Chem 1000: Gases in Chemical Reactions

LAW OF COMBINING VOLUMES

The Law of Combining Volumes states that the volume ratio of the gases consumed and produced in a chemical is the same as the mole ratio, provided the volumes are all measured at the same temperature and pressure.

MIXTURES OF GASES

Simple gas laws and the ideal gas law equation apply to a mixture of nonreactive gases as well as individual gases. The simplest approach to working with gaseous mixtures is just use the value of n the total number of moles of the gaseous mixture (Ntot). These include:

1. For a mixture of gases in a vessel of fixed volume and temperature, the total pressure of the mixture is determined by the total number of moles:
   
2. For fixed values of temperature and pressure, the total volumes of a mixture of gases is determined by the total number of moles:
1. V_tot= n_totRT/P

 DALTON'S LAW OF PARTIAL PRESSURE

John Dalton proposed that in a mixture, each gas expands to fill the container and exerts the same pressure referred to as partial pressure that it would if it were left alone in the container. DALTON'S LAW OF PARTIAL PRESSURE states that the total pressure of a mixture of gases is the sum of the partial pressures of the components of the mixtures. For a mixture of "dry" gases, A, B and so on, :

In a gaseous mixture of moles of A, moles of B and so on, the volume each gas would individually occupy at a pressure equal to the total pressure is:

               V_tot=n_ART/P_tot

 The total volume of the gaseous mixture is:

               V_tot=V_A+V_B ....

The expression percent by volume is:

    Volume % A = V_A/V_tot

Using all the following ratios, we can derive a useful expression as :

n_A/n_tot=P_A/P_tot=V_A/V_tot=X_A

Note that n_A/n_tot is given a special name as the mole fraction of A,X_A. The mole fraction of a component in a mixture is the fraction of all the molecules in the mixture contributed by a component. The sum of all the mole fractions in a mixture is one.

 

"WET GAS"-WHAT IS THAT?

 

A gas collected in a pneumatic trough filled with water is said to be collected over water and is wet. It is ideally a mixture of two gases- the desired gas and water vapour. The gas being collected expands to fill the container and exerts its partial pressure Pgas. Water vapour formed by the evaporation of liquid water, also fills the container and exerts a partial pressure PH20. The pressure of the water depends on temperature.

 

According to Dalton's law, the total pressure of wet gas is the sum of the two partial pressures. The total pressure must be made equal to the prevailing pressure of the atmosphere. Therefore the equation is: