COLLISION THEORY, ACTIVATION ENERGY, AND ACTIVATED COMPLEX (TRANSITION STATE)
THE COLLISION THEORY, ACTIVATION ENERGY AND ACTIVATED COMPLEX (TRANSITION STATE)
Particles (ions/atoms/molecules) must collide with one another for a reaction to take place; i.e reactant forming product. When reactant particles continuously collide with one another, the already existing chemical bonds holding the atoms together break and new bonds are formed as product. No chemical reaction can occur if the particles of the reactants do not collide with one other. Just like the principle of kinetic theory which states that “particles of matter are in continuous random motion bombarding/colliding with one another”, particles of matter are incessant; they are always in motion non-stop.
Arrhenius, a Swedish scientist, proposed a theory called ‘The Collision Theory’, which states that two or more particles of reactants must collide before the products can be formed. Collision theory explains why different reactions occur at different rates. This collision theory is based on the principle of kinetic theory which was stated earlier.
It is good to know that it is the frequency of collision between particles influence the rate of a reaction. The more effective particles collide with one another, the more effective the reaction rate and vice versa.
However, it is not all collisions between particles are effective to initiate a reaction i.e. to be able to form product. For a collision to be effective, the particles must:
- Be properly aligned (arranged/oriented).
Particles need to be in a correct orientation so that the correct bonds are broken or formed. The particles need to be moving towards each other and fully facing each other so that the particles do not simply bounce off from each other resulting in an unsuccessful collision.
- Possess the minimum kinetic energy, and this minimum kinetic energy is called the Activation Energy, EA.
For instance, assuming that a healthy energized full grown man of a good physique could create a crack on a wall just by hitting the wall with his powerful fist, but then a 15 years boy could not create a wall crack after several trials. And for the 15 years old boy to be able to create a crack, he has to possess up to the same amount of energy or power the other man has, to be able to create a wall crack with his fist. This energy or power is likened to Activation energy.
Activation energy, EA is the minimum amount of energy a particle must possess for a collision to be effective. When particles that do not have or have less than this activation energy collide, no tangible change occurs because the particles will bounce back apart from each other. But when particles have equal or greater amount of this activation energy, on collision, the bonds between atoms/particles break and form fresh bonds between other particles.
Once a reaction starts, the reacting particles (reactants) start to absorb energy from their surrounding, which makes their energy increased to a higher energy level, until a maximum energy level is attained, where they form the transition state or activated complex. This energy absorbed corresponds to the activation energy, EA which is found by subtracting the potential energy of reactants, HR from the activated complex, AC. It is after the activated complex that the system (reactants) releases energy to the surrounding in order to form product.
EA = AC – HR
Activated complex, AC, can be described as the state of particles when the reaction is at the point of activation energy. The reaction is unstable and that is because it is the point in time when the reactants are about to form/transit into products. It is when the bonds of the reactants and the product are simultaneously breaking and forming respectively.
Activated complex, AC and transition state are often used interchangeably, but they represent different concepts. Transition states only represent the highest potential energy configuration of the atoms during the reaction, while activated complex refers to a range of configuration near the transition state. In a reaction coordinate/energy profile diagram, the transition state is the configuration at the maximum of the diagram while the activated complex can refer to any point near the maximum. The reactants are first transformed into the activated complex before breaking into the products.
When the energy absorbed by the reactants to form activated complex (i.e activation energy) is less than the energy released by the activated complex to form product, then, the potential energy of product, HP be lower than the potential energy of reactants, HR. Under this condition, the enthalpy change, ∆H for the reaction is negative (-), and the reaction is therefore exothermic.
∆H = HP – HR = -ve
However, when the activation energy is greater than the energy released to form the products, then, the potential energy of product, HP will be higher than the potential energy of reactant, HR. Under this condition, the enthalpy change, ∆H for the reaction is positive (+), and the reaction is endothermic.
∆H = HP – HR = +ve
Try studying the two energy profile diagrams shown above carefully. I tried making them self explanatory with different colours. It would surely give you a deep understanding on this topic.