Topic 6

• Chemical kinetics


  The study of the rate of chemical reactions
• Many chemical reactions occur very quickly, such as the rapid inflation of airbags in cars, while others such as rusting take place over a period of years.
• Rate of reaction


  The change in concentration of reactants or products per unit time
• Units of rate of reaction

  mol dm-3 s-1, mol dm-3 min-1, etc.
• Experimental measurements of reaction rates

  1. Monitor a property that will change when the reactants are converted into products

  2. Examples: change in pH, change in conductivity, change in mass or volume, change in colour
• Determining the rate of reaction

  1. Plot a graph of concentration (or the property associated with concentration) versus time

  2. The rate of reaction is determined from the slope or gradient of the tangential line at time t
• Average rate


  A measure of the change in concentration of reactant or product in a given time interval
• Instantaneous rate


  The rate at a specific point in time, given by the limit of Δc/Δt as Δt approaches 0
• Initial rate


  The instantaneous rate at the start of the reaction (t=0)
• Determining initial rate


  Draw a tangent to the curve at t=0 and find the slope/gradient
• Determining instantaneous rate


  Draw a tangent to the curve at the desired time point and find the slope/gradient
• Rate equation


  A mathematical differential expression showing rate in terms of concentration
• Kinetic molecular theory of gases

  • Gases consist of a large number of particles moving at high velocities in random directions
  • The size of gaseous particles is negligible
  • Collisions between gaseous particles are completely elastic
  • The average kinetic energy of the particles is proportional to the absolute temperature in kelvin
• Occam's razor


  A principle stating that the simplest explanation that fits the observed facts should be preferred
• Collision theory


  • For a reaction to occur, the reacting particles must collide with each other, have the correct mutual orientation, and have sufficient kinetic energy
• Activation energy


  The minimum energy that colliding particles need for a reaction to occur
• Activation energy


  Analogous to the minimum energy required to push a rock over a hill
• Transition state/Activated complex


  The arrangement of atoms at the crest of the potential energy profile
• Catalysts


  Can be homogeneous or heterogeneous
• Ion


  Particle with a net positive or negative charge
• Orientation is favourable, so may result in reaction if there is sufficient kinetic energy
• Effective collision


  A collision that results in a reaction
• Transition state


  The arrangement of atoms at the crest of the energy profile
• Catalyst


  A substance that increases the rate of a chemical reaction, but is not consumed in the reaction itself
• Catalysts


  • Provide an alternative pathway for the reaction and lower the activation energy
• Homogeneous catalyst


  A catalyst in the same physical phase or state as the reactants
• In the stratosphere, ozone in the ozone layer absorbs over 95% of the UV radiation reaching Earth from the sun, protecting us from this harmful radiation
• Catalytic destruction of ozone

  1. Chlorine atoms produced from CFCs with UV light

  2. Chlorine radicals attack ozone

  3. Chlorine acts as a catalyst and is regenerated
• Depletion of the ozone layer


  More UV radiation can reach the Earth's surface, increasing risk of skin cancers and cataracts
• Heterogeneous catalyst


  A catalyst in a different phase or state from the reactants, typically solid while reactants are liquid or gas
• Catalytic converter reactions


  Converts CO, NO, and unburned hydrocarbons to CO2, H2O, and N2
• Leaded petrol (gasoline) poisons catalytic converters
• Maxwell-Boltzmann energy distribution curve

  Plot of the fraction of particles with a given kinetic energy versus kinetic energy
• Increasing temperature


  Increases the proportion of particles with sufficient kinetic energy to overcome activation energy barrier
• Typically with a 10°C increase in temperature, the reaction rate will double
• Kelvin scale


  Measure of the average kinetic energy of particles, with 0 K being the lowest theoretically attainable temperature
• Physical properties like temperature are discovered, not invented
• Factors that affect the rate of a chemical reaction

  • Increasing temperature
  • Addition of a catalyst
  • Increasing reactant concentration
  • Decreasing particle size of solid reactants
• Factors that can increase the rate of a chemical reaction

  • Increasing the temperature at which the reaction is conducted
  • Addition of a catalyst
  • Increasing the concentration of the reactants
  • Decreasing the particle size of reactants in the solid phase
• Increasing the temperature at which the reaction is conducted

  Increases the rate of reaction