Chemical kinetics

Subject classification: this is a chemistry resource.

Chemical kinetics is the study of the rate of a chemical reaction. For any given reaction: A + B → Products Kinetics can answer:

How quick is A consumed?
How quick are the Products produced?
What can we learn about the reaction mechanism?

Chemical Kinetics

When we say "chemical kinetics", we mean that we are studying the rates of reaction, and what affects the rates. Chemists have been able to establish a fairly accurate way of predicting how long it will actually take for a reaction to move to completion (meaning there is no reaction going in the direction of the products. To visualize this, we look at the decomposition of dinitrogen pentoxide (N₂O₅):

2N₂O₅ (g) → 4NO₂ (g) + O₂ (g)

So when we say "move to completion," we mean how long it takes for all possible nitrogen dioxide (NO₂) and oxygen gas (O₂) molecules to be formed from the reaction process.

Reaction Rates

The rate of a reaction is basically the change in quantity of a substance over the change in time. The quantity of the substance is often measured in litres, grams, moles, or moles per litre. Time, on the other hand, can be measured using any applicable unit such as seconds, minutes, hours, or even years. The unit that is the most often used for measuring reaction rates is the mole per litre-second (mol/L•s).

To monitor how fast the above reaction is going we could, for example, look at how much Dinotrogen pentoxide (N₂O₅) is left as a function of time. Alternatively, we could keep an eye on how much nitrogen dioxide (NO₂) is formed. The stoichiometric coefficients allow us to relate these different approaches by defining the reaction rate (r_t) as:

r_t = - 1/a d[A]/d[t] = - 1/b d[B]/d[t] = + 1/c d[C]/d[t]

The above equation is used to determine the general reaction rate of a chemical reaction of the type:

a A + b B → c C

For the general reaction rate equation, "r_t" is the general reaction rate, "A" and "B" are reactants and "a" and "b" are their respective stoechiometric coefficients, and "C" is the product and "c" is its coefficient. The lower case "d" represents a variation. In the above example with the decomposition of dinitrogen pentoxide, we obtain:

r_t =-1/2 d[N₂O₅ _(g)]/d[t] =1/4 d[NO₂ _(g)]/d[t]= d[O₂ _(g)]/d[t]

In the study of kinetics often one of the concentrations is measured over time, eg, by looking at the optical absorption by one of the species. However, it is also possible to look at overall effects of the reaction, such as the amount of heat generated or the change in conductivity of the reaction mixture. Provided we use properly balanced equations all these can be related to the rate r_t.

What affects reaction rate?

So what affects reaction rates? First, we might do well to remember what is required for a chemical reaction to take place:

Two or more molecules must collide.
They must collide with enough kinetic energy to react. We associate this with the activation energy (denoted as E_a).
When the collision occurs, the particles must have the right orientation.

If all of the above requirements are met, an inelastic collision, one in which products will be formed, will occur. In any other case, products will not be formed since an elastic collision will take place between particles.

Increasing reaction rate

To improve the rate of a reaction, the conditions can be altered.

To improve the likelihood of a collision, you may increase the concentration, the surface area, the temperature, or the pressure if you are dealing with a reactant that is in the gaseous phase. A catalyst, which decreases the activation energy of the reaction, can also be added to increase the number of inelastic collisions per unit of time, and therefore the reaction rate.

An increase in the concentration of the reactants means that there are more particles present per unit volume. Statistically, an increase in the number of molecules translates into an increase in the number of collisions. If the amount of general collisions increase, the number of effective collisions will also increase. Thus, the reaction rate, that is, the number of inelastic collisions per unit time, increases.

Increasing the temperature of the system causes an increase in the agitation of particles. as a matter of fact, the amount of motion, the velocity of the molecules, increases. This results in an increase in kinetic energy. Since there are more particles containing an amount of energy greater than or equal to the activation energy, more particles are capable of passing the energy barrier. Statistically, this increase in the number of particles with sufficient energy to react translates into an increase in the number of particles that undergo an inelastic collision. As a result, the amount of effective collisions per unit time, the reaction rate, increases.

If at least one of the reactants is in the gas phase, an increase in pressure, which causes a decrease in volume, will result in an increase in the concentration of the gaseous particles. There are more molecules per unit volume, and, statistically, the number of collisions increases. If the general amount of collisions increases, the number of effective collisions also increases, and the reaction rate acquires a greater value.

If one of the reactants is a solid, an increase in surface area would result in a greater number of collisions between reactants. Statistically, the number of inelastic collisions per unit time increases. The rate of the reaction therefore increases. This concept explains why it is easier to ignite small pieces of wood when doing a campfire. The surface area of the small pieces of wood allows more oxygen gas molecules to collide with the fuel (wood). If a big wooden log was dropped in an unlit campfire and someone tried to set it on fire, the task would be much harder because the surface area is smaller. Another example would be the danger posed by dust particles in the air. If the concentration of dust particles is high, there is a strong risk of fire because the oxygen particles in the air are colliding with the dust, which has a great surface area.

If a catalyst is added to a chemical reaction, the activation energy required to trigger a reaction will be reduced. Most of the time, the addition of a catalyst causes an increase in the number of steps required to complete the reaction. Therefore, reaction intermediates make their appearance. Since the activation energy is lowered, more particles have sufficient energy to react, and the number of inelastic collisions per unit of time increase. The reaction rate, that is, the rate at which the reactants are transformed into products, increases.

Decreasing reaction rate

To decrease the rate of a chemical reaction, one can either decrease the concentration of the reactants, decrease the temperature of the system, decrease the surface area of a solid reactant, decrease the pressure if at least one of the reactants is in the gas phase, or add an inhibitor in the reaction.