Unit 6 · Topic 6.2 Beta

Energy Diagrams

An energy diagram shows reactants and products at their enthalpy levels.

Practice 3: Representing Data and PhenomenaPractice 4: Model Analysis

Question set for this topic

Part 1 · Hook

Why this matters

A hiking map shows the trail's ups and downs; an energy diagram does the same for a reaction. Where the trail starts and ends tells you whether energy is released or absorbed overall. The hill in between tells you how hard it is to get going. Both readings come from one picture.

Part 2 · Before you start

What this builds on

Part 3 · Prerequisite check

Quick check before you start

1. On a reaction energy profile, what is the activation energy?

  1. The climb from the reactants to the top of the hump
  2. The drop from the hump to the products
  3. The difference between reactants and products
  4. The height of the products above the axis
Show the answer

Ea is the minimum energy the colliding particles need: the climb from the reactant level to the transition state.

  • Correct: The climb from the reactants to the top of the hump:
  • The drop from the hump to the products:
  • The difference between reactants and products:
  • The height of the products above the axis:

2. In an exothermic process, energy moves

  1. from the system to the surroundings
  2. from the surroundings to the system
  3. in neither direction
  4. only as light
Show the answer

Exothermic means energy leaves the system, so ΔH is negative.

  • Correct: from the system to the surroundings:
  • from the surroundings to the system:
  • in neither direction:
  • only as light:

Part 4 · See it

See it first

Two energy diagrams, enthalpy on the vertical axis and progress of the reaction on the horizontal axis. Exothermic: products lower than reactants, ΔH arrow points down, ΔH < 0. Endothermic: products higher than reactants, ΔH arrow points up, ΔH > 0. In both, the hump above the reactants is the activation energy, Ea.
Read the end levels for ΔH and the hump for the activation energy. Products lower: exothermic; products higher: endothermic. LevlPrep original diagram.

Part 5 · Step by step

How it works, step by step

  1. Each substance has an enthalpy set by its bonds and attractionsreactants and products sit at definite levels on the diagram
  2. ΔH = H(products) − H(reactants)products below reactants give a negative ΔH (exothermic); products above give a positive ΔH (endothermic)
  3. Bonds must partly break before new ones forma hump between the levels marks the activation energy
  4. Running the reaction backward reads the same diagram right to leftΔH flips sign and the activation energy is measured from the other side

Part 6 · Key ideas

Key ideas

  • An energy diagram plots enthalpy against the progress of the reaction.
  • ΔH = H(products) − H(reactants): products lower means exothermic (ΔH < 0); products higher means endothermic (ΔH > 0).
  • The hump gives the activation energy, the climb from the starting level to the top. It controls speed, not ΔH.
  • Reverse reaction: same picture read backward; ΔH changes sign and Ea(reverse) = Ea(forward) − ΔH.
  • A catalyst lowers the hump and leaves both end levels, and ΔH, unchanged.

Part 7 · Misconception

A common mistake

The wrong idea: A reaction with a tall hump must be endothermic, because it takes a lot of energy.

What actually happens: The hump is the activation energy. Whether a reaction is endothermic depends only on whether the products end above the reactants.

Part 8 · Check yourself

Check yourself

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

Graph

Energy diagram for a gas-phase reaction

The diagram shows the enthalpy of a reacting system as the reaction X(g) + Y(g) → Z(g) proceeds, per mole of reaction. The three marked levels are: reactants 125 kJ, top of the hump 214 kJ, products 38 kJ.

05010015020002468Progress of the reactionEnthalpy (kJ)
Data table
Progress of the reactionX + Y → Z
0125
1125
2150
3200
3.5214
4200
5110
650
6.538
838

1. Use the diagram to determine ΔH for the reaction X + Y → Z, in kJ/mol.

Type a number and its unit.

Show the answer

ΔH = H(products) − H(reactants) = 38 kJ − 125 kJ = −87 kJ per mole of reaction. Products sit below reactants, so the sign is negative.

  • Answer: -87 kJ/mol

2. Use the diagram to determine the activation energy of the forward reaction, in kJ/mol.

Type a number and its unit.

Show the answer

Ea(forward) = H(peak) − H(reactants) = 214 kJ − 125 kJ = 89 kJ/mol.

  • Answer: 89 kJ/mol

3. Which statement about the reverse reaction, Z → X + Y, is supported by the diagram?

  1. It is exothermic, with ΔH = −87 kJ/mol and an activation energy of 89 kJ/mol.
  2. It is endothermic, with ΔH = +87 kJ/mol and an activation energy of 89 kJ/mol.
  3. It is endothermic, with ΔH = +87 kJ/mol and an activation energy of 176 kJ/mol.
  4. It is endothermic, with ΔH = +87 kJ/mol and an activation energy of zero.
Show the answer

For the reverse reaction, ΔH = 125 − 38 = +87 kJ/mol and Ea(reverse) = 214 − 38 = 176 kJ/mol. The diagram is read right to left.

