Energy Diagrams: Study Guide (Chemistry 6.2)

What an energy diagram shows

In Topic 6.1 you described processes in words: energy moves out of the system (exothermic) or into it (endothermic). An energy diagram shows the same idea as a picture.

• The vertical axis is the energy of the system, usually in kilojoules per mole, kJ mol⁻¹. “Per mole” means per mole of the process as written in the equation.
• The horizontal axis shows the progress of the process: start on the left, finish on the right. It has no scale and no unit.
• A horizontal line (level) marks the energy of each stage: reactants, products, or an intermediate stage.
• A vertical arrow from the starting level to the final level shows the energy change. In this unit it is written ΔH, the enthalpy change. (Topic 6.6 defines enthalpy properly; for now, read ΔH as the energy change of the system when the process happens at constant pressure.)

The surroundings are not drawn. They are implied: whatever energy the system loses goes to them, and whatever it gains comes from them.

Reading endothermic and exothermic from the levels

Feature	Exothermic	Endothermic
Products compared with reactants	lower	higher
Arrow from reactants to products	points down	points up
Sign of ΔH	negative	positive
Energy transfer	system → surroundings	surroundings → system
Effect on the surroundings	warm up	cool down

ΔH is always found as final level minus starting level:

ΔH = E(products) − E(reactants)

If the products are lower, this difference is negative, which matches energy leaving the system.

Scale and units

An energy diagram is a graph, so it follows graph rules. The vertical axis needs:

1. A label with a quantity and a unit, such as “Energy (kJ mol⁻¹)”.
2. Even tick spacing (for example every 50 kJ mol⁻¹), with the numbers written on the ticks.
3. A range that includes every level you need to draw, with a little space above and below.

Where you put zero is a free choice. Energies of substances are not measured on an absolute scale here; only the differences between levels have meaning. Most students put the reactants at 0, which makes ΔH equal to the products’ level.

Figure 1 shows why the scale matters. The two arrows are a similar length, but the energy change on the left is about 150 times larger. Each diagram is correct for its own axis. When you compare two processes, draw them on the same axis.

Drawing an energy diagram, step by step

1. Write the equation with state symbols, and note the value and sign of ΔH.
2. Draw and label the vertical axis: quantity, unit, even ticks, a range that fits all levels.
3. Draw the starting level (often at 0) on the left and label it with the reactants.
4. Draw the final level on the right, at the correct height on your scale, and label it with the products.
5. Draw the arrow from the starting level to the final level, pointing in the direction of change. Label it with ΔH, including its sign and unit.
6. Check: products lower and ΔH negative go together; products higher and ΔH positive go together.

Diagrams for physical processes

Energy diagrams are not only for reactions. Any change of a system can be drawn.

Phase changes. For one substance, the gas has the highest energy, then the liquid, then the solid, because attractions between particles lower the energy as the particles come together. So melting and boiling arrows point up (endothermic), and freezing and condensing arrows point down (exothermic). The right panel of Figure 1 is a phase-change diagram.

Dissolving. In Topic 6.1 you split dissolving into three steps. A step diagram shows each step as its own arrow:

• Step 1, separating solute particles: up.
• Step 2, separating solvent molecules: up.
• Step 3, forming solute–solvent attractions: down.

The overall arrow, from the first level to the last, shows whether dissolving is endothermic or exothermic. Worked example 2 draws one.

Diagrams with a hump

In Topic 5.6 you drew energy profiles with a curve that rises to a transition state before falling to the products, and in Topic 5.11 you saw a catalyst lower that hump. A profile is still an energy diagram. The hump shows the activation energy, but the endothermic or exothermic nature depends only on where the curve starts and ends. A catalyst changes the hump, not the two end levels, so it does not change ΔH. If a question asks only “is this exothermic?”, a two-level diagram like Figure 2 is enough.

Worked example 1: drawing a diagram to scale

Question. The fictional reaction J(g) + K₂(g) → JK₂(g) has ΔH = −184 kJ mol⁻¹. (a) Draw an energy diagram to scale. (b) Describe the diagram for the reverse reaction.

