Rate equations

• 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 are m and n in the rate equation?


  Reaction orders
• What does a reaction order of 0 indicate?


  The concentration of that reactant has no effect on the rate of reaction
• What does a reaction order of 1 indicate?


  The rate of reaction is directly proportional to the concentration of that reactant
• What does a reaction order of 2 indicate?


  The rate of reaction is proportional to the concentration of that reactant squared
• What does the notation [A] represent?


  The concentration of A
• 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 significance of the value of k in a rate equation?


  • Independent of concentration and time
  • Constant at a fixed temperature
  • Increases with an increase in temperature
• What is a continuous rate method?


  It involves following one experiment over time and recording the change in concentration
• 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 of the reaction, where the gradient is steepest
• What does it mean when the graph becomes horizontal?

  The reaction has stopped
• How is the initial rate calculated?


  From the gradient of a continuous monitoring concentration vs time graph at time = zero
• What is the typical method for measuring the rate of reaction with magnesium and hydrochloric acid?


  1. Measure 50 cm³ of 1.0 mol dm^-3 hydrochloric acid.
  2. Weigh 0.20 g of magnesium.
  3. Add magnesium to the acid and start the timer.
  4. Record the volume of hydrogen gas collected every 15 seconds for 3 minutes.
• What is the reaction between hydrogen peroxide and iodide ions?


  H₂O₂(aq) + 2H⁺(aq) + 2I^-(aq) → I₂(aq) + 2H₂O(l)
• What happens when I₂ produced reacts with thiosulfate ions?


  2S₂O₃^2-(aq) + I₂(aq) → 2I^-(aq) + S₄O₆^2-(aq)
• What indicates that excess I₂ remains in solution during the clock reaction?


  The reaction with starch forms a dark blue-black color
• What is varied in the series of experiments involving iodide ions?


  The concentration of iodide ions
• What is measured in each experiment with iodide ions?


  The time taken for the reaction mixture to turn blue