Energy diagrams and thermodynamics
Major concepts
 Although many mechanisms may be proposed, a mechanism is only feasible if it is energetically
feasible
 Energy diagrams show the energy of a reaction as the function of the reaction coordinate, and
are useful tools in analyzing a reaction
 A reaction is spontaneous only if it is thermodynamically downhill
 The relative stability of starting materials and products determine the thermodynamics of a
reaction
 The thermodynamics of an ionic reaction can be estimated from pKa values.
 A tenfold difference in equilibrium constant is a 1.4 kcal difference in energy at room
temperature.
 The thermodynamics of a non-ionic reaction can be estimated from bond energies
 A catalyst does not change the thermodynamics of a reaction.
 Thermodynamics can be shifted by adding or removing reactants or products
Vocabulary
 Energy diagram
 Thermodynamics
 Exothermic/endothermic
 catalyst
Students should be able to:
 Draw an energy diagram for an acid/base reaction and estimate the equilibrium constant and
difference in energies
 Use bond energy principles to predict whether a non-ionic reaction is thermodynamically
favorable or unfavorable
 Predict qualitiatively whether a reaction is uphill or downhill
Daily Problems
1A. Would you expect this addition reaction to be thermodynamically uphill or downhill?
B. A catalyst is added to the reaction. Is it thermodynamically uphill or downhill? Explain.
2. What are the approximate Keq for these reactions? Draw energy diagrams.
3. What are the approximate energy differences between starting materials and products in the
reactions from problem #1?
4. Are these reactions favorable or unfavorable? (You don’t have to determine quantitatively.)
Cumulative problems
5. Predict the approximate equilibrium constant for these acid/base reactions and describe them as
“uphill” or “downhill.” (Hint: You already did a lot of the work for this problem in problem #4 of the pKa
homework.)
Extension problem
6. So far, we have only drawn energy diagrams for one-step reaction mechanisms, like acid/base
reactions. What about the reaction of HCl with propene, which is a two-step mechanism? Draw an
energy diagram that takes into account two steps, with the carbocation intermediate being very
unstable.