Unit 5 · Topic 5.6 Beta

Reaction Energy Profile

An energy profile shows potential energy along the reaction coordinate.

Practice 3: Representing Data and PhenomenaPractice 4: Model Analysis

Question set for this topic

Part 1 · Hook

Why this matters

A ball at the top of a hill will roll down to the valley on the other side, but only if someone first pushes it over the small bump at the hilltop edge. Chemical reactions have a bump like that too. An energy profile is the map of the hill: where the reactants start, how high the bump is, and where the products end up.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. What is the minimum energy a collision needs to cause a reaction called?

  1. The activation energy
  2. The kinetic energy
  3. The bond energy
  4. The thermal energy
Show the answer

A collision needs at least the activation energy, Ea, to react.

  • Correct: The activation energy:
  • The kinetic energy:
  • The bond energy:
  • The thermal energy:

2. Raising the temperature speeds up a reaction mainly because:

  1. more collisions have at least Ea
  2. Ea becomes smaller
  3. the molecules become larger
  4. fewer collisions happen
Show the answer

The energy distribution shifts so a larger fraction of collisions clears the same barrier.

  • Correct: more collisions have at least Ea:
  • Ea becomes smaller:
  • the molecules become larger:
  • fewer collisions happen:

Part 4 · See it

See it first

Potential energy on the vertical axis, reaction progress on the horizontal axis. The curve starts at the reactant level, rises to a peak labeled transition state, and falls to a product level lower than the reactants. A double arrow from the reactant level to the peak is the activation energy. A double arrow from the reactant level down to the product level is the energy change of the reaction; here energy is released because the products are lower.
Activation energy is the climb from reactants to the transition state; the energy change is products minus reactants. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Bonds must partly break before new ones formpotential energy rises to a peak, the transition state
  2. The climb from reactants to that peak is the activation energya higher peak means fewer collisions succeed and a smaller k
  3. The products sit at their own level, above or below the reactantsproducts minus reactants gives the energy released or absorbed
  4. A reaction and its reverse share one transition statethe difference between their barriers equals the energy change

Part 6 · Key ideas

Key ideas

  • An energy profile plots potential energy against reaction progress, from reactants through the transition state to products.
  • Ea = E(transition state) − E(reactants). Energy change = E(products) − E(reactants).
  • Energy change = Ea(forward) − Ea(reverse).
  • A higher barrier means a smaller k and a slower reaction at the same temperature. The overall energy change does not set the rate.

Part 7 · Misconception

A common mistake

The wrong idea: A reaction that releases a lot of energy must be fast.

What actually happens: The rate depends on the barrier height, not on how much energy is released. A reaction can release a lot of energy and still be very slow if its activation energy is high.

Part 8 · Check yourself

Check yourself

Exam-style questions. Anything you miss goes into your review queue.

Graph

Energy profile for a one-step reaction

The graph is the energy profile for a one-step (elementary) gas reaction, A + BC → AB + C. Reaction progress has no units.

0255075100125150175012345678910Reaction progressPotential energy (kJ/mol)
Data table
Reaction progressEnergy
040
140
240
367.5
4122.5
5150
6115
745
810
910
1010

1. What is the activation energy of the reaction as written, in kJ/mol?

Type a number in kJ/mol.

Show the answer

Ea (forward) = E(transition state) − E(reactants) = 150 kJ/mol − 40 kJ/mol = 110 kJ/mol.

  • Answer: 110 kJ/mol

2. What is the energy change for the reaction (products minus reactants), in kJ/mol?

Type a number in kJ/mol.

Show the answer

Energy change = E(products) − E(reactants) = 10 kJ/mol − 40 kJ/mol = −30 kJ/mol. The negative sign means the reacting particles give off energy.

  • Answer: -30 kJ/mol

3. What is the activation energy for the reaction run in reverse, AB + C → A + BC, in kJ/mol?

Type a number in kJ/mol.

Show the answer

Run in reverse, the reaction starts at the products and climbs to the same transition state: Ea (reverse) = 150 − 10 = 140 kJ/mol. Check: Ea(reverse) − Ea(forward) = 140 − 110 = 30, the size of the energy released.

  • Answer: 140 kJ/mol

4. What does the highest point of the curve represent?

  1. The transition state, with bonds half broken and half formed
  2. A stable molecule that can be collected
  3. The products, which have the most energy
  4. The average energy of the reactant molecules
Show the answer

The peak is the transition state, the highest-energy arrangement along the path. It lasts for about one molecular vibration and cannot be isolated.

  • Correct: The transition state, with bonds half broken and half formed: Right: the unstable arrangement at the top of the barrier.
  • A stable molecule that can be collected: Nothing can be collected at the peak; it is the least stable point on the path.
  • The products, which have the most energy: The products are at the right-hand end, here at 10 kJ/mol.
  • The average energy of the reactant molecules: The reactants are the left-hand plateau, not the peak.

Graph

Two reactions at the same temperature

Energy profiles for two different one-step reactions, P and Q, run at the same temperature with the same starting concentrations. Assume their orientation needs are similar.

0255075100125150175012345678910Reaction progressPotential energy (kJ/mol)

Reaction PReaction Q

Data table
Reaction progressReaction PReaction Q
05050
15050
25050
362.577.5
487.5132.5
5100160
693.8125
781.355
87520
97520
107520

5. Which reaction is faster, and why?

  1. P, because its activation energy is smaller
  2. Q, because it releases more energy
  3. Q, because its products are lower in energy
  4. They are equally fast, because they start at the same energy
Show the answer

P has Ea = 100 − 50 = 50 kJ/mol; Q has Ea = 160 − 50 = 110 kJ/mol. At the same temperature, a larger fraction of collisions clears the smaller barrier, so P is faster. How much energy a reaction releases does not set its rate.

  • Correct: P, because its activation energy is smaller: Right: the barrier height controls the rate.
  • Q, because it releases more energy: Releasing more energy says nothing about how fast the reaction starts; Q has the much higher barrier.
  • Q, because its products are lower in energy: The product level sets the overall energy change, not the rate.
  • They are equally fast, because they start at the same energy: Equal starting energies do not mean equal barriers.

6. For a one-step reaction, the forward activation energy is 48 kJ/mol and the reverse activation energy is 75 kJ/mol. What is the overall energy change, products minus reactants, in kJ/mol?

Type a number in kJ/mol.

Show the answer

Energy change = Ea(forward) − Ea(reverse) = 48 − 75 = −27 kJ/mol. The products lie 27 kJ/mol below the reactants.

  • Answer: -27 kJ/mol

Part 9 · Summary

Summary

An energy profile shows potential energy along the reaction coordinate. The activation energy is the rise from the reactants to the transition state, and the energy change is products minus reactants. At the same temperature, a higher barrier means a smaller rate constant, whatever the overall energy change.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections