Solubilities and Thermodynamic of Thiourea in Methanol

PHYSICAL CHEMISTRY COMMUNICATIONS Volume 1 Issue 1, October 2014 www.bacpl.org/j/pcc

Solubilities and Thermodynamic of Thiourea in Methanol Hua Li1*, Xingli Jiao1, Xiaoshuang Chen2 School of Chemical and Energy Engineering, Zhengzhou University, Zhengzhou, China, 450001 School of Energy and Power Engineering, Xiʹan Jiaotong University, Xiʹan, Shaanxi, China, 710049

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lihua@zzu.edu.cn; jiaoxingli123@163.com; 653852518@qq.com

Received 2 October 2013; Revised 26 November 2013; Accepted 13 February 2014; Published 17 October 2014 © 2014 BIOLOGICAL AND CHEMICAL PUBLISHING

Abstract: The solubilities of thiourea in methanol were correlated with the modified Apelblat equation. The dissolution enthalpy and dissolution entropy were calculated from the experimental data. The mutual interaction between solvent and solute were discussed in brief. The data can be used for the synthesis of isopropyl mercaptan.

Key words: Solubility; Thiourea; Dissolution Enthalpy; Dissolution Entropy; Methanol

Introduction Isopropyl mercaptan is an important pharmaceutical inter‐mediate and chemical material with wide use and optimum application prospect. For the synthesis purification process of isopropyl mercaptan with thiourea method, the solvent selection is very important. To choose the proper solvent for the synthesis of isopropyl mercaptan, in this study, the solubilities of thiourea in methanol and temperature range from 290K to 330K, have been determined. Moreover, there are few essays which explain dissolution enthalpy and dissolution entropy of thiourea in methanol. Therefore, we calculated the data of dissolution enthalpy and dissolution entropy for thiourea dissolving in methanol, also the reason for the dissolving phenomenon has been explained in brief.

Experimental Section Materials Thiourea, and methanol were of analytical grade, all obtained from Shanghai Chemical Reagent Co. and had the mass fraction purities of 0.995.

Solubility Measurement The solubilities of thiourea in methanol were measured by a synthetic method at atmospheric pressure which can be seen in the previous article [1].

Test of Apparatus To prove the feasibility and the uncertainty of the measurement, the solubility of NaCl in water was measured and compared with the values reported in the literature [2]. The experimental data agreed with the reported values with a mean relative deviation of 0.23%. The measured values are listed in Table 1.

TABLE 1 SOLUBILITY OF NACL IN 100g WATER

T/K xc x[3] 100 RD 293.15 0.0996 0.0998 ‐0.20 313.15 0.1015 0.1013 0.20 333.15 0.1033 0.1030 0.29 353.15 0.1061 0.1058 0.28 373.15 0.1090 0.1092 ‐0.18

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Result and Discussion The measured solubilities of thiourea in methanol can be seen in the Table 2.

TABLE 2 SOLUBILITIES OF THIOUREA IN METHANOL

T 298.65 301.45 304.35 307.85 309.05

x 0.05336 0.05652 0.06059 0.06429 0.06671

xc 0.05326 0.05674 0.06053 0.06536 0.06708

100 RD 0.1905 ‐0.3890 0.1006 ‐1.6642 ‐0.5592

T 311.25 314.55 319.45 321.25

x 0.07024 0.07608 0.08210 0.08716

xc 0.07033 0.07543 0.08353 0.08667

100 RD ‐0.1326 0.8479 ‐1.7437 0.5642

The temperature dependence of thiourea in methanol is described by the modified Apelblat equation [4].

(2)

Where x is the mole fraction solubility of thiourea, T is the absolute temperature, and A, B, C are the model parameters, which can be obtained from optimization and fitting. The values are listed in Table 3.

TABLE 3 PARAMETERS OF THIOUREA IN METHANOL

A

B

C

R2

103RMSD

102RAD

‐17.7878

‐1066.9938

3.2328

0.9965

0.6716

0.6880

The definition of RD, RAD and RMSD can be seen in the previous article [1]. Relative deviations(RD) between the experimental values and the calculated values can be seen in Table 2 and root mean square deviation (RMSD), relative average deviation (RAD) are listed in Table 3. It can be seen from Table 3 that the RMSD is just 0.6716‰, the relative average deviation (RAD) is only 0.688%, which indicates that the experimental data show good agreement with the calculated solubilities and modified Apelblat Equation is right fit for thiourea dissolving in methanol.

