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HL5-6

Lattice Enthalpy 15.2.1. HL5-6.ppt. 15.2.1 – Define and apply the terms lattice enthalpy and electron affinity. Q – Which one of these are you already familiar with?. First ionization energy ( Δ H i ˚). Earlier we learned that metals and non-metals tend to form ionic compounds.

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HL5-6

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  1. Lattice Enthalpy 15.2.1 HL5-6.ppt

  2. 15.2.1 – Define and apply the terms lattice enthalpy and electron affinity. • Q – Which one of these are you already familiar with?

  3. First ionization energy (ΔHi˚) • Earlier we learned that metals and non-metals tend to form ionic compounds. • Q – Who loses and who gains electrons? • ΔHi˚ refers to the energy needed to form the positive ion. • Na(g)  Na+(g) + e-(g) ΔHi˚= +496 kJ/mol • Q – Why is this value >0? • Q – Which element would have a larger ΔHi˚ Na or Mg?

  4. First electron affinity (ΔHe˚) • On the other hand, the non-metal elements in ionic compounds attract electrons. • ΔHe˚ refers to the enthalpy change when one mole of gaseous atoms attracts one mole of electrons. • Values can be found in Table 7 of the IB Data Booklet. • Cl(g) + e-(g)  Cl-(g) ΔHe˚= -349 kJ/mol • Q – Is this process endo or exothermic?

  5. Lattice Enthalpies • So what if we put those two processes together? • Na(g) + Cl(g)  Na+(g) + Cl-(g) • ΔH˚= ΔHe˚ + ΔHi˚ = -349 + 496 = +147 kJ/mol • We can see that e- transfer process is endothermic, yet we know that the formation of salt is highly exothermic. Something else must be at play. • Q - In reality, what do those oppositely charged gas ions do?

  6. Lattice Enthalpies • The oppositely charged gas ions come together to form an ionic lattice (framework). • Q – Is this process endo or exothermic? • This process is highly exothermic as there is a strong attraction between the ions. • Na+(g) + Cl-(g)  NaCl(s) • ΔH˚= -790 kJ/mol • Generally, most endothermic reactions do not readily occur (like the transfer of electrons noted before) but this last step explains the readiness of metals and non-metals to form ionic compounds.

  7. Lattice Enthalpies • The lattice enthalpy ΔHlat˚ expresses this enthalpy change in terms of the reverse endothermic process. • It can also be described as the enthalpy related to the formation of gaseous ions from one mole of a solid crystal breaking into gaseous ions. • NaCl(s)  Na+(g) + Cl-(g) ΔHlat˚= +790 kJ/mol

  8. Review of ΔH variables • ΔH˚ = ? • ΔHlat˚ = ? • ΔHi˚ = ? • ΔHe˚ = ? • ΔHc˚ = ? • ΔHf˚ = ?

  9. HW • Work on your calorimetry IA

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