Gases Chapter 10

What is a gas? Definition of a gas A gas is a substance that has no well defined boundries but diffuses rapidly to fill any container in which it is placed .

Kinetic theory of gases To study the action of the molecules, we must use a theoretical model. The model, called the kinetic theory of gases, assumes that the molecules are very small relative to the distance between molecules. The molecules are in constant, random motion and frequently collide with each other and with the walls of any container.

Units of Temperature, Pressure and Volume Temperature(T) Kelvin Scale O Celsius = 273k 100 Celsius = 373k 0 kelvin =Absolute zero Pressure(P) Newton per Meter squared(Nm-2) = Pascal Normal Atmospheric pressure = 1 x 105 Pa = 100 kPa Volume(V) Cubic Meter = m3

Standard Temperature and Pressure(STP) Temperature = 273 K or (0C) Pressure = 100 kPa or (1 x 105 Pa)

Boyle’s Law Boyles Law-- At constant Temperature, the volume of a fixed mass of gas is inversely proportional to its pressure. P1.V1 = P2.V2

Charles Law Charles law At constant pressure, the volume of a fixed mass of Gas is directly proportional to its temperature on the Kelvin scale. V1/T1 = V2/T2

General Gas Law General Gas Law-combining Boyles and Charles Law. P1.V1 = P2.V2 T1 T2 See Book Page 113

Guy Lussac’s Law Of Combining Volumes. In a reaction between gases, the volumes of reacting gases and products are in the ratio of small whole numbers provided the volumes are measured at the same temperature and pressure.

Avogedro’s Law Avogadro’s law Avagadro’s law states that equal volumes of gases contain equal numbers of molecules under the same conditions of temperature and pressure. At STP one mole of any gas occupies a volume of 22.4 Litres.

The Mole 1 MOLE Relative Molecular mass 6 x 1023 atoms or molecules 22.4 L at S.T.P.

The Kinetic Theory Of Gases The Kinetic theory was developed to explain the gas laws. Matter was imagined as being made up of tiny particles. One of the first pieces of evidence to support the idea of particles in motion came from Robert Brown.The random movement of tiny particles suspended in a liquid or gas is called Brownian motion.

Assumptions of the Kinetic Theory The kinetic theory of gases was developed by James Clerk Maxwell and Ludwig Boltzmann. 1. Gases are made of particles that are in continuous motion. 2. There are no attractive or repulsive forces between the molecules. 3. Actual volume of gas negligible compared with the space the gas occupies. 4. Collisions between the molecules are perfectly elastic. 5. The average kinetic energy of the molecules in a sample of gas is proportional to the temperature measured on the absolute scale.

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.

Why do real gases deviate? Intermolecular forces are present. (Such as dipole – dipole, Van der Waals, etc.,) Molecules have volume. Collisions are not perfectly elastic.

Limitations of the kinetic theory. There are attractive and repulsive forces between molecules. The volume of the molecules is not negligible compared with the space they occupy.

An Ideal Gas An ideal gas is one that obeys all the assumptions of the Kinetic theory of gases under all conditions of temperature and pressure.(does not exist) A real gas behaves like an ideal gas at low pressure and high temperature. Real gases differ from ideal gases because forces of attraction and repulsion between the molecules The volume of the molecules is not negligible(especially at high pressure)

Equation of state of an Ideal gas P = pressure(Nm) n = no of moles V = volume(m3) T = temperature(K) R = Universal Gas Constant (8.31 Nm mol-1 K-1) Experiment: To calculate the Relative molecular mass of a volatile liquid. ( See page 122 BOOK) PV=nRT

Question Question: Write down the equation of state for an ideal gas. A mass of 6.4g of a gaseous diatomic element occupies a volume of 9.96 x 10–3m3 at 327°C and a pressure of 1.0 x 105Nm–2 (Pa). Find the relative molecular mass of the element and give its name.

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