Kinetics Mechanism and Thermodynamic Study of the Oxidation of Iodide Ion by Dichromate Ion in Acidic Medium

This work introduces a newly developed kinetic mechanism and thermodynamic study that gives a clear theoretical insight between the iodine and chromium in acid solution. The paper mainly focuses on evaluating kinetic mechanism and thermodynamic properties of the oxidation of iodide by dichromate ion in acidic medium. This study was investigated using iodometric method. The rate has been studied by appropriate choice of concentrated conditions at constant temperature and ionic strength. The kinetic studies revealed half order reaction with respect to dichromate ion, first order for iodide ion and second order for hydrogen ion concentration. The dependence of reaction rate on the temperature was monitored in order to generate some thermodynamic parameters, which can be used in predicting the direction and spontaneity of the reaction. The thermodynamic studies of this reaction revealed negative values for ΔG, ΔS and positive values for ΔH which is an indication that the reaction was spontaneous and endothermic. The oxidant chromium (vi) exist in acidic media as H2CrO4. The effect of ionic strength on the rate of redox reaction was investigated. The ionic strength was adjusted by varying the concentration of the nitrate to maximum. The effect of temperature was also studied to evaluate the thermodynamic parameters. Transition metals should be use as catalyst for future purpose to monitor the rate of redox reaction. The techniques employed on this research are very useful in determining the rate of a redox reaction. DOI: https://dx.doi.org/10.4314/jasem.v24i5.14 Copyright: Copyright © 2020 Nyong et al. This is an open access article distributed under the Creative Commons Attribution License (CCL), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Dates: Received: 16 March 2020; Revised: 11 April 2020; Accepted: 29 April 2020

Kinetics provides useful information about the mechanism and rate of chemical reaction, which helps to run a chemical reaction successfully by a way of selecting optimum condition as to get maximum yield. Kinetic study also helps us to Study the factors which influence the rate of reaction like temperature, pressure, substrate concentration, oxidant concentration, composition of reaction mixture and catalyst while Mechanistic study gives a clear theoretical insight between the iodine and chromium (vi) solution in the presence of acid solution, specifically kinetics plays a very important role in the investigation of the reaction of oxidation of iodide compound carried out by oxidizing agent like potassium dichromate (VI) (Fawzy et al., 2014a ;Sayyed et al., 2011). According to the literature survey, considerable amount of work has been done on the oxidation of organic compounds (Sayyed et al., 2011;Fawzy et al., 2014b;Sayyed et al., 2010;Odebunmi et al., 2009;Anweting et al., 2012;Goel, 2010, Khan;Id-Din, 2001) by potassium permanganate, but scanty on the kinetics oxidation of iodide ion by dichromate ion in acidic medium (Ignaz et al., 1997;Isa and kolo, 2015). The objective of this study is to propose kinetic mechanism and evaluate the thermodynamic properties of the oxidation of iodide ion by dichromate ion in acidic medium.

MATERIALS AND METHODS
All the chemicals used were of AR grade, and were prepared and estimated by standard method. The oxidation of iodide ion by dichromate ion in the acidic medium in was investigated idometrically in the temperature range 10 -35 o C at constant ionic strength of 0.0641.
Rate Law Determination: Seven burettes were used to dispense the reagents, 50 ml each of trioxonitrate (v) acid solution and Potassium dichromate solution were measured into the first 250 ml beaker. 50ml each of Potassium iodide and sodium thiosulphate and 10 ml sodium sulphate solution were measured into the second beaker and 15ml 2% starch solution was added. The two 250 ml beakers were placed side by side on the display table and stirring rod was place in the second beaker and quickly the content of the first beaker was poured into the second beaker and stirred gently to increase mixing of the two solutions. The time for the reaction was noted from time of mixing to the first appearance of the blue colour.

