Cambridge O Level Chemistry · Syllabus 5070 · Chemical Energetics
Reaction Pathway Diagram
What is Reaction Pathway Diagram?
A diagram with energy on the vertical axis and progress of reaction on the horizontal axis, showing the reactants level, a peak, and the products level, on which the activation energy and the enthalpy change of a reaction can be labelled.
This definition is part of the Chemical Energetics chapter in Cambridge O Level Chemistry.
Common mistakes with Reaction Pathway Diagram
- 16. “I added a second, lower curve to show the catalysed route.” Repair Catalysts are Topic 6, not Topic 5. Drawing one here answers a question that was not asked, and in a “draw the reaction pathway diagram” question the extra curve can only confuse the labelling of the one that was wanted. Keep to a single pathway.
Questions students ask about Reaction Pathway Diagram
Why is the enthalpy change negative for an exothermic reaction?
Because the sign is a direction label, not a statement that energy is negative. There is no negative energy. The minus sign records that thermal energy left the system and went into the surroundings. If a reaction has \(\Delta H = -184\;\mathrm{kJ\,mol^{-1}}\), then 184 kJ of thermal energy is transferred to the surroundings for every mole of reaction; the sign says which way it went and the number says how much. On a reaction pathway diagram the products sit below the reactants.
What is activation energy, and how is it shown on a reaction pathway diagram?
Activation energy, \(E_\mathrm{a}\), is the minimum energy that colliding particles must have to react. Every reaction, exothermic or endothermic, has this barrier. On a reaction pathway diagram it is the arrow drawn from the reactants level up to the peak of the curve, never from zero and never from the products level. The enthalpy change, \(\Delta H\), is a different arrow: it runs between the reactants level and the products level, and the peak plays no part in it. Same start, different finish.