5.1.1 - how fast

Cards (37)

  • Rate of reaction is change in concentration of a reactant or product per unit time
  • At a given instant you can calculate the rate of reaction by
    rate = change in concentration of reactants or products / time
  • if the order is 0 with respect to a reactant changing the concentration of the reactant has no effect on the rate.
  • in a first order reaction, rate is directly proportional to the concentration
  • in a second order reaction, change in rate is equal to the change in concentration squared.
  • Generic rate equation:
    rate = k[A]x[B]y
  • in a generic rate equation, k is the rate constant for the reaction
  • in a generic rate equation, [A] and [B] are the concentrations of the reactants A and B
  • in a generic rate equation x and y are the orders of reaction with respect to A and B
  • Zero order reactants dont appear on rate equations because they have no effect on the rate
  • the overall order of a reaction is calculated by the sum of individual orders.
  • to calculate units for the rate constant, use the units for rate ( moldm-3s-1) and the units for concentration ( moldm-3)
    rearrange the equation to get k = … then cancel out any repeated units.
  • If the overall order is 3, the units for rate constant are
    dm6mol-2s-1
  • If the overall order is 2, the units for rate constant are
    dm3mol-1s-1
  • if the overall order is 1, the units for rate constant are
    s-1
  • if the overall order is 0, the units for rate constant are
    moldm-3s-1
  • To measure the rate of a 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 or measure the change in mass of the reaction mixture
  • Half life is the time taken for concentration of a reactant to decrease by half.
  • The symbol for half life is t1/2
  • concentration-time graph for a zero order reactant
  • concentration-time graph for a first order reactant
  • first order reactants have constant half lives
  • The equation used to determine rate constant using half life is:
    k = ln(2) / t 1/2
  • To draw a rate concentration graph plot [A] against time. Draw tangents at different values to calculate the rate and then draw a secondary Graph of rate against [A].
  • Rate is directly proportional to 1/t
  • Rate concentration graph for a zero order reactant
  • Rate concentration graph for a first order reactant
  • Rate concentration graph of a second order reactant
  • To determine the rate constant from a rate concentration graph of first order:
    k = rate / concentration
  • The rate determining step is the slowest step in a multi step mechanism
  • Any species involved in the rate determining step appear in the rate equation. Species only involved after the rate determining step do not appear in the rate equation.
  • For a reactant in the rate equation, the order of the reactant indicates how many molecules of that reactant are involved in the rate determining step.
  • the rate equation of a reaction is rate = k[NO]2
    therefore there will be 2 molecules of NO present in the rate determining step.
  • Temperature affects the value of the rate constant for a given reaction
  • The rate of reaction is roughly doubled for a 10 degree temperature increase
  • The Arrhenius equation is
    k = Ae (-Ea/RT)
    k: rate constant
    A: pre-exponential factor
    R: gas constant
    T: temperature (in kelvin)
    Ea: activation energy for reaction (in joules)
  • The Arrhenius equation can be converted into a useful form for plotting a graph which is:
    lnK = -Ea/RT + lnA
    graph of lnK against 1/T is a straight line where the gradient is -Ea/R and the y intercept is lnA