AP® Chemistry review sheet from Aim for Five (aimforfive.com/chem/units/6/6-2)
Unit 6 · Topic 6.2
6.2 Energy Diagrams
An energy diagram shows the energy of the reactants and products of a change side by side, so you can see at a glance whether the change releases or absorbs energy. You should be able to read one, draw one with correct labels, and use the same idea for physical changes like melting and dissolving.
Key terms
- energy diagram
- enthalpy change (ΔH)
- potential energy
- exothermic
- endothermic
What an energy diagram shows
An energy diagram has energy (enthalpy, H) on the vertical axis. The horizontal axis shows the progress of the change, from reactants on the left to products on the right. The reactants sit at one energy level and the products at another.
The enthalpy change is the difference between the two levels: ΔH = H(products) − H(reactants). Draw it as an arrow that starts at the reactant level and ends at the product level. You rarely know the absolute height of either level; only the difference matters.
Exothermic and endothermic shapes
Exothermic: the product level is lower than the reactant level. The system lost energy to the surroundings, so the ΔH arrow points down and ΔH is negative.
Endothermic: the product level is higher than the reactant level. The system gained energy, so the ΔH arrow points up and ΔH is positive.
A quick way to remember: on an energy diagram, the system's energy goes down when energy goes out.
Adding the activation energy hump
Many energy diagrams also show a hump between reactants and products. The top of the hump is the transition state, and the height from the reactant level to the top is the activation energy, Ea (see topic 5.6).
Two quantities can be read from the same diagram, and they are easy to confuse. ΔH is reactants-to-products. Ea is reactants-to-peak. For the reverse reaction, the activation energy is measured from the products up to the same peak, so Ea(reverse) = Ea(forward) − ΔH.
A catalyst lowers the hump but leaves the reactant and product levels exactly where they were. That's why a catalyst never changes ΔH.
Diagrams for physical changes
The same picture works for phase changes. For water, the liquid level sits 6.01 kJ/mol above the solid level (enthalpy of fusion), and the gas level sits 40.7 kJ/mol above the liquid level at 100 °C (enthalpy of vaporization). Going up the diagram is endothermic (melting, boiling); coming down is exothermic (condensing, freezing).
Dissolving can be drawn in steps. Start at the solid plus water. Separating the solute particles moves you up. Separating some water molecules moves you up again. Forming solute–solvent attractions moves you down. If the final level ends below the start, dissolving is exothermic; if it ends above, it's endothermic.
Drawing one for full credit
- Label the vertical axis as energy (or enthalpy) and the horizontal axis as reaction progress.
- Label each level with the formulas and phases, such as N₂(g) + O₂(g).
- Put the products clearly above or below the reactants to match the sign of ΔH.
- Draw and label the ΔH arrow from reactants to products; add Ea from reactants to the peak if asked.
- If values are given, keep the relative heights roughly to scale.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Reading values off a diagram
On an energy diagram, the reactants are at 50 kJ, the top of the hump (transition state) is at 130 kJ, and the products are at −30 kJ. Find ΔH for the forward reaction, the forward activation energy, and the reverse activation energy.
Show the solutionHide the solution
- Step 1: ΔH = H(products) − H(reactants) = (−30 kJ) − (50 kJ) = −80 kJ. Products are lower, so the reaction is exothermic.
- Step 2: Forward Ea is measured from the reactants up to the peak: 130 kJ − 50 kJ = 80 kJ.
- Step 3: Reverse Ea is measured from the products up to the same peak: 130 kJ − (−30 kJ) = 160 kJ.
- Step 4: Check with Ea(reverse) = Ea(forward) − ΔH = 80 − (−80) = 160 kJ. It matches.
Answer: ΔH = −80 kJ; Ea(forward) = 80 kJ; Ea(reverse) = 160 kJ.
- Example 2
Trap: sketching an endothermic reaction
For N₂(g) + O₂(g) → 2NO(g), ΔH° = +180.6 kJ. Describe a correct energy diagram, and explain what would change if a catalyst were added.
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- Step 1: ΔH is positive, so the products must be drawn higher than the reactants. Drawing 2NO(g) below N₂(g) + O₂(g) is the classic mistake.
- Step 2: Label the left level N₂(g) + O₂(g) and the right level 2NO(g). Draw an upward arrow between them labeled ΔH = +180.6 kJ.
- Step 3: Draw a hump between them whose peak is above the product level. Every reaction has an activation energy, and for an endothermic reaction Ea must be at least as big as ΔH.
- Step 4: With a catalyst, draw a lower hump. The two levels, and so the +180.6 kJ, stay exactly the same.
Answer: Products 2NO(g) drawn 180.6 kJ above reactants N₂(g) + O₂(g), with an upward ΔH arrow and a hump higher than the product level; a catalyst lowers only the hump.
Common mistakes
- Drawing endothermic products below the reactants. Positive ΔH means products are higher in energy.
- Reading ΔH from the reactants to the top of the hump. That distance is the activation energy; ΔH is reactants to products.
- Saying a catalyst makes a reaction more exothermic. A catalyst only lowers the activation energy; the reactant and product levels don't move.
- Leaving off labels. An unlabeled pair of lines doesn't show which side is reactants or what the arrow means.
On the exam
- You may be asked to sketch an energy diagram from a given ΔH, or to pick the diagram that matches a temperature observation. Make the relative heights and the ΔH arrow direction obviously correct.
- Questions often combine this topic with kinetics: read both ΔH and Ea from one diagram, and explain what a catalyst does and doesn't change.
Connected topics
Videos
Check yourself
4 questions on 6.2 Energy Diagrams. Pick an answer to see if you got it, and why.
Which of the following correctly describes an energy diagram for the process H₂O(s) → H₂O(l)?
A process takes place in two steps, A → B (step 1) and B → C (step 2).
An energy diagram for the process shows A at 0 kJ/mol, B at +40 kJ/mol and C at −25 kJ/mol. (The energy peaks between the levels are not shown.)
Described energy diagram
What is ΔH for the overall process A → C?
Which of the following correctly describes the two steps?
An energy diagram for the reaction P → R shows R 60 kJ/mol below P. The highest point on the path between them is 150 kJ/mol above P. What is ΔH for the reverse reaction, R → P?
0 of 4 answered