Collision Model

The Collision Model describes the process of how reactions take place.
It states that for a reaction to occur, reactant particles (atoms, molecules, or ions) must collide with each other with enough force and in the correct orientation.
A successful collision that leads to a chemical reaction is termed an effective collision.

Factors Influencing Collision Frequency

Concentration: When the concentration of reactants is increased, the likelihood of collisions happening is also increased, thus speeding up the reaction rate.
Surface Area: By increasing the surface area of a reactant, more particles are exposed for collisions, leading to an increased reaction rate.
Pressure: Increasing pressure (in reactions involving gases) effectively increases the concentration, leading to more frequent collisions and a faster reaction rate.

Not every collision results in a reaction. The particles must collide with a certain minimum energy called the activation energy (Ea).
Activation energy is the minimum energy that needs to be surpassed during a collision for a reaction to take place.
If the energy of the colliding particles is less than the activation energy, they will simply bounce off each other without reacting.

Temperature influences the reaction rate by affecting both the frequency and the energy of the collisions.
Higher temperatures increase the kinetic energy of the particles, leading to more collisions as well as giving more particles the requisite activation energy for effective collisions.

Many complex reactions take place in stages, with each stage involving a different, simpler reaction. Each of these stages is called an elementary step.
The sequence of elementary steps that make up the overall reaction is called the reaction mechanism.
Some elementary steps involve the collision and reaction of two particles, known as bimolecular steps, while others involve just one, known as unimolecular steps.