Question

A catalytic poison renders the catalyst ineffective because: (a) It is preferentially adsorbed on the catalyst (b) It adsorbs the molecules of the reactants (c) It combines chemically with the catalyst (d) It combines chemically with one of the reactants

Short answer

(a) It is preferentially adsorbed on the catalyst. This option directly describes catalytic poisoning by blocking the active sites on the catalyst.

Step-by-step solution

Understanding Catalytic Poisoning

Catalytic poisoning refers to the phenomenon where a substance, called a poison, attaches to the surface of the catalyst and decreases its activity. When a poison is preferentially adsorbed on the catalyst, it blocks the active sites where the reactants would normally adsorb and react. This means the catalyst is no longer able to facilitate the reaction effectively.

Analyzing the Options

Evaluate each option against the definition of catalytic poisoning. If the substance adsorbs reactant molecules or chemically combines with one of the reactants, it is not directly poisoning the catalyst but rather interacting with the reactants. If it chemically combines with the catalyst, it may deactivate the catalyst but by a process different than preferential adsorption.

Identifying the Correct Option

We need to choose the option that best describes a direct interaction with the catalyst that would lead to a decrease in its effectiveness due to blocking of its active sites.

Key concepts

Catalyst

A catalyst is a substance that speeds up a chemical reaction without being consumed by the reaction itself. It works by providing an alternative reaction pathway with a lower activation energy than the uncatalyzed reaction. Catalysts are essential in many industrial processes, like the synthesis of chemicals and the treatment of exhaust gases, because they make reactions more efficient and cost-effective.

Catalysts function by creating a surface where the reactants can come together to react more easily. This process involves the formation of intermediate species that are less stable than the reactants or products, leading to a faster overall reaction. A catalyst is not altered in the process, allowing it to be used repeatedly until it becomes ineffective due to various factors, including poisoning.

Adsorption

Adsorption is a surface phenomenon where molecules (adsorbates) adhere to the surface of a solid or a liquid (adsorbent), forming a film. In the context of catalysis, the reactants are adsorbed onto the catalyst's surface, allowing them to come into close proximity and react more readily.

The process relies on physical forces (such as Van der Waals forces) or chemical bonds. The strength and nature of adsorption can profoundly affect the reaction rate. Adsorption is reversible and differs from absorption, where the substance is taken up by the volume, not just the surface, of a material. In catalysis, the effectiveness of a catalyst can be measured by the extent and nature of adsorption of reactants on its surface.

Reactants

Reactants are substances initially present in a chemical reaction that are consumed to form products. During a catalyzed reaction, reactants interact with the catalyst's surface through a process of adsorption, allowing for the formation of transitional complexes. These complexes are less stable than the reactants or the products, resulting in a lower activation energy for the reaction.

The nature and concentration of reactants can significantly influence the rate and outcome of the catalytic reaction. The presence of impurities or compounds that can act as catalytic poisons may also impact the efficiency of the catalyst by modifying the availability of the active sites required for the transformation of reactants into products.

Catalytic Activity

Catalytic activity refers to the ability of a catalyst to increase the rate of a chemical reaction. This activity is dependent on the properties of the catalyst, such as the number of active sites, and their accessibility to the reactants. The size, shape, and electronic properties of the catalyst play an important role in its effectiveness.

For maximum catalytic activity, the surface of the catalyst needs to be free of contaminants or poisons that can block active sites and reduce efficiency. Over time, catalysts can become less active due to physical changes or chemical reactions with substances other than the intended reactants, a process known as deactivation. Measures are often taken to protect the catalyst and maintain its activity for as long as possible, preserving the reaction's efficiency.