Medium effects in water—ethylene carbonate mixtures

Abstract

Electromotive force measurements of the cell of the type: ▪ in the temperature range 25 °C to 45 °C at 5 °C intervals have been used to derive the change in the standard free energy (Δ G° t), enthalpy(Δ H° t), and entropy (Δ S° t) of transfer of HCl from water to water—EC mixtures. Figure 1 shows that at 25 °C, Δ G° t changes its sign from negative values at concentration of the organic solvent less than 40 wt.%, to positive values at higher percentages of EC. This behaviour is explained on the basis of the change in the internal structure of the mixed solvent. The electrostatic contribution of the Gibbs free energy of transfer is evaluated according to Born's model. The results show that the electrostatic part of the free energy change plays an important role in the total value of Δ G° t at concentration of EC < 27 wt.%. At higher percentages, the nonelectrostatic free energy change start to have an appreciable effect on Δ G° t. Values of the standard entropy and enthalpy of transfer are found to be always negative and decrease with increasing EC content. This clarifies that the net effect is to increase the entropy loss, and suggests that, in the transfer process, the net amount of order created by HCl as a solute is more marked in the mixed solvent than in pure water. The entropy change of mixed hydration and solvation Δ G° h,s,H + ,is evaluated in each mixed solvent. It can be divided into a hydration component Δ S° h,H + , and a solvation component Δ S° s,H + , according to the relation: ▪ where, x 2 and x 2 are the mole fractions of water and EC respectively. t001 Hydration and Solvation Numbers in H 2O—Ethylene Carbonate Solvent Mixtures. EC wt% Δ S° h,s,H + Δ S° h,H + n h Δ S° s,H + n s 0.00>C>−32.03 −32.03 5.4 − − 9.50 −32.49 −31.46 5.2 −1.90 0.3 20.16 −35.49 −30.46 ]5.1 −4.41 0.7 28.98 −34.92 −29.56 4.9 −6.93 1.2 39.96 −38.49 −28.19 4.7 −10.80 1.8 49.75 −40.46 −26.62 4.4 −15.21 2.5 61.35 −43.34 −24.15 4.0 −22.14 3.7 78.12 −56.85 −18.42 3.1 −38.24 6.4 100.00 − − − −89.98 15.0 ▪ Fig. 1. Variation of Δ G° t and Δ G° el with thewt.% of ethylene carbonate. Table I illustrates the calculated values of the hydration and solvation numbers in each of water— EC solvent mixture. The high value of the solvation number in 100 wt.%EC may be due to the primary solvation layer together with some contribution from the secondary solvation layer.