Kinetics

• Kinetics


  The rate of a reaction is a measure of the speed at which reactants are changed into products
• Reactions


  • Sodium and water (very fast)
  • Ripening of fruits (very slow)
• Rate of reaction


  1. Change in concentration / Time for change to happen

  2. Nreactant / Atime

  3. Aproduct / Atime
• Collision Theory


  The collision theory states that for a reaction to occur the reacting particles must collide together with enough energy to react
• Activation energy


  The minimum amount of energy required for a reaction to occur
• Successful collisions are ones that result in a reaction, and they take place when reactant particles collide with the activation energy
• Factors affecting the rate of a chemical reaction

  • Concentration of reactants in solution
  • Size of the particles of a solid reactant (surface area)
  • Pressure of gaseous reactants
  • Temperature at which the reaction is carried out
  • Addition of a catalyst
• Concentration of reactants in solution

  Increasing the concentration of reactants increases the rate of reaction. There are more particles present in the same volume, leading to more successful collisions between particles in a given time, giving a faster rate of reaction
• Size of the particles of a solid reactant (surface area)

  If one of the reactants is powdered, then a larger surface area is created. This means that there will be a greater surface area so there are more particles on the surface exposed to the other reactant, resulting in more successful collisions (with the activation energy) between particles in a given time (the successful collisions are more frequent) giving a faster rate of reaction
• Pressure of gaseous reactants

  Increasing the pressure in a gaseous reaction increases the number of particles in a given volume. Pressure, like concentration, leads to a greater number of effective collisions and a faster reaction rate. The particles are pushed closer together, the same number of particles occupy a smaller volume, resulting in more successful collisions (with the activation energy) between particles in a given time (the successful collisions are more frequent) giving a faster rate of reaction
• Temperature at which the reaction is carried out


  Increasing temperature increases rate of reaction. Particles are moving faster so there are more frequent collisions. The particles have more energy so more of the particles have the activation energy and react when they collide, resulting in more successful collisions (with the activation energy) between particles in a given time (the successful collisions are more frequent) giving a faster rate of reaction
• Addition of a catalyst

  A catalyst provides a different pathway of lower activation energy. As a result there are more particles with greater energy than the activation energy, resulting in more successful collisions (with the activation energy) between particles in a given time (the successful collisions are more frequent) giving a faster rate of reaction
• Practical techniques to measure reaction rate


  • Measure the rate at which a reactant is used up
  • Measure the rate at which a product is formed
• Measuring the change in mass at a given time

  1. The change in mass during a reaction can be recorded at various time intervals using apparatus with a cotton wool plug to allow gas out but prevent liquid loss

  2. A graph of loss in mass against time, or mass against time can be plotted
• Measuring the volume of gas produced per unit time

  1. The amount of gas produced at different time intervals can be measured using an apparatus with a plunger that moves out as gas is produced

  2. The reaction is finished when no more gas is produced because one of the reactants is used up

  3. A curve is normally obtained with the steeper the curve indicating the faster the reaction
• Maxwell-Boltzmann distribution curve


  A plot of the number of particles having each particular energy in any system. The most common energy is represented by the peak value. The area under the curve gives the total number of particles. The number of particles with energy greater than the activation energy (Ea) is shaded, as only these particles have enough energy to react
• Increasing temperature


  Increases the rate of most chemical reactions dramatically. This is due to more molecules having energy greater than the activation energy, resulting in more successful collisions (with the activation energy) between particles in a given time (the successful collisions are more frequent) giving a faster rate of reaction
• Presence of a catalyst


  Provides an alternative pathway of lower activation energy, so there are more particles with greater energy than the activation energy
• Reaction profile for a catalysed and uncatalysed reaction

  The catalyst provides an alternative pathway with lower activation energy
• Types of catalysts


  • Heterogeneous catalyst (different state from reactants)
  • Homogeneous catalyst (same state as reactants)
• Heterogeneous catalysts


  • Reactants adsorb (stick) to the surface of the catalyst at active sites
  • Bonds in the reactant molecules weaken
  • Products desorb (leave) the active site
• Catalytic converters


  Fitted to vehicle exhausts to remove pollutants by getting them to react with one another to form harmless products. Consist of a ceramic honeycomb coated with metal catalysts like platinum, palladium and rhodium
• Catalytic converters have some drawbacks - they are expensive, the vehicle must run on lead-free fuel or the catalyst will be 'poisoned', and they are only effective at temperatures over 400°C