ENTHALPY Activation energy, the activated complex, and the effect of a catalyst

Learning Objectives The activated complex is an unstable arrangement of atoms formed at the maximum of the potential energy barrier, during a reaction. The activation energy is the energy required by colliding molecules to form an activated complex. The activation energy can be calculated from potential energy diagrams. The effect of a catalyst can be explained in terms of alternative reaction pathways with lower activation energy. A potential energy diagram can be used to show the effect of a catalyst on activation energy.

ACTIVATION ENERGY

Reaction Progress Energ y Reactants Products

Reaction Progress Energ y Reactants Products HH EAEA

Reaction Progress Energ y Reactants Products HH

Reaction Progress Energ y Reactants Products HH

Reaction Progress Energ y Reactants Products HH EAEA

Reaction Progress Energ y Reactants Products HH EAEA What happens here?

THE ACTIVATED COMPLEX

Reactant A

Reactant B

Activated complex

Product C

Reaction Progress Energ y Reactants A + B Product C HH EAEA Activated Complex

THE EFFECT OF A CATALYST

MOLECULAR ENERGY EaEa NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

EaEa EXTRA MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER MOLECULAR ENERGY NUMBER OF MOLECUES WITH A PARTICULAR ENERGY

Reaction Progress Energ y Reactants Products HH Effect of catalyst Without catalyst EAEA

Reaction Progress Energ y Reactants Products HH Effect of catalyst With catalyst

Reaction Progress Energ y Reactants Products HH Effect of catalyst With catalyst EAEA

Reaction Progress Energ y Reactants Products HH Effect of catalyst Without catalyst EAEA

Reaction Progress Energ y Reactants Products HH Effect of catalyst With catalyst EAEA

Reaction Progress Energ y Reactants Products HH Effect of catalyst Without catalyst With catalyst EAEA EAEA