Lesson 7: Kinetics

• Kinetics

  The study of reaction rates
• Reaction Kinetics

  The speed or rate at which a chemical reaction occurs, also called reaction rate
• Reaction rate

  • Can be defined qualitatively (description of how "fast" or "slow" a reaction takes place)
  • Can be defined quantitatively (measurements of certain factors)
• Qualitative examples

  • Iron rusting is slow
  • Combustion is fast
• Quantitative definition of reaction rate

  Rate = change in quantity / change in time
• Factors that can be measured to determine reaction rate

  • Concentration (solution or gas)
  • Pressure (gas)
  • Volume (solution, liquid or gas)
  • Mass (solid, liquid or gas)
  • pH (acid or base)
  • Conductivity (electrolytes)
  • Color intensity (colored solution)

  
• Concentration

  How much solute per solvent (mol/L)
• Secant

  Slope connecting 2 points, average rate of change
• Tangent

  Slope touching one point, instantaneous rate of change
• Reaction rates are not constant, they change over time because initially there are more reactants but later on there are less, which is why it starts off fast but gets slower towards the end
• Reaction rate in terms of disappearance of reactant

  Rate = - ∆[N2O5] / ∆t
• Reaction rate in terms of appearance of product

  Rate = + ∆[NO2] / ∆t or + ∆[O2] / ∆t
• You won't have to add the products because they are created in the same molar ratio, you can calculate for only one of the products, and it is positive because they are being produced
• Stoichiometry


  For every 2 mol of N2O5 gas consumed, 1 mol of O2 gas is produced and 4 mole of NO2 gas are also produced
• Stoichiometric relationship


  ∆[O2] = - 1/2 ∆[N2O5] or ∆[O2] = 1/4 ∆[NO2]
• Note on concentration change

  [C] = Cfinal - Cinitial
• Kinetic Molecular Theory (KMT)

  • All matter is made up of particles that are in constant motion because they possess kinetic energy
  • The speed at which the particles move and the distance between the particles determine the physical state of the substance
• Evidence to support KMT

  • Diffusion
  • Pressure (gases)
• States of motion

  • Rotational
  • Translational
  • Vibrational
• Solid particles

  Close together and exude vibrational motion
• Liquid particles

  Some distance apart and do vibrational, translational, and rotational motion
• Gas particles

  Far apart and do translational motion
• Example of KMT

  • Perfume being sprayed and the smell spreading across a room
• Collision Reaction Theory(CRT)

  • Particles in motion must collide
  • Particles must collide with a certain minimum amount of energy (Activation Energy, EA)
  • Particles must collide in the correct orientation
• Potential energy

  Energy that is stored
• Potential Energy Diagram

  A diagram that shows the change in potential energy as a reaction progresses

  
• Exothermic reaction

  Energy of reactants > energy of products, net amount of energy is released to surroundings

  
• Endothermic reaction

  Energy of reactants < energy of products, net amount of energy is absorbed from the surroundings

  
• Region A represents the potential energy of the reactants

  
• Region B represents the increase in potential energy that occurs when reactants collide, which is the Activation Energy (for the forward reaction)

  
• Region C represents the point at which the bonds of the colliding particles are stretched to the breaking point, forming an unstable Activated Complex
• Region D represents the energy released when new bonds form between particles to make products, which is the Heat of Reaction (∆H)

  
• Region E shows the potential energy of the products

  
• Heat of Reaction (∆H)

  ∆H = Ep of products - Ep of reactants

  
• For an exothermic reaction, ∆H is given a negative sign, for an endothermic reaction, ∆H is given a positive sign

  
• The energy of the activated complex = ↑ A + ↑ B
• The activation energy for the reverse reaction = ↑ B + ↑ D and it is endothermic