The Chemistry of Gases, Part 1 - General Properties and Reaction Kinetics

The Gas State of Matter

The gas phase is perhaps the most mysterious of states because it is not generally readily observable.  Gas phase properties and reaction kinetics are, arguably, the most difficult to learn due to the dynamic nature of the state.

 

On the macroscopic level, a gas is characterized by the following fundamental properties:

• Pressure - The collective force of gas particles striking a unit area, P
• Volume - The space occupied by a gas, V
• Temperature - A measure indicating the relative (to Absolute Zero, 0 K) average kinetic energy of gas particles, T
• Amount - The number of gas particles present, measured in moles (mol), n

On the molecular level, an ideal gas has the following fundamental properties:

• Fully elastic collisions between molecules and between molecules and the walls of a container
• Volume occupied by gas molecules is negligible relative to the total volume occupied
• Motions of gas molecules are completely random
• Speed of molecular motion is a function of the total thermal energy of a "system" (closed container)
• Average force of collisions between molecules and the container wall are a function of molecular velocity and molecule-wall collision frequency
• Molecule-wall collision frequency is dependent on the average molecular velocity and molar gas concentration

The Ideal Gas Law Formula

There is an ideal gas law constant, R, which has a value calculated from standard conditions of the four fundamental gas properties.  The standard condition values are V (1 Standard Molar Volume) = 22.414 Liters, P = 1 Atmosphere, n = 1 mole, and T = 273.15 K.

The ideal gas law formula is PV = nRT.  The ideal gas law constant, R, has a value of
0.08206 L-atm/mol-K determined from the ratio, (1 atm)(22.414 L)/(1 mol)(273.15 K).

Fundamental Gas Phase Reaction Kinetics

The rate of any chemical reaction depends on certain events occurring on the molecular scale called rate factors.  The rate factors are:

• Collision Frequency, Fc (mol/s)
• Probability of Collisions With Enough Energy to Break Chemical Bonds, Pe
• Probability of Collisions With the Proper Orientation to Start a Reaction, Po

The actual reaction rate in mol/s is the mathematical product, Po x Pe x Fc.  It is important to understand that the calculated reaction rate is only valid for a particular gas phase concentration of a particular set of reactants at a particular temperature.  The order of a chemical reaction, which is the increase in reaction rate relative to a doubling of reactant concentration at a particular temperature, cannot be accurately predicted by theoretical calculations and therefore must be experimentally determined.

The Practical Importance of Gas Phase Reaction Kinetics

Reaction kinetics is very economically important for the production of a certain chemical product on an industrial scale.  A Chemical Engineer uses his/her knowledge of reaction kinetics to design a synthesis process which will maximize the yield of an industrial-scale reaction in the safest manner possible.  The scientific application of reaction kinetics is incredibly valuable for the profitability of a chemical manufacturing company!

 

That's all for this post.  My next post will explain the order of gas-phase reactions in more detail.

 

Have a good one!