Chemical Kinetics and Catalysis -Absolute Reaction Rate Theory in thermodynamic terms – Significance of entropy and volume of activation

Absolute Reaction Rate Theory in thermodynamic terms – Significance of
entropy and volume of activation. Ionic reactions – primary and secondary salt
effects - Acid-base catalysis – Bronsted relations, catalytic coefficients and their
determination. Enzyme catalysis - Michaelis-Menten equation – Heterogeneous
catalysis-Langmuir-Hinshelwood and Eley- Rideal mechanisms General features of
fast reactions – Study of fast reactions by flow methods.

1. Unit –I : Chemical Kinetics and Catalysis Dr. R. Valliappan Professor of Chemistry Annamalai University Annamalai Nagar 608 002

2. Partition function physical meaning is the following: It expresses the number of thermally accesible states that a system provides to carriers (e.g. electrons). Thermodynamic functions are obtained in the usual way as the derivatives of the chemical potential with respect to temperature or pressure.

3. Volume of activation

4. Ionic reaction is generally the reaction of ions that are anions and cations to form a compound in a medium in which they are dissolved. When ions of water-soluble salts interact with each other in aqueous medium results in the formation of water-insoluble salts.

5. Influence of Ionic strength – primary salt effect The introduction of the salt not containing common ions with components of the mixture changes the ionic strength of the solution, as a result of which degrees of dissociation of substances change. This phenomenon is referred to as primary effect. 2. Primary salt effect is involved mostly in non-catalytic reaction. Ionic strength can impact the behavior of proteins resulting in salting in (increased solubility) or salting out (decreased solubility). The decrease in solubility with an increase in ionic strength is usually attributed to the colloidal stability of a protein. If one of the reactants is uncharged, zAzB is zero and the rate constant is independent of the ionic strength. B. This change of k with I is called the primary kinetic salt effect.

6. It is the concept of solution kinetics discussed under the banner of the effect of adding non-reacting ions on the kinetics of reactions. This effect of adding species and observing their influence on the reaction kinetics is termed the salt effect. It is of two types, depending on the nature of the change it brings to the system; the primary salt effect and the secondary salt effect. The primary salt effect is defined as the influence of electrolyte concentration, or more precisely, ionic strength, on the activity coefficients and hence the rate of the reaction. Mathematically, it can be expressed by Bronsted-Bjerrum’s equation: 𝐼logk r = log k + 1.02 Z A Z B μ Equation (14) is known as Bronsted – Bjerrum equation.  

7. Secondary Salt effect It refers to the actual change in the concentration of reacting ions by the addition of electrolyte. The degree of dissociation of the reacting electrolyte varies with the ionic strength of the medium. Hence the ionic strength influences the concentration of the reacting ions. If these are produced by strong acid (or) base, the secondary salt effect is negligible. It however becomes important when ions are produced by the dissociation of a weak electrolyte. Eg. Hydrolysis cane sugar by a weak acid. The secondary salt effect refers to the actual change in concentration of reactive ions caused by the addition of electrolytes. The addition of salt changes the concentration of H + or OH - ions in a process where H + or OH - ions generated by a weak acid or weak base function as a catalyzing agent. Because the rate of reaction is determined by the concentration of H + or OH - , the salt concentration will have an impact on the outcome. This is known as the secondary salt impact.

8. Acid – base catalysis A catalyst is a substance that speeds up a chemical reaction, or lowers the temperature or pressure needed to start one, without itself being consumed during the reaction. Catalysis is the process of adding a catalyst to facilitate a reaction. Acid – base catalysis A large number of reactions are catalysed by either acid or base. These reactions are known as acid – base catalysis. The three main types of acid base catalysis are a) Specific hydrogen ion catalysis These are the reactions which are catalysed by H+ ion only. Example is the inversion of cane sugar by H+ ion only b) Specific hydroxyl ion catalysis These are the reactions which are catalysed by OH¯ ion only. Example, Conversion of acetone into diacetone is catalysed by OH¯ ions only, c) Hydrogen and hydroxyl ion catalysis These are the reactions in which both H+ and OH¯ ions simultaneously act as catalysts. Example, Hydrolysis of ester is catalysed by H+ as well as by OH_x0002_ions.

9. Catalytic Coefficients Catalytic coefficient are regarded as a measure of the effectiveness of any acid base catalyst. Actually the reaction is effected by all the species present in the solution. For example. if a reaction is catalysed by hydrogen ion, the rate constant of a reaction is proportional to– [H + ], ie., k = kH  [H + ] …(1) Where kH + is the catalytic coefficient of the hydrogen ion.

10. Bronsted relations Acid – base catalysis has been a common feature of organic reactions. In reactions subject to acid – base catalysis, the first step of a reaction would be the transfer of a proton from acid to the substrate molecule (or) from substrate molecule to the base. Bronsted proposed a satisfactory relationship between the acid catalytic coefficient k a and dissociation constant of acid K a i.e., k a = G a K a  α …(1) Where G a and  α are constants characteristic of the reaction temperature and solvent. The value of α  is less than unity (between 0.3 to 0.9). Similarly for a base catalysis, Bronsted and Pederson gave the equation k b = G b K b  β …(2) Where k b rate constant of the base catalysis, K b is the dissociation constant of the base and again G b and  β are constants characteristic of the reaction.

11. Enzyme catalysis Enzymes are complex organic compound which are produced by living plants and animals. Basically enzyme are proteins but associated with non-protein substance (Coenzymes) that are essential for enzyme action. The catalytic activity of an enzyme is due to a relatively small region of the protein molecule known as active centre. The kinetic behaviour of enzyme catalysis is similar to that of heterogeneous process, hence enzyme catalysis has been referred to as “micro heterogeneous catalysis”. Each enzyme can catalyse a specific reaction Example: 1. The enzyme zymase is able to convert Glucose into ethyl alcohol,

12. The Michaelis–Menten equation Michaelis-Menten kinetics is a model of enzyme kinetics which explains how the rate of an enzyme-catalysed reaction depends on the concentration of the enzyme and its substrate. The Michaelis–Menten equation is mainly used to characterize the enzymatic rate at different substrate concentrations, but it is also widely applied to characterize the elimination of chemical (the first-order kinetics) compounds from the body.

13. Factors Governing the rate of an enzyme reaction Concentration: The rate of reaction is approximately proportional to the concentration of enzyme. pH: The pH of the solution has a remarkable effect on the rate of an enzyme catalysed reaction. Temperature: The enzyme catalysed reactions frequently pass through a maximum as the temperature is raised. The temperature at which the rate is maximum is often referred to as the optimum temperature.

14. Heterogeneous catalysis

15. Unimolecular surface Reactions

16. BIMOLECULAR SURFACE REACTIONS Bimolecular surface reactions may occur via Langmuir – Hinshelwood mechanism (or) Langmuir – RiedalElay mechanism. a) Langmuir – Hinselwood Mechanism The reaction between two reactants (A and B) may involve adsorption of both the reactants on neighbouring sites, and the probability of this happening is proportional to the individual concentrations of adsorbed A and adsorbed B. Reaction takes place between two molecules, adsorbed on adjacent sites. The Langmuir – Hinshelwood mechanism for a reaction A and B is formulated as follows

17. Flow methods

18. UNIT-II: CHEMICAL DYNAMICS