1.9 - 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 does a reaction order of 0 indicate?

  No effect of concentration on reaction rate
• How is the total order of a reaction calculated?

  By adding individual orders m and n
• What does a first order reaction imply about the rate?

  Rate is directly proportional to concentration
• What does a second order reaction imply about the rate?

  Rate is proportional to concentration squared
• How does the value of k change with temperature?

  It increases with an increase in temperature
• 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 is the process of continuous monitoring in rate equations?

  • Follow one experiment over time
  • Record change in concentration
  • Gradient represents rate of reaction
  • Reaction rate 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?

  The gradient decreases as reactants are used
• What is a common method for measuring gas volume in reactions?

  Using a gas syringe
• What is the initial rate of a reaction?

  The rate at the start of the reaction
• Why is the initial rate preferable for calculations?

  Concentrations are known at the start
• What does it mean if a reactant is in large excess?

  It appears not to affect the rate
• What factors affect the rate of reaction?

  • Concentration of reactants
  • Temperature
  • Surface area
  • Presence of catalysts
• How can the initial rate be calculated from a graph?

  From the gradient at time = zero
• What is a clock reaction?

  A reaction with a sudden color change
• What happens when thiosulfate ions react with iodine?

  They prevent a color change until exhausted
• How is the order of reaction determined in clock reactions?

  By varying the concentration of one reactant
• What is the overall rate equation for the reaction A + B + 2C → D + 2E?

  r = k[A][B]²
• How is the order of reactant A determined from experiments?

  By comparing experiments with varying A
• What does it indicate if the rate quadruples when concentration doubles?

  Order is 2 for that reactant
• What is the significance of the overall rate equation?

  It summarizes the relationship between reactants and rate
• What is the process for determining reaction orders from experimental data?

  1. Change initial concentrations one at a time
  2. Measure initial rates for each change
  3. Compare rates to determine order
• How can the order be calculated graphically?

  By plotting log(rate) vs log[reactant]
• What does the gradient of a log(rate) vs log[reactant] graph represent?

  The order of the reactant
• What happens when both reactant concentrations are changed simultaneously?

  Overall change on rate is multiplied
• What are the steps to derive the rate equation from experimental data?

  1. Conduct experiments varying one reactant
  2. Measure initial rates
  3. Determine orders for each reactant
  4. Write overall rate equation
• What is the equation for log rate?

  Log rate = log k + n log [Y]
• What does a graph of log rate vs log [Y] yield?

  A straight line with gradient n
• What does the gradient represent in the rate equation?

  The order n of the reaction
• What happens to percentage errors at high concentrations?

  They have the biggest percentage errors