STD IX – STUDY OF GAS LAWS – (Online)

The behavior of gases under changes of temperature and pressure

In order to understand the behaviour of gases at the molecular level, scientists developed a theoretical model called Kinetic Molecular Theory (KMT) or Microscopic Model of Gases.

The kinetic theory describes a gas as a large number of submicroscopic particles (atoms or molecules), all of which are in constant rapid motion that has randomness arising from their many collisions with each other and with the walls of the container.

Relation between Kelvin and Celsius scale

The primary difference between the centigrade, or Celsius, scale and the absolute, or Kelvin, the scale is in their respective starting points.

The Celsius scale sets the freezing point of pure water at sea level as 0, while 0 degrees Kelvin is absolute zero, the coldest temperature theoretically possible.

K = °C + 273

Standard temperature and Pressure

Since temperature and air pressure may vary from place to place it is necessary to have standard values of pressure and temperature for carrying out experimental procedures.

STP or Standard Temperature and Pressure are used when performing calculations on gases.

Example: Gas density. The standard temperature is 273 K (0° Celsius) and the standard pressure is 1 atm pressure. At STP, one mole of gas occupies 22.4 L of volume (molar volume).

Charles’ law (Remember it as CP – “Charles law – Pressure constant”)

Charles’ Law expresses the relationship between volume and temperature.

Pressure remaining a constant, the volume and absolute temperature of a certain quantity of a gas are directly proportional to each other.

Boyle’s law, also called Mariotte’s law – P

Boyle’s law, also called Mariotte’s law, a relation concerning the compression and expansion of a gas at constant temperature. This empirical relation, formulated by the physicist Robert Boyle in 1662, states that the pressure (p) of a given quantity of gas varies inversely with its volume (v) at constant temperature, i.e., in equation form, pv = k, a constant.

The law can be derived from the kinetic theory of gases assuming a perfect (ideal) gas (see perfect gas). Real gases obey Boyle’s law at sufficiently low pressures, although the product pv generally decreases slightly at higher pressures, where the gas begins to depart from ideal behaviour.