reaction rates and equilibrium

• a reaction will take place if the right conditions are met

  collide with correct orientation

  have enough energy to overcome the activation energy barrier
• following the progression of a reaction

  monitoring the removal of a reactant

  following the formation of the products
• methods to determine the rate of a reaction that produces a gas

  monitor volume of gas produced in regular intervals

  monitoring loss of mass of reactants
• what does a catalyst do

  substance that changes the rate of a reaction without undergoing any permanent change itself
• catalysts


  not used up in the chemical reaction

  may react with a reactant to form an intermediate or provide a surface for the reaction to take place

  catalyst is regenerated at the end of the reaction
• homogeneous catalysts

  same physical state as the reactants. the reactants react with the catalyst to form an intermediate which breaks down to give the products and regenerates the catalyst
• heterogeneous catalysts

  catalyst with a different physical state to the reactants
• heterogenous catalysts are usually solids in contact with gaseous reactants or reactants in solution. reactant molecules are adsorbed onto the surface of the catalyst where the reaction takes place. after the reaction, the product molecules leave the surface by desorption.
• adsorption


  weakly bonded
• autocatalysis

  if a reaction product acts as a catalyst for the reaction. will typically start slow and speed up as more products are formed
• boltzmann distribution


  spread of molecular energies in gases
• features in the boltzmann distribution

  molecules have zero energy - curve starts at the origin

  area under curve is total to the number of molecules

  no maximum energy - the curve does not meet the x axis at high energy
• increasing temperature affects the boltzmann distribution


  peak of curve shifts right and lowers

  more particles have energy greater or equal to activation energy

  total area of curve remains constant
• effect of catalyst on boltzmann distribution


  lowers activation energy, so more particles have sufficient energy to react
• large Kc

  equilibrium favours products
• small Kc

  equilibrium favours reactants
• Kc reaction


  aA + bB $\Leftrightarrow$ cC + dD