5.1.1 - how fast

• 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