Gas Laws A gas is defined as a substance that has no well defined boundaries but diffuses rapidly to fill any container in which it is placed Three main properties of a fixed amount of gas are.. Temperature Pressure Volume

Particle Diagrams

Temperature Degree of hotness of an object Measured in Celsius (centigrade) scale or the Kelvin (absolute) scale Celsius scaled has two fixed points ( freezing point and boiling point) Kelvin scales zero is the temperature at which a gas would occupy no volume, if it could be cooled indefinitely without becoming a solid or a liquid (absolute zero)

0 deg Celsius= 273K 50 deg Celsius= 323 K Temperature can be converted from the Celsius scale to the Kelvin scale by adding 273 0 deg Celsius= 273K 50 deg Celsius= 323 K

Pressure The pressure of a gas is the force that the gas exerts on each unit area of its container SI unit is newtons per metre squared (Nm-2 or Pascal) Atmospheric pressure 1 X 105 Nm-2 or 100 kPa Volume Cubic metres M3 Usually use smaller unit such as cm3 or litres 1 litre = 1000cm3 = 1 cubic decimetre dm3

S.T.P. (Standard, Temperature and Pressure) Scientists who first studied gases soon realised that the pressure and temperature controlled the volume observed for a gas sample. Therefore to compare different gas samples, they defined a set of reference conditions = STP. Standard Temperature = 273 K Standard Pressure = 1 x 105, 101kPa, 101,325 Pa or 760 mm Hg p.g. 64 log book

Gases can be very different when the are compared to each other at S.T.P. but most gases obey certain laws…

Boyle’s Law At a constant temperature, the volume of a given mass of any gas is inversely proportional to the pressure of the gas.

Boyles Law

of gas is inversely proportional to its pressure Boyles law states that, at constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure Boyle found that this relationship was true for all gases. P x V = constant (k)

Charles’ Law At a constant pressure, the volume of a fixed mass of any gas is directly proportional to the temperature on the Kelvin scale.

Gay – Lussac’s Law of Combining Volumes When gases react, the volumes of reacting gases and volumes of any gaseous product are in a simple whole number ratio to each other, if all volumes are measured under the same conditions of temperature and pressure. 1 : 3 : 2

Equipment used to study the volumes of gases that react with each other

Avogadro’s Law Equal volumes of (ideal) gases, contain the same number of particles, or molecules, under the same conditions.

Avogadro’s Law

This theory was not believed initially because Daltons Theory did not allow for molecules Avogadro was the scientist to first introduce the idea of molecules

Kinetic Theory Of Gases Brownian Motion: Robert Brown observed pollen grains in drop of water. He noticed that they were in continuous irregular motion because they were bombarded by the moving water molecules . This evidence supports the Particle theory

The Kinetic Theory of Gases The kinetic theory of gases was developed by James Clerk Maxwell and Ludwig Boltzmann. This theory assumes that: Gases are made up of particles whose diameters are negligible compared to the distances between them. There are no attractive or repulsive forces between these particles. The particles are in constant rapid random motion, colliding with each other and with the walls of the container. The average kinetic energy of the particles is proportional to the Kelvin temperature. All collisions are perfectly elastic .

Limitations to these gas laws 2. It is not valid to say that there are no forces of attraction between gases. Covalent bonds that are polar have attractive forces and there are weak attractive forces also between non polar molecules called Van der Waals forces 1. Under high pressure (or low temperatures) the diameters of the gas molecules is not negligible compared with the distances between them 3. Collisions between molecules are not perfectly elastic in reality as they lose energy to surroundings

Ideal gases don’t exist An Ideal Gas is one that obeys all the assumptions of the Kinetic theory of gases under all conditions of temperature and pressure Ideal gases don’t exist A real gas behaves similar to an ideal gas at low pressure (molecules widely spaced) and at high Temperatures (when molecules are moving rapidly and intermolecular forces of attraction are small)

At high pressure and low temperatures the ordinary gases, we cant assume there are no attractive forces between molecules and the volumes of the molecules cannot be ignored

Ideal Gases versus Real Gases An ideal gas is one which obeys all the gas laws and under all conditions of temperature and pressure. No such gases exists, but real gases behave most like an ideal gas at high temperatures and at low pressures. Under these conditions, the particles of a real gas are relatively far away from each other, and the assumptions of the kinetic theory are reasonably valid.

By rearranging Charles Law, Boyles law and Avogadros law we can get the following equation R is a constant of proportionality called the Universal Gas Constant 8.31 cm 3/ 1000 to change to M3 M3 not litres Pa not KPa Note units

Equation of State for an Ideal Gas

All the Law’s Combined Charles’ Law Boyle’s Law Ideal Gas Law PV = nRT Combined Gas Law

The Combined Gas Laws Combined Gas Law P1, V1, and T1 are the initial pressure, volume and Kelvin temperature. P2, V2 and T2 are the final pressure, volume and Kelvin temperature. Pressure can be in any units as long as it’s the same for P1 and P2. Volume can be in any units as long as it’s the same for V1 and V2. Temperature must be in Kelvin’s for T1 and T2. To convert from degrees to Kelvin’s add on 273. For example 25o = 25 + 273 = 298 K

Important points to note Since the combined gas law is derived from Charles Law, you must convert all temperature scales to Kelvin The units on either side of the equation must be consistent

To measure the relative molecular mass, Mr, of a volatile liquid Weigh conical. rubber band & tin foil Pour approx. 10cm3 of volatile liquid (propanone) Seal conical and put pinhole in aluminium lid Immerse in boiling water (don’t assume it is 100°C) and allow any excess vapour to escape through pinhole. Use thermometer to measure temperature that all liquid has formed gas Cool flask till liquid condenses Dry outside of flask and weigh flask with aluminium & rubber band (to find mass of liquid subtract the mass of empty conical with aluminium and rubber band) Find complete volume of flask by filling with water and pouring water into graduated cylinder Propanone is a volatile liquid (low boiling point) Barometer is used to measure pressure Standard temperature = 273 K Standard pressure = 760 mm Hg PV = nRT Mass spectrometer can also be used to find relative molecular mass

To measure the relative molecular mass of a volatile liquid