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
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atomization of Na
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atomization of Cl
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first ionization of Na
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electron affinity of Cl
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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)
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
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always endothermic
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ex. M (s) → M (g)
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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
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depends on the strength of the bond:
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magnitude of charge of ions: if |charge|↑ then lattice enthalpy ↑
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size of ions: if size ↑ then lattice enthalpy ↓
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smaller ions have greater attraction because of their higher charge density
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rule of thumb: if the charge is the same, the smaller ion wins
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larger lattice enthalpies → higher melting points (extra energy required to separate oppositely charged ions)
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
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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
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type of ion formed affects the bond strength
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can be either + or –
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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
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type of ion formed affects the bond strength
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always – because it is exothermic
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gas + water → aqueous
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if the solvent ≠ water → enthalpy of solvation
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∆Hºhyd is greater when...
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size ↓
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charge ↑
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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.
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nature goes towards high entropy
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entropy increases with temperature
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can be determined by the change in the number of gaseous particles
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measured in J/K*mol
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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.
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if Gº is...
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positive: non-spontaneous reaction
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negative: spontaneous reaction
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zero: in equilibrium
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measured in kJ/K*mol
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must convert entropy to kJ/K*mol before using the following equation:
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∆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)
From Ms. Fu's powerpoint
From Ms. Fu's powerpoint
From Ms. Fu's powerpoint
Pearson textbook pg. 253