1. Classify. ΔH is negative, so the reaction is exothermic. The product level must be below the reactant level.
2. Choose the axis. Put the reactants at 0. The product level is −184, so the axis must reach below −184. A range from +50 to −200 kJ mol⁻¹ with ticks every 50 works and leaves space above the reactants for a label.
3. Place the levels. Reactants on the 0 line. Products at −184, which is about two-thirds of the way from the −150 tick to the −200 tick (34 out of 50).
4. Draw and label the arrow. Downward, from 0 to −184, labelled “ΔH = −184 kJ mol⁻¹”.

5. (b) The reverse reaction, JK₂(g) → J(g) + K₂(g), uses the same two levels. It now starts at the JK₂ level and ends at the J + K₂ level, so the arrow points up and ΔH = +184 kJ mol⁻¹. The reverse reaction is endothermic.

Check. Energy is conserved: the 184 kJ released per mole when JK₂ forms must be supplied to break it apart again.

Worked example 2: a step diagram for dissolving

Question. A fictional salt MQ dissolves in water. Illustrative energy changes for the three steps, per mole of MQ, are: separating the ions, +690 kJ; separating water molecules to make room, +60 kJ; forming ion–water attractions, −770 kJ. (a) Draw a step diagram. (b) Is dissolving MQ endothermic or exothermic? (c) Predict what happens to the temperature of the water.

1. Start at 0 for solid MQ and water, separately.
2. Step 1 (up 690): the level for “ions separated” is at +690 kJ mol⁻¹.
3. Step 2 (up 60): the level for “ions and water molecules separated” is at 690 + 60 = +750 kJ mol⁻¹.
4. Step 3 (down 770): the final level, MQ(aq), is at 750 − 770 = −20 kJ mol⁻¹.

5. (b) The final level (−20) is below the starting level (0), so ΔH = −20 kJ mol⁻¹: dissolving MQ is exothermic. The energy released by forming ion–water attractions (770 kJ) is larger than the energy needed for steps 1 and 2 (750 kJ).
6. (c) Energy moves from the dissolving system into the water, so the temperature of the water rises, though only a little, because the overall change is small.

Point to notice. The overall change is a small difference between two large numbers. If step 3 had been −700 kJ mol⁻¹ instead, the final level would be +50 kJ mol⁻¹ and the same salt would dissolve endothermically. That is why some salts warm water and others cool it.

Worked example 3: a phase-change diagram

Question. A fictional substance K condenses with ΔH = −32 kJ mol⁻¹ and freezes with ΔH = −9 kJ mol⁻¹ (both at the same temperature). Place K(g), K(l) and K(s) on one energy diagram, and find ΔH for K(s) → K(g).

1. Choose a zero. Put K(g) at 0.
2. Condensing goes down 32: K(l) is at −32 kJ mol⁻¹.
3. Freezing goes down a further 9: K(s) is at −32 − 9 = −41 kJ mol⁻¹.

Level	Energy with K(g) at 0 (kJ mol⁻¹)	Energy with K(s) at 0 (kJ mol⁻¹)
K(g)	0	+41
K(l)	−32	+9
K(s)	−41	0

4. Sublimation, K(s) → K(g), starts at the solid level and ends at the gas level: up 41, so ΔH = +41 kJ mol⁻¹ (endothermic).

Check. The second column moves every level up by 41 but keeps every gap the same. Changing the zero never changes an energy change.

Common misconceptions

• “The arrow goes from the higher level to the lower level.” The arrow always goes from the start of the process to the end. For an endothermic process it points up.
• “The horizontal axis is time.” It only shows the order of stages. A diagram says nothing about how fast a process is.
• “A longer arrow always means a bigger energy change.” Only on the same axis. Read the numbers on the scale (Figure 1).
• “The products must be at a negative number for an exothermic reaction.” Only the products’ position relative to the reactants matters. Reactants at 100 and products at 40 is exothermic.
• “A catalyst makes a reaction more exothermic.” It lowers the hump, not the end levels, so ΔH is unchanged.
• “The diagram shows the energy of the surroundings.” It shows the system. The surroundings change by the same amount in the opposite direction.
• Labelling the vertical axis “Temperature”. The axis is energy, with units of kJ mol⁻¹.

Where this leads

Next, Topic 6.3 explains how energy moves as heat through particle collisions. Energy diagrams return in Topic 6.5 (phase changes), Topic 6.7 (bond enthalpies) and Topic 6.9 (Hess’s law), where step diagrams like Figure 3 help you add energy changes. Try the practice questions now, then use the revision notes and the checklist to consolidate.