Given below are two statements for catalytic properties of transition metals. Statement I : First row transition metals which act as catalyst utilise their 3d electrons only for formation of bonds between reactant molecules and atoms on the surface of catalyst. Statement II : There is increase in the concentration of reactants on the surface of catalyst which strengthens the bonds in reacting molecules. In the light of the above statements, choose the correct answer from the options given below :

Both Statement I and Statement II are incorrect

Both Statement I and Statement II are correct

Statement I is correct but Statement II is incorrect

Statement I is incorrect but Statement II is correct

A paper dipped in a dil. H_{2}{SO}_{4} solution of ’ X ’ upon treatment with {SO}_{2} gas turns into green. The compound ’ X ’ is :

Pb\left( {CH}_{3}COO \right)_{2}

Which of the following is NOT a physical or chemical characteristics of interstitial compounds?

They are very soft and ionic in nature.

They have high melting points, higher than those of pure metals.

They retain metallic conductivity.

They are chemically inert and usually non-stoichiometric.

JEE Main · Chemistry · Chapter 16

The d and f Block Elements JEE Main PYQs (2020–2026)

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JEE Main 2026MediumMagnetic Properties, Catalytic Properties & Coloured Ions
Q1
Given below are two statements for catalytic properties of transition metals.

Statement I : First row transition metals which act as catalyst utilise their 3d electrons only for formation of bonds between reactant molecules and atoms on the surface of catalyst.

Statement II : There is increase in the concentration of reactants on the surface of catalyst which strengthens the bonds in reacting molecules.

In the light of the above statements, choose the correct answer from the options given below :
- ABoth Statement I and Statement II are correct
- BBoth Statement I and Statement II are incorrect
- CStatement I is correct but Statement II is incorrect
- DStatement I is incorrect but Statement II is correct
Show answer & solution
Correct answer: (B)
The first-row transition metals use both their 3d and 4s electrons to form bonds with reactant molecules on the surface of the catalyst.

∴ Statement I is incorrect.

When reactants are adsorbed on the catalytic surface, their concentration on the surface increases, but this adsorption actually weakens the bonds within the reactant molecules. This helps in forming an activated complex and reduces the activation energy of the reaction.

∴ Statement II is incorrect.
JEE Main 2026EasyPreparation, Properties & Reactions of K2Cr2O7
Q2
A paper dipped in a dil. $H_{2} SO_{4}$ solution of ’ $X$ ’ upon treatment with $SO_{2}$ gas turns into green. The compound ’ $X$ ’ is :
- A $K I - s t r a c h$
- B $K M n O_{4}$
- C $P b (C H_{3} COO)_{2}$
- D $K_{2} C r_{2} O_{7}$
Show answer & solution
Correct answer: (D)
The correct answer is Option D: $K_{2} C r_{2} O_{7}$ .

A paper dipped in dilute $H_{2} SO_{4}$ solution of $X$ turns green when treated with $SO_{2}$ gas.

This means that $X$ must be a substance which changes to green on reduction by $SO_{2}$ .

Now, $SO_{2}$ is a reducing agent.

Acidified potassium dichromate, that is, $K_{2} C r_{2} O_{7}$ in dilute $H_{2} SO_{4}$ , is orange in colour and gets reduced to $C r^{3 +}$ , which is green.

The reaction is:

$C r_{2} O_{7}^{2 -} + 3 SO_{2} + 2 H^{+} \to 2 C r^{3 +} + 3 SO_{4}^{2 -} + H_{2} O$

So, the orange dichromate paper turns green.

Now check other options briefly:

A: $K I$ -starch

This is used to detect oxidising agents; it gives blue colour due to iodine-starch complex, not green.

B: $K M n O_{4}$

Acidified $K M n O_{4}$ gets decolourised by $SO_{2}$ , not green.

C: $P b (C H_{3} COO)_{2}$

Lead acetate paper is used for detecting $H_{2} S$ , giving black colour.

Hence, the compound $X$ is

$K_{2} C r_{2} O_{7}$
JEE Main 2026EasyGeneral Trends in 3d Series: Electronic Config & Radii
Q3
Which of the following is NOT a physical or chemical characteristics of interstitial compounds?
- AThey have high melting points, higher than those of pure metals.
- BThey are very soft and ionic in nature.
- CThey retain metallic conductivity.
- DThey are chemically inert and usually non-stoichiometric.
Show answer & solution
Correct answer: (B)
The main physical and chemical properties of interstitial compounds are :

They have very high melting points, even higher than the pure metals from which they are formed.

They are extremely hard. For example, some borides can be almost as hard as diamond.

They continue to show metallic conductivity, meaning they can conduct electricity like metals.

They are chemically inactive (inert) and often do not have a fixed composition, which is called being non-stoichiometric.