How Fast

• What is the definition of the rate of reaction?

  Change in concentration of a reactant or a product per unit time
• How can you calculate the rate of reaction at a given instant?

  Rate of reaction = change in concentration of reactants or products / time
• What does it mean if the order is zero with respect to a reactant?

  Changing the concentration of the reactant has no effect on the rate
• What is the effect on rate in a first order reaction?

  Rate is directly proportional to the concentration
• What is the effect on rate in a second order reaction?

  Change in rate = change in concentration squared
• What is a generic rate equation and what does each term mean?

  • Rate = k [X]^x [Y]^y
  • k = rate constant for the reaction
  • [X] and [Y] = concentrations of species X and Y respectively
  • x and y = orders of reaction with respect to X and Y
• Do zero order reactants appear in the rate equation? Why?

  No, because they don’t have an effect on the rate
• How is the overall order of a reaction calculated?

  Sum of individual orders
• How could you measure the rate of reaction experimentally?

  • Use a colorimeter at suitable intervals if there is a colour change
  • If gas is evolved, use a gas syringe to collect volume of gas evolved
  • Measure the change in mass of the reaction mixture
• What does half-life mean?

  The time taken for concentration of a reactant to decrease by half
• What is the symbol for half-life?

  t 1/2
• What is the relationship between first order reactions and half-life?

  • First order reactants have constant half-lives
• What is the equation used to determine the rate constant using half-life in a first order reaction?

  k = ln2 / t 1/2
• How would you draw a rate concentration graph?

  1. Plot [A] against time
  2. Draw tangents at different values
  3. Draw a secondary graph of rate against [A]
• What is the relationship between rate and time?

  Rate ∝ 1/t
• How to determine the rate constant from a rate concentration graph of first order?

  k = rate / concentration
• What is the rate determining step?

  The slowest step in a reaction with multiple steps
• How does the rate determining step link to the species involved in the rate equation?

  Any species involved in the rate determining step appear in the rate equation
• For a reactant in the rate equation, what indicates how many molecules of that reactant are involved in the rate determining step?

  The order of the reactant
• In the rate equation of a reaction, rate = k[NO]², how many molecules of NO will be present in the rate determining equation?

  2
• What affects the value of the rate constant for a given reaction?

  Temperature, nothing else
• What is the effect of a 10℃ temperature increase on the rate of reaction, roughly?

  Doubles rate of reaction
• What is the Arrhenius equation and what does each term mean?

  • Arrhenius equation: k = A e^{-Ea/RT}
  • k = rate constant
  • A = frequency factor
  • Ea = activation energy
  • R = universal gas constant
  • T = temperature in Kelvin
• How can you convert the Arrhenius equation into a useful form for plotting a graph?

  • ln k = -Ea / RT + ln A
  • Graph of ln k against 1/T is a straight line
  • Gradient = -Ea/R
  • y-intercept = ln A