5.1.1 HOW FAST

• rate of reaction = change in concentration of reactant or product / time
• the order with respect to a reactant is the power to which the concentration of the reactant is raised in the rate equation
• the overall order of a reaction is the sum of the individual orders, m + n
• the rate constant, k, is the constant that links the rate of reaction with the concentrations of the reactants raised to the powers of their orders in the rate equation
• the time taken for the concentration of a reactant to decrease by half is known as the half-life of the reactant
• the rate-determining step is the slowest step of a multi-step reaction
• deducing order of reaction from concentration time graph shape

  
  

  a zero order reaction produces a straight line graph, with the concentration decreasing at a constant rate. the half life decreases with time
• deducing order of reaction from concentration time graph shape

  
  

  a first order reaction produces a curved graph with constant half-life and halving of the reactant concentration occurring at equal time intervals
• calculating reaction rates from gradient measurement of graphs

  rate = change in conc / time for the change to take place

  i.e change in y / change in x
• from a concentration-time graph of a first order reaction, measurement of constant half-life. t 1/2

  in an example, the initial conc is 1.000 mol dm-3

  half of the conc is 0.500 mol dm-3

  the conc occurs at 54s so the half life is 54s

  in a first order reaction the half life is constant so it will always be 54s
• because first order reactions have constant half-lives, the value of the half-life can be used to determine the value of the rate constant

  k = ln2 / t 1/2
• deducing order of reaction from rate time graph shape

  zero order w respect to a given reactant, the rate-concentration graph will appear as a plateaued line

  the graph shows rate rate ∝ [A]^0

  changes in the conc of this reactant have no effect on the rate
• deducing order of reaction from rate time graph shape

  1st order w respect to a given reactant, the rate-concentration graph will appear as a linear line, positive correlation

  graph shows that rate ∝ [B]^1

  if conc of reactant B is doubled, rate will double

  if conc of reactant B is tripled, the rate will triple
• deducing order of reaction from rate time graph shape


  2nd order w respect to a given reactant, the rate-concentration graph will appear as a curved line, positive correlation

  the graph shows that rate ∝ [C]^2

  if conc of reactant C is doubled, rate will increase by 2^2

  if conc of reactant C is tripled, rate will increase by 3^2
• continuous monitoring of rate

  concentration-time graphs can be plotted from continuous measurements taken during the course of a reaction
• two methods for continuous monitoring of reactions that produce a gas as one of the products

  monitoring by gas collection

  monitoring by mass loss