Chemical Reaction Rates: Factors, Catalysis, and Key Concepts

Chemical Reaction Rates: Fundamentals and Catalysis

Key Concepts in Chemical Kinetics

• Rate Law

  The speed of a reaction is directly proportional to the concentrations of the reactants raised to some exponent.

• Collision Theory

  This theory states that a greater concentration of reactants leads to more frequent collisions, thereby increasing the reaction rate.

• Reaction Profile

  A diagram that illustrates the progress or development of a chemical reaction.

Factors Influencing Chemical Reaction Rates

The speed of a chemical reaction depends on several critical factors:

• Nature of Reactants: The inherent properties of the reacting substances.
• Concentration: Higher concentrations generally lead to faster reactions.
• Physical State of Reactants: This factor involves two situations:
  • Homogeneous Reactions: When all reactants are in the same phase (e.g., all gases or all liquids).
  • Heterogeneous Reactions: When reactants are in different phases (e.g., a solid reacting with a gas).
  • Surface Area: For heterogeneous reactions involving solids, a larger surface area (e.g., smaller particle size) increases the reaction rate.
• Temperature: Higher temperatures typically accelerate reactions.
• Presence of Catalysts: Substances that alter reaction rates without being consumed.

Temperature and Reaction Speed

Why are chemical reactions faster at higher temperatures?

At higher temperatures, reactant molecules possess more kinetic energy, leading to more frequent and energetic collisions. This increases the likelihood of successful collisions that overcome the activation energy, resulting in a faster reaction rate.

Catalysis: Definition and Classification

Catalysis is the process by which the rate of a chemical reaction is increased or decreased by a substance known as a catalyst.

Types of Catalysis:

• Enzyme Catalysis: Involves enzymes, which are specialized proteins that significantly increase reaction rates in biological systems.
• Heterogeneous Catalysis: Occurs when the catalyst is in a different phase from the reactants (e.g., a solid catalyst in a gas-phase reaction).
• Homogeneous Catalysis: Occurs when the catalyst is in the same phase as the reactants (e.g., a liquid catalyst in a liquid-phase reaction).

Key Terms in Reaction Kinetics

• Activated Complex

  A transient, high-energy state that the reacting molecules must reach for a chemical reaction to occur. It represents the peak of the energy curve in a reaction profile.

• Activation Energy

  The minimum amount of energy required for a chemical reaction to proceed. Reactants must overcome this energy barrier to transform into products.

Important Chemical Applications and Processes

• Catalytic Converter

  A device used in vehicles to reduce harmful emissions. Its primary function is to reduce nitrogen oxides (NO, NO₂) to nitrogen (N₂) and oxygen (O₂), and to oxidize carbon monoxide (CO) and unburnt hydrocarbons to carbon dioxide (CO₂) and water (H₂O).

• Rhodium

  A precious metal often used as a catalyst in platinum alloys, particularly in catalytic converters.

• Ammonia

  A vital chemical compound with significant use as a fertilizer, increasing soil nitrogen levels.

• Haber-Bosch Process

  An industrial method for synthesizing ammonia directly from nitrogen and hydrogen gases (N₂ + 3H₂ → 2NH₃).