# S-Cool Revision Summary

## S-Cool Revision Summary

The kinetic theory of matter states that all matter is made up of particles and exists in one of three states, solid, liqiud or gas.

The order of the particles decreases as you change from solid, liquid to gas - due to decrease in forces between particles.

To change a substance from a solid to finally a gaseous state, energy must be supplied in order to overcome these forces of attractions between particles. As a change of state occurs the temperature of the substance remains constant as the energy supplied is used to overcome these attractive forces.

The kinetic theory of ideal gases makes two major assumptions.

1. Gases do have a volume.

2. Intermolecular forces of attraction do exist.

Real gases deviate from ideal behaviour at low temperatures and high pressure.

There are three gas laws that govern the behaviour of gases with regards to changes in temperature, pressure and volume.

1. Boyle's law: For a fixed mass of gas, the pressure is inversely proportional to the volume, if temperature remans constant.

pV = constant

2. Charles' law: For a fixed mass of gas, the volume is proportional to the absolute temperature, if the pressure remains constant.

V/T= constant

3. Pressure law: for a fixed mass of gas, the pressure is proportional to the absolute temperature, if the volume remains constant.

p/T - constant

The gas constant depends on the amount of gas, therefore is wriiten as nR, where n = no. of moles. R = 8.314 JK-1mol-1.

The ideal gas equation can now be written as:

pV = nRT

Units used must be SI.

Alternative uses of this equation are:

P1V1/T1 = P2V2/T2

where 1 represents the gas conditions before any change, 2 represents gas conditions after a change.

To calculate molecular mass: Mr,

Mr = mRT/pV

There are three types of intermolecular forces:

1. van der Waal's:

Caused by non-polar molecules having temporary dipoles (due to movement of electrons) that cause an imbalance of electrons in neighbouring molecules. Hence, creating electrostatic attractions.

Example: methane CH4

2. Permanent dipole:

A polar molecule contains permanent dipoles, due to the molecule being unsymmetrical in terms of shape or type of atom present. The size of this force is determined by the electronegativities of the atoms present.

Solids whose particles are held by permanent dipoles have greater boiling points than those held by van der Waal's due to their permanent nature.

Example: HCl

3. Hydrogen bond:

A strong electrostatic attraction between the poorly shielded proton of the hydrogen atom bonded to a small highly electronegative atom, such as N, O or F and a lone pair of electrons on a neighbouring molecule.

Example: water - H2O.

Solids

Can be classified as one of five types:

1. Metallic:

Atoms held together by electrostatic forces between pseudo cations and delocalised electrons. Have high melting points and are good conductors of heat and electricity.

2. Giant ionic:

Ions held in a giant lattice due to electrostatic attraction between cations and anions. Soluble in water, good conductors when dissolved or in molten state, brittle.

3. Giant covalent:

In general each atom (C or Si) can be imagined situated in the centre of a tetrahedron strongly bonded to four other atoms. Covalent linking of these atoms occurs throughout the lattice.

Diamond has C atoms with the above arrangement, leading to its properties of poor conductor of electricity and heat, hard, very high melting point.

Graphite also has this giant linkage of covalent bonds between carbon atoms, however only three bonds are made by each atom, leaving a delocalised electron on each atom.

The carbon atoms are arranged in flat parallel layers. Between layers are weak van der Waal forces. Graphite is hard, a good conductor of heat and electricity with a very high melting point.

4. Simple molecular:

Weak van der Waal forces hold molecules in lattice (e.g. iodine) They have low melting points, are non-conductors of electricity and are insoluble in polar solvents such as water.

5. Hydrogen bonded:

High melting point in comparison to similar compounds due to presence of strong intermolecular forces. Ice has a less dense solid than liqiud due to solid structure having much more free space between molecules. 