Problem 107 - Chemistry and Chemical Reactivity

Question

Determine if each of the following statements is true or false (a) For a reaction that can yield more than one product, the most stable product is always formed in the greatest amount. (b) For a reaction that can yield more than one product, the pathway that has the lowest activation energy always occurs more quickly. (c) Reactions that are thermodynamically favored are always fast. (d) Diamond is always more stable than graphite.

Step-by-Step Solution

Answer

False, True, False, False.

Step 1: Analyze Statement (a)

Consider the principle of kinetic versus thermodynamic control. The most stable product is associated with thermodynamic control, which may not always be formed in the greatest amount when kinetic factors dominate. Therefore, statement (a) is false.

Step 2: Analyze Statement (b)

Examine the concept of reaction kinetics. A reaction pathway with lower activation energy indeed occurs more quickly because it requires less energy to reach the transition state. Thus, statement (b) is true.

Step 3: Analyze Statement (c)

Consider the distinction between thermodynamics and kinetics. A thermodynamically favored reaction means it releases energy, but it does not imply that the reaction rate is fast. Many thermodynamically favored reactions can be slow. Therefore, statement (c) is false.

Step 4: Analyze Statement (d)

Recognize the stability of carbon forms. Graphite is more stable than diamond under normal conditions due to its lower Gibbs free energy. Therefore, statement (d) is false.

Key Concepts

Kinetic and Thermodynamic Control

In chemical reactions, understanding kinetic and thermodynamic control is crucial. These concepts help predict which product will form under various conditions. When a reaction is under kinetic control, the product is formed rapidly and may not be the most stable. This happens because the reaction pathway has a lower activation energy, allowing the product to form quickly.
Thermodynamic control, on the other hand, leads to the most stable product being formed. Although this product may not appear as rapidly, it's favored at equilibrium due to its lower energy state. At higher temperatures, products under thermodynamic control become more prevalent because molecules have enough energy to reach this stable state.

• Kinetic Control: Quick formation, not always stable.
• Thermodynamic Control: Slow formation, most stable product.

These concepts explain why the most stable product isn't always the most abundant.

Reaction Kinetics

Reaction kinetics focuses on the speed at which a reaction proceeds. A key factor here is the activation energy needed to reach the transition state from the reactants. The lower the activation energy, the faster the reaction occurs, as molecules easily achieve the energy necessary to react.
Time plays a crucial role in kinetics, helping determine how quickly products form after reactants interact. Catalysts can be used to lower activation energy, increasing the reaction rate without affecting the final products.

• Lower activation energy = faster reaction.
• Catalysts enhance rate, no effect on products.

This principle underscores the truth of the statement that pathways with lower activation energy occur more quickly.

Thermodynamic Favorability

Thermodynamic favorability concerns whether a reaction will naturally occur. A reaction favored by thermodynamics releases energy, moving toward a more stable, lower-energy state. However, favorability does not guarantee a fast reaction.
Many reactions that are thermodynamically favorable are still slow due to high activation energy barriers. These reactions need more energy input to start and proceed. This is why some processes happen very gradually despite being energetically favorable.

• Favorable = energy released, stable products.
• High activation barriers can slow down reactions.

The distinction between thermodynamics and kinetics explains why not all energy-releasing reactions are rapid.

Stability of Carbon Allotropes

Carbon exists in several forms, known as allotropes, with diamond and graphite being among the most well-known. Graphite is more stable than diamond under normal conditions.
This stability arises because graphite has a lower Gibbs free energy, meaning it's at a lower energy state compared to diamond. Although diamond is hard and durable, making it useful for various applications, it is not as thermodynamically stable.

• Graphite: Lower energy, more stable.
• Diamond: Higher energy, less stable.

This difference in stability is due to differences in atomic structure and bonding within each carbon form.