Chemical Reaction Rates and Equilibrium Explained

Chemical Reaction Rates: Fundamentals

Chemical reaction rate: The amount of substance consumed or produced per unit time.

Chemical Kinetics: The study of the rates of chemical reactions and the mechanisms by which they occur.

Collision Theory Explained

Collision theory: States that chemical reactions occur through effective collisions between reactant particles, whether atoms, molecules, or ions. For this to occur, particles must have appropriate orientation and sufficient activation energy.

Factors Influencing Reaction Rates

• Nature of Reactants

  Depends on the inherent reactivity of each substance itself.

• Physical State of Reactants

  The smaller the particle size of a material (or the more finely divided it is), the more rapid the reaction rate.

• Concentration of Reactants

  Even with the same total quantity of material, a more concentrated solution will react faster.

• Temperature Effects

  As temperature increases, the reaction rate accelerates because the number of effective collisions between particles is greater. If the temperature is too low, the reaction rate may become negligible, or reactions may cease.

Note: The original document contained an unclear formula "Vr = Mol Ins Ins". Assuming "Vr" refers to Reaction Rate, a typical unit is Moles per Liter per Second (mol/L·s).

Chemical Equilibrium Principles

Types of Chemical Reactions

• Reversible Reactions

  Chemical reactions where products can react to reform the original substances.

• Irreversible Reactions

  Chemical reactions where products cannot easily revert to the original substances.

Understanding Chemical Equilibrium

Chemical equilibrium: The state where the concentrations of reactants and products have stabilized at specific values. At equilibrium, the rates of the forward and reverse reactions are equal, and the concentrations of reactants and products remain constant.

The Equilibrium Constant (K)

The equilibrium constant (K) is a measure of the equilibrium position.

• If K is small, the equilibrium is shifted to the left (favoring reactants).
• If K is close to 1, then both reactants and products are present in appreciable concentrations at equilibrium.
• If K is large, the equilibrium is shifted to the right (favoring products).

Le Chatelier's Principle

If a change in temperature, pressure, or concentration is applied to a system at equilibrium, the system will shift in a direction that counteracts the applied change.

Example: N₂O₄ Equilibrium

Consider the reaction: N₂O₄(g) + 14 kcal ⇇ 2NO₂(g)

• If the temperature is raised, the equilibrium shifts to the right (favoring products) because the forward reaction is endothermic (absorbs heat).
• If the pressure increases, the equilibrium shifts to the left (favoring reactants) because the left side has fewer moles of gas (1 mole N₂O₄ vs 2 moles NO₂).
• If the concentration of N₂O₄ is increased, the equilibrium shifts to the right (favoring products) to consume the added N₂O₄.

Factors Modifying Chemical Equilibrium

• Temperature Changes: Van 't Hoff's Law

  When the temperature of a system at equilibrium is increased, the reaction that absorbs heat (endothermic direction) is favored.

  • A + B + 6.00 kcal ⇇ C (Endothermic: promoted directly by absorbing heat; increasing temperature favors product formation).
  • A + B ⇇ C + 22,000 kcal (Exothermic: increasing temperature favors the reverse reaction, which absorbs heat).

• Pressure Changes

  When pressure is increased in a system at equilibrium, the reaction that reduces the total volume (fewer moles of gas) is favored. Conversely, reducing the pressure favors the reaction that increases the total volume. Pressure changes have no effect on reactions where there is no change in the number of moles of gas.

• Role of Catalysts

  A catalyst helps the system reach equilibrium more quickly in a closed container, without affecting the final equilibrium concentrations of reactants or products, thus having no effect on the position of equilibrium.

Equilibrium Constant Expression Example

For the reaction: 2AB ⇇ C₂ + D₂

The equilibrium constant expression is: K = [C₂][D₂] / [AB]²

Note on units: For this specific reaction, if concentrations are in mol/L, the units cancel out, making K dimensionless:

K = (mol/L) × (mol/L) / (mol/L)² = (mol/L)² / (mol/L)² = Dimensionless