Rate equations

• What is the rate of a reaction?


  How fast a product is produced in a given time.
• What are the 4 ways a reaction can be monitored and how do we measure these?

  1) Colour change (calorimeter)

  2) Collecting gas (gas syringe)

  3) Temperature change (thermometer)

  4) Change in mass (balance)
• What are 5 ways we can speed up a reaction?


  Increase temperature

  Increase pressure

  Increase concentration

  Increase surface area

  Catalyst
• Acid 1 : V = 50cm^3

  c = 0.1 moldm^-3

  n = 0.005 moles

  
  

  Acid 2 : V = 25cm^3

  c = 0.2 moldm^-3

  n = 0.005 moles

  What happens on graph?

  Same volume produced (due to the same moles) but 2 has a higher concentration so faster rate.
• Acid 1 : V = 50cm^3

  c = 0.1 moldm^-3

  n = 0.005 moles

  
  

  Acid 2 : V = 25cm^3

  c = 0.1 moldm^-3

  n = 0.0025 moles

  What happens on graph?

  Acid 2 has half the volume produced (due to it having half the moles) but same rate as they have the same concentration.
• Draw a rate of reaction graph

  
• What is the initial rate of reaction?


  When t = 0

  Rate at the start of reaction
• What is the rate equation?


  Rate = k [A]^x [B]^y
• What is k in the rate equation?


  The rate constant
• What are the orders of a reaction?


  How changing the concentration of a particular reactant affects the rate of the reaction (independent of other reactants)
• How do you find the overall order of the reaction?


  Add the indices in the rate equation.
• In the equation rate = k [A]^x [B]^y

  What are A and B?

  Reactants
• In a rate equation, what are the only possibilities that the indices can be?


  0, 1 or 2
• In the rate equation, what are the indices known as?


  Orders e.g. x is the order of the reaction with respect to A
• If doubling reactant A doubles the rate, what does this mean?


  Directly proportional relationship.

  The order with respect to A is 1
• If doubling reactant B quadruples the rate, what does this mean?


  Square relationship (2^2 = 4)

  The order with respect to B is 2
• If doubling C does not affect the rate, what does this mean?


  No relationship

  The order with respect to C is 0
• What is the Arrhenius equation?


  k=Ae^(-Ea/RT)
• What is each thing in the Arrhenius equation and its units?


  Ea = Activation energy (J/mol or KJ/mol)

  R = gas constant

  T = temperature in Kelvin

  A = Arrhenius constant

  k = rate constant
• How do you get the power in the Arrhenius equation out of the power?


  lnk = lnA - Ea/RT
• Compare arrhenius equation to the straight line equation


  y = lnk

  m = -Ea/R

  x = 1/T

  c = lnA
• Draw the Arrhenius graph

  
• What is the equation for the gradient when you have Arrhenius equation?


  -Ea/R
• What is the equation for the activation energy when you have Arrhenius equation graph?


  -R x gradient
• According to Arrhenius, if we increase the activation energy, what happens?


  Rate constant (k) will decrease (less particles with sufficient energy to react)
• According to Arrhenius, if we increase the temperature, what happens?


  Rate constant (k) will increase (particles have more energy so increased chance of successful collisions)
• What is the RDS?


  The rate determining step - the step that is slower than all the others
• When given the mechanism of a reaction, what is the rate equation based on?


  The slow step (RDS)
• Based on the mechanism below, what is the rate of reaction?

  A + 2B + C ---> D + E

  Mechanism:

  1) A + B ---> X + D Slow step

  2) X + C ---> Y

  3) Y + B ---> E

  Rate = k [A]^ 1 [B]^ 1 [C]^0