2.1.3 Ionic Lattices and Properties ionic compounds

Ionic crystals ( Ionic compounds ) are formed in a lattice-structure

Lattice: Continous three-dimesional networks of repeating positive & negative ions. These networks are distinct that heavily depends on the size and ratio of ions.

The formula of the Ionic compound can indicate this ratio. ( $NaCl$) , indicates 1 : 1 ratio.

Note that Ionic Bonds are non-directional, meaning an anion attracts to all the cations surrounding it w/ the attraction being divided equally amongst the cations. This happens w/ the same thing with cations but with anions. This makes the forces of attraction extremely strong..

A lattice is actually 3D so it is now 2D for simplication.

Lattice Enthalpy($\Delta H^{(-)}_{lattice}$)

• Tells one how strong the ionic bonds are

• It is a measure of the standard Enthalpy change ( heat ) that happens when one mole of the solid lattice turns to a gaseous-ion, because it is only then that all the electrostatic-ally attracted bond are broken.

The process is endothermic

• This means an increased Ionic radius results in a decrease in Lattice Ethalphy

$\Delta H^{(-)}_{lattice} \propto \frac{1}{IR}$

• This means an increased Ionic charge yields a increase in Enthalphy

  $\Delta H^{(-)}_{lattice} \propto Z_{eff}$

Properties

These properties can be determined by their structural feature of strong ionic bonds held together electrostatically.

1. Volatility

• Tendency of substance to turn to gas

• Due to ions attraction, volatility low ( non-volatile )

  • Means have high boiling points and typically high melting points

1. Electrical Conductivity

• In order to conduct electricity, must contain charged particles that can move

  • The charged metric is fulfilled

  • But the crystalline structure only lets these ions vibrate not change position

    • Though when molten or aqueous, these ions and cations can move freely and thus is conductive ( mobile ions )

1. Solubility

• Solube in polar solvents ( 2.2 )

  • $H_2O$(water)

• insolube in non-polar solvents

  • $C_6H_{14}$(hexane)

Polar Solvents, in summary is that $\because$of the difference in Electronegativity between atoms in a compound, it results in the solvent having a partial charge.

When an Ionic Compound goes into water, the cations of one substance goes towards the anions of water and vice versa, pulling apart the lattice. IN the case of a non-polar, there is no attraction between the ions of the ionic compound and the solvent, meaning the ions remain in the lattice.

This is not black and white though... As dissolvabiity depends on two factors.

1. Ionic bonds between cations & ions in lattice

2. and using example of water: The attraction of the latter ions an d cations with the partial charge of the water molecules.

So if the attraction between the cations and ions in the lattice exceeds the association between the ions and the partial charge of the water molecule, the ionic Compound is not solube.