Which one of the following characteristics of the transition metal is associated with their catalytic activity?

Variable oxidation states

The correct answer is option 1. variable oxidation states.

Let us delve into why variable oxidation states are crucial for the catalytic activity of transition metals.

Transition Metals and Catalytic Activity

1. Variable Oxidation States:

Transition Metals: Transition metals are elements found in the d-block of the periodic table. These metals have partially filled d-orbitals, which give them the ability to exhibit a range of oxidation states. This is because the energy difference between the d-orbital and the s-orbital electrons is relatively small, allowing electrons to be easily removed or added.

Catalytic Activity: Catalysis involves speeding up a chemical reaction without being consumed in the process. Transition metals often act as catalysts by participating in redox (reduction-oxidation) reactions. Their ability to exist in multiple oxidation states allows them to alternately donate and accept electrons during a reaction. This process can lower the activation energy of the reaction and facilitate the conversion of reactants to products.

Example of Catalytic Activity:

Hydrogenation Reactions: In the hydrogenation of alkenes (adding hydrogen to unsaturated compounds), a transition metal like nickel, platinum, or palladium is used as a catalyst. These metals can switch between different oxidation states (e.g., \(Ni^{2+}\) to \(Ni^0\)), facilitating the breaking of hydrogen molecules \((H_2)\) and their addition to the alkene.

Oxidation of Alcohols: In the oxidation of alcohols to aldehydes or ketones, transition metals such as chromium or manganese can switch between different oxidation states (e.g., \(Cr^{3+}\) to \(Cr^{6+}\)) to facilitate the reaction.

Why Not the Other Characteristics

2. Colour of Hydrated Ions: The color of transition metal complexes arises from d-d transitions, where electrons move between different d-orbitals. While this property is characteristic of transition metals, it is not directly related to their ability to act as catalysts.

3. Paramagnetic Behaviour: Paramagnetism is due to unpaired electrons in the d-orbitals of transition metals. Although it is a notable property, it does not directly impact the metal's catalytic activity.

4. High Enthalpy of Atomization: The high enthalpy of atomization indicates strong metal-metal bonds in the solid state, reflecting the energy required to separate metal atoms. This property affects the stability of the metal in its elemental form but does not directly influence its catalytic properties.

Conclusion: Transition metals' ability to exhibit multiple oxidation states is crucial for their catalytic activity because it allows them to engage in redox reactions and facilitate various chemical transformations. This property enables them to donate and accept electrons efficiently, making them effective catalysts in numerous reactions.