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What is chemical kinetics?

The chemical kinetics is the branch of chemistry that is responsible for studying the rate of chemical reactions, which factors affect this speed and the mechanisms or through individual steps which are chemical reactions occur.

In addition, it allows to understand many aspects of chemical reactions such as chemical balance and activation energy, and studies the processes of catalysis. The latter makes it especially important in multiple applications, from biochemistry to industrial engineering.

At the center of kinetics is the reaction rate, which can be understood as the rate at which reactants are transformed into products. However, in chemical kinetics a much more precise definition is given.

Definition of reaction rate

In this reaction, A and B represent the reactants, C and D represent products, while a, b, c and d represent the respective stoichiometric coefficients.

The reaction rate (represented by the symbol v ) indicates how fast reactants (A or B) are consumed or how fast products (C or D) are produced in a chemical reaction. Mathematically, this is expressed as follows:

Where Δ [X] represents the change in molar concentration of species X (A, B, C or D) in the time interval Δt.

It is clear that the faster any one of the reagents is consumed, the faster the other reagents will be consumed and the faster the products will be produced. For this reason, it is only necessary to define the speed in terms of one of them.

To ensure that the result of the velocity calculation is always the same, regardless of the species according to which the velocity is defined, it must always be divided by the stoichiometric coefficient and place a minus sign before the equation if it is a reactant. . Namely:

Example of definition of reaction speed

Given the following chemical reaction:

Write the definition of the reaction rate as a function of each species involved.

Solution:

In terms of H 2 :

In terms of I 2 :

In terms of HI:

The law of speed and the order of reaction

Reactions occur when atoms and molecules collide with each other with sufficient energy and in the proper orientation. The probability of this happening increases as the atoms and molecules become more concentrated.

The mathematical equation that relates the speed of a reaction with the concentration of the different species involved is called the “ Speed ​​Law ” and, in the simplest cases, it has the following form:

Where k is a constant of proportionality called the rate constant , and the exponents of A, B and any other species that appear in the equation are called reaction orders .

According to the sum of all the reaction orders (which is called the global order), different types of reactions can be distinguished. These differ in how much the concentration affects the speed, in the formula for the time it takes to consume half of the reagents (half-life time) and in the way in which the concentration of the reagents changes over time ( graphs [ A] vs t).

  • Reactions of order 0

When all exponents in the speed law are 0, the speed law equation reduces to:

In other words, they are reactions that occur at a constant rate and in which the rate does not depend on the concentration of any reactant or product.

Charts of order 0

The graph of [A] vs t of the reactions of order 0 are descending straight lines.

LEGEND: The concentration vs. time graph of a reaction of order 0 gives a straight line.

Half-life time for order 0

For a reaction of order 0, the half-life time ( 1/2 ) is given by:

  • 1st order reactions

In these reactions the rate varies linearly with the concentration. The first order speed law is:

First order graphs

The graph of [A] vs t of the first order reactions are descending hyperbolas. However, if the natural logarithm of the concentration (ln [A]) vs time is plotted, a straight line is obtained.

The graph of concentration vs. time of a reaction of order 1 gives a hyperbola, but the graph of the natural logarithm of concentration vs. time gives a straight line

Half-life for first-order reactions

For a reaction of order 1, 1/2 does not depend on the initial concentration and is given by:

  • Second order reactions

A reaction can be second order with respect to a single reactant, or first order with respect to two reactants. In the first case, the speed law is given by:

Second order graphs

The graph of [A] vs t of the second order reactions are descending hyperbolas. However, if the inverse of concentration (1 / [A]) vs time is plotted, a straight line is obtained.

The graph of concentration vs time of a reaction of order 2 gives a curve similar to a descending hyperbola, but the graph of the inverse of concentration vs time gives a straight line

Half-life time for second-order reactions

For a reaction of order 2, 1/2 is given by:

Factors Affecting Reaction Rate

Temperature

Temperature always increases the rate at which reactions occur, as it increases the frequency and energy with which molecules collide with each other. The dependence of the reaction rate with temperature is given by the Arrhenius equation.

Pressure

Increasing the pressure is equivalent to increasing the concentration of all species in the reaction medium (for gas phase reactions), so increases in pressure tend to increase the speed of the reactions.

Catalysts

Catalysts are chemicals that are added to the medium just to increase the reaction rate. Most catalysts work by changing the reaction mechanism to one that requires lower activation energy.

Common examples of catalysts are enzymes in living systems and some metals such as platinum and palladium.

Inhibitors

Inhibitors are the opposite of catalysts. They are substances that make reactions happen more slowly. Anticorrosives are examples of inhibitors that seek to slow down corrosion reactions to make metal surfaces last longer.

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