Chemical Kinetics

Chemical kinetics explores various aspects of reactions, including reaction mechanisms, reaction orders, rate laws, and the effect of temperature, concentration, catalysts, and other factors on reaction rates. By studying chemical kinetics, scientists can gain insights into the fundamental nature of reactions, predict reaction outcomes, and design reaction conditions to optimize desired processes in fields ranging from industrial chemistry to environmental science. It is primarily knowledge of chemical kinetics and reactor design that distinguishes the chemical engineer from other engineers. The functions of the chemical engineer therefore in the field of chemical reaction engineering are to specify the size of and geometry of reactors for the production of a given amount of a particular chemical and to appraise the performance of existing reactors.

The study of chemical reaction engineering (CRE) combines the study of chemical kinetics with the reactors in which the reactions occur. Chemical kinetics and reactor design are at the heart of producing almost all industrial chemicals. Let’s get started!

Table of Contents

What is Chemical Kinetics?
Chemical Identity
Terminology in Chemical Kinetics
Reaction Rate
Order of Reaction
Molecularity and Mechanism of Reaction
Rate Equation (Rate Law) and Specific Reaction Rate (Rate Constant Or Reaction Velocity)
Temperature Dependence of Rate Constant (K)
Stoichiometry of Reaction
Solved Questions
Quiz

What is Chemical Kinetics? - Click to download it

Chemical kinetics is the branch of chemistry that deals with the study of the speed or rates at which chemical reactions occur and the factors that influence these rates. It involves the investigation of how reactants transform into products over time, and it seeks to understand the underlying mechanisms and principles governing these transformations.

Simply, Chemical kinetics is the study of the rate of reaction and mechanism by which one chemical species is converted to another.

Chemical Identity

A chemical species is said to have reacted when it has lost its chemical identity. The identity of a chemical species is determined by the kind, number, and configuration of that species’ atoms. The three (3) basic ways a chemical species may undergo conversion are;

Terminology in Chemical Kinetics

Limiting reactant: if in the reactant system the reactants are not present in the proportions required by the stoichiometric equation then the reactant which is not in excess is the limiting reactant.
Fractional conversion (x): this refers to the fraction of a reactant that has undergone a chemical change at a particular stage of a reaction process.

Practically, the fractional conversion of the limiting reactant is usually considered. Fractional conversion usually ranges from 0-1 (i.e. 0-100%)

Yield: it is generally defined with respect to a basis e.g.

If ‘a’ moles represent the amount of alcohol fed to the reactor of which a total of ‘q’ moles react to give ‘r’ moles of the aldehyde and s moles of the ethylene. The total conversion is then q/a. the aldehyde yield is either r/a or r/q, depending upon whether one chooses as basis of total alcohol fed or that part of the alcohol which reacts. Similarly the ethylene yield is s/a or s/q

Reaction Rate (Rate Law rA)

The rate of reaction is the mass or moles of a product produced or reactants consumed per unit time. The rate of chemical reaction is only a function of the properties of the reacting material e.g. species concentration, temperature, pressure or type of catalyst used and it is independent of the type of reactor used (batch or continuous reactant). If the reactants are not well stirred then the rate depends on the position in the reactor. Thus, we may write for component A as:

rA = f (state of the system) = f ( temperature, pressure, concentration)

Let’s consider this reaction below;

Order of Reaction

Generally, the order of reaction refers to the powers to which the concentrations are raised in the rate law. Let’s consider this reaction expressed by:

The rate of conversion of A per unit volume of mixture is

Where α is reaction order with respect to A and β is reaction order with respect to B. the overall order N = α + β. The overall order need not be an integer; it may be 0 or a small positive number which rarely exceeds 3.

Elementary Reaction

A reaction order for which each species is identical to its Stoichiometric coefficient as shown:

Where a and b represent the reaction order with respect to the reactant A and B respectively, over all reaction order (n) = a + b

Let’s consider this example below;

The reaction is second order with respect to NO and first order with respect to O2 and overall order of 3.

NB: α and β may or may not be equal to their respective stoichiometric numbers, a and b.

Other examples are shown below:

Reaction rate constant, k will vary with the order of the reaction as shown:

A → Products

Another example of elementary reaction; reversible second order:-

Where Kc is equilibrium constant

All reversible reaction rate laws must reduce to the thermodynamic relationship relating reacting species concentrations at equilibrium. At equilibrium, the net rate of reaction is zero for all species involved in the reaction

Non-Elementary Reaction

Do not follow the Stoichiometric coefficients for the overall reaction

Let’s consider this Homogeneous Reactions below:

Also let’s consider Heterogeneous Reactions

Heterogeneous reaction and corresponding rate law is the hydrodemethylation of toluene (T) to form benzene (B) and methane (M) carried out over a solid catalyst.

Molecularity and Mechanism of Reaction

The term molecularity refers to number of atoms, ions, or molecules involved in the rate-limiting step of the reaction.

Unimolecular – one reactant involved in reaction
Bimolecular – two reactants must collide to react
Termolecular – three reactants must interact for reaction to occur

The mechanism of a reaction is the sequence of individual chemical events whose overall results produce the observed reaction. The molecularity describes the mechanism of chemical reaction and must be an integer. Thus the number of molecules which react at the rate determining step gives the molecularity of the reaction. Consider the overall reaction.

The Molecularity of the above reaction is 2, because the rate determining step involves the reaction between 1 mole of A and 1 mole of B.

Rate Equation (Rate Law) and Specific Reaction Rate (Rate Constant or Reaction Velocity)

Rate Law or rate equation is an algebraic equation that relates the rate of reaction to the species concentration in the kinetic expression.

K = specific reaction rate or rate constant or rate coefficient or velocity constant. Its dimensions vary with the order of the reaction.

Units of K

Let’s deduce the units of K for an nth order reaction

Also let’s consider Rate equation using Partial Pressures

Temperature Dependence of Rate Constant (K)

K(T) is not a true constant but mainly independent of the concentration of species involved in the reaction. It is strongly dependent upon temperature but other variable such as pressure and catalyst weight will exhibit much less effect on the rate constant.

Rate constant dependent on temperature is given by;

· Arrhenius equation base on the collision theory of molecules