Energy Level and Reaction Profile Diagrams

FREE CHEMISTRY REVISION · TOPIC 39

Energy Level and Reaction Profile Diagrams

Pearson BTEC Level 3 AAQ Applied Science
Unit 2: Principles and Applications of Chemistry · D1.3.2

What you will learn

Read energy-level diagrams and reaction profiles, identify exothermic and endothermic changes, and calculate forward and reverse activation energies. Explain catalysis using unchanged reactant and product levels and a lower-barrier pathway.

1. Two diagrams with different jobs

An energy-level diagram compares the enthalpies of the initial reactants and final products. It shows the overall change, ΔH. A reaction profile additionally sketches the barrier along a reaction pathway, allowing activation energies to be identified. Neither graph shows temperature against time.

The vertical axis is enthalpy or relative energy. Use the stated units: for a molar reaction profile these are commonly kJ mol⁻¹ for the reaction as written. The horizontal axis is reaction progress or reaction coordinate: an abstract description of changes in bonding and arrangement, not distance travelled or elapsed time. The zero of relative energy is arbitrary; differences carry the meaning.

Compare the same amounts, physical states and conditions at each end. On these simple single-step schematics the peak represents a transition state, not an isolable product. A real mechanism can have several peaks and intermediates.

2. Energy levels and the sign of ΔH

Reactants at 80, products at 20 relative energy units. Enthalpy change -60. No activation barrier is shown. Original schematic; reaction progress is not time. Vertical differences represent energies, and the arbitrary zero is not an absolute enthalpy. Energy levels alone do not give activation energies.

Reactants at 20, products at 80 relative energy units. Enthalpy change +60. No activation barrier is shown. Original schematic; reaction progress is not time. Vertical differences represent energies, and the arbitrary zero is not an absolute enthalpy. Energy levels alone do not give activation energies.

ΔH = H(products) − H(reactants). Exothermic: products lower, ΔH negative, enthalpy-change arrow points down from reactant level to product level. Endothermic: products higher, ΔH positive, arrow points up. This arrow joins the two levels; it does not go to the peak.

Energy is conserved. A fall in system enthalpy corresponds to heat transferred to surroundings under constant-pressure, pressure–volume-work-only conditions. A rise requires heat input under those conditions. An energy-level diagram alone contains no peak, so it cannot tell you Eₐ.

3. Forward and reverse activation energies

Reactants at 80, products at 20 relative energy units. Enthalpy change -60.Uncatalysed peak 140, catalytic peak 110; both pathways have identical reactant and product levels. Original schematic; reaction progress is not time. Vertical differences represent energies, and the arbitrary zero is not an absolute enthalpy. Activation-energy arrows refer to the solid uncatalysed pathway; the dashed pathway has smaller forward and reverse barriers. A real catalytic mechanism may have several steps.

Reactants at 20, products at 80 relative energy units. Enthalpy change +60.Uncatalysed peak 140, catalytic peak 110; both pathways have identical reactant and product levels. Original schematic; reaction progress is not time. Vertical differences represent energies, and the arbitrary zero is not an absolute enthalpy. Activation-energy arrows refer to the solid uncatalysed pathway; the dashed pathway has smaller forward and reverse barriers. A real catalytic mechanism may have several steps.

For a single-step pathway with the same transition-state peak in both directions: Eₐ(forward) = H(peak) − H(reactants); Eₐ(reverse) = H(peak) − H(products). Both activation-energy arrows point upwards from their own starting level to the peak.

Subtracting gives ΔH(forward) = Eₐ(forward) − Eₐ(reverse), or Eₐ(reverse) = Eₐ(forward) − ΔH(forward). In an exothermic forward reaction, the reverse barrier is larger. In an endothermic forward reaction, it is smaller. This simple relationship refers to the same single-step profile; measured effective activation energies for complex mechanisms need more careful interpretation.

4. What a catalyst changes

A catalyst provides an alternative pathway with a lower activation barrier. On a schematic overlay, keep reactant and product levels identical and lower the peak. The catalyst is regenerated overall; it does not change the initial and final chemical states used to define ΔH. Therefore ΔH stays unchanged.

In the paired profiles above, the uncatalysed peak is 140 relative units and the catalytic peak is 110. Both forward and reverse barriers fall by 30 relative units. Their difference stays unchanged. Catalysis speeds approach to equilibrium in both directions without changing the equilibrium position. It does not make an endothermic reaction exothermic.

A real catalytic pathway may contain multiple smaller barriers rather than a single smaller hump. Draw the form requested by the question and label pathways clearly. Do not lower the product level or use the ΔH arrow as an activation-energy arrow.

5. Worked diagram interpretations

Example 1 — interpret an exothermic profile

Reactants are at 80, products at 20 and the peak at 140 kJ mol⁻¹ on a supplied relative scale. ΔH = 20 − 80 = −60 kJ mol⁻¹; Eₐ(forward) = 140 − 80 = 60 kJ mol⁻¹; Eₐ(reverse) = 140 − 20 = 120 kJ mol⁻¹. The downward ΔH arrow and upward barrier arrows describe different quantities.

Example 2 — interpret an endothermic profile

Reactants are at 20, products at 80 and the peak at 140 kJ mol⁻¹. ΔH = +60; forward Eₐ = 120; reverse Eₐ = 60 kJ mol⁻¹. The reverse reaction is exothermic with ΔH = −60 kJ mol⁻¹.

Example 3 — find a reverse barrier from two values

For a single-step profile, forward Eₐ = 95 kJ mol⁻¹ and forward ΔH = −35 kJ mol⁻¹. Reverse Eₐ = 95 − (−35) = 130 kJ mol⁻¹. The negative enthalpy change makes the reverse barrier greater, not smaller.

Example 4 — calculate a catalytic comparison

Use Example 1 but replace the peak 140 by 110 kJ mol⁻¹. Catalysed forward Eₐ = 110 − 80 = 30; reverse Eₐ = 110 − 20 = 90 kJ mol⁻¹. ΔH remains 20 − 80 = −60 kJ mol⁻¹. Both barriers fall by 30.

Example 5 — identify an impossible annotation

A single-step drawing labels forward Eₐ = 50 kJ mol⁻¹ and forward ΔH = +70 kJ mol⁻¹, with the peak above both ends. It would imply reverse Eₐ = 50 − 70 = −20 kJ mol⁻¹, contradicting that drawing. Check the arrow endpoints or numbers; the product level cannot lie above the stated peak in this schematic.

Example 6 — explain ignition

A combustion profile can have low products but a high peak. A spark helps some particles overcome the initial barrier. The net enthalpy change is still negative. Needing energy to start is not evidence that the overall reaction is endothermic.

Common misconceptions

Exothermic means a downward curve everywhere: a profile can rise to a barrier before ending lower.
ΔH is the peak height: it is the product–reactant level difference.
Reverse Eₐ is negative: on these profiles it is the upward gap from products to peak.
A catalyst lowers ΔH: only the pathway changes; endpoints remain fixed.
Reaction progress means time: a profile is not a rate-versus-time graph.

Exam tips

Label both axes, reactants, products, ΔH and the relevant Eₐ.
Start each arrow at the level belonging to its definition.
Use products minus reactants, then include the sign and units.
For catalyst overlays, align both pairs of endpoints exactly.
Check Eₐ(forward) − Eₐ(reverse) against ΔH for a simple shared-peak profile.

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