Rate equations

• What does the rate equation relate mathematically?

  The rate of reaction to reactant concentration
• What is the generalised rate equation for the reaction aA + bB → products?

  r = k[A]m[B]n
• What is the unit of r in the rate equation?

  mol dm-3 s-1
• What do the square brackets [A] represent in the rate equation?

  The concentration of A in mol dm-3
• What is the rate constant denoted by in the rate equation?

  k
• What are m and n in the rate equation?

  Reaction orders for reactants A and B
• What integer values can reaction orders typically take?

  0, 1, or 2
• How are reaction orders determined?

  They are worked out experimentally
• How is the total order of a reaction calculated?

  By adding all individual orders together
• What does zero order mean for a reactant's concentration?

  It has no effect on the rate of reaction
• What is the rate equation for a zero-order reaction with respect to A?

  r = k
• What does first order mean for a reactant's concentration?

  The rate is directly proportional to concentration
• What is the rate equation for a first-order reaction with respect to A?

  r = k[A]
• What does second order mean for a reactant's concentration?

  The rate is proportional to concentration squared
• What is the rate equation for a second-order reaction with respect to A?

  r = k[A]2
• How do the units of k depend on the overall order of reaction?

  They must be worked out from the rate equation
• What is true about the value of k?

  It is independent of concentration and time
• How does temperature affect the value of k?

  It increases with higher temperature
• What is the unit of k for a first-order overall reaction?


  s-1
• What is the unit of k for a second-order overall reaction?

  mol-1 dm3 s-1
• What is the unit of k for a third-order overall reaction?

  mol-2 dm6 s-1
• What is the process of continuous monitoring in rate equations?

  • Follows one experiment over time
  • Records change in concentration
  • Gradient represents the rate of reaction
  • Reaction rate drops as reactants are used up
• What does the gradient of a concentration vs. time graph represent?

  The rate of reaction
• When is the reaction fastest according to the concentration vs. time graph?

  At the start where the gradient is steepest
• What happens to the graph as the reaction proceeds?

  It becomes horizontal as the reaction stops
• How can the change in volume of a gas be measured in a reaction?

  Using a gas syringe
• What is a limitation of a typical gas syringe?

  It only measures up to 100 ml of gas
• What is the typical method for measuring gas volume in a reaction?

  Measure hydrochloric acid and magnesium
• How is the initial rate of reaction calculated?

  From the gradient at time = zero
• What does a large excess of one reactant imply?

  It appears not to affect the rate
• What is the term for a reactant that does not affect the rate due to excess?

  Pseudo-zero order
• What factors affect the rate of reaction?

  • Concentration of reactants
  • Temperature
  • Surface area
  • Presence of catalysts
• How can initial rates be calculated from clock reactions?

  By measuring time to reach fixed concentration
• What happens when I2 reacts with thiosulfate ions in a clock reaction?

  It forms a dark blue-black color
• How can the order of reaction be determined in clock reactions?

  By varying the concentration of a reactant
• What is the relationship between initial rate and time in clock reactions?

  Initial rate is represented as (1/t)
• How can the order of a reactant be calculated from experimental data?

  By comparing experiments with varying concentrations
• What is the overall order of the reaction A + B + 2C → D + 2E?

  3rd order overall
• How is the unit of the rate constant k derived?

  From the rate equation and its units
• What is the graphical method for determining reaction order?

  • Change concentration of one reagent
  • Measure reaction rate
  • Log both sides of rate equation
  • Plot log rate vs log [Y]
  • Gradient equals order n