Temperature of Study:
The effect of temperature on the rate of redox reaction was also considered in this study. The same procedure of the previous experiment was employed, except for the variation of temperature which ranges from 10, 15, 20, 25, 30 and 35 o C This reaction can been represented as equation 1.
Cr2O7 2-+6I -+14H + →2Cr 3+ +3I2+7H2O 1 The rate of this reaction can be monitored by the rate of formation of Iodine. Experimental evidence has revealed that as the concentration of reactants increase, the number of molecule or ionic collision increases with a corresponding increase in reaction rate (Isa and kolo, 2015;Atkins and Depaula, 2006)   The results shows that the reaction rate increases with increase in the concentration of dichromate ion and the order of reaction was obtained by the linear plot of Figure 1, the plot gave a slope of 0.54 which confirmed half order reaction in dichromate ion concentration (Atkins and Depaular, 2006). That means the rate of the effect of dichromate ion concentration reaction was increased by half when the concentration of dichromate ion was doubled (Cherkupally and Padma, 2010).

Effect of [I -] on rate of iodine formation:
The result of iodide concentration on the rate of iodine formation has been presented Figure 2. It was also observed from the results that the reaction rate increased as the concentration of iodide ion increases. From the Figure  2 it is observed that linear plot gave a slope of approximately 1 which indicated that the order of reaction with respect to iodine ion was first order reaction (Paricia et al., 2009).

Effect of [H + ] on rate of iodine formation:
The dependence of rate of reaction on the concentration of hydrogen ion is display in Figure 3 and it revealed that  Figure 3. The plot was linear and the slope of the plot was 2 confirming second order reactions for hydrogen ion concentration Isa and Kolo, 2015;Sayyed et al., 2010).
Effect of temperature on the rate of redox reaction: The temperature dependence of reaction rates can be studied by plotting a graph between experimental rate constant against the inverse of temperature for different reactions perform. The values of the rate constant presented in Table 1 were all evaluated from equation 5 using the data from Table 1. This shows the effect of temperature on the experimental rate constant. The results illustrated that the experimental rate constant, kexp increases as the temperature increases (Eddy et al., 2018, Atkins andDepaula, 2006). The calculated activation energy was given as 17.7kjmol -1 or 4.71kcalmol -1 while the pre-exponential factor A was calculated as 3.709.

Effect of Ionic Strength on the Rate of Redox Reaction:
The effect of ionic strength on the rate of redox reaction was investigated; the ionic strength was adjusted by varying the concentration of the dichromate ion, hydrogen ion and iodide to maximum. Since increase in concentration of a reactant increases ionic strength.  (Khalid, 2007).
Ionic species for dichromate ion Cr2O7 2 is represented as A with charge of -2 and B represent ionic ionic species of acid H + with a charge of + 1. Therefore primary elementary step of this species are illustrated in equation 7-8.
A 2-+B + → Intermediate → Products 9 Cr2O7 2-+ H2O HCrO4 -10 The possible explanation for ZAZB is -2 for equation 7 and the primary elementary step show the protonation of dichromate ion followed by the addition of OHto form 2HCrO4 -.

12
Where N is the Avogadro's number, h is the plank constant, ΔH is the enthalpy of the reactions, R is the gas constant and T is the absolute temperature. The application of transition state equation to redox reaction of iodide was confirmed by the high degree of linearity displayed by the plot of log Kexp versus 1/T as shown in Figure 4. Table 2 contains values of ΔH that were calculated from the slope of Figure 4, Using equation 13 = − ∆ 13 \While the entropy was evaluated by equation 14.

∆ = ∆ − ∆ 14
It is seen from the listed results that change in enthalpy is generally positive. Predicting the reaction to be endothermic. Negative values obtained for change in entropy confirm that there is association of the reaction between the iodide and chromate in acidic media, while free energy of reaction shows that the reaction was spontaneous (Eddy et al., 2018;Ikpi et al., 2017;Abeng et al., 2017). Conclusions: The reaction between iodine and chromium (vi) was investigated in trioxonitrate (v) acid. The oxidant chromium (vi) exist in acidic media as H2CrO4 which takes part in the chemical reaction. The propose mechanism for the reaction confirm the kinetic studies of redox reaction with half order reaction for dichromate ion, first order for iodide ion and second order for hydrogen ion concentration. The effect of ionic strength on the rate of redox reaction was investigated. The ionic strength was adjusted by varying the concentration of the nitrate to maximum. The effect of temperature was also studied to evaluate the thermodynamic parameters.