Problem 20 - A Complete Resource Book in Chemistry for JEE Main

Question

Which one of the following characteristics of the transition metal is associated with their catalytic activity? (a) variable oxidation states (b) colour of hydrated ions (c) paramagnetic behaviour (d) high enthalpy of atomization

Step-by-Step Solution

Answer

(a) variable oxidation states.

Step 1: Understanding Transition Metals

Transition metals are elements that have partially filled d orbitals. This gives them unique chemical and physical properties that are not seen in s or p block elements. They are known for their tendency to lose electrons and form positive ions, which greatly contributes to their chemical reactivity.

Step 2: Identifying Relevant Properties

Out of the options given, we need to identify which property is most associated with catalytic activity of transition metals. Let's consider what catalytic activity involves: the ability to increase the rate of a chemical reaction by lowering the activation energy, typically through the formation of temporary intermediate compounds.

Step 3: Analyzing the Options

Let's examine each option: (a) Variable oxidation states - Transition metals often exhibit multiple oxidation states which allow them to form various intermediate complexes and act as catalysts in redox reactions by temporarily gaining or losing electrons. (b) Colour of hydrated ions - This is due to electronic transitions and does not directly relate to catalysis. (c) Paramagnetic behaviour - While interesting, this is related to unpaired electrons and does not directly affect catalytic ability. (d) High enthalpy of atomization - This relates to the energy required to break all atoms in the solid phase and doesn't directly affect their catalytic properties.

Step 4: Selecting the Best Answer

Based on the analysis, the ability of transition metals to exhibit variable oxidation states (option a) is most closely related to their catalytic activity. It allows them to engage in various oxidation-reduction reactions by easily changing their oxidation states, which is crucial for catalytic activity.

Key Concepts

Variable Oxidation States

Transition metals are renowned for having variable oxidation states. This means that the same metal can participate in reactions at different levels of oxidation. Essentially, they can "juggle" electrons between different energy levels. This versatility is key to their role in catalysis. Transition metals can engage in redox (reduction-oxidation) reactions by changing their oxidation states.

• For example, in industrial processes like the Haber process for ammonia production, Iron (Fe) can switch between +2 and +3 oxidation states.
• This ability allows transition metals to form temporary intermediates, making them excellent catalysts.

When a transition metal changes its oxidation state during a reaction, it provides a pathway with lower activation energy, speeding up the reaction process. This is why variability in oxidation states is crucial to understanding their effectiveness in catalysis.

Transition Metals

Transition metals are those positioned in the central block of the periodic table (d-block), including well-known elements such as Iron (Fe), Nickel (Ni), Copper (Cu), and more. They have partially filled d orbitals, a unique feature contributing to their diverse chemical properties.

• These metals can form complex ions by sharing electrons with ligands, aiding in catalytic activities.
• They often showcase distinct colors when hydrated, a result of electron transitions within their d orbitals, though this is unrelated to their catalysis properties.
• Their paramagnetic qualities, due to the presence of unpaired d electrons, add another layer of intriguing features.

Furthermore, these metals are essential in forming alloys, enhancing the materials’ strength and resistance to corrosion. Transition metals' unique electron configuration makes them flexible in their chemical behavior, crucial for their catalytic roles.

Catalytic Activity

Catalytic activity refers to the ability of substances to increase the rate of a chemical reaction without themselves being consumed. Transition metals are particularly known for their catalytic prowess due to several properties:

• They can easily change oxidation states, allowing them to facilitate redox reactions by providing essential intermediate complexes.
• Their ability to adsorb and activate molecules on their surface makes them advantageous in processes like hydrogenation and oxidation reactions.

When a transition metal acts as a catalyst, it often forms a temporary bond with the reactants, lowering the activation energy needed for the reaction. This temporary interaction enables the bond rearrangements necessary for the reaction to proceed. Overall, the structural and electronic properties of transition metals make them uniquely equipped to serve as effective catalysts in various industrial and biological processes.