Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 2 - 7.6 Solvent choice and intermolecular forces-Study Notes - New Syllabus

7.6 Factors Influencing Choice of Solvents 

(i) Water as a Solvent for Ionic Compounds

Key Idea: Hydration of Ions

Water dissolves ionic compounds because ions become surrounded by water molecules (hydration).

Explanation 

• Water is a polar molecule:
  • Oxygen has \( \delta^- \) charge
  • Hydrogen has \( \delta^+ \) charge

• When an ionic compound dissolves:
  • Positive ions attract the \( \delta^- \) oxygen end
  • Negative ions attract the \( \delta^+ \) hydrogen end

• This forms ion–dipole interactions.
• Ions become hydrated and separated from the lattice.

Example

\( \mathrm{NaCl(s) \rightarrow Na^+(aq) + Cl^-(aq)} \)

Key Conclusion

Water dissolves ionic compounds due to strong ion–dipole attractions (hydration).

(ii) Water as a Solvent for Simple Alcohols

Key Idea: Hydrogen Bonding

Simple alcohols dissolve in water because they can form hydrogen bonds with water molecules.

Explanation 

• Alcohols contain –OH group.
• This group is polar.

• Hydrogen bonding occurs between:
  • \( \mathrm{H^{\delta+}} \) of –OH
  • Lone pair on oxygen of water

• These strong interactions allow alcohol molecules to mix with water.

Effect of Chain Length

• Small alcohols (e.g. methanol, ethanol) → fully miscible.
• Larger alcohols → less soluble.
• Due to increasing non-polar hydrocarbon chain.

Example

\( \mathrm{C_2H_5OH(l) + H_2O(l)} \) → hydrogen bonding between molecules

Key Conclusion

Alcohols dissolve in water due to hydrogen bonding between –OH groups and water molecules.

Summary 

• Ionic compounds dissolve due to hydration (ion–dipole forces).
• Alcohols dissolve due to hydrogen bonding.

Therefore, solubility in water depends on the ability to form strong interactions with water molecules.

(iii) Water as a Poor Solvent for Some Compounds

Key Idea: Inability to Form Hydrogen Bonds

Water is a poor solvent for many organic compounds because they cannot form hydrogen bonds with water.

Explanation 

• Water is polar and forms strong hydrogen bonds.
• Some compounds (e.g. halogenoalkanes):
  • May be slightly polar
  • But do not form hydrogen bonds with water
• Therefore:
  • Weak interactions with water
  • Cannot replace strong water–water hydrogen bonds
• Result: substance is insoluble or only slightly soluble.

Example

Halogenoalkanes (e.g. \( \mathrm{CH_3Cl} \)) are not soluble in water.

Conclusion

If a substance cannot form hydrogen bonds with water, it is unlikely to dissolve.

(iv) Non-aqueous Solvents and “Like Dissolves Like”

A substance dissolves best in a solvent that has similar intermolecular forces.

Explanation 

• Non-aqueous solvents (e.g. hexane) are non-polar.

• They interact via London dispersion forces.

• Non-polar solutes:
  • Also have London forces
  • Therefore dissolve easily

• Polar solutes:
  • Do not dissolve well in non-polar solvents

Examples

• Iodine dissolves in hexane (both non-polar).
• Oil dissolves in organic solvents, not in water.

Summary 

• Water cannot dissolve substances that cannot hydrogen bond.
• Solubility depends on similarity of intermolecular forces.
• “Like dissolves like” explains solvent choice.

Therefore, solvent selection depends on intermolecular interactions between solute and solvent.