Rate equations

• What does the rate equation mathematically relate?

  The rate of reaction to reactant concentration
• For the reaction aA + bB -> products, what is the generalized rate equation?

  r = k[A]m[B]n
• What symbol is used for rate?

  r
• What is the usual unit for rate (r)?

  mol dm-3 s-1
• What does [A] mean?

  Concentration of A
• What is the unit for concentration in rate equations?

  mol dm-3
• What is 'k' called in the rate equation?

  Rate constant
• What are 'm' and 'n' called in the rate equation?

  Reaction orders
• What values are orders usually?

  Integers 0, 1, 2
• What does a reaction order of 0 mean with respect to that reactant?

  The concentration has no effect on rate
• What does a reaction order of 1 mean?

  First order
• What does a reaction order of 2 mean?

  Second order
• How are reaction orders determined?

  Experimentally
• How is the total order for a reaction calculated?

  Adding all the individual orders together
• For a zero-order reaction, what is the relationship between the concentration of A and the rate?

  Concentration of A has no effect
• If r = k[A]1, how does the rate of reaction relate to the concentration of A?

  Directly proportional
• If r = k[A]2, how does the rate of reaction relate to the concentration of A?

  Proportional to the concentration of A squared
• On what does the units of k depend?

  The overall order of reaction
• How do you work out the units of k?

  From the rate equation
• What two factors is the value of k independent of?

  Concentration and time
• What condition must be fixed for k to remain constant?

  Fixed temperature
• How does temperature affect the value of k?

  It increases if temperature increases
• For a 1st order overall reaction, what is the unit of k?

  s-1
• For a 2nd order overall reaction, what is the unit of k?

  mol-1dm3s-1
• For a 3rd order overall reaction, what is the unit of k?

  mol-2dm6s-1
• If Rate = k[A][B]0, what are the values of m and n?

  m = 1 and n = 0
• If Rate = k[A][B]0, what is the order with respect to A and B?

  First order in A and zero order in B
• If Rate = k[A][B]0, what is the overall order?

  1
• If Rate = k[A][B]0, how is this usually written?


  Rate = k[A]
• How are reaction orders determined - by looking at what?

  Experiment
• How do you calculate the units of k?

  1. Rearrange rate equation to give k as subject

  k = Rate / [A]

  1. Insert units and cancel

  k = mol dm-3s-1 / mol dm-3

  Unit of k = s-1
• Write the rate equation for a reaction between A and B where A is 1st order and B is 2nd order.

  r = k[A][B]2
• If r = k[A][B]2, what is the overall order?

  3
• Calculate the unit of k where r = k[A][B]2

  1. Rearrange rate equation to give k as subject

  k = Rate / [A][B]2

  1. Insert units and cancel

  k = mol dm-3s-1 / mol dm-3.(moldm-3)2

  1. Simplify fraction

  k = s-1 / mol2dm-6
• What is it called when you follow one experiment over time recording the change in concentration?

  Continuous rate method
• What does the gradient represent on a continuous rate method graph?

  The rate of reaction
• When is the reaction fastest on a continuous rate method graph?

  At the start, steepest gradient
• Why does the rate drop during a continuous rate method experiment?

  Reactants get used up and concentration drops
• What does a horizontal line on a concentration vs time graph represent?

  The reaction has stopped
• What type of reaction can change in volume of a gas be used to measure?

  Reactions with change in gas moles