F322: Reaction Rate

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Introduction

  • "The change in concentration of a reactant or a product in a given time".
    E.g. Combustion = Fast reaction     Rusting of Fe = Slow reaction
  • For most reactions:
    -The rate is fastest at the start, the reactant concentration is at is highest.
    -As the reaction proceeds, the rate decreases.
    -When one of the reactants is used up, the rate becomes zero.
  • Factors affecting rate:
    -Temperature
    -Pressure
    -Concentration
    -Surface Area
    -Catalyst
  • For chemical reactions to occur, particles must collide; with sufficient energy.
  • ACTIVATION ENERGY.
  • The molecules must also collide with the correct orientation. 
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Factor: Concentration

  • Increasing concentration of a reactant will increase the rate of reaction:
    -Number of particles in a given volume increases.
    -Number of collisions will increase.
    -Number of SUCCESSFUL collisions will increase.
    -More reactions occur. 
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Factor: Pressure

  • Increasing pressure will increase the rate because it effectively increases concentration.
  • Increase rate because:
    -The particles are closer together.
    -More frequent collisions.
    -Number of successful collisions increases.
    -More reactions will occur. 
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Factor: Temperature

  • Must consider activation energy: "Energy required to start a reaction by breaking of bonds".
  • Increasing temperature increases rate because:
    -Molecules move faster as they have more kinetic energy.
    -Number of successful collisions occurs, as they have energy greater than activation energy.
    -More reactions will occur. 
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Factor: Temperature (Cont.)

  • We can show this on the Boltzmann distribution curve.

    -The graph shifts to the right and the peak is lower. 
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Factor: Catalysts

  • "A catalyst speeds up a reaction without being consumed in the overall reaction".
  • They provide an alternative route for reaction to go through, with lower activation energy.
  • Increase rate because:
    -Lower activation energy; so more molecules have sufficient energy.
    -Number of successful collisions increase.
    -More reactions will occur.
     
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Factor: Catalysts (Cont.)

  • The Boltzmann distribution shows that more molecules have the Ea required.
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Importance Of Catalysts

  • Used in industrial processes: 
    E.g. Fe in the Haber process to make NH3, Ziegler-Natta in production of polyethene, and Ni in hydrogenation of oils to make margarine.
  • They:
    -Can be reused.
    -They lower the activation energy, reducing energy and costs.
    -They means less fossil fuels are needed.
    -Better atom economy and less hazardous waste.
  • Enzymes are a biological homogenous catalyst, they are large protein molecules that are in the same phase as the reactants.
  • Industrially useful because:
    -Lower temperature and pressure needed, so less energy costs.
    -Produce single pure product, good atom economy.
    -They are biogradable, so can be disposed of. 
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