Lecture on Kinetics

Introduction to Kinetics

• Definition: The study of reaction rates, i.e., how fast reactions occur.
• Comparison to Thermodynamics:
  • Thermodynamics tells us whether a reaction should happen (spontaneity, endothermic/exothermic nature, entropy changes).
  • Kinetics tells us the speed of the reaction.

Example of Thermodynamics vs. Kinetics

• Example: Paper reacting with O₂ in the air
  • Thermodynamics: Reaction should happen (paper should burn when exposed to air).
  • Kinetics: High activation energy prevents it from occurring spontaneously at room temperature without additional energy (e.g., a flame or heat from an oven).

Rate Expressions

Definition

• Rate expressions are used to express the rate of a chemical reaction.
• Example: A reaction between nitrogen and hydrogen.

Measuring Reaction Rates

• Methods:
  • Measure the rate of disappearance of reactants.
  • Measure the rate of appearance of products.
• For the given reaction:
  • Measure the change in concentration of nitrogen, hydrogen, or ammonia over time.

Stoichiometry in Rate Expressions

• Rates of reactants and products must account for stoichiometric coefficients.
• Example:
  • Hydrogen is produced 3 times faster than nitrogen.
  • Proper rate expression compensates for stoichiometry by dividing by the coefficient.

Sign Convention

• Reactants: Rates are negative due to decrease in concentration.
• Products: Rates are positive.

Applying Rate Expressions

Exam Questions

• Identify valid rate expressions for a given reaction.
• Compare numerical rates of production or consumption for reactants/products.

Example Problem

• Given: Rate of change of hydrogen = 12 mol/min
• Find: Rate of production of nitrogen and consumption of ammonia.
  • Nitrogen rate: 4 mol/min (3 times less than hydrogen).
  • Ammonia rate: -8 mol/min (consumption, hence negative).

Terminology

• Rate of Consumption: Only the absolute value is given, no negative sign.
• Rate of Change: Indicate direction (negative for reactants, positive for products).

Classroom Example

• Anecdote involving Shahab, a student and hypothetical gambler, illustrating the concept of negative values in context (e.g., loss = negative change).

Key Takeaways

• Understand the distinction between thermodynamics and kinetics.
• Be able to write and interpret rate expressions accounting for stoichiometry.
• Be aware of how terminology affects whether to include negative signs in rates.