J. Chem. SOC., Faraday Trans. I , 1982, 78, 1533-1538 Kinetic Studies of the Formation of Copper(1) in Water + Acetonitrile Mixtures at 25 O C from the Reversible Reaction Cu2+ + Cuo s 2Cu-t BY D I P SINGH GILL* AND REETA SRIVASTAVA Department of Chemistry, Himachal Pradesh University, Simla 17 1005, India Received 18th June, 1981 The rate of the forward reaction of the reversible reaction Cu2+ + Cuo G 2Cu+ has been measured at 25 OC in 150 cm3 of water+ acetonitrile (H20 +AN) mixtures containing 5,10,20 and 40% (v/v) acetonitrile (AN) together with 0.032, 0.064, 0.16 and 0.32 mol dm-3 copper sulphate, 0.070 mol dmP3 H,SO, and 1, 4 and 8 g of copper crystals 2-30 pm in size. The rate of the reaction is strongly influenced by the concentrations of copper sulphate and AN and by the amount and particle size of the copper crystals.The rate of the reaction in an atmosphere of nitrogen is not affected by up to 0.47 mol dm-3 H,SO,. The reaction under investigation is reasonably fast and in most cases goes almost to completion in 3-5 h. By reversing the reaction, the backward reaction becomes faster than the forward reaction and pure copper powder starts separating out from the solution. The present investigation is thus very useful for developing a cheap and quick method for the recovery of copper from crude samples. Copper(1) salts are unstable in aqueous solutions at 25 OC.l They can, however, be stabilized in water containing nitriles, ammonia, organic bases, cyanides, halides and gases like CO and C2H4.2t Electrochemical studies by Parker and coworkers2-* have shown that electrorefining of copper by the electrolysis of copper(1) salts involves the consumption of less electricity and hence less cost as compared with the electrolysis of copper(I1) salts.The electrorefining method for the purification of copper is, however, expensive. We have developed in our laboratory a simpler, quicker and cheaper method for the purification of copper. This method involves the reversible reaction cu2+ + CUO * 2cu+. (1) Under certain conditions the forward reaction is much faster than the backward reaction and under other conditions only the backward reaction is significant. In order to discover the optimum conditions under which the forward and the backward reactions become predominant, we have measured the rates of the reactions under different conditions at 25 O C .Kinetic studies of the forward reaction are reported in this paper and those of the reverse reaction will be given in a subsequent paper. EXPERIMENTAL Doubly distilled water (distilled over acidified KMnO,) with a specific conductance of 1-3 x S cm-l was used for the preparation of all solutions. Acetonitrile (AN) (E. Merck) of 99% purity was used without further purification. Extra pure copper sulphate crystals (Sarabhai M. Chemicals) of 99.5% purity were used as received. Copper crystals stored under nitrogen gas were sieved and those particles between 2 and 30 pm in size were selected for use. For each kinetic measurement 150 cm3 of the reaction solution was used. This solution was prepared by adding AN, corresponding to 5, 10,20 and 40% (v/v), and 0.070 mol dm-3 H,SO, to the balance of distilled water.The amount of copper sulphate required to obtain 0.032,0.064, 15331534 c U 2 + + c U o $2cU+ IN WATER + ACE TONI T R I LE 0.16 and 0.32 rnol dm-3 solutions was accurately weighed and dissolved in the appropriate H,O+AN mixture. In order to remove the dissolved air from the solution, nitrogen gas (saturated with the same H,O + AN mixture) was bubbled through the solution for 20-30 min. The kinetic measurements were made at 25k0.5 OC under nitrogen gas atmosphere. The rcaction was initiated by adding 1, 4 and 8 g of copper crystals to the reaction solution and the solution was vigorously stirred. The rate of the reaction was effected drastically by changing the stirring speed and for the efficient formation of copper(1) a high stirring speed was required. Therefore, we stirred the reaction solution with a magnetic stirrer operating at a speed of 800 r.p.m.The amount of copper(1) formed as function of time was determined by KMnO, titration. The reaction kinetics in all the cases with 1 g of copper crystals was studied at 15 min time intervals while with 4 and 8 g of copper crystals 5 min time intervals were used because the reaction in these two cases was faster. In every case the first reading was, however, taken one minute after the initiation of the reaction. In all cases kinetic measurements