AP Chemistry Name: Potential Energy Diagrams Potential Energy Diagrams

• You may represent the PE Diagrams we made during Thermochemistry. The reality is that most reactions have PE diagrams that are slightly more complicated.

• Potential Energy diagrams represent changes in the potential energy of the reacting particles forming products when they are colliding.

a) Activation Energy for the Forward Reaction o energy needed to form the activated complex. (or the energy needed

to be overcome to form products). Can be calculated by taking the difference between the energy of the activated complex and the energy of the reactants

b) Activation Energy for the Reverse Reaction

Notice that the activation energy for the forward and reverse reactions are not the same, even though the PE of the Activated Complex is the same.

c) Change in Enthalpy of the Reaction (∆H) o Energy of Products – Energy of Reactants o If ∆H is a positive number then reaction is endothermic o If ∆H negative number then reaction is exothermic

AP Chemistry Name: Potential Energy Diagrams Potential Energy Diagrams

• Activated Complex – point at which the reactants form a high energy, unstable intermediate compound before forming either reactants or products

• Kinetic Energy Diagrams From Thermodynamics, you may briefly remember seeing a Maxwell-Boltzmann Distribution Curve.

o Not all particles have the same amount of energy; therefore, do not travel at the same speed. So travel slower (have less energy) while others travel faster. A Maxwell-Boltzmann Distribution Curve can represent the energy distribution of particles at different temperatures.

AP Chemistry Name: Potential Energy Diagrams Potential Energy Diagrams

Comparing Kinetic vs. Potential Energy Diagrams

o Notice…the activation energy (P.E. diagram) and threshold energy (K.E. diagram) are the same for the given reaction

Effect of a Catalyst & Inhibitor

o A catalyst increases the rate of a chemical reaction without being used up by causing the reaction to proceed by a different reaction mechanism (or pathway) with a lower activation energy

o An inhibitor slows down a reaction by creating a pathway with a higher activation energy (ex. preservatives)

AP Chemistry Name: Potential Energy Diagrams Potential Energy Diagrams

o Generally, if a reaction requires a low activation energy, it is considered to be a spontaneous reaction (i.e. It is generally a fast reaction)

• Reaction Mechanism

o Elementary Process • A process in which a reaction occurs in one step (usually occurs

between 2 particles)

Ternary Process • A termolecular collision (collision of 3 or more m/c) happening in a

single collision is a very unlikely event (like a 3 car pile up). Most of these reactions occur as a series of collisions rather than just a single one.

• For these types of reactions, there must be a slow step with the

highest Ea. This slow step is called the rate-determining step (or R.D.S.). All other steps in the reaction are considered fast. The P.E. diagram for these types of reaction is identified by having at least two ‘bumps’

Example 2 NO + H2(g) → N2O + H2O + heat Step #1 NO + NO → N2O2 (fast) Step #2 N2O2(g) + H2(g) → N2O + H2O (slow)

AP Chemistry Name: Potential Energy Diagrams Potential Energy Diagrams

Activated Complex #1 Activated Complex #2

• In the diagram above, the second step has the highest activation energy relative to the reactants, it is considered to be the activation energy for the overall reaction.

• Between the first and second bumps, there is a valley. In this

location, reaction intermediates are found. Reaction intermediates are substances created in an early step, but used as a reactant in a later step.

Example 2 NO + H2(g) → N2O + H2O + heat Step #1 NO + NO → N2O2 (fast) Step #2 N2O2(g) + H2(g) → N2O + H2O (slow)

• To recognize a catalyst in a reaction, it is used in an early step and

produced in a later step and is not used again.

• Both reaction intermediates and catalysts are not written in an overall net reaction.

• Reaction Intermediate ≠ Activated Complex

AP Chemistry Name: Potential Energy Diagrams Potential Energy Diagrams

• Check your understanding (don’t forget Units and Signs)

• 1. What is the PE of the Activated Complex: ______________

2. What is the Activation Energy for the forward reaction? ________

3. What is the Activation Energy for the reverse reaction? ________

4. What is the delta H for the forward reaction? _____________

5. What is the delta H for the reverse reaction? _____________

6. Is the reverse reaction endothermic or exothermic? ____________

Now try this one:

The following graph represents a reaction in which when a catalyst is added, the activation energy of the original reaction is now lower, and there are also now Reaction intermediates.

1. There are 4 steps to the reaction with the catalyst. Which one is the rate determining step? ____________

2. Write the overall

reaction:

AP Chemistry Name: Potential Energy Diagrams Potential Energy Diagrams

Potential Energy Diagram Worksheet

1. Which of the letters a–f in the diagram represents the potential energy of the products? _______

2. Which letter indicates the potential energy of the activated complex? ________

3. Which letter indicates the potential energy of the reactants? ________

4. Which letter indicates the activation energy? _____

5. Which letter indicates the heat of reaction? ______

6. Is the reaction exothermic or endothermic? ______

7. Which letter indicates the activation energy of the reverse reaction? ________

8. Which letter indicates the heat of reaction of the reverse reaction? ________

9. Is the reverse reaction exothermic or endothermic? ___

1. The heat content of the reactants of the forward reaction is about ______ kilojoules.

2. The heat content of the products of the forward reaction is about _______kilojoules.

3. The heat content of the activated complex of the forward reaction is about ______ kilojoules.

4. The activation energy of the forward reaction is about ______ kilojoules.

5. The heat of reaction (¨H) of the forward reaction is about ______ kilojoules.

6. The forward reaction is _______________ (endothermic or exothermic).

7. The heat content of the reactants of the reverse reaction is about ________ kilojoules.

8. The heat content of the products of the reverse reaction is about _______ kilojoules.

9. The heat content of the activated complex of the reverse reaction is about _______kilojoules.

10. The activation energy of the reverse reaction is about _______ kilojoules.

11. The heat of reaction (¨H) of the reverse reaction is about _______ kilojoules.

12. The reverse reaction is __________________ (endothermic or exothermic).

AP Chemistry Name: Potential Energy Diagrams Potential Energy Diagrams Name ______________________________________ Date ______________ Period _________

Energy Ws #1: Reaction Rates1. Chemical reactions occur when reactants collide. For what reasons may a collision

fail to produce a chemical reaction?

2. If every collision between reactants lead to a reaction, what determines the rate atwhich the reaction occurs?

3. What is the activation energy of a reaction, and how is this energy related to theactivated complex of the reaction?

4. What happens when a catalyst is used in a reaction?

5. Name 4 things that will speed up or slow down a chemical reaction.

6. Draw an energy diagram for a reaction. (label the axis)Potential energy of reactants = 350 KJ/moleActivation energy = 100 KJ/molePotential energy of products = 250 KJ/mole

7. Is the reaction in # 6 exothermic or endothermic? Explain.

8. How could you lower the activation energy for the reaction in #6?