Chemistry MCQs for NEET — Practice Questions with Answers

The NCERT text explicitly lists '$CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)$' as an example under '1. Combination reactions'. Although it looks like a combustion reaction, combustion reactions are a type of combination reaction, especially when involving elemental dioxygen as mentioned in the text: 'All combustion reactions, which make use of elemental dioxygen, as well as other reactions involving elements other than dioxygen, are redox reactions.'

The NCERT text states, 'It may carefully be noted that there is no change in the oxidation number of hydrogen in methane under combination reactions and that of potassium in potassium chlorate in reaction (7.28). This may also be noted here that all decomposition reactions are not redox reactions. For example, decomposition of calcium carbonate is not a redox reaction.' In $CaCO_3 \rightarrow CaO + CO_2$, the oxidation states are: Ca (+2, +2, +2), C (+4, +4), O (-2, -2, -2). Since no oxidation states change, it is not a redox reaction.

The NCERT text states, 'A combination reaction may be denoted in the manner: A + B \rightarrow C. Either A and B or both A and B must be in the elemental form for such a reaction to be a redox reaction.' This implies changes in oxidation states are necessary. The formation of an oxide is a common example but not a necessary condition. For instance, $3Mg(s) + N_2(g) \rightarrow Mg_3N_2(s)$ is a redox combination reaction but does not produce an oxide.

The NCERT text explicitly states, 'Either A and B or both A and B must be in the elemental form for such a reaction to be a redox reaction.' This covers the conditions under which a combination reaction typically becomes a redox reaction due to changes in oxidation states from the elemental state (0).

The NCERT text defines decomposition reactions as 'opposite of combination reactions. Precisely, a decomposition reaction leads to the breakdown of a compound into two or more components at least one of which must be in the elemental state.' If the elemental state is achieved from a compound, a change in oxidation state occurs, making it a redox reaction.

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.