Kinetics

• Collision theory

  1. For a reaction to occur the reactants must:

  2. Collide with each other

  3. Collide with sufficient energy to break the bonds between them

  4. Collide with the correct orientation

  5. The energy of the collisions must be equal to or greater than the activation energy

  6. Only a small proportion at a given temperature will have sufficient kinetic energy to overcome this activation energy barrier
• Activation energy

  Minimum amount of energy required for a reaction to occur
• The activation energy will be different for different reactions
• A low activation energy means that most collisions will result in a reaction and vice versa
• Increasing reaction rate

  Two main ways of increasing the rate of a reaction:

  2. Increase the frequency of collisions

  3. Increase the amount of kinetic energy of the collisions so that a greater number of collisions lead to a reaction
• Higher concentration (or pressure for gases) on reaction rate

  • Higher concentration = more particles per unit volume
  • High frequency of collisions so initial rate of reaction is greater
  • Graph flattens out in shorter time because reaction is completed in shorter time
  • Amount of CO2 produced remains the same
• Effect of temperature on reaction rate

  • Average particle kinetic energy is increased
  • Increased kinetic energy means increased speeds and so an increased frequency collision
  • A greater proportion of collisions have sufficient energy to overcome the energy of the activation barrier
  • The reaction rate is approximately doubled if the temperature is increased by 10 degrees Celsius
  • Initial reaction rate is greater at higher temperature because a greater proportion of particles have energies in excess of the activation energy
  • The proportion of successful collisions is increased
  • Graph flattens out in a shorter time because the reaction is completed in a shorter time
  • The amount of CO2 produced remains the same (limiting reagent)
• Effect of surface on reaction rate

  • Solids with a smaller particle size (e.g powders or small chips) react more quickly than solids with a larger particle size
  • More of the surface area is exposed and so the frequency of collisions increases
• Maxwell Boltzmann curve demonstrates how changes in temperature affect the rate of reaction within a curve
• Curve at lower temperature v/s higher temperature


  • Important points:
  • Neither curve is symmetrical
  • Both start at the origin and finish near the axis
• Small fraction of particles have either very low or very high velocities
• Most of the particles have intermediate velocities
• Curve at lower temperature v/s higher temperature


  Neither curve is symmetrical
• Curve at lower temperature v/s higher temperature

  • Both start at the origin and finish near the axis (not touching it)
  • Area under each curve is the same (as the number of molecules has not changed)
  • The peak will change to left or right depending on whether temperature is going up or down
• Catalysts

  • A substance that increases the rate of a chemical reaction without being changed in chemical composition or amount
  • Work by providing an alternative reaction route of lower activation energy
  • Lower the activation energy and more reactant particles have sufficient energy to overcome activation energy barrier
• How do catalysts work?

  1. Heterogeneous catalysts are in a different state to the reactants -> the solid weakens the bonds in the reactants by adsorption

  2. Homogeneous catalysts are in the same state as the reactants -> either enhance the electrophilic or nucleophilic properties of the reagent
• Biological catalysts

  • enzymes are natures catalyst
  • They are specialised proteins that speed up biochemical reactions
  • Each enzyme will only work for specific reactions e.g amylase catalyses the hydrolysis of starch into sugars
• Catalysts in the industry

  • Significantly important
  • e.g Ni in hydrogenation of alkenes, Pt in catalytic converters, etc…
• Economic benefits to catalysts

  • Increase rate = more product can be made in a shorter time
  • Reactions can take place at lower temperatures = energy costs are lowered too
• Calculate the rate of reaction

  1. Time taken for a reaction = 1/time

  2. Gradient of suitable graph (by drawing a tangent, either for initial rate or a time (t)

  3. To find the rate of a reaction you should measure - Amount of reactant used up per unit time or Amount of product made per unit time e.g. mL of gas made in one minute
• Measuring rates of reaction

  1. Rate is a measure of the change that happens in a single unit of time

  2. To find the rate of a reaction you should measure - Amount of reactant used up per unit time or Amount of product made per unit time e.g. mL of gas made in one minute
• What does the collision theory state ?

  for a reaction to occur the reactants must:

  • collide with each other
  • collide with sufficient energy to break the bonds between them
  • collide with the correct orientation
  • energy of the collisions must be equal to or greater than the activation energy
  • only a small proportion at a given temperature will have sufficient kinetic energy to overcome this activation energy barrier
• explain this diagram

  Activation energy

  A) number of particles

  B) particles have insufficient energy to initiate reaction

  C) particles have enough energy to react

  D) activation energy

  E) particle kinetic energy

  
• define activation energy

  minimum amount of energy will be different for a reaction to occur.
• define a catalyst

  substance that increases the rate of a chemical reaction without being changed in chemical composition or amount
• how does a catalyst work ?

  work by providing an alternative reaction route of lower activation energy
• how does the activation energy affect the maxwell Boltzmann curve ?

  lower the activation energy and more reactant particles have sufficient energy to overcome the activation energy barrier
• What are the different types of catalysts?

  heterogeneous catalysts -> in a different state to the reactants, the solid weakens the bonds in the reactants by adsorption.

  
  

  homogeneous catalysts -> in the same state as the reactants, either enhance the electrophilic or nucleophilic properties of the reagent.
• Define biological catalysts

  enzymes are natures catalyst

   

  they are specialised proteins that speed up biochemical reactions

   

  each enzyme will only work for specific reactions e.g amylase catalyses the hydrolysis of starch into sugars.

   
• what are the different types of catalysts in the industry ?

  Nickel in hydrogenation of alkenes

  Pt in catalytic converters
• How do you calculate the rate of reaction?

  
  

  1. Time taken for a reaction = 1/time
  2. Gradient of suitable graph (by drawing a tangent, either for initial rate or a time (t).