Problem 74 - Chemistry The Molecular Science

Question

Which of these statements is (are) true? If a statement is false, reword it so that it becomes true. (a) The concentration of a homogeneous catalyst may appear in the rate law. (b) A catalyst is always consumed in the reaction. (c) A catalyst must always be in the same phase as the reactants. (d) A catalyst can change the course of a reaction and allow different products to be produced.

Step-by-Step Solution

Answer

Statements (a) is true; (b), (c), (d) are false and need rewording.

Step 1: Examine Statement (a)

Statement (a) claims that the concentration of a homogeneous catalyst may appear in the rate law. This statement is true because a homogeneous catalyst participates in the reaction and can affect the rate, hence its concentration can be part of the rate law.

Step 2: Examine Statement (b)

Statement (b) asserts that a catalyst is always consumed in the reaction. This is false because a catalyst is not consumed in a reaction; it is regenerated at the end and facilitates the process without being used up. The reworded true statement should be: 'A catalyst is not consumed in the reaction; it remains unchanged after the reaction.'

Step 3: Examine Statement (c)

Statement (c) suggests that a catalyst must always be in the same phase as the reactants. This is false because a catalyst can be in a different phase from the reactants, known as a heterogeneous catalyst (e.g., a solid catalyst working with gaseous reactants). The true statement is: 'A catalyst may be in the same or different phase compared to the reactants.'

Step 4: Examine Statement (d)

Statement (d) indicates that a catalyst can change the course of a reaction and allow different products to be produced. This is false because a catalyst does not change the thermodynamic path or products of a reaction; it only changes the reaction pathway to increase the reaction rate. A correct version would be: 'A catalyst speeds up a reaction by providing an alternative pathway, but it does not change the products of the reaction.'

Key Concepts

Homogeneous Catalyst

A homogeneous catalyst exists in the same phase as the reactants it is acting upon, usually in a liquid or gas phase.
This means that the catalyst is evenly distributed throughout the reaction mixture, facilitating a more uniform interaction with the reactants.
Key features of homogeneous catalysts include:

• Their ability to increase the reaction rate by lowering the activation energy, making it easier for reactants to convert into products.
• The catalyst's concentration can indeed appear in the rate law, as it actively participates in the reaction mechanism.
• Because homogeneous catalysts are mixed at a molecular level with reactants, the reaction pathway they provide tends to be more specific and selective.

A classic example of a homogeneous catalyst is sulfuric acid in the esterification process, where it accelerates the reaction while remaining in the solution.

Heterogeneous Catalyst

Heterogeneous catalysts differ from homogeneous catalysts as they exist in a different phase than the reactants, often as solids working in a liquid or gaseous environment.
The catalyst provides a surface where the reaction can take place, making the transition between reactants and products easier.
Here are some important characteristics of heterogeneous catalysts:

• They offer the advantage of easy separation from the reaction mixture post-reaction, which simplifies the purification process.
• Typically, the surface area of the catalyst is crucial; a larger surface area provides more active sites for the reaction.
• Commonly used in industrial processes, such as the Haber process for ammonia synthesis, where catalysts like iron play a crucial role.

Heterogeneous catalysis is often favored in large-scale industrial applications due to practicality and cost-effectiveness.

Reaction Rate

The reaction rate is a measure of how fast or slow a chemical reaction occurs.
It is influenced by several factors, one of which includes the presence of a catalyst. Catalysts work by providing an alternative pathway with lower activation energy, leading to a faster reaction without being consumed by it.
Key points about reaction rate include:

• For reactions with catalysts, the rate law can include terms for the concentration of catalysts, especially if it's homogeneous dealing with reaction intermediates.
• The rate of a reaction can also be affected by other factors like temperature, concentration of reactants, and surface area in the case of heterogeneous catalysts.
• An increased reaction rate means the reactants are converted into products more quickly, which can be crucial in both industrial and laboratory settings.

Understanding the factors that control reaction rate can help in designing processes that are more efficient and economical.