Inorganic Chemistry For Dummies
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Kinetics is the study of the speed of a chemical reaction. Some chemical reactions are fast; others are slow. Sometimes chemists want to speed the slow ones up and slow the fast ones down.

There are several factors that affect the speed of a reaction:

  • Nature of the reactants

  • Particle size of the reactants

  • Concentration of the reactants

  • Pressure of gaseous reactants

  • Temperature

  • Catalysts

If you want to produce as much of a product as possible as fast as possible in a chemical reaction, you must consider the kinetics of the reaction.

Nature of chemical reactants

In order for a reaction to occur, there must be a collision between the reactants at the reactive site of the molecule. The larger and more complex the reactant molecules, the less chance there is of a collision at the reactive site.

Sometimes, in very complex molecules, the reactive site is totally blocked off by other parts of the molecule, so no reaction occurs. There may be a lot of collisions, but only the ones that occur at the reactive site have any chance of leading to chemical reaction.

In general, the reaction rate is slower when the reactants are large and complex molecules.

Particle size of chemical reactants

Reaction depends on collisions. The more surface area on which collisions can occur, the faster the reaction. You can hold a burning match to a large chunk of coal and nothing will happen. But if you take that same piece of coal, grind it up very, very fine, throw it up into the air, and strike a match, you’ll get an explosion because of the increased surface area of the coal.

Concentration of chemical reactants

Increasing the number of collisions speeds up the reaction rate. The more reactant molecules there are colliding, the faster the reaction will be. For example, a wood splint burns okay in air (20 percent oxygen), but it burns much faster in pure oxygen.

In most simple cases, increasing the concentration of the reactants increases the speed of the reaction. However, if the reaction is complex and has a complex mechanism (series of steps in the reaction), this may not be the case. Determining the concentration effect on the rate of reaction can give you clues as to which reactant is involved in the rate-determining step of the mechanism.

You can do this by running the reaction at several different concentrations and observing the effect on the rate of reaction. If, for example, changing the concentration of one reactant has no effect on the rate of reaction, then you know that reactant is not involved in the slowest step (the rate-determining step) in the mechanism.

Pressure of gaseous reactants

The pressure of gaseous reactants has basically the same effect as concentration. The higher the reactant pressure, the faster the reaction rate. This is due to the increased number of collisions. But if there’s a complex mechanism involved, changing the pressure may not have the expected result.

How temperature affects chemical reaction rate

Increasing the temperature causes molecules to move faster, so there’s an increased chance of them colliding with each other and reacting. But increasing the temperature also increases the average kinetic energy of the molecules.

The following figure shows an example of how increasing the temperature affects the kinetic energy of the reactants and increases the reaction rate.

The effect of temperature on the kinetic energy of reactants.
The effect of temperature on the kinetic energy of reactants.

At a given temperature, not all the molecules are moving with the same kinetic energy. A small number of molecules are moving very slow (low kinetic energy), while a few are moving very fast (high kinetic energy). A vast majority of the molecules are somewhere in between these two extremes.

In fact, temperature is a measure of the average kinetic energy of the molecules. As you can see in the figure, increasing the temperature increases the average kinetic energy of the reactants, essentially shifting the curve to the right toward higher kinetic energies.

But also notice the minimum amount of kinetic energy needed by the reactants to provide the activation energy (the energy required to get a reaction going) during collision. The reactants have to collide at the reactive site, but they also have to transfer enough energy to break bonds so that new bonds can be formed. If the reactants don’t have enough energy, a reaction won’t occur even if the reactants do collide at the reactive site.

Notice that at the lower temperature, very few of the reactant molecules have the minimum amount of kinetic energy needed to provide the activation energy. At the higher temperature, many more molecules possess the minimum amount of kinetic energy needed, which means a lot more collisions will be energetic enough to lead to reaction.

Increasing the temperature not only increases the number of collisions but also increases the number of collisions that are effective — that transfer enough energy to cause a reaction to take place.

How catalysts increase chemical reaction rate

Catalysts are substances that increase the reaction rate without themselves being changed at the end of the reaction. They increase the reaction rate by lowering the activation energy for the reaction.

In the preceding figure, if you shift to the left that dotted line representing the minimum amount of kinetic energy needed to provide the activation energy, then many more molecules will have the minimum energy needed, and the reaction will be faster.

Catalysts lower the activation energy of a reaction in one of two ways:

  • Providing a surface and orientation

  • Providing an alternative mechanism (series of steps for the reaction to go through) with a lower activation energy

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