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6.2Thermochemistry

Energy Diagrams — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 8 parts Lesson 6 of 14 1 quiz

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Lesson summary

6.2 Energy Diagrams — AP CHEMISTRY notes: Draw and interpret energy diagrams for exothermic and endothermic processes, including activation energy and the effect of a catalyst.Axes — potential energy against progress of the reactionOverall energy change ΔH — the difference betwee…

  • Activation energy Ea
  • Forward Ea
  • Reverse Ea
  • Heat added to raise the temperature of water
  • Transition state
  • Overall energy change ΔH — the difference between the energy of the products and the reactants

Section 1 of 8Reading an energy diagram

Axes
  • Axes potential energy against progress of the reaction
Overall energy change ΔH
  • Overall energy change ΔH the difference between the energy of the products and the reactants
Activation energy Ea
  • Activation energy Ea the difference between the reactants and the peak
    • (transition state)

Section 2 of 8Exothermic and endothermic diagrams

FeatureExothermicEndothermic
Products compared with reactantsLowerHigher
ΔHNegativePositive
Energy in equationProduct side, as in 2H2 + O2 → 2H2O + energyReactant side, as in 2HgO + energy → 2Hg + O2
Energy supplied from surroundingsNo, energy releasedYes, equal to the energy difference

Section 3 of 8Activation energy

Forward Ea
  • Forward Ea measured from the reactants to the peak
Reverse Ea
  • Reverse Ea measured from the products to the peak, equal to Ea(forward) − ΔH
Rate link
  • Rate link a higher Ea means a slower reaction

Section 4 of 8Effect of a catalyst

Activation energy
  • Activation energy a catalyst lowers Ea for both forward and reverse reactions
ΔH
  • ΔH it is unchanged because reactant and product energies are the same

Section 5 of 8Interpreting heating of water

  • Heat added to raise the temperature of water
    • the process is endothermic

Section 6 of 8More from the specification

More from the specification
Ionic precipitation into crystalline solid
  • Ionic precipitation into crystalline solid mixing stoichiometric quantities of barium chloride and sodium sulfate produces barium sulfate as a dense white solid.
Lattice
  • A regular, repeating three-dimensional arrangement of particles.
Combustion
  • Burning in oxygen
Kinetic energy
  • The energy of motion of a particle.
Lattice energy
  • Enthalpy change when one mole of an ionic compound forms from its gaseous ions.
Heating curve
  • A graph of temperature against time as a substance is heated.
Cooling curve
  • A graph of temperature against time as a substance cools.
System
  • The reacting chemicals.
Substance
  • A material with only one type of particle.
Dissolution
  • Process of a solute dissolving in a solvent.
Hydration
  • The addition of steam to C=C to make an alcohol.
Particle
  • A tiny portion of matter, too small to see.

Section 7 of 8Key terms

TermMeaning
Energy diagramPlot of potential energy against reaction progress.
Activation energyEnergy barrier between reactants and the peak.
ΔHEnergy of products minus energy of reactants.
Transition stateHighest-energy point on the diagram.

Section 8 of 8Exam technique

Avoid these mistakes
Drawing the peak at the product level — the peak is always above both reactants and products.
Saying a catalyst changes ΔH — it only changes the height of the barrier.
Placing energy on the wrong side of the equation — it is a product in exothermic and a reactant in endothermic reactions.

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