Pressure of gaseous reactants
What concentration is to a solution, pressure is to a gas. In reactions involving gaseous reactants, the rate of reaction can be increased by increasing the pressure.
A — low pressure conditions
B — high pressure conditions
Figure 17.1
Cases A and B hold the same mass of reactant — eight particles in each. But the volume in B has been reduced, so the pressure of the reactants in B is greater than the pressure in A.
Which means
In B the number of reactant particles colliding with one another in unit time is greater, so the rate of reaction is greater. It is the concentration argument over again — for a gas it simply shows up as pressure.