6.2 - Collision theory
6.2.1: Describe and explain the collision theory. Students should know that not all collisions lead to a reaction.
Collision theory says that particles must collide in order to react. This is because chemical reactions involve breaking and making bonds. Breaking bonds is not possible without collisions.
Collision theory then says that collisions need to have a minimum energy for a process to be possible. Once again this is common sense; a car is not going to get damaged if it hits a tree at 1 km/hour. However, if it collides with a tree at 100 km/hr ...
Even if two particles collide with sufficient energy to cause reaction it does not necessarily mean that reaction will take place. The collision could have the wrong orientation, i.e. the collision could take place on a part of the molecule where bonds cannot be broken.
6.2.2: Define activation energy (Ea) and explain that reactions occur when reacting species have E < Ea. Molecules must have a minimum energy and appropriate collision geometry in order to react.
The activation energy is the minimum amount of energy required for reaction to take place. It is also called the activation energy barrier to reaction.
The distribution of energy over the particles of a substance is statistical and follows the Maxwell Boltzmann distribution.
Only a certain proportion of the particles of a sample will have the required activation energy.
6.2.3: Predict and explain, using collision theory, the qualitative effect of particle size, temperature, concentration and catalysts on the rate of a reaction. Increasing the temperature increases the frequency of collisions but, more importantly, the proportion of molecules with E < Ea increases.
Particle size refers to the bulk particles of a substance. For example, a powder has smaller particle size than crystals.
The effect of changing bulk particle size by making the bulk particles smaller is to expose more reactive sites to the reacting compounds. This increases the possibility of succesful collision and rate of reaction.
Powders react faster than lumps. Solutions have particles dissolved that are almost as small as possible. Gases are the limit in terms of minimum particle size, hence gases react faster than other phases.
This has two effects on reaction rate. On one hand the particles travel faster and collide more frequently. This has the effect of increasing the rate, BUT is not as important as the change in distribution of particles as the temperature increases. This causes more particles to have the required activation energy. This is far more important.
Increasing the concentration of reactants (or pressure in the case of gases) increases the number of collisions per unit time and hence the rate.
These provide an alternative route to the products which has a lower activation energy. This means that there are greater numbers of particles with the required lower activation energy and hence a faster reaction rate is possible.
6.2.4: Explain that reactions can occur by more than one step and that one step can determine the rate of reaction.
Few reactions involve just one step although one step in the reaction, the rate determining step, determines the reaction rate. Orders of reactions and rate laws are not required.
The actual process of a chemical reaction often involves many collision steps. This is called the mechanism of the reaction.
Usually these steps happen at very different rates. The slowest step has
a greater influence on the overall rate and is consequently called the "rate