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Topic 15. Energetics/thermochemistry HL

15.1 Energy cycles

The Born-Haber Cycle is a diagrammatic representation of the formation of ionic compounds, starting from atoms, moving to gaseous atoms to gaseous ions to a solid compound. It is used to determine lattice enthalpies.

ex) Born-Haber Cycle for NaCl

  1. atomization of Na

  2. atomization of Cl 

  3. first ionization of Na

  4. electron affinity of Cl

  5. formation of NaCl

Na (s) + ½ Cl2 (g)

Na (g) + ½ Cl2 (g)

Na (g) + Cl (g)     ½ bond enthalpy of Cl2

Na⁺ (g) + Cl (g)

Na⁺ (g) + Cl⁻ (g)

NaCl (s)

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From Ms. Fu's powerpoint


The enthalpy of atomization (∆Hºat) is the energy change required to change one mole of atoms from their standard state to a gaseous state

  • always endothermic

  • ex. M (s) → M (g)

  • ex. ½ X₂ (g) → X (g)

Ionization requires energy → endothermic

Electron affinity → exothermic

The lattice enthalpy (∆Hºlat) is the energy required to convert one mole of an ionic substance into gaseous ions

  • depends on the strength of the bond:

    • magnitude of charge of ions: if |charge|↑ then lattice enthalpy ↑​

    • size of ions: if size ↑ then lattice enthalpy ↓

      • smaller ions have greater attraction because of their higher charge density​​

  • rule of thumb: if the charge is the same, the smaller ion wins

  • larger lattice enthalpies → higher melting points (extra energy required to separate oppositely charged ions)

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From Ms. Fu's powerpoint

Covalent character increases down a group and across a period. Covalent character increases as the difference between experimental and theoretical values of lattice enthalpy increases. (Not that important).

∆Hºsol = ∆Hºlat + ∆Hºhyd

Solvation is when IMFs form between a solvent and solute

  • type of ion formed affects the bond strength

The enthalpy of solution (∆Hºsol) is the energy change that occurs when one mole of solute dissolves in excess solvent to form a dilute solution

  • type of ion formed affects the bond strength

  • can be either + or –

  • solid → aqueous 

The enthalpy of hydration (∆Hºhyd) is the energy change that occurs when one mole of gaseous ions are added to water to form a dilute solution

  • type of ion formed affects the bond strength

  • always – because it is exothermic

  • gas + water → aqueous

  • if the solvent ≠ water → enthalpy of solvation

  • ∆Hºhyd is greater when...

    • size ↓​

    • charge ↑

    • because there is a stronger interaction with polar water molecules

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Same reaction, different way to illustrate.

15.2 Entropy and spontaneity

Entropy (s) is a measure of distribution of total available energy among particles. It measures "how disordered" a system is.

  • nature goes towards high entropy

  • entropy increases with temperature

  • can be determined by the change in the number of gaseous particles

  • measured in J/K*mol​​​

  • gases have the most entropy, solids have the least

∆Sº = ∑Sº (products) – ∑Sº (reactants)

Gibbs free energy (G) is the energy available to do work.

  • if Gº is...

    • positive: non-spontaneous reaction​

    • negative: spontaneous reaction

    • zero: in equilibrium

  • measured in kJ/K*mol​​​

    • must convert entropy to kJ/K*mol before using the following equation:

∆Gº = ∆Hº – T∆Sº


∆Gº: change in Gibbs free energy (kJ/m0l)

∆H: enthalpy change (kJ/mol)

T: temperature (K)

∆Sº: entropy (convert to kJ/K*mol)

Screen Shot 2022-05-11 at 8.08.20 PM.png

From Ms. Fu's powerpoint

From Ms. Fu's powerpoint

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From Ms. Fu's powerpoint

Pearson textbook pg. 253

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