Reaction Pathway Diagrams
Advanced Pathways Past Papers — Drawing, Labelling & Interpretation
Arrow Placement
5 Questions
Drawing
5 Questions
Interpretation
5 Questions
On a reaction pathway diagram, where exactly does the arrow for activation energy (Eₐ) start and where does it end?
💡 Hint: Think about the definition: the minimum energy needed to START a reaction.
On a reaction pathway diagram for an exothermic reaction, where does the ΔH arrow start and end? In which direction does it point?
💡 Hint: ΔH = H(products) − H(reactants). For exothermic, ΔH is negative.
On an endothermic reaction pathway diagram, in which direction does the ΔH arrow point, and what is the sign of ΔH?
💡 Hint: Products have MORE energy than reactants in an endothermic reaction.
A catalyst is added to an exothermic reaction. On the pathway diagram, how would you show the effect of the catalyst? Which arrows change position?
💡 Hint: A catalyst provides an alternative reaction pathway with a lower activation energy.
On a reaction pathway diagram, a student draws the ΔH arrow starting from the peak and ending at the products line. Is this correct? Explain.
💡 Hint: What does ΔH actually measure?
Draw and fully label a reaction pathway diagram for an EXOTHERMIC reaction. Your diagram must include: the reaction curve, labelled Reactants line, labelled Products line, ΔH arrow (negative), Eₐ arrow, and the peak labelled as 'Transition State'.
💡 Hint: Reactants are HIGHER than products. The peak is ABOVE the reactants line.
Draw and fully label a reaction pathway diagram for an ENDOTHERMIC reaction. Include: reaction curve, Reactants line, Products line, ΔH arrow (positive), Eₐ arrow, transition state peak.
💡 Hint: Products are HIGHER in energy than reactants. The peak still rises ABOVE the products line.
Sketch the reaction pathway diagram for the thermal decomposition of calcium carbonate: CaCO₃(s) → CaO(s) + CO₂(g). Is this reaction exothermic or endothermic? Justify your answer from the diagram.
💡 Hint: Thermal decomposition always requires a continuous supply of heat energy.
Draw TWO curves on the same reaction pathway diagram to show an exothermic reaction (a) without a catalyst and (b) with a catalyst. Clearly label both curves, and indicate which ΔH and Eₐ values change.
💡 Hint: A catalyst lowers the activation energy but does not change the overall energy change.
A student draws a reaction pathway diagram and labels the x-axis as 'Time'. Identify the error and explain what the x-axis should be labelled.
💡 Hint: Reaction pathways show energy changes along the reaction coordinate, not over real time.
A reaction pathway diagram shows: Reactants line at 50 kJ, Peak at 150 kJ, Products line at 20 kJ. Calculate (a) the activation energy Eₐ and (b) the enthalpy change ΔH.
💡 Hint: Eₐ = Peak − Reactants. ΔH = Products − Reactants.
Diagram data: Reactants = 40 kJ, Peak = 180 kJ, Products = 90 kJ. (a) Calculate Eₐ. (b) Calculate ΔH. (c) Is the reaction exothermic or endothermic? Justify.
💡 Hint: Check whether products are higher or lower than reactants.
For a catalysed reaction, the diagram shows: Reactants = 80 kJ, Uncatalysed peak = 200 kJ, Catalysed peak = 130 kJ, Products = 50 kJ. By how much does the catalyst lower the activation energy?
💡 Hint: Calculate each Eₐ separately, then find the difference.
A student reads a diagram and states: 'The activation energy is 200 kJ/mol because the peak is at 200 kJ.' The reactants line is at 75 kJ. Identify and correct the student's error. Calculate the correct Eₐ.
💡 Hint: Activation energy is measured FROM the reactants level, not from zero.
Using the concept of activation energy from a reaction pathway diagram, explain why (a) increasing temperature increases the rate of reaction, and (b) a catalyst increases the rate of reaction.
💡 Hint: Think about how many molecules have enough energy to react.
📋 Examiner's Golden Rules for Pathway Diagrams
• Eₐ arrow: Always starts at the reactants energy level — NEVER from zero on the y-axis
• ΔH arrow: Always from reactants to products — NEVER from peak to products
• Catalyst effect: Lowers Eₐ ONLY — ΔH is completely unchanged
• X-axis label: "Reaction Progress" or "Reaction Coordinate" — NEVER "Time"
• Units: Always state kJ/mol for energy values
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