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### Gases

Need to know……

A gas

- is a substance that has no well-defined boundaries but diffuses rapidly to fill any container in which it is placed.

a few numbers

- Kelvin = Celsius + 273
- standard temp = 273 K
- standard pressure = 1X105 Pa
(100kPa)

m3 = litres X10 -3

m3 = cm3 X10 -6

(1 litre = 1000cm3)

Boyle’s Law

- states that:
at constant temperature,

the volume of a fixed mass of gas is

inversely proportional to its pressure.

Charles’ Law

- states that:
at constant pressure,

the volume of a fixed mass of a gas

is directly proportional to its temperature measured on the Kelvin scale.

General Gas Law

- P1 X V1 = P2 X V2
T1 T2

Temp in Kelvin

Units for volume same each side

Units for pressure same each side

Gay Lussac’s law of Combining Volumes

- the volumes of the reacting gases and the volumes of any gaseous products are
in the ratio of small whole numbers

provided the volumes are measured at the same temp and pressure

Avogadro’s Law

- states that
equal volumes of gases contain

equal numbers of molecules under the same conditions of temp. and pressure

Exam Q (Hons)

‘07/Q10b

Molar Volume

- At s.t.p
one mole of any gas

occupies 22.4 litres

- Remember to watch out for r.t.p in questions
- room temp. and press = as given in Q
- (often 24 litres)

Ideal Gas

Exam Q (Hons)

’06/Q11a

- is one which perfectly obeys
all the gas laws and all the assumptions* of the kinetic theory of gases

under all conditions of temperature and pressure.

- (Know the assumptions)

Ideal Gas Assumptions

- Gas molecules are in constant rapid random straight line motion & collide with each other and walls of container.
- No forces of attraction or repulsion between gas molecules.
- The molecules are so small and far apart that their volume is negligible in comparison to the distances between them and the volume they occupy
- Collisions are perfectly elastic - no loss of energy.
- Average kinetic energy is proportional to the temperature (in Kelvin).

Real v.Ideal Gas Assumptions

- No forces of attraction or repulsion
- Attraction forces DO EXIST
- Polar gases (NH3) - far from ideal
- Non polar gases closest to ideal but Van derWaals forces do exist Cl2
- Volume of molecules is negligible
- Molecules close together at high pressure

Real v. ideal gas

- Real gases differ from ideal gases at high pressure and low temp. because
- there are forces of attraction/repulsion between the molecules*
- the volume of the molecules is not negligible compared to the distances between them
(*know examples of real gases and the forces involved)

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