Rates of reaction
- The rate of reaction is the change in concentration of reactant or product per unit time.
Different chemical reactions occur at different speeds. For chemical reactions, we use the term rate instead of speed. The study of rates of reaction is known as chemical kinetics. The rate of reaction is found by measuring the amount of a reactant used up (or product formed) per unit of time. The amount is often expressed in terms of concentration:
In another variable, such as mass or volume is measured, the rate can be expressed in corresponding units such as g s^-1 or cm^3s^-1.
It's obvious from watching a reaction that the instantaneous rate changes as the reaction proceeds. Usually for reactions:
- The rate is the fastest at the start of the reaction since each rectant has its greatest concentration
- The rate slows down as the reaction proceeds since the concentration of the reactants decreases
- The rate becomes zero when the reaction stops, i.e. when one of the reactants has been used up
We follow the rate of a reaction by measuring the concentration of a reactant (or product) over a period of time. The results obtained are plotted to give a graph. To find the initial rate it's necessary to find the initial slope of the line. To find the gradient on the straight line draw a horizontal line to the y axis, and draw a vertical line from that to the beginning of the slope.
To find out the relationship between initial rate and the initial concentrations of the reactants, a series of experiments in which the concentration of one reactant is changed at a time, must be performed.
The explanation of rates of reaction is based on collision theory. This says that for a reaction between two molecules to occur, an effective collision must take place. The reaction rate is a measure of how frequently successful collisions occur.
Not all collisions between molecules result in reactions, however, the greater the number of collisions, the higher the chance that some of them will be successful. For a collission to be successful the molecules must collide in the correct oritentation and with enough energy to react. Any factor that increases the rate of successful collisions will also increase the rate of the reaction.
Factors that affect the rate of reaction
1) Concentration (pressure for gases)
Increasing the concentration of reactants increases the rate of reaction. For reactions involving gases, increasing pressure will increase the rate of reaction, as pressure is proportional to concentration. If there's an increase in concentration of reactants there are more molecules in a given volume. Distances between the molecules are reduced so there's an increase in the number of collisions per unit time. This means there's a greater chance that the number of effective collisions increases, the rate of reaction increases. For a gaseous reaction, increasing the pressure is the same as increasing the concentration.