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 does the symbol r represent in the rate equation?

  Rate of reaction
• What is the unit of r in the rate equation?

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

  The concentration of A in mol dm^-3
• What is k in the rate equation?

  The rate constant
• What are m and n in the rate equation?

  Reaction orders
• What do reaction orders indicate?

  The effect of concentration on the rate

  
• How are reaction orders determined?

  Experimentally, not from stoichiometric coefficients
• How is the total order of a reaction calculated?

  By adding individual orders m and n
• What does zero order mean for a reactant?

  No effect on the rate of reaction
• What is the rate equation for a zero-order reactant A?

  r = k[A]⁰ = k
• What does first order mean for a reactant?

  Rate is directly proportional to concentration
• What is the rate equation for a first-order reactant A?

  r = k[A]¹
• What does second order mean for a reactant?

  Rate is proportional to concentration squared
• What is the rate equation for a second-order reactant A?

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

  They vary based on the overall order
• What is the unit of k for a first-order reaction?

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

  mol^-1dm³s^-1
• What is the unit of k for a third-order reaction?

  mol^-2dm⁶s^-1
• What does the value of k refer to?

  A specific temperature for the reaction
• How does temperature affect the value of k?

  It increases with higher temperature
• What is the continuous rate method in experiments?

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

  The rate of reaction

  
• What happens to the gradient as the reaction proceeds?

  It decreases as reactants are used up
• What does a horizontal line on a concentration vs. time graph indicate?

  The reaction has stopped

  
• What is a common method for measuring gas volume in reactions?

  Using a gas syringe
• What is the typical setup for measuring gas volume in a reaction?

  Gas syringe in a stand with reactants
• How should quantities of reactants be calculated for gas reactions?

  Carefully, to avoid exceeding gas syringe volume
• What is the initial rate of a reaction?

  The rate at the start of the reaction
• How can the initial rate be calculated from a graph?

  From the gradient at time = zero
• What happens when one reactant is in large excess?

  It appears to have no effect on rate
• What is the effect of a pseudo-zero order reactant?

  Its concentration stays virtually constant
• What factors affect the rate of reaction?

  • Concentration of reactants
  • Temperature
  • Surface area
  • Presence of catalysts
• How does the initial rate method work?

  Calculates rate from the gradient at time = zero
• What is a common clock reaction example?

  Hydrogen peroxide with iodide ions
• What indicates the end point in a clock reaction?

  A sudden colour change occurs
• How can the order of reaction be determined in clock reactions?

  By varying the concentration of one reactant
• What does the initial rate represent in clock reactions?

  It can be represented as (1/t)
• How can the order of reactants be calculated from experimental data?

  By comparing experiments with varying concentrations