3.1.9 rate equations

• What is the formula for the rate of a reaction?

  Rate = change in concentrations / time
• What does the order of reaction indicate?

  It indicates the power of the reaction term in the rate equation
• What characterizes a zero-order reaction?

  The rate is unaffected by changes in [A]
• How is a first-order reaction defined?

  The rate is proportional to [A]
• What defines a second-order reaction?

  The rate is proportional to [A]^2
• What is the general rate equation format?

  Rate = k [X]^A [Y]^B
• What does the variable k represent in the rate equation?

  k is the rate constant of the reaction
• How does conc change affect the rate constant k?

  If temperature remains constant, changing the concentration of reactants will have no effect on k
• What happens to the rate of reaction when temperature increases?


  The rate of reaction increases as more particles have energy greater than the activation energy. As the LHS of the reaction increases, so must the RHS and the only value on the RHS that is able to change is k.

  AS RATE INCREASES, SO DOES K
• In the reaction 3A + B → 2C + D, how is the order with respect to A determined?


  By comparing experiments where [B] is constant and observing changes in [A] and rate
• What is the rate determining step in a reaction?


  • The slowest step in a reaction mechanism
  • The equation includes all reactants up to and including this step
  • Intermediates are not included in the rate equation
• What are the rules for writing rate equations?

  1. Full equation ≠ rate equation
  2. Include reactants in all steps up to the slow step, excluding intermediates
  3. Catalysts can be in the rate equation but not the overall equation
  4. If there’s 1 molecule of reactant up to the slow step, it’s first order, etc.
  5. None of the species after the slow step are in the rate equation
• How do concentration-time graphs indicate the order of a reaction?

  • Zero Order: Straight line graph, conc changes but rate's constant, K=gradient
  • First Order: Straight line graph through the origin
  • Second Order: Exponential graph starting at the origin
• What is the Arrhenius equation?

  k = A e^{-E_a / RT}
• What does the variable Ea represent in the Arrhenius equation?

  Ea is the activation energy in J mol-1
• What is the gas constant R in the Arrhenius equation?


  R = 8.32 J K-1 mol-1
• What does the variable A represent in the Arrhenius equation?

  A is the Arrhenius constant, which has the same units as k
• What is the relationship between k and the rate of reaction?

  A larger value of k indicates a faster rate of reaction
• What are the units of k dependent on?

  The units of k depend on the order of the reaction
• What is the mathematical quantity E in the Arrhenius equation?


  E is the base of the natural logarithm
• Arrhenius equation

  $k=$$Ae^{\frac{-Ea}{RT}}$