Chemical Reaction Rates: Factors and Catalysis
Chemical Reaction Rate
Experience indicates that the rate at which reactants are transformed into products varies greatly from one reaction to another. Some reactions are extremely slow, while others are almost instantaneous. The rate of a reaction is a positive quantity that expresses the change of concentration of a reactant or product over time.
Measuring the Rate of Reaction
The rate can be found at a particular moment, called the instantaneous rate. The rate of a reaction at a given instant is equal to the slope of the tangent to the concentration-time curve on reactants or products, at the corresponding point.
Reaction Order and Molecularity
The reaction rate can be expressed as: rate = K [A]m [B]n. This equation is called the rate equation or the rate law. The constant of proportionality, K, is called the rate constant but does vary with the temperature. The exponent to which the concentration of a reactant is raised in the rate equation is called the order of reaction with respect to that reactant. The sum of all exponents is called the total order. The rate equation of a chemical reaction must be determined experimentally.
Factors Affecting the Rate of Reaction
1) Nature of the reagents: The rate of interplay varies widely depending on the nature of the reagents.
2) Ease of reagents to contact: Most reactions require two or more reagents to come into contact. This is one reason why reactions are carried out in the gas phase or in a liquid solution. When the size of solid particles is reduced, their surface area increases, so that the contact area with the other reactants increases. This translates into increased reaction rate. If you want to increase the reaction rate when a reagent is solid, grind it to reduce it to a fine powder.
3) Concentration of the reactants: The more we increase the number of molecules confined in a given volume, the greater the frequency with which these collide. Therefore, the rate of a reaction usually increases with increasing concentration of the reactants.
4) Temperature of the system: The rate of almost all chemical reactions increases when increasing the temperature. As a rough rule of thumb, an increase in the temperature of 10°C doubles the rate of reaction. The increased temperature of a system increases the frequency with which these collide, resulting in the increase in the rate of reaction. For example, gasoline does not burn at environmental temperature, but it does when heated.
5) Occasionally, certain substances called catalysts, added to the reactants, significantly increase the rate of reaction. Besides, at the end of the catalytic reaction, they are intact.
The Collision Theory
It is based on the idea that for a reaction to take place, the molecules of the reactant substances must collide in advance between themselves. The following must be met: the rate of a reaction is proportional to the number of collisions between reagent molecules. According to the collision theory, any factor increasing the frequency at which such collisions occur should increase the rate. Not all collisions are effective. If so, liquid or gas reactions would all be lightning quick, and in the media, there are many collisions. Very few reactions occur at high rates. Most are slow because the molecules of the reactants collide without altering their links.
Effect of the Orientation or Steric Effect
For the formation of products, it is not sufficient that the molecules of the reactants collide with adequate guidance. It is also required to have a certain minimum energy, given that any reaction requires an input of energy. Therefore, colliding particles must have enough power so that we can break those links. For a collision to be effective and lead to a reaction, the molecules involved must meet the following conditions: a) they should be geared adequately, b) they must have sufficient energy.
Activation Energy
The intermediate state of the system, which has the maximum energy, is called the transition state or activated complex. The energy necessary to go from reactants to the transition state is called the activation energy, Ea. The reactants must overcome the activation energy barrier to become products. The peak corresponds to the barrier. Normally, the activated complex, the kinetic energy of the reactant molecules supplies the activation energy. The lower the Ea, the higher the number of molecules with sufficient energy to surmount the barrier and react. The rate of reaction increases with decreasing activation energy. In practice, increasing the reaction rate is very much exceeded by the increased number of clashes between molecules because the kinetic energy of molecules increases with T, and many more molecules overcome the activation energy.
Catalysis
A catalyst is a substance that affects the rate of a chemical reaction and is incorporated at the end of it (although it may appear in another physical state). The term catalyst is reserved for substances that accelerate the rate. If a substance decreases the rate of reaction, it is called an inhibitor. The action of a catalyst is called catalysis. The catalyst does not appear in the net reaction equation, which is regenerated during the same. The catalyst plays an active role: changing the mechanism of reaction. The catalyst provides a path whose activation energy is lower than that corresponding to the same reaction without catalysis. Catalysts increase the reaction rate because they decrease the activation energy. The presence of a catalyst does nothing to the heat or the spontaneity of the process. ΔH, ΔS, and ΔG values are unaffected by the presence of a catalyst.
Catalysis Classes
Different types of catalysis are:
- Homogeneous catalysis: The catalyst is in the same phase as the reactants.
- Heterogeneous catalysis: The catalyst is in a phase different from that in which the reactants are. Its most common mechanism is based on the absorption of reactant molecules.
- Enzyme catalysis: The substances that catalyze biochemical reactions are called enzymes. The reagent or substrate fits in a specific point on the surface of the enzyme, kept by intermolecular forces. After this, the configuration of the enzyme may vary, weakening the key link of the substrate and thereby increasing the reaction rate. Sometimes there are several possible reactions from some of these reactants. In the same cases, products are obtained that form rapidly, and this may depend on the catalyst used.
The nature of the catalyst can determine the reaction products.