Reaction rates
THE RATE of a reaction is the change in the amount of reactant or product with time. The mass or volume of the product formed or the loss of the reactant can be measured. A typical reaction to collect a gaseous product is shown. The reaction rate can be shown on a graph. If the course of a reaction is followed over time, the concentration of the reactants decreases while the concentration of the products increases gradually.
The initial part of the reaction is the fastest, after which the reaction rate slows and eventually stops. This is shown as a curve which straightens and becomes constant after no more product is formed.
Several factors such as concentration, pressure, temperature, particle size and catalysts affect reaction rates.
The collision theory states that particles must collide in order to react, and they must do so with a minimum energy to break the bonds. Most factors work by increasing the number of collisions in the reaction.
An increase in concentration will result in more particles in the same volume, which will cause more collisions, increasing the reaction rate. Increasing pressure will also cause an increase in rate, as more particles are in a smaller volume, leading to more collisions.
Smaller pieces of reactants can also lead to an increase in reaction rate as more surface area becomes available for the reaction to take place. Smaller particles will increase the number of collisions, which will increase the reaction rate. Larger pieces lead to a slower reaction.
The effect of temperature is similar to the effect of concentration and pressure, but faster. When temperature is increased, the reaction particles gain more kinetic energy, which causes them to collide even more. This increases the rate of the reaction tenfold. Whenever a reaction rate increases, the slope of the graph increases, meaning more product is formed faster at the beginning of the reaction. This causes the slope of the graph to get steeper. The graph of a slower reaction is not as steep.
In a reaction between marble chips and hydrochloric acid, the amount of carbon dioxide produced increases as the acid concentration increases. The slope of the graph gets steeper. So, a less concentrated acid will result in a slower reaction, seen by a gentle slope. As the acid concentration increases, the reaction rate increases, as seen in the steeper slope.
Catalysts also affect the rate of a reaction. Catalysts speed up the reaction rate, allowing more product to be formed at the initial stage of the reaction. A reaction that uses a catalyst has a steeper slope. Remember: the catalyst is unchanged at the end of the reaction and only affects the rate. For example, in the decomposition of hydrogen peroxide, more catalyst (manganese IV oxide) produces more product (O2) as reaction rate increases.
Francine Taylor-Campbell is an independent contributor. Send questions and comments to kerryann.hepburn@gleanerjm.com