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CI 4.5

CI 4.5. Energy changes in solutions. Why do some ionic substances dissolve in water, whilst others are insoluble?. If there is enough energy to separate the ions in the lattice, the substance will be soluble. Ions in solid lattice. Ions in solution. - H LE is a positive value.

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CI 4.5

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  1. CI 4.5 Energy changes in solutions

  2. Why do some ionic substances dissolve in water, whilst others are insoluble? • If there is enough energy to separate the ions in the lattice, the substance will be soluble Ions in solid lattice Ions in solution

  3. -HLE is a positive value Lattice enthalpy, HLE • The enthalpy change when 1 mole of solid is formed from the separate ions • This is always an exothermic process • HLE is always negative • The energy to break up a lattice = - HLE

  4. Which will give more negative HLE? • Small ionic charge • Large ionic charge • Small ionic radius • Large ionic radius    

  5. 3- 3+ Where does the energy come from to break up the lattice? • Hydration

  6. Exothermic – because bonds are made Is hydration exothermic or endothermic? Enthalpy of hydration,Hhyd • Hhyd is the enthalpy change when a solution of ions is made from 1 mole of gaseous ions

  7. Example of hydration • Na+(g) + aq Na+(aq) • Hhyd = - 406 kJ mol-1

  8. Different ions • An ionic compound contains cations and anions • Total Hhyd = Hhyd (cation) + Hhyd (anion) • When NaCl is dissolved: • Total Hhyd = Hhyd (Na+) + Hhyd (Cl-)

  9. Which will give more negative Hhyd? • Small ionic charge • Large ionic charge • Small ionic radius • Large ionic radius    

  10. Water is not the only solvent • Enthalpy of solvation, Hsolv , is used for other solvents

  11. Enthalpy change of solution • This is the enthalpy change when 1 mole of a solute dissolves to form an infinitely dilute solution • Hsolution = Hhyd (cation) + Hhyd (anion) - HLE • If Hsolution is negative, it is more likely that the substance will dissolve

  12. What decides solubility? • Energy is put in to separate the ions in the lattice [ - HLE] • Energy is released by hydration of the ions • If more energy is released than used up, then Hsolution will be negative and the solute is more likely to dissolve. [Hhyd (cation) + Hhyd (anion)]

  13. Enthalpy cycle for solution Hsolution Ionic lattice + solvent solution - HLE Hhyd(cation) + Hhyd (anion) Gaseous ions + solvent Hsolution = Hhyd (cation) + Hhyd (anion) - HLE

  14. Enthalpy level diagrams • Make it easier to compare the sizes of the enthalpy changes

  15. Is ΔHsolution Endothermic or exothermic ? ΔHsolution = -ΔHLE + ΔHhyd(cat) + ΔHhyd(an) enthalpy Gaseous ions -ΔHLE ΔHhyd(cat) ΔHhyd(an) Solute + solvent ΔHsolution Solution

  16. Is ΔHsolution exothermic or endothermic? ΔHsolution = -ΔHLE + ΔHhyd(cat) + ΔHhyd(an) enthalpy Gaseous ions ΔHhyd(cat) -ΔHLE ΔHhyd(an) Solution ΔHsolution Solute + solvent

  17. Solubility • The more negative the value for ΔHsolution the more likely the solute is to dissolve. • If ΔHsolutionis very large and positive, the solute will not dissolve. • If ΔHsolution is small and positive, the solute may dissolve, if there is sufficient increase in entropy.

  18. Entropy • An increase in entropy favours dissolving – even if a little energy is needed. • Substances with a small positive ΔHsolution can still dissolve if there is a favourable entropy change. • CaCO3 has a small negative ΔHsolution but a large entropy decrease so it is insoluble. Why is there a decrease in entropy?

  19. Is ΔHsolution exothermic or endothermic? ΔHsolution = -ΔHLE + ΔHSolv(cat) + ΔHsolv(an) enthalpy Gaseous ions ΔHsolv(cat) ΔHsolv(an) -ΔHLE Solution ΔHsolution Solute + solvent

  20. Over to you! ? • Time to try the problems on page 82: • 1 • 2 • 5 • 6

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