Draw and label reaction pathway diagrams for exothermic and endothermic reactions using information provided, to include: (a) reactants (b) products (c) enthalpy change of the reaction, $Delta H$ (d) activation energy, $E_{mathrm{a}}$

Chemical Energetics – Exothermic & Endothermic Reactions

What’s Happening?

When atoms rearrange to form new bonds, energy can be released (exothermic) or absorbed (endothermic). Think of it like a roller‑coaster: the track’s shape (energy diagram) shows how high the car climbs (activation energy) and how much energy is left at the bottom (enthalpy change).

Exothermic Reactions 🔥

Reaction Pathway Diagram

Reactants
|
|  Ea
v
Transition State
|
|  -ΔH
v
Products

In an exothermic reaction the product side is lower in energy – the car rolls down and releases energy as heat.

Example: Combustion of Methane

ReactantsProductsΔHEa
CH₄ + 2O₂CO₂ + 2H₂O\$-890\ \text{kJ mol}^{-1}\$\$200\ \text{kJ mol}^{-1}\$

ΔH is negative, meaning energy is released. The car (reaction) climbs a small hill (Ea) before dropping into a lower valley (products).

Endothermic Reactions ❄️

Reaction Pathway Diagram

Reactants
|
|  Ea
v
Transition State
|
|  +ΔH
v
Products

Here the product side is higher in energy – the car climbs a hill and needs extra energy to keep moving.

Example: Photosynthesis (simplified)

ReactantsProductsΔHEa
6CO₂ + 6H₂O + lightC₆H₁₂O₆ + 6O₂\$+2800\ \text{kJ mol}^{-1}\$\$400\ \text{kJ mol}^{-1}\$

ΔH is positive – energy is absorbed. The car climbs a steep hill and needs the light energy to keep going.

Comparing the Two

  • ΔH – Exothermic: negative (energy released), Endothermic: positive (energy absorbed)

  • Energy Diagram – Products lower than reactants, Products higher than reactants

  • Real‑world analogy – Fireworks (exothermic), Ice melting (endothermic)

Exam Tips for IGCSE Chemistry 0620

Key Points to Remember

  1. Write ΔH in the correct sign: negative for exothermic, positive for endothermic.

  2. Always include the activation energy (Ea) as a positive value.

  3. Use the reaction pathway diagram to show the relative heights of reactants, transition state, and products.

  4. When drawing tables, keep columns aligned and use clear labels.

  5. Explain the diagram in words – e.g., “the reaction climbs a hill of height Ea before dropping into a lower valley (exothermic) or climbing a higher valley (endothermic).”

Remember: clarity and correct sign conventions are worth full marks!