were repeated at least twice to see the effect of heterogeneity of the copper crystals on the reaction rate. It was found that the reproducibility from two independent sets of experiments was within 10%.RESULTS AND DISCUSSION The forward reaction of the reversible reaction (1) is extremely slow in aqueous solutions at 25 OC because copper(]) salts are unstable in aqueous solutions.' This reaction in H,O +AN mixtures in the presence of a small quantity of an acid becomes very fast (the equilibrium constant changes from a value of mol dm-3 in aqueous solutions to a value of 1O1O mol dm-3 in H20+AN mixtures).2*3 This is due to the preferential solvation of Cu+ ions by AN which stabilizes copper(1) salts in H 2 0 +AN mixtures. In H20+AN mixtures the rate of the forward reaction of the reversible reaction (1) is thus significant and the rate of the backward reaction as compared with the forward reaction is negligible. We have measured the rate of the forward reaction of the reversible reaction (1) at 25 O C in 150 cm3 of H20+AN mixtures containing 5, 10, 20 and 40% (v/v) AN together with 0.032, 0.064,O.16 and 0.32 mol dm-3 copper sulphate, 0.070 mol dm-3 H2S0, and 1, 4 and 8 g of 2-30 pm in size copper crystals. The amount of copper(1) formed in mol dm-3 ( M ) as a function of time in minutes ( t ) gave parabolic plots passing through the origin at zero time in all cases, with a steep linear rise to the plots at the beginning of the reaction. The plots slowly smoothed out after 50% of the reaction time. The amount of copper(1) formed in a fixed time was a maximum in case of 40% AN and decreased in the order 40% > 20% > 10% > 5%.In cases with 0.16 and 0.32 mol dm-3 copper sulphate the reaction kinetics could not be studied in the 40% AN mixture because in this mixture the required amount of copper sulphate did not dissolve. The rate of the reaction R (= dM/dt) was determined in all cases by the empirical differentiation method, i.e. by evaluating the slope of the linear parts of the plots of M against t at the initial stage of the reaction. The values of the rate thus obtained for various systems are reported in table 1. The uncertainty in these values is 10%. A perusal of table 1 shows that the rate of the reaction is strongly influenced by the concentration of copper sulphate, AN and by the amount of copper crystals. Increasing the concentration of copper sulphate, AN and the amount of copper crystals increased the rate of the reaction. EFFECT OF PARTICLE SIZE AND THE AMOUNT OF COPPER ON THE REACTION RATE In order to study the effect of the particle size of copper on the reaction rate, kinetic measurements were also made under similar conditions using 200 mesh copper powder as well as copper crystals between 30 and l00pm in size.The rate of the reaction with 200D. S. GILL A N D R. SRIVASTAVA 1535 mesh copper powder was 5-10 times faster than the rate with 2-30 pm copper crystals and the rate of the reaction with 30-100pm copper crystals was extremely slow as compared with the rate using 2-30 pm copper crystals. Also, the rate with 8 g of copper was more than with 4 g, and with 1 g of copper it was the lowest. This is expected because in heterogeneous surface reactions the rate of the reaction depends upon the surface area of the solid phase and the larger the surface area, the faster the r e a ~ t i o n .~ ~ TABLE ACE RATE (R) OF THE FORWARD REACTION OF THE REVERSIBLE REACTION Cu2+ + Cuo e 2Cu+, IN WATER + ACETONITRILE MIXTURES AT 25 OC R/ mol dm-3 rnin-ld concentration 5% AN 10% AN 20% AN 40% AN of c u s o , /moldm-3 la 2b 3" la 2b 3" l a 2b 3" l a 2' 3' 0.032 0.50 1.37 1.89 0.55 1.67 2.43 0.71 1.72 3.00 0.73 2.57 3.86 0.064 0.58 2.96 4.58 0.79 4.66 6.20 1.85 6.09 11.70 2.72 6.33 12.50 0.16 1.91 6.67 8.29 2.82 8.00 11.60 4.17 9.47 17.60 - - - 0.32 2.41 8.57 12.90 4.10 14.00 17.00 6.67 20.00 31.30 - - - a With 1 g copper crystals; with 4 g copper crystals; with 8 g copper crystals. All these rate values have an uncertainty of & 10%.EFFECT OF THE CONCENTRATION OF H2S04 O N THE REACTION RATE Copper(1) salts are easily hydrolysed by water in neutral medium. The possibility of hydrolysis in acidic medium, however, is eliminated. Therefore, the kinetic study of the present reaction was made in the presence of an acid; dilute H2S04 was found to be the most suitable. In some cases kinetic studies were attempted without H2S0, and in all those cases precipitation occurred after a few minutes. Kinetic measurements made under similar conditions using 