WJEC Chemistry for AS Level Student Book: 2nd Edition (Draft)

 1.3 Chemical calculations

Stretch & challenge

The ideal gas equation is derived from the gas laws.

Boyle’s law In 1662, Robert Boyle published his work on the compressibility of gases. He stated that: At a constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure.

It can be written as V ∝ 1/P or PV = constant. Graphs illustrating Boyle’s law are shown below:

P

P

1

V

V

Charles’ law In 1787, Jacques Charles stated his law on the effect of temperature on the volume of a gas. He stated that: The volume of a fixed mass of a given gas, at constant pressure, is directly proportional to its temperature in kelvins. It can be written as V ∝ T  or   = constant A graph illustrating Charles’ law is shown below: V T

V

Stretch & challenge The kinetic theory of gases puts forward a model that explains the gas laws. It makes the general assumption that for an ideal gas: ▪ The molecules in a gas are in continuous random motion. ▪ There are no intermolecular forces, so the only interactions between molecules are collisions. ▪ All collisions are perfectly elastic, i.e. the molecules bounce off each other without their total kinetic energy changing. ▪ The molecules themselves have no size, i.e. they occupy zero volume. Can you explain why real gases depart from ideal behaviour at high pressures and low temperatures?

100

–300

–200

–100

0

200

300

Temperature / ˚C

Avogadro’s principle In 1811, Amadeo Avogadro proposed the following hypothesis about gases: Equal volumes of different gases, measured at the same temperature and pressure, contain the same number of molecules. Another way of putting this is that the volume of a gas depends on the amount of moles or V ∝ n 41 DRAFT