# Chemistry, O Revision Notes

###### Contents:
• 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

## O1

### 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.