Rates of reaction

  • Created by: dgavan
  • Created on: 22-05-19 11:22
View mindmap
  • Rates of Reaction
    • Orders of reactants
      • Zero order: changing [reactant] has no effect on rate
        • NOT included in the rate eqn
      • First order: change in [reactant] is directly proportional to change in rate
        • Included in rate eqn
          • Rate eqn:  rate = k[A]^m [B]^n
      • Second order: rate is multiplied by (change in [reactant]) squared
        • Included in rate eqn
          • Rate eqn:  rate = k[A]^m [B]^n
      • Order of reactants can be determined through exp.al data given in a table
      • Overall order: the sum of all the orders of reactants e.g. if you had one zero order, one first order and two second order, then overall = 0 + 1 + 2 + 2 =5
    • Concentration-time graphs
      • Must be produced from continuous monitoring  e.g. vol of gas collected, mass lost, pH and colorimetry
      • Method: (1) Pick a conc for the reactant being investigated (2) Start the exp (3) Measure [R] by taking samples at regular time intervals
        • Processing the data: (1) Plot a graph of conc vs time (2) Draw a line of best fit (3) Draw a tangent and find gradient to get rate at that point
      • Orders
        • Zero order reactants have a linear (straight-line) conc-time graph going down
        • First order reactants have a constant half life (t1/2); time taken for [R] to halve. It's a curved graph
          • You can find rate by drawing a tangent then measuring gradient, and you  can then rearrange to get k
          • OR... you can just do k = ln2/half life
        • You don't need to know how to calc rate from a second order graph
    • Rate-concentration graphs
      • Must be produced from initial rates; the conc of 1 thing is varied and everything else stays the same.
        • It's the time taken for "something" to happen e.g. iodine clock (time taken for colourless -> blue/black once all the thiosulphate ions are used up). Could also be time taken for ppte to form, a set vol of gas to be produced etc
      • Rate = 1/time i.e. rate in s^-1
      • Orders
        • Zero order is just a flat horizontal line so k = y-intercept
        • First order is a straight line going diagonally up; k = gradient
          • MUST PASS THROUGH THE ORIGIN
        • You don't need to know how to calc k for a 2nd order reactant, but the graph is a curve going up
          • MUST PASS THROUGH THE ORIGIN
    • Arrhenius eqn
      • Only temp can affect k (rate constant)
      • k = Ae^-(Ea/RT)
        • So... lnk = (-Ea/R) (1/T) + lnA
          • i.e. y = mx + c
          • Forms a straight line graph (remember if lnA = y-int, then you've gotta un-ln it to get A, i.e. A = e^y-intercept)
        • k= rate constant
        • A = pre-exponential frequency factor (relates to no of successful collisions
        • e = exponential factor (just the e button on your calc)
        • Ea = activation energy (kJmol^-1)
        • R = gas constant (8.314 mol^-1K^-1)
        • T = temp (K)
    • Rate-determining step
      • Steps happen at different speeds; the slowest one dictates the rate i.e. RDS
      • Everything in the rate eqn must be in the RDS
      • You can use the info from the RDs, the rate eqn and the overall eqn to piece together a potential mechanism

Comments

No comments have yet been made

Similar Chemistry resources:

See all Chemistry resources »See all Rates resources »