Rates -4

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  • Created by: Shannon
  • Created on: 02-04-15 13:32
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  • Rates
    • Rate determing step
      • The overall rate of a multi-step reaction is determined by the slowest step
      • Only molecules involved in the rate determining step appear in the rate equation
    • Rate of reaction
      • The rate of a chemical reaction is the change in concentration over a given time
      • The initial rate of a reaction is when the reactant are first mixed
      • Increased temperature will increase the rate of a reaction
    • Rate equation
      • Links the rate of a reaction to the concentration of the reactants
        • Done by either investigating increase or decrease in concentration of a reactant
      • e.g r =k[A][B]2
        • r = rate of reaction (moldm-3s-1)
        • k = rate constant
        • [ ] = concentration (moldm-3)
    • Orders of reaction
      • Orders tell you how much the concentration of a reactant affects the rate
        • Order can be zero if the rate is unaffected
      • First order
        • Rate is directly proportional to concentration of reactant e.g as [A] doubles so does the rate
      • Second order
        • Rate is proportional to the square of the concentration of the reactant e.g tripling [A] would increase the rate 3x
      • The overall order of the reaction is found by adding up the individual orders
    • Measuring the rate of a chemical reaction
      • Mass change - due to gas evolving
      • Colorimetry
      • Temperature change
      • Volume of gas given off
      • pH change
      • Titration
      • Conductimetric analysis
    • Half life
      • The time for the concentration of a reaction to half
      • The half life of a 1st order reaction is constant
      • The half life of a 2nd order reaction increases as conc falls
    • Reaction mechanisms: hydrolysis of halogenoalkanes
      • SN2
        • Favoured by primary haloalkanes
        • 2 species involved in rate determining step, 1 step mechanism
        • Nucleophile attacks from opposite side of halogen atom
          • Unstable transition complex formed
      • SN1
        • Favoured by tertiary haloalkanes because of steric hinderance
        • 1 species involved in rate determining step, 2 step mechanism
        • In SN1 reaction, a stable carbocation intermediate is formed - stable due to inductive effect


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