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 standard temp = 273 K standard pressure = 1X10 5 Pa (100kPa) m 3 = litres X10 -3 m 3 = cm 3 X10 -6 (1 litre = 1000cm 3)

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 P 1 X V 1 = P 2 X V 2 T1T2T1T2 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 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) Exam Q (Hons) ’06/Q11a

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 (NH 3 ) - far from ideal Non polar gases closest to ideal but Van der Waals forces do exist Cl 2 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)