3.5 Kinetics

  • Created by: holly_b
  • Created on: 04-01-17 09:40
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  • Kinetics
    • Experimental Methods
      • Iodine Clock
        • When the reaction first begins the iodine that is formed reacts with sodium thiosulfate.
          • Only when the thiosulfate is used up does the iodine produce a dark blue colour with starch.
        • The time taken from mixing the reactants to the formation of the dark blue starch complex gives a measure of the initial rate of reaction.
        • Initial rate is proportional 1/t
      • Measure volume of gas formed over time.
      • Measure change in mass over time.
      • Measure concentration of acid by sampling at set times intervals.
      • Some reactions cannot be easily monitored by methods throughout the entire chemical reaction so sampling and quenching may be used.
        • Sampling
          • A small amount of the sample is removed at regular intervals.
          • If the catalyst is homogenous then it is sampled too so it needs to be quenched.
        • Quenching
          • The sample is placed into iced water to cool and dilute the mixture which stops the reaction.
        • Pros
          • Accurate
          • Can be applied to a large number of chemical reactions
        • Cons
          • Time consuming
          • Labour internsive.
          • Homogenous mixture
    • The Rate Equation
      • Order of Reactions
        • Order tells you how much the concentration of a reactant affects the rate.
        • Individual order: the power to which a concentration is raised in the rate equation
        • Overall order: the sum of all the individual orders on the rate equation, e.g. if m = 2 and n = 1, the overall order of reaction would be 2 + 1 = 3
        • Graphical Determination
        • For a reaction to occur reacting particles must collide with sufficient activation energy.
      • Units of the Rate Constant
        • Zero order: Unit =
        • First order: Unit =
        • Second order: Unit =
      • Rate Determining Step
        • Most reactions take place by a number of simple steps, the sequence being known as the mechanism. The reaction cannot proceed faster than the slowest of these steps.
    • The Arrhenius Equation
      • On the straight line graph of lnk (y) and 1/T (x), the gradient is -Ea/R and the y intercept is lnA.


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