Chemistry: Animation, Video, and Laboratory Activity

Although the factors affecting rates were discussed in the previous section, no consideration was given to what was actually happening at a molecular level during a reaction. The chemical equation for a reaction indicates the nature of the reactants and products but provides no information about the way in which thereactants are converted to products. For example, the decomposition of hydrogen iodide is represented by the following equation.

2HI → H2 + I2

This equation indicates that for every two molecules of HI which decompose, one molecule of H2 and one molecule of I2 are produced. However, the equation does not indicate how HI molecules are converted to H2 and I2. The first step in a chemical reaction is thought to involve a collision between the reactant particles. This idea is part of the collision theory of reaction undergo an
appropriate collision.
From the kinetic theory, the particles in a gas are in a continuous state of random straight-line motion. While most of the particles have energies that are close to the average for all the particles in the system, a small fraction have energies much lower or much higher than the average. Because of this range of kinetic energies, collision between HI molecules will occur with differing energies. A collision between reactant molecules does not necessarily mean that a
reaction will take place. In fact, most collisions do not bring about a reaction. The collision theory requires that for a collision between reactant particles to lead to a chemical reaction the following conditions must be fulfilled.
1. The molecules must collide with sufficient energy to disrupt the bonds of the reactant molecules.
2. The molecules must collide with an orientation that is suitable for the breaking
For a reaction to occur between reactant molecules they must collide with a certain minimum energy. Unless this minimum collision energy is exceeded, the colliding molecules will simply rebound and move away from each other. The minimum energy that is required for a collision to result in a reaction is known as the activation energy for the particular reaction. Some reactions have relatively low activation energies and so react at a significant rate at room temperature. For these reactions, a noticeable reaction occurs as soon as the reactants are mixed. For example, a piece of sodium metal placed in water at room temperature produces a violent reaction almost instantaneously. Other reactions occur to an almost insignificant extent at room temperature. In such reactions the activation energy is so high that it is very unlikely that reactant molecules will collide with sufficient energy to undergo reaction. For example, methane will not react with oxygen unless the mixture is ignited. There is a relatively high activation energy for the reactions there would be in the order of 1010 collisions per second between
reactant molecules, virtually none of these would have sufficient energy for a reaction to take place.
As well as needing a minimum amount of energy, successful collisions also often have an orientation requirement. The relative orientations of the reactant molecules during a collision must be favorable for the breaking of particular bonds in the reactants and the formation of new bonds in the products. This factor also contributes to the fact that many collisions between reactants are unsuccessful in producing a reaction.