Thermodynamic functions related with solubility are mainly dissolution enthalpy and dissolution entropy, namely ΔsolH and ΔsolS, which can be obtained through a series of thermodynamic equations. Then we’ll obtain the expressions of ΔsolH and ΔsolS as follows.

The dissolution process of solid S, in liquid W, can be expressed as S+W=SW according to a pseudo chemical reaction process [3]. And the relationship between its dissolution equilibrium and activities can be expressed as: Ki 

ai (3) as  aw

Where ai stands for the activity of thiourea in the solution, as and aw represent the activities of pure solid S and pure liquid W, respectively. Due to the standard states of S and W, as and aw are constants. The relationship between the activity ai and the mole fraction xi can be shown as below: ai   i  xi (4)

Therefore Equation (3) can be written as: Ki 

 i  xi as  aw

(5)

Where γi is the activity coefficient of thiourea, and xi is the mole fraction of thiourea in the solution.

Equation (5) can be changed into Equation (6) by logarithmic treatment: ln Ki  ln xi  J (6)

Where J=lnγi‐ln (as *aw), and J is a constant independent off temperature.

The equation for calculating molar enthalpies of dissolution ΔsolH can be obtained as follows according to Gibbs equation and the modified Van’t Hoff method [5].

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PHYSICAL CHEMISTRY COMMUNICATIONS Volume 1 Issue 1, October 2014 www.bacpl.org/j/pcc

 sol H   R 

d ln Ki dT 1

(7)

Namely

 sol H   R 

d ln xi dT 1 (8)

Equation (2) is used to obtain the derivation and then substituted into Equation (8), thus Equation (9) is given below.  sol H  R  T  (C 

B ) (9) T

According to the related equations between enthalpy and entropy [6], dissolution ΔsolS can be obtained as below:  sol S  R  (C 

B ) (10) T

R is the molar gas constant, whose value 8.314 J∙mol‐1∙K‐1; According to the parameters of modified Apelblat equation listed in Table 3, ΔsolH and ΔsolS can be calculated by Equation (9) and Equation (10), which is listed in Table 4.

TABLE 4 ΔSOLH(KJ MOL‐1)AND ΔSOLS(J MOL‐1 K‐1) FOR THIOUREA IN METHANOL T/K 298.65 301.45 304.35 307.85 309.05

ΔsolH (kJ mol‐1) 16.896 16.971 17.049 17.143 17.175

△solS (J mol‐1 K‐1) 56.57 56.30 56.02 55.69 55.57

T/K 311.25 314.55 319.45 321.25

ΔsolH (kJ mol‐1) 17.235 17.323 17.455 17.503

△solS (J mol‐1 K‐1) 55.37 55.07 54.64 54.48

From Table 4, it is seen that dissolution enthalpies ΔsolH of thiourea in methanol are positive and they increase with the increase of temperature; and that the dissolution entropies ΔsolS are positive, too. Besides that, dissolution entropies will increase with increase of temperature. The positive ΔsolS for the process indicates that the dissolving process is entropy‐driven. As known, entropy is a measurement of chaos for the system [7]. The solvent of methanol is quite in order when it hasn’t mingled with thiourea solute while if added the solute into the orderly solvent, the aligment of the solvent molecules is disrupted, thus the chaos degree of the system is enhanced, and dissolution entropy for the process is positive.

The positive ΔsolH shows the process is endothermic, which isn’t beneficial to dissolving. This might be because the interactions between thiourea and the solvent are more powerful than those between the solvent molecules. When the temperature increases, molecular motion will be explosive, and molecular of the same kind will absorb more heat to destroy the crystal lattice, hydrogen bond and other intermolecular interactions and accelerate the dissolving process. Therefore, the solubilities will increase and the dissolution enthalpies will increase, too.

Conclusion In our study, the solubilities of thiourea in methanol were measured, and the experimental data were correlated with the Apelblat equation, also the dissolution enthalpies ΔsolH and dissolution entropies ΔsolS were estimated.

The positive ΔsolH and ΔsolS revealed that the process is endothermic and entropy‐driven. The experimental solubility and correlation equation in this work can be used for the synthesis of isopropyl mercaptan, and the calculated dissolution enthalpies ΔsolH and dissolution entropies ΔsolS can provide some essential data for the analysis of energy consumption in the chemical process.

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