MODULE No. 24: Solution Kinetics Part - III

Transcription

1 Subject Paper No and Title Module No and Title Module Tag 6 and PHYSICAL CHEMISTRY-II (Statistical 24 and Solution Kinetics - III CHE_P6_M24

2 TABLE OF CONTENTS 1. Learning outcomes 2. Introduction 3. Primary salt effect 4. Secondary salt effect 5. Summary

3 1. Learning Outcomes After studying this module, you shall be able to Learn about primary salt effect Learn about secondary salt effect 2. Introduction In our last two modules, we took up solution kinetics in detail. You now know that solution kinetics is the kinetic study of chemical reactions taking place in solution phase and the nature of solvent has a predominant influence on the kinetics of such reactions. In general, the solution phase reactions can be classified as Ionic reactions and Non Ionic reactions. And here, we are concerned with the kinetic study of ionic reactions. A + B P Rate = k[a][b] (1) (2) where k is the rate constant for the reaction. We found that only Classical Thermodynamic approach of transition state theory can be utilized to determine the rate constant of ionic reactions or solution phase reactions and studied the effect of solvent on rate constant. In this module, focus will be laid on primary salt effect and secondary salt effect. 3. Primary salt effect It has been observed that the rate of a reaction can be altered by the presence of non-reacting or inert ionic species in the solution. This effect is profound when the reaction takes place between ions, even at low concentrations. This influence of charged species on the rate of the reaction is referred to as salt effect. The salt effect is classified as Primary salt effect Secondary salt effect

4 Both effects are important in the study of ionic reactions in solutions. Over here, we will first take up primary salt effect. The primary salt effect takes into account the influence of electrolyte concentration on the activity coefficient and hence the rate of the reaction. Let us consider a reaction with transition state complex, A + B [AB] P...(3) Where A + B [AB] has k is the equilibrium rate constant and is given by k = a = [AB]. γ a A a B [A][B] γ A γ B (4) Or [AB] = k [A][B] γ Aγ B γ (5) The rate of such a reaction is given by the expression, r = k BT h [AB] (6) r = k BT h k [A][B] γ Aγ B (7) γ Experimentally the rate of such a reaction is given by the expression, Rate = k[a][b] (8) On comparing equation (7) and (8), we get k = k BT h k γ Aγ B (9) γ k = k 0 γ A γ B γ (10) We did derive the same expression in our earlier module where we considered the concentration terms in place of activity for the sake of simplicity. Over here, we have considered the same derivation in terms of activity. So, k 0 becomes my rate constant when all the activity coefficients are unity. Taking logarithm on both sides of equation (10) gives, log k = log k 0 + log γ A + log γ B log γ (11)

5 Activity coefficient varies with the concentration especially in the presence of an added electrolyte. We did study a term called IONIC STRENGTH. This term was introduced by Lewis and Randall and is a measure of the intensity of the electric field due to the ions in a solution. Ionic strength is the cumulative measure of both charge on the ion as well as its concentration in the solution. It is expressed as, I = 1 2 c iz i 2 i (12) According to Debye - Huckel limiting law, the activity coefficient of an ion is related to the ionic strength as, log γ = QZ 2 I where Q is a constant (approx..51 for aqueous solution at 25 C) (13) Since the charge on an activated complex is Z = Z A + Z B, the above equation can be rewritten as, log k = log k 0 Q[Z A 2 + Z B 2 (Z A 2 + Z B 2 + 2Z A Z B )] I log k = log k 0 + Q[2Z A Z B ] I log k = log k [Z A Z B ] I (14) (15) (16) Equation (16) has been tested for a number of times. A plot of log k versus I gives a straight line with slope equal to 1.02[Z A Z B ]. Fig. 1: Plot of log k vs I

6 Sign of Z A Z B determines the direction of slope. If one of the reactant is a neutral molecule, Z A Z B is zero and rate constant is expected to be independent of ionic strength. If the reactions involve similarly charged ions, Z A Z B will be positive and slope of straight line will be positive, i.e. rate of reaction will increase on addition of electrolyte. On the other hand, if the reaction involves oppositely charged species in rate determining step, Z A Z B will be negative and the rate of reaction will decrease on increasing µ, i.e. on adding salt in reaction mixture. Examples: So, in primary salt effect, addition of an electrolyte (salt) or variation of ionic strength affects the activity coefficients and hence the rate of reaction. 4. Secondary salt effect The secondary salt effect is the actual change in the concentration of the reacting ions resulting from the addition of electrolytes. In a reaction where H + or OH ions produced from a weak acid or weak base act as catalyting agent, the addition of salt influences the concentration of H + or OH ions. Since the rate of reaction depends upon the concentration of H + or OH, it will be affected by the salt concentration. This phenomenon is known as secondary salt effect. Let us consider a reaction which is catalyzed by H + produced by weak acid HA. The dissociation constant of acid is given as, HA H + + A (17) k a = a H +a A = [H+ ][A ] a HA [HA]. γ H +γ A (18)

7 [H + ] = k a [HA] [A ] γ H +γ A (19) [H + ] = k γ H +γ A where k = k a [HA] [A ] = constant (20) because [HA] will remain constant for a given acid salt mixture. [A ] Comparing equation (20) with equation (8) gives, k = k 0 γ H +γ A (21) where k 0 is the rate constant in absence of secondary salt effect and includes the primary salt effect. Equation (21) at 25 C can take up the form, log k = log k I (22) So, when ionic strength increase, the concentration of H + increases and therefore, the rate of acid catalyzed reaction also increases. Similar results were obtained for reactions catalyzed by hydroxyl ions. The ionic strength must be kept constant while examination of general acid/base catalyzed reaction because, if ionic strength is not kept constant the rate constant will change. 5. Summary Solution kinetics is the kinetic study of chemical reactions taking place in solution phase and the nature of solvent has a predominant influence on the kinetics of such reactions. In this module, focus was laid on primary salt effect and secondary salt effect. It has been observed that the rate of a reaction can be altered by the presence of non-reacting or inert ionic species in the solution. This effect is profound when the reaction takes place between ions, even at low concentrations. This influence of charged species on the rate of the reaction is referred to as salt effect. The salt effect is classified as Primary salt effect and Secondary salt effect. The primary salt effect takes into account the influence of electrolyte concentration on the activity coefficient and hence the rate of the reaction. The secondary salt effect is the actual change in the concentration of the reacting ions resulting from the addition of electrolytes.