Topic 5.6 Notes – Reaction Energy Profile

1. What a Reaction Energy Profile Shows

A reaction energy profile is a graph of potential energy (y-axis) vs. reaction coordinate (x-axis).

It’s usually drawn for an elementary reaction, which means:

• It happens in one single step.
• Bonds are broken and formed in the same molecular event.
• The energy diagram represents that one step directly.

Reaction Coordinate (x-axis)

This does not mean time.

It represents the progress of bond rearrangement as reactants turn into products. Think of it as tracking how atoms move and how bonds stretch, break, and reform.

Potential Energy (y-axis)

This includes the energy stored in chemical bonds.

As bonds are stretched or broken, potential energy increases. As stronger bonds form, potential energy decreases.

So the graph is a map of energy changes during the reaction.

2. The Three Key Points on the Diagram

Every elementary reaction energy profile has three critical regions.

Reactants

• Located on the left side
• Have a defined potential energy
• Bonds are intact as written in the balanced equation

This is your starting energy level.

Transition State (Activated Complex)

• The highest point on the graph
• Extremely unstable and short-lived
• Bonds are partially broken and partially formed

It is not a substance you can isolate. It’s a fleeting arrangement at maximum energy.

Here’s what a typical energy profile looks like. Focus on the single peak, which represents the transition state.

Exothermic reaction energy profile diagram

Products

• Located on the right side
• Have their own potential energy
• Bonds are fully formed in their new arrangement

In this example, the products sit lower than the reactants, indicating an exothermic reaction. Comparing product energy to reactant energy tells you the overall energy change.

3. Activation Energy and Overall Energy Change

Two vertical energy differences matter.

Activation Energy (Ea)

$$E_a = E_{\text{transition state}} - E_{\text{reactants}}$$

• Measured from reactants up to the peak
• Represents the minimum energy needed to reach the transition state

This is the energy barrier.

Key connection to rate:

• Higher $E_a$ → fewer collisions can overcome it → slower reaction
• Lower $E_a$ → more successful collisions → faster reaction

Students often confuse this with ΔH. They measure different things.

Overall Energy Change (ΔH)

$$\Delta H = E_{\text{products}} - E_{\text{reactants}}$$

This compares start and finish.

Exothermic

• Products lower than reactants
• $\Delta H < 0$
• Energy released to surroundings

Endothermic

• Products higher than reactants
• $\Delta H > 0$
• Energy absorbed from surroundings

Notice that a reaction can be highly exothermic and still slow if its activation energy is large. Rate and thermodynamics are separate ideas.

4. Why Activation Energy Controls Reaction Rate

Collision theory explains this clearly.

For a reaction to occur:

1. Particles must collide.
2. They must collide with correct orientation.
3. They must have energy ≥ $E_a$.

Only collisions with enough kinetic energy can reach the transition state.

Temperature and Rate

When temperature increases:

• Average kinetic energy increases.
• A larger fraction of molecules have energy ≥ $E_a$.
• More collisions form the transition state.
• Reaction rate increases.

The Arrhenius equation connects the rate constant $k$, activation energy $E_a$, and temperature $T$. You will not calculate with it on the AP exam, but conceptually:

• Larger $E_a$ → reaction is more sensitive to temperature changes.
• Smaller $E_a$ → temperature has a smaller effect.

AP questions often show two energy diagrams and ask which reaction speeds up more when heated. The one with the larger activation energy changes more dramatically.

5. What You Should Be Able to Do

Given an energy profile, you should confidently:

• Label reactants, products, and transition state
• Identify $E_a$ for the forward reaction
• Determine if the reaction is endothermic or exothermic
• Compare two diagrams and decide:
  • Which is faster (lower $E_a$)
  • Which has larger magnitude of $\Delta H$
• Explain temperature effects using the idea of collisions reaching the transition state

When you look at the graph, always think:

• Height of peak above reactants → rate.
• Products relative to reactants → ΔH.
• Temperature changes how many particles can climb the peak.