0.18, 0.28 and 0.47 mol dm-3 H,SO, instead of the 0.070 mol dm-3 which we used in all of our measurements showed that the rate of the reaction in an atmosphere of nitrogen gas was not much effected by the amount of H,S04 added.Dilute H,S04 has been found not to react with copper metal in the absence of oxygen.'9 In the present measurements all the dissolved oxygen was removed by bubbling oxygen-free nitrogen gas through the mixture and performing the measurements in an atmosphere of nitrogen gas; therefore, the chances of a side reaction of copper metal with H2S04 were eliminated. DEPENDENCE OF RATE O N CU2+ CONCENTRATION From table 1 it is also evident that the increased concentration of copper sulphate and hence Cu2+ increased the rate of the reaction. The dependence of rate on Cu2+ concentration has been determined by plotting log (rate) from table 1 against [Cu2+] for 1, 4 and 8 g of copper crystals in mixtures containing 5, 10 and 20% AN, [Cu2+] being the molar concentration of Cu2+.In all cases the plots were linear and the slope of these plots gave an average value of l.O+O.O5 as the dependence of the rate on [Cu2+]. This observation is confirmed in the next section. PLOTS USING DIMENSIONLESS PARAMETERS In order to find out whether the increased AN concentration changed the order of the reaction or only influenced the rate of the reaction by bringing about changes1536 c U 2 + -f- c U o $2cU+ I N WATER -k ACE TONITRI LE in the solvent composition (medium effects),s plots using dimensionless parameters in the form recommended by Powell and extended by Frost and Pearsong were made in all cases. The relative concentration, a, for the dimensionless parameter plots was calculated in each case from the ratio of the concentration of Cu2+ remaining at a particular time, i.e.Co - x, to the initial concentration of Cu2+ taken, i.e. Co. The values of x were calculated at various times from the amount of Cu+ formed using the relation Cu2+ = +Cu+. For a given amount of copper crystals (1, 4 and 8 g) with 5 , 10 and 20% AN (including 40% in cases of 0.064 and 0.032moldm-3 copper sulphate solutions) for each of the copper sulphate solutions studied, the plots using dimensionless parameters were all of the same type and comparable to the theoretical curve for the first-order rea~tion.~ This showed that the basic type of curve and hence the order of the reaction was not effected by increasing the AN concentration. Similarly, the basic type of curve was not effected when either 1, 4 or 8 g copper crystals were used.In all these cases also the plots were comparable to the theoretical curve for the first-order reaction. This showed that increasing the amount of copper crystals did not effect the order of the reaction, though an increased amount of copper crystals increased the rate of the reaction due to the increased surface area of the heterogeneous phase. For illustration the plots using dimensionless parameters for 0.16 mol dmP3 copper sulphate 1, 4 and 8 g copper crystals (2-30 pm) with 5 , 10 and 20% AN are shown in fig. 1-3. log t FIG. 1 .-Fraction of Cu2+ remaining (a) as a function of time parameter (log t) in 150 cm3 of the reaction solution consisting of 0.16 mol dmP3 CuSO, .5H,O, 1 g copper crystals (2-30 pm), x , 5 , 0, 10 and a, 20% AN and 0.070 mol drn+ H,SO, at 25 OC.EFFECT OF ACETONITRILE O N THE REACTION RATE In the light of the discussion in the previous section, it can be emphasised that for a given concentration of Cu2+ and a given amount of copper crystals an increase in the rate of the reaction with increasing AN concentration in H20+AN mixtures (table 1) can only be due to changes in the medium effects8 The medium effects influence the rate of the reaction either by change in the dielectric constant of the medium or by changes in the solvation effects.l0V1l With an increase in AN concentration the dielectric constant of the H,O + AN mixture decreases12 and one expects the formation of more ion-pairs, leading to a decrease in free Cu2+. This would lead to a decrease in the rate of the reaction.There is actually an increase in the rate with increasing AN concentration (table 1). Therefore, the increase in the rate is possibly due to the increased solvation effects.I0 Studies by Parker and coworkers2 confirm this assumption. They have reported that Cu2+ ions are preferentially hydrated and Cu+ ions are preferentially solvated by AN in H20+AN mixtures.2 Their studies have also shown that the free energy of transfer for Cu+ from pure waterD. S. G I L L A N D R. SRIVASTAVA 1537 1 I 1 0 1 2 3 0.31 log t FIG. 2.