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6.1Thermochemistry

Chemical Energy — AP CHEMISTRY Revision Notes & Topical Questions

5 min read 10 parts Lesson 5 of 14 1 quiz

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Lesson summary

6.1 Chemical Energy — AP CHEMISTRY notes: Explain energy changes in terms of bond and intermolecular force breaking and forming, and classify processes as exothermic or endothermic.First law — energy lost by the system is gained by the surroundings: qsystem = −qsurroundingsEnthal…

  • Enthalpy change
  • Open system
  • Closed system
  • Isolated system
  • Origin of energy changes
  • Bond breaking

Section 1 of 10Conservation of energy

1First law
  • First law energy lost by the system is gained by the surroundings: qsystem = −qsurroundings
2Enthalpy change
  • ΔH = Hproducts − Hreactants
  • ΔH < 0 for exothermic and ΔH > 0 for endothermic processes.

Section 2 of 10System and surroundings

System and surroundings
System
  • System the reactants and products being studied;
    • surroundings — everything else, such as the solvent, container and air
Open system
  • Open system exchanges both matter and energy with the surroundings
  • An open beaker is an example.
Closed system
  • Closed system exchanges energy but not matter
  • A sealed flask is an example.
Isolated system
  • Isolated system exchanges neither matter nor energy
  • An ideal insulated calorimeter is an example.

Section 3 of 10Energy changes from bonds and forces

Origin of energy changes
  • Origin of energy changes bonds or intermolecular attractions are broken and/or formed in every chemical and physical change
Bond breaking
  • Bond breaking always endothermic; bond forming — always exothermic
Net effect
  • Net effect if forming new bonds releases more energy than breaking old bonds absorbs, the reaction is exothermic
  • If breaking absorbs more than forming releases, the reaction is endothermic.

Section 4 of 10Heat and temperature

Heat
  • Heat energy in transit because of a temperature difference; it flows from hot to cold
Temperature
  • Temperature a measure of the average kinetic energy of the particles
Thermometer reading
  • Thermometer reading a thermometer measures the surroundings, so a rise in temperature means the process is exothermic

Section 5 of 10Phase changes

ProcessEnergyReason
Melting, vaporisation, sublimationAbsorbed (endothermic)Intermolecular forces are overcome
Freezing, condensation, depositionReleased (exothermic)Intermolecular forces form
During any phase changeTemperature constant at constant pressureEnergy changes potential energy, not kinetic

Section 6 of 10Dissolving

1
Steps
  • Steps energy is absorbed to separate solute particles and solvent particles, and released when solute-solvent attractions form
2
Sign
  • Sign dissolving is endothermic if the energy absorbed is greater and exothermic if the energy released is greater
  • Ionic solidΔHsolution = ΔHlattice + ΔHhydration.
  • NaOH and CaCl2 dissolve exothermically; NH4NO3 dissolves endothermically and is used in cold packs.

Section 7 of 10Common examples

Exothermic
  • Exothermic combustion, respiration, neutralisation and many dissolving processes
Endothermic
  • Endothermic photosynthesis, thermal decomposition and dissolving NH4NO3

Section 8 of 10More from the specification

Substance
  • A material with only one type of particle.
Dissolution
  • Process of a solute dissolving in a solvent.
Heating curve
  • A graph of temperature against time as a substance is heated.
Energy diagram
  • Plot of potential energy against reaction progress.
Cooling curve
  • A graph of temperature against time as a substance cools.

Section 9 of 10Key terms

TermMeaning
Enthalpy change (ΔH)Heat change at constant pressure, Hproducts − Hreactants.
Open systemSystem exchanging matter and energy with surroundings.
Closed systemSystem exchanging energy but not matter.
Isolated systemSystem exchanging neither matter nor energy.
ExothermicΔH < 0; heat leaves the system.
EndothermicΔH > 0; heat enters the system.

Section 10 of 10Exam technique

Avoid these mistakes
Saying breaking bonds releases energy — breaking bonds always absorbs energy.
Saying a reaction is exothermic because its products are less stable — exothermic products are more stable, with lower energy.
Believing equal temperature means equal thermal energy — thermal energy also depends on mass.

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