4.3 Factors Influencing Reaction Rate: Concentration & Pressure
Increasing the concentration of any one or more of the reactants will usually (but not always) increase the rate of a reaction if the reactants are all in the same phase (solid, liquid, gas, or aqueous).
Why? Concentration is a measure of how many particles are in a given volume (see note). By packing more particles in the same space, collisions will occur more often, thus increasing the rate of the reaction.
Remember our thought experiment with blindfolded students in a room? You would demonstrate how concentration increases reaction rate by packing more blindfolded students into that room.
The concentration of gases is typically increased by decreasing the volume (making the container smaller). This forces the particles closer together, thus increasing their concentration.
Changing the concentration, or increasing the pressure, does not affect the amount of energy particles collide with. It simply increases the rate by increasing the frequency of collisions.
Changes in concentration also explain why the rate of a reaction slows down as a reaction proceeds. As the reactant particles collide with one another to form products, there are fewer and fewer reactant particles as the reaction continues. This lowers their concentration (fewer particles in a given volume), thus the rate declines.
Complete the Practice Questions and check your answers. Then complete the assignment.
