Understanding the factors that influence the rate
of a chemical reaction is a fairly complex, but completely logical affair. Students readily accept the idea that molecules must collide if a reaction is to occur. However, many students have the misconception that every collision leads to the formation of product, when the truth of the matter
is that many collisions do not go anywhere. A huge percentage of molecules within a sample may collide without ever turning into products. When creating a mental picture of a successful collision, students must envision molecules that possess both proper orientation
and a sufficient amount of energy
- The colliding molecules must collide with the correct orientation. Unless the alignment is favorable, the reaction will not be successful. Of course, as the overall number of collisions increases, it becomes more likely that a collision with a favorable orientation will occur.
- The molecules must collide with enough energy to overcome the activation energy barrier. The activation energy barrier can be lowered when a catalyst is present. Increasing the temperature gives the reactants molecules more kinetic energy, thus more high energy collisions can occur.
This topic of reaction rate
is especially amendable to having students create their own experiment.
Here is a possible list of variables to investigate:
- changing the concentration of one of the reactants
- changing temperature
- use of a catalyst
- changing the surface area (i.e. using a powdered vs. granular form of reactant).
Reactions in which a gaseous product
forms are particularly easy for students to monitor. Students can measure the reaction rate
- monitoring the volume of gas produced in a determined amount of time.
- monitoring the change in mass of the system in a set amount of time.
Two possible reactions to investigate:
- Baking soda and vinegar ?
This uses cheap, easily obtained reactants that are very safe to use.
- Magnesium and hydrochloric acid ?
Magnesium comes in powdered, granular, and ribbon form. This reaction can be catalyzed using a strip of copper metal. However, HCl must be used with care. The hydrogen gas
produced is flammable, no open flames can be allowed.