Reaction Energy Profiles (College Board AP Chemistry)

• Collision theory explains some important features of an elementary reaction but does not explain the role of activation energy
• In elementary reactions, the energy from the collision of reactant molecules is used to break bonds before products are formed
• Bond breaking and bond making, in relation to activation energy, are described by the transition state theory of reaction rate

Transition State Theory

• Transition-state theory explains the reaction resulting from the collision of two molecules in terms of an activated complex
  • An activated complex is an unstable, high-energy species that must be formed before the reaction can occur
• The transition state describes an intermediate where bonds in the reacting molecules have not been completely broken and new bonds in products have not completely formed
• For example in the reaction between carbon monoxide, CO, and nitrogen dioxide, NO₂, the activated complex is made up of CO and NO₂ molecules in close contact
  • In this activated complex, the N—O bond in the NO₂ molecule has been partially broken and a new bond between carbon and oxygen has started to form
  • The dotted lines stand for “partial bonds” in the activated complex

Activated Complex

A diagram showing the reaction path involving the formation of activated complex

• One advantage of the transition state theory is that it explains why the activation energy of reactions is much smaller than the energy required to break the bonds in reacting molecules
  • This is because the formation of an activated complex requires the absorption of relatively little energy needed to weaken the bonds rather than breaking them
  • In the reaction above, the activation energy is 134 kJ/mol
  • This is considerably smaller than the amount of energy required to break the bonds in the reactants:
    • CO 1075 kJ
    • N  O 607 kJ
    • N – O 222 kJ

• The changes in the energies of the reactants during collision can be described by an energy profile
  • This can also be known as a potential energy diagram
• Based on the transition state theory, the collision of reactant molecules results in a change in their kinetic and potential energies
  • As molecules collide, their kinetic energy is converted to potential energy
  • After collision, the products formed recoil and the potential energy is reconverted to kinetic energy
• An energy profile plots the changes in potential energy as the reaction proceeds

Energy Profile Diagram

A diagram showing the changes in the energy for the reaction between CO and NO₂.  During the reaction initiation, the activation energy (E_a) of 134 kJ must be given to the reactants for every mole of CO that reacts

• From the energy profile diagram for the reaction between CO and NO₂ shown above:
  • The reactants are at a higher energy level than the products, which means the reaction is exothermic
    • The overall difference in this potential energy is known as the enthalpy change, ΔH, of the reaction
    • When reactants have less energy than products, the reaction is said to be endothermic
  • The difference between the maximum energy and the energy of the reactant indicates the activation energy, E_a of the forward reaction
  • The top of the activation energy barrier represents the transition state
    • The chemical species that exists at this transition point is called the activated complex

Activation Energy and Reaction Rate

• The rate of a reaction is dependent on the magnitude of E_a and not on the enthalpy change
  • Slow reactions have high activation energy
  • Fast reactions have low activation energy
• This is because, for a slow reaction, relatively few reactants have sufficient kinetic energy for a successful reaction
• There is no way to predict the activation energy of a reaction from its enthalpy change
  • A highly exothermic reaction may be very slow because it has a high activation energy
  • On the other hand, an endothermic reaction may be very fast because it has a low activation energy
• The activation energy for the reverse reaction is simply the difference between the potential energy of products and the maximum energy of the curve
  • For example, for the reaction between CO and NO₂:

                        E_a (forward) = +134 kJ

                        ΔH = -226 kJ

                        E_a (reverse) = 134 -(-226)

                        E_a (reverse) = 360 kJ