Simulic
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Dissolving at particle level – hydration, like dissolves like and enthalpy of solution

ChemistryEnergetics & ThermodynamicsAges 15–16

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Particles screen: water molecules pull ions off a sodium chloride or potassium nitrate lattice and surround them with hydration shells, sucrose dissolves through hydrogen bonds, and iodine dissolves in hexane but hardly in water; add solute until the solution is saturated and change the temperature to read the solubility curve. Energy screen: a cycle of the lattice dissociation and hydration enthalpies gives the enthalpy of solution of six salts, ΔsolG = ΔsolH − TΔsolS shows the role of entropy, and a virtual calorimeter measures the temperature change.

Lesson: Solutions and solubility: intermolecular forces, hydration, enthalpy and entropy of solution

What it shows

Dissolving is a competition between attractions. A solid dissolves when the attractions between solute and solvent particles make up for breaking the solute's own attractions and some of the solvent's. Water dipoles surround ions (ion–dipole forces) and polar molecules (hydrogen bonds), while non-polar iodine mixes with non-polar hexane: like dissolves like. The enthalpy of solution is the small difference between the lattice dissociation enthalpy and the hydration enthalpies, so it can be positive or negative; the entropy change then decides, through ΔG = ΔH − TΔS, whether dissolving is spontaneous.

How to use

On Particles, choose a Solute and a Solvent, press +10 g until undissolved solid remains, then move the Temperature slider and read the solubility from the curve. Compare NaCl in water and in hexane, and iodine in both solvents. On Energy of dissolving, choose each Salt, compare the cycle with the measured ΔsolH, change the Temperature to follow the sign of ΔG, then press Add salt and stir and Record to calculate ΔsolH from ΔT.

Parameters you can change

  • Screen Particles, Energy of dissolving
  • Solute (particles) NaCl (sodium chloride), KNO₃ (potassium nitrate), C₁₂H₂₂O₁₁ (sucrose), I₂ (iodine)
  • Solvent (particles) water (H₂O), hexane (C₆H₁₄)
  • Temperature 0–100 °C
  • Solute added per 100 g of solvent 0–600 g
  • Salt (energy) NaCl (sodium chloride), KCl (potassium chloride), LiCl (lithium chloride), CaCl₂ (calcium chloride), KNO₃ (potassium nitrate), NH₄NO₃ (ammonium nitrate)
  • Mass of salt in the calorimeter 1–20 g

Questions to explore

  1. Why does sodium chloride dissolve in water but not in hexane?
  2. Why can a salt such as ammonium nitrate dissolve spontaneously even though dissolving it is endothermic?
  3. Why is the enthalpy of solution from the cycle less reliable than the measured value?

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