  • It is exothermic, with ΔH = −87 kJ/mol and an activation energy of 89 kJ/mol.: This copies the forward values. Reversing the direction flips the sign of ΔH and changes which side the climb starts from.
  • It is endothermic, with ΔH = +87 kJ/mol and an activation energy of 89 kJ/mol.: The sign is right, but the reverse climb starts at the products, 38 kJ, not at the reactants.
  • Correct: It is endothermic, with ΔH = +87 kJ/mol and an activation energy of 176 kJ/mol.: Right: going backward climbs from 38 kJ to 214 kJ, then ends at 125 kJ, higher than it started.
  • It is endothermic, with ΔH = +87 kJ/mol and an activation energy of zero.: The reverse reaction passes over the same hump; climbing it from 38 kJ to 214 kJ takes 176 kJ/mol.

4. A catalyst is added to the reaction mixture. How does the diagram change?

  1. The peak is lower, and the products move down, so ΔH becomes more negative.
  2. The peak stays the same, and the products move down, so more energy is released.
  3. The peak is higher, so the reaction absorbs extra energy and ΔH becomes positive.
  4. The peak is lower; the reactant and product levels, and so ΔH, stay the same.
Show the answer

ΔH depends only on where the reactants and products sit. A catalyst lowers the hump between them.

  • The peak is lower, and the products move down, so ΔH becomes more negative.: The products are the same substances, so their enthalpy is unchanged.
  • The peak stays the same, and the products move down, so more energy is released.: A catalyst changes the path, which is the peak, not the end points.
  • The peak is higher, so the reaction absorbs extra energy and ΔH becomes positive.: A catalyst lowers the activation energy; it does not raise it, and it does not change ΔH.
  • Correct: The peak is lower; the reactant and product levels, and so ΔH, stay the same.: Right: a catalyst offers a path with a lower activation energy but starts and ends at the same substances.

5. A diagram for the reaction 2 H₂O₂(aq) → 2 H₂O(l) + O₂(g), which warms the solution, is drawn by a student with the products higher than the reactants. What is wrong?

  1. The products belong at the same level as the reactants, because atoms are conserved.
  2. A reaction that warms its surroundings releases energy, so its products belong below the reactants.
  3. Nothing is wrong, because a reaction that warms its surroundings gains enthalpy.
  4. The hump is missing, because a reaction that releases energy has no activation energy.
Show the answer

The diagram shows the system. An exothermic system ends with less enthalpy, so the products sit below the reactants.

  • The products belong at the same level as the reactants, because atoms are conserved.: Atoms are conserved, but energy is not stored the same way in different bonds; the levels differ.
  • Correct: A reaction that warms its surroundings releases energy, so its products belong below the reactants.: Right: warming the solution means the system lost energy, so ΔH < 0 and the products sit lower.
  • Nothing is wrong, because a reaction that warms its surroundings gains enthalpy.: The surroundings gain energy; the system, whose enthalpy is drawn, loses it.
  • The hump is missing, because a reaction that releases energy has no activation energy.: The student's hump is not the issue, and exothermic reactions do have activation energies.

6. Which energy diagram matches an endothermic reaction that is fast at room temperature?

  1. Products well below the reactants, with a small hump
  2. Products a little above the reactants, with a very tall hump
  3. Products a little above the reactants, with a small hump
  4. Products level with the reactants, with no hump
Show the answer

Two separate readings: the end levels give the sign of ΔH; the hump gives the activation energy, which governs speed (5.6).

  • Products well below the reactants, with a small hump: A small hump fits a fast reaction, but products below reactants make it exothermic.
  • Products a little above the reactants, with a very tall hump: Endothermic is right, but a tall hump means a large Ea and a slow reaction.
  • Correct: Products a little above the reactants, with a small hump: Right: products above reactants means endothermic; a small hump means a small Ea and a fast reaction.
  • Products level with the reactants, with no hump: Level ends give ΔH = 0, and a reaction that rearranges bonds has a hump.

7. Methane burns in oxygen with ΔH = −890 kJ/mol, yet a gas stove does not light until a spark is applied. Which energy diagram explains both facts?

  1. Products far below reactants, with a hump that the spark helps the molecules climb
  2. Products far above reactants, so the spark supplies the extra energy stored in the products
  3. Products far below reactants, with no hump, so the spark is not really needed
  4. Products level with reactants, with a hump that the spark climbs
Show the answer

ΔH (end levels) tells how much energy is released; Ea (the hump) tells whether molecules have enough energy to start. Once burning, the energy released keeps lighting new molecules.

  • Correct: Products far below reactants, with a hump that the spark helps the molecules climb: Right: the low products make it strongly exothermic; the hump (Ea) is why it needs a start.
  • Products far above reactants, so the spark supplies the extra energy stored in the products: A ΔH of −890 kJ/mol means the products are far below the reactants.
  • Products far below reactants, with no hump, so the spark is not really needed: Without a hump, methane would burn the moment it met air; the spark is needed.
  • Products level with reactants, with a hump that the spark climbs: Level ends would mean ΔH = 0, not −890 kJ/mol.

Part 9 · Summary

Summary

An energy diagram shows reactants and products at their enthalpy levels. The vertical gap between the ends is ΔH, negative when products are lower (exothermic) and positive when higher (endothermic). The hump between them is the activation energy, which a catalyst lowers without changing ΔH.

Part 10 · Up next

What comes next

Part 11 · Connections

Connections