AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/5/5-6)
Unit 5 · Topic 5.6
5.6 Reaction Energy Profile
A reaction energy profile plots energy along the path from reactants, up to the transition state and down to products. The climb from reactants to the peak is the activation energy, and the difference between reactants and products is the overall energy change.
Key terms
- reaction coordinate
- transition state
- activation energy
- energy profile
- Arrhenius equation
What the graph shows
In an elementary reaction, some bonds break and new ones form. The reaction coordinate, on the x-axis, represents progress along that change, from reactants on the left to products on the right. It isn't time; it's a summary of all the atomic motions involved. Energy is on the y-axis.
The highest point on the path is the transition state (also called the activated complex). It's a fleeting arrangement in which old bonds are partly broken and new ones partly formed. It can't be isolated.
Reading energies off the profile
An exothermic profile ends lower than it starts. An endothermic profile ends higher. Either way, there's a hump to get over.
- Activation energy for the forward reaction, Eₐ: energy of the transition state minus energy of the reactants.
- Overall energy change, ΔE (or ΔH): energy of the products minus energy of the reactants. Negative means energy is released (exothermic); positive means energy is absorbed (endothermic).
- Activation energy for the reverse reaction: energy of the transition state minus energy of the products. It equals the forward Eₐ minus ΔE.
Why there is a hump at all
Even in a reaction that releases a lot of energy overall, bonds in the reactants have to start breaking before new bonds can form. Stretching and breaking bonds costs energy, so the energy rises first. Past the transition state, new bonds form and energy is released, so the energy falls.
For an endothermic reaction, the products end up higher than the reactants, and the forward activation energy is always at least as large as ΔH, since the peak has to be at or above the products. For any reaction, you can't reach the products without first getting over the peak.
Activation energy and rate
A larger Eₐ means fewer collisions have enough energy to reach the transition state, so the reaction is slower at a given temperature. A smaller Eₐ means a faster reaction.
Activation energy and the overall energy change are independent. A very exothermic reaction can still be extremely slow if its Eₐ is high. A mixture of hydrogen and oxygen gas can sit unreacted at room temperature until a spark provides enough energy to get some molecules over the hump.
The Arrhenius equation, qualitatively
The rate constant k depends on temperature because temperature controls what fraction of collisions carry enough energy to get up to the transition state. The Arrhenius equation describes this: k increases as temperature increases, and a larger Eₐ makes k more sensitive to temperature.
You need the idea, not the math. Calculations with the Arrhenius equation aren't assessed on the AP exam.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Reading a profile
On an energy profile, the reactants are at 50 kJ/mol, the transition state at 130 kJ/mol and the products at 20 kJ/mol. Find the forward activation energy, the overall energy change and the reverse activation energy. Is the reaction exothermic or endothermic?
Show the solutionHide the solution
- Step 1: Forward Eₐ = 130 − 50 = 80 kJ/mol.
- Step 2: ΔE = 20 − 50 = −30 kJ/mol. Negative, so exothermic.
- Step 3: Reverse Eₐ = 130 − 20 = 110 kJ/mol. Check: forward Eₐ − ΔE = 80 − (−30) = 110 kJ/mol.
Answer: Forward Eₐ = 80 kJ/mol; ΔE = −30 kJ/mol (exothermic); reverse Eₐ = 110 kJ/mol
- Example 2
Exothermic doesn't mean fast (classic trap)
Reaction X has ΔH = −400 kJ/mol and Eₐ = 150 kJ/mol. Reaction Y has ΔH = −20 kJ/mol and Eₐ = 40 kJ/mol. At the same temperature, which is likely faster?
Show the solutionHide the solution
- Step 1: Rate depends on the activation energy, not on how much energy the reaction releases overall.
- Step 2: Y has a much smaller Eₐ, so a far larger fraction of collisions can get over its barrier.
- Step 3: X releases more energy, but that has nothing to do with how quickly it starts.
Answer: Reaction Y, because it has the lower activation energy.
Common mistakes
- Measuring Eₐ from zero on the y-axis instead of from the reactants' energy.
- Calling the transition state an intermediate. A transition state is a peak; an intermediate sits in a valley (topic 5.10).
- Assuming a more exothermic reaction is faster.
- Reading the reaction coordinate as time.
On the exam
- Expect to sketch or label an energy profile: mark reactants, products, the transition state, Eₐ (an arrow from reactants up to the peak) and ΔH (from reactants to products).
- Questions may ask how a profile changes with a catalyst or with temperature. Temperature doesn't change the profile itself; it changes how many particles can get over it.
Connected topics
Videos
Check yourself
4 questions on 5.6 Reaction Energy Profile. Pick an answer to see if you got it, and why.
An energy profile for the one-step reaction A + B → C + D is described below.
The reactants are at 30 kJ/mol. The energy rises to a single peak at 110 kJ/mol, then falls to the products at 70 kJ/mol.
Described energy profile
Which of the following gives ΔH and the activation energy of the forward reaction?
Which of the following best describes the species at the peak of the profile?
Two reactions are run at the same temperature. Their reactant molecules collide equally often and have similar orientation requirements. Reaction 1 has Eₐ = 40 kJ/mol and reaction 2 has Eₐ = 80 kJ/mol. Which of the following is correct?
An energy profile for the one-step reaction X + Y → Z shows the reactants at 50 kJ/mol, the transition state at 150 kJ/mol and the product at 20 kJ/mol.
Described energy profile
What is the activation energy of the reverse reaction, Z → X + Y?
0 of 4 answered