Chemistry Rate Laws and Mechanisms

Review of Rate Laws and Mechanisms

• Mechanisms describe the step-by-step process of a reaction
• The rate-determining step (slowest step) is critical in determining the overall reaction rate law
• Intermediates are formed and consumed during the reaction but are not part of the overall balanced equation.

Mechanisms: Example

• The reaction between NO and H₂ is a three-step process, with each step indicated, showing the sequence of molecular interactions.
  • NO + NO <=> N₂O₂ (fast)
  • N₂O₂ + H₂ => N₂O + H₂O (slow)
  • N₂O + H₂ => N₂ + H₂O (fast)
  • The slow step involves N₂O₂, determining the rate law for the reaction.

Rate Laws From Mechanisms

• Consider the example mechanism for Cl₂ + CHCl₃ → HCl + CCl₄:
  • Step 1: Cl₂ <=> 2Cl(g) (fast)
  • Step 2: Cl(g) + CHCl₃(g) => HCl(g) + CCl₃(g) (slow)
  • Step 3: Cl(g) + CCl₃(g) => CCl₄(g) (fast)
• The rate law is determined by the slow step(s)

Collision Theory

• Reactions are the result of effective collisions between reacting molecules.
• Factors affecting reaction rate include collision frequency, collision energy, and correct collision orientation.
• These factors appear in the rate law through the rate constant (k), activation energy (Ea), and temperature (T).

Temperature, Rate, Ea, and k

• Temperature affects the rate of reactions by impacting the fraction of molecules possessing the necessary activation energy for reaction.
• Higher temperatures lead to a higher proportion of molecules having the required kinetic energy.
• The activation energy (Ea) is the minimum energy required for a reaction to occur.

Relating k to Ea, T, and A

• The relationship between the rate constant (k) and the activation energy (Ea) is given by the Arrhenius equation

• k = Ae^(-Ea/RT)

• where:

  • k = rate constant
  • A = pre-exponential factor, related to collision frequency and orientation.
  • Ea = activation energy
  • R = ideal gas constant
  • T = absolute temperature

Activation Energy and Catalysts

• Catalysts lower the activation energy required for a reaction, increasing the reaction rate without being consumed in the overall process,
• During a reaction, bonds are broken in the reactants, and bonds are formed in the products to create new bonds.

Energy Profiles

• Energy profiles demonstrate the energy changes during a reaction.
• Endothermic reactions absorb energy.
• Exothermic reactions release energy.

Energy Profiles with Catalysts

• Catalysts lower the activation energy without changing the overall enthalpy change or energy level of reactants and products.

Review for Test Prep

• Review past quizzes and lab feedback
• Work through practice problems (FRQs)

Test your understanding of rate laws and reaction mechanisms with this quiz. Explore the intricacies of reaction steps, rate-determining steps, and the role of intermediates. Challenge yourself with examples and collision theory concepts to enhance your chemistry knowledge.