-Fraction of Cu2+ remaining (a) as a function of time parameters (log t ) in 150 cm3 of the reaction solution consisting of 0.16 mol dm-3 CuSO, .5H,O, 4 g copper crystals (2-30 pm), x , 5 . 0, 10 and 0 , 2 0 % AN and 0.070 mol dm+ H2S04 at 25 O C .0 1 2 3 log t FIG. 3.-Fraction of Cu2+ remaining (a) as a function of time parameter (log 1 ) in 150 cm3 of the reaction solution consisting of 0.16 mol dm+ CuSO, . 5H20, 8 g copper crystals (2-30 pm), x , 5 , 0, 10 and 0,20% AN and 0.070 mol dm-3 H,SO, at 25 OC.1538 c U 2 + + c U o 2cU+ I N WATER + A C ETONI TR I LE to H 2 0 + AN mixtures becomes more negative while for Cu2+ it becomes more positive as the amount of AN in the mixture increases, and the effects for Cu+ are much more pronounced than the effects for Cu2+. This indicates that increasing the concentration of AN in the mixture increases the solvation effects for Cu+ much more strongly than it decreases the hydration of Cu2+. The increased solvation effects for Cu+ stabilize these ions much more in H,O + AN mixtures with higher AN concentration and hence there is an increase in the rate of the reaction. Single-ion solvent activity coefficients for SO:-, Cu2+, Cu+ and the activated complex formed in the present reaction are not available in the literature for H20+AN mixtures and, therefore, a quantitative account of the changes of the solvation effects in terms of the solvent activity coefficientsll could not be made.But from the work reviewed by Parker1' it has been reported that due to the increased solvation effects the rate of the reaction is usually faster in dipolar aprotic solvents and in protic +dipolar aprotic solvent mixtures as compared with the rate in protic solvents. MECHANISM OF THE REACTION As the rate of the reaction was drastically increased by increasing the stirring speed, this indicated that the reaction under investigation was transport contr01led.l~ The reaction is assumed to take place not through a single step but through a number of different steps: (i) Diffusion of the preferentially hydrated Cu2+ ions from the bulk of the solution to the copper surface.(ii) Diffusion of the solvated SO:- ions from the bulk of the solution to form the negative side of the solvent-separated electrical double layer near the solid surface.14 (iii) Electron transfer from a copper atom at the surface to a Cu2+ ion to form two Cu+ cations. (iv) Solvation of Cut ions preferentially by AN to form the stable Cu+ complex. (v) Diffusion of the preferentially solvated Cu+ complex out of the electrical double layer to the bulk of the solution.Steps (iii) and (iv) are fast and should not be effected by changing the stirring speed. It is thus any one or all of the other steps which determine the rate of the reaction. We have found that under certain other conditions the backward reaction of the reversible reaction (1) becomes the significant reaction and pure copper powder starts separating out from the solution. The details of the kinetics of the backward reaction are currently under study and the results will be presented in the next paper of this series. We thank the C.S.I.R., New Delhi for a research grant to carry out this project. F. A. Cotton and G. Wilkinson, Advanced Inorganic Chemistry (Interscience, New York, 2nd edn, 1966). I. D. Macleod, D. M. Muir, A. J. Parker and P. Singh, Aust. J. Chem., 1977, 30, 1423. A. J. Parker, D. A. Clarke, R. A. Couche, G. Miller, R. A. Tilley and W. E. Waghorne, Aust. J. Chem., 1977, 30, 1661. D. M. Muir, A. J. Parker, J. H. Sharp and W. E. Waghorne, Hydrometallurgy, 1975, 1, 61, 155. K. J. Laidler, Chemical Kinetics (McGraw-Hill, New York, 2nd edn, 1965). Physical Chemistry of Process Metallurgy, Part 2, ed. G. R. St Pierre (Interscience, New York, 1961). R. G. Bates, in The Chemistry of Non-aqueous Solvents, ed. J. J. Lagowski (Academic press, New York, 1966), vol. 1 . A. A. Frost and R. G. Pearson, Kinetics and Mechanism (Wiley Eastern, New Delhi, 2nd edn, 1970), p. 14. ' R. B. Heslop and P. L. Robinson, Inorganic Chemistry (Elsevier, New York, 3rd edn, 1967). lo L. P. Hammett, Physical Organic Chemistry (McGraw-Hill, New York, 3rd edn, 1970). 'l A. J. Parker, Chem. Rev., 1969, 69, 1 . l2 A. D'Aprano and R. M. Fuoss, J. Phys. Chem., 1969, 73, 400. l 3 L. L. Bircumshaw and A. C. Riddiford, Q. Rev. Chem. SOC., 1952, 6, 157. l4 J. H. de Boer, The Mechanism of Heterogeneous Catalysis (Elsevier, Amsterdam, 1959). (PAPER 1 /995)