features of transition metals as catalysts

• Define: catalyst


  A catalyst is a substance that increases the rate of a chemical reaction but is not used up in the reaction. A catalyst lowers the activation energy by providing an alternative pathway (doesn't affect ∆G/∆S/∆H).
• Why are transition metals good catalysts?

  They have variable oxidation states so can provide alternative pathways
• What are the benefits of using catalysts in industry?

  Lower temperatures and pressures needed (saves energy and resources)
• Define: heterogenous catalyst

  A catalyst that is in a different phase from the reactants which speeds up the reaction without getting used up.
• Describe the stages involved when a heterogenous catalyst is used.

  The reaction occurs at active sites on the surface of the catalyst .Reactants adsorb onto the catalyst surface and are held in the correct orientation to react -This weakens bonds in the reactants and new bonds are formed The product desorbs from the surface
• Explain why heterogenous catalysts cannot adsorb a) too strongly or b) too weakly

  a) Reactants cannot move on the surface or products are not desorbed

  b) Reactants are not brought together/ no increase in reactant concentration on catalyst surface/ reactants not held long enough for a reaction to occur/ reactant bonds not weakened
• How can you minimise the cost of a catalyst?

  Spread the powdered catalyst onto a support medium to maximise the surface area.
• How do heterogenous catalysts lose their efficiency?

  The finely divided catalyst is lost from the support medium .Heterogeneous catalysts can become poisoned by impurities that block the active sites, this decreases the catalyst's surface area
• How can the poisoning of a catalyst be minimised?

  Treat reactants to remove impurities
• What name is given to a reaction in which one of the products acts as a catalyst?

  autocatalysis
• describe and explain the main stages in heterogenous catalysis?

  1. adsorption

  at least one of the reactants weakly adsorbs onto the surface of the catalyst (this increases the concentration of reactant particles for gas phase reactants)

  2. reaction

  some bonds may have been weakened and atoms get rearranged to form products

  3. Desorption

  products molecules are released from the surface of the catalyst
• define autocatalysis

  A reaction that produces its own catalyst
• why would a support medium be used in addition with a catalyst?

  to increase the surface area and minimise the cost