Chemistry, O Revision Notes

  • 1 O1
    • 1.1 Ions in Solution
      • 1.1.1 Ionic Solids
      • 1.1.2 Ionic Substances in Solution
    • 1.2 Energy Changes in Solutions
      • 1.2.1 Lattice Enthalpy


Ions in Solution

Ionic Solids

  • Ionic solids are held together by opposite charges

    • Anions and Cations form giant ionic lattice

    • Eg. \(NaCl\)

    • Each \(Na^{+}\) ion surrounded by six \(Cl^{-}\) ions and vice versa

  • Overall attraction within lattice greater than repulsion

  • Strong melting and boiling points

Ionic Substances in Solution

  • Many ionic substances dissolve readily in water

  • Ions becomes surrounded by water and spread out

  • Ions lose regular arrangement - becomes random

  • Ions behave independently of each other

Energy Changes in Solutions

Not all ionic substances dissolve readily in solution, as energy changes are an important factor.

Lattice Enthalpy

Before an ionic solid can dissolve, the ions must be separated from their lattice. Energy must be supplied to overcome the electrostatic attractions, making it an endothermic process.

\(\Delta_{LE}H\) is the enthalpy change when one mole of a solid is formed from its separated ions in their gaseous states.

  • Energy required to form a lattice is negative

  • Energy required to break a lattice is therefore \(-\Delta_{LE}H\)

Lattice enthalpy is affected by both the charge and radii of the ions.

  • A greater charge will increase the magnitude of the lattice enthalpy

  • A greater atomic radii will decrease the magnitude of the lattice enthalpy

    • Larger separation between charges leads to smaller attraction

  • The magnitude of the lattice enthalpy will increase with greater charge density

  • Thus, substances with large enthalpies are normally insoluble

Hydration and Solvation

Many ionic substances dissolve despite their value of \(\Delta_{LE}H\).

  • Many ionics form solutions with water as solvent

    • Covalent bonds in water are polar due to electronegativity differences in oxygen and hydrogen

    • Bent shape means whole molecule is polar and acts as a dipole

  • Charges on water are attracted to charges on ions

    • Ion-Dipole interactions

  • Molecules interact with surface of ionic solid, separating ions from lattice

  • Ions are now surrounded by water molecules and in solution

  • Ion is now hydrated, as they are bound to water molecules

  • Strength of interaction determines hydration

  • Extensively hydrated ions will bind strongly to water molecules

  • Strongest hydration will lead to many attached water molecules

  • Higher charge density ions will attract more water molecules

    • A small ion can become a large hydrated ion

  • Energy is released when ions are hydrated

  • May be sufficient to hydrate another ion from the lattice

  • However, hydrogen bonds between water must be broken

    • Allows water molecules to rearrange around ions

    • Requires energy

\(\Delta_{hyd}H\) is the enthalpy change when one mole of gaseous ions dissolve is sufficient water to form a solution of infinite dilution, and are always negative as it is an exothermic process.

  • \(|\Delta_{hyd}H|\) is greatest with a large charge density

    • Smallest radii and largest charge

When a solvent other than water is used, the enthalpy of solvation, \(\Delta_{solv}H\) is used. For example, Ethanol has polar molecules that can bond to ions.