LEARNING OBJECTIVES
By the end of this chapter, you should be able to:
- distinguish exothermic and endothermic reactions
- interpret and draw reaction pathway diagrams
- define enthalpy change and activation energy
- explain energy change through bond breaking and making
- calculate enthalpy change from bond energies
THE BIG IDEA
Track energy transfers using temperature changes, reaction pathways, activation energy and bond energies.
Chemical reactions transfer energy because bonds in reactants are broken and new bonds in products are made.
The sign of enthalpy change describes the reaction system: energy leaving the system gives a negative value, while energy entering gives a positive value.
SECTION 01
Exothermic and endothermic reactions
An exothermic reaction transfers thermal energy to the surroundings, raising their temperature. Combustion and neutralisation are common examples.
An endothermic reaction takes thermal energy from the surroundings, lowering their temperature. Thermal decomposition and some dissolving processes are examples.
KEY IDEAS
- Exothermic: ΔH < 0.
- Endothermic: ΔH > 0.
- Temperature change describes the surroundings, not whether the reacting mixture contains heat.
SECTION 02
Reaction pathways and activation energy
A reaction pathway diagram plots energy against reaction progress. The vertical difference between reactants and products is ΔH.
Activation energy, Eₐ, is the minimum energy colliding particles need to react. A catalyst provides a different pathway with lower Eₐ but does not change reactant energy, product energy or ΔH.
KEY IDEAS
- Exothermic products lie below reactants.
- Endothermic products lie above reactants.
- The peak above reactants represents activation energy.
SECTION 03
Bond energies
Breaking bonds requires energy and is endothermic. Making bonds releases energy and is exothermic.
Average bond energies estimate enthalpy change. Count every bond using the balanced equation before adding energies.
Estimating ΔH for H₂ + Cl₂ → 2HCl
- Break one H–H bond: 436 kJ/mol.
- Break one Cl–Cl bond: 243 kJ/mol.
- Make two H–Cl bonds: 2 × 431 = 862 kJ/mol released.
- ΔH = 436 + 243 − 862.
Answer: ΔH = −183 kJ/mol, so the reaction is exothermic.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Exothermic reactions warm the surroundings.
- Endothermic reactions cool the surroundings.
- Catalysts lower activation energy without changing ΔH.
- Bond breaking absorbs and bond making releases energy.