' This order indicates that Zinc is the strongest reducing agent among the three, followed by Copper, and then Silver.

Decomposition reactions involve a compound breaking down, and for it to be a redox reaction, at least one product must be in its elemental form (and thus change its oxidation state). In option 3, water (a compound) decomposes into elemental hydrogen ($H_2$) and oxygen ($O_2$), where the oxidation state of H changes from +1 to 0 and O changes from -2 to 0. Options 1, 2, and 4 are combination reactions where elements combine.

In this reaction, the oxidation state of Mg changes from 0 (in Mg(s)) to +2 (in $Mg(OH)_2(s)$). An increase in oxidation state signifies oxidation. A species that undergoes oxidation is a reducing agent. Therefore, Mg is a reducing agent undergoing oxidation. Hydrogen's oxidation state changes from +1 (in $H_2O$) to 0 (in $H_2$), indicating reduction, so $H_2O$ acts as the oxidizing agent.

The introductory paragraph states, 'Of late, environmental issues like Hydrogen Economy (use of liquid hydrogen as fuel) and development of ‘Ozone Hole’ have started figuring under redox phenomenon.' Redox reactions inherently involve changes in oxidation states (electron transfer). The formation and depletion of ozone in the atmosphere ($O_3$) involve oxygen species in different oxidation states and transformations between them through oxidation and reduction processes which are crucial to understanding the 'Ozone Hole'.

The NCERT text, under '7.2 REDOX REACTIONS IN TERMS OF ELECTRON TRANSFER REACTIONS,' explains that reactions like the formation of NaCl can be seen as two separate steps: 'one involving the loss of electrons and the other the gain of electrons.' It defines 'half reactions that involve loss of electrons are called oxidation reactions' and 'half reactions that involve gain of electrons are called reduction reactions.' The overall redox reaction involves both simultaneously, leading to changes in oxidation states.

D wrong (H–H dissociation enthalpy is the highest). E wrong (H₂ reduces oxides of metals LESS active than Fe, e.g., Cu, Ag).

Cr₂O₇²⁻ gains 6e (a=1); SO₃²⁻ → SO₄²⁻ loses 2e (b=3); charge balance with H⁺: c = 8.

Fe₂O₃ → FeO occurs in the upper, cooler part of the furnace (≈500–800 K), not in the 900–1500 K zone.