Solutions

Students Study material

1. I B.Sc., Chemistry General Chemistry – III UNIT-IV: Solutions of Non-Electrolyte Dr. D.THIRUPPATHI Department of Chemistry Vivekananda College Tiruvedakam West – 625 234

2. UNIT- IV : SOLUTIONS OF NON-ELECTROLYTE Solutions of liquids in liquids: Concentration terms: molarity, molality, normality, mole fraction. Ideal solutions: Raoult's law and Henry's law – non-ideal solutions – vapour pressure-composition curves, vapour pressure-temperature curves. Azeotropes: HCl-H 2 O systems and ethanol-water system – fractional distillation – steam distillation Partially miscible liquids: phenol-water, trimethylamine-water and nicotine-water systems. Solutions of gases in liquids Factors influencing the solubility of a gas Syllabus

3. A  solution  is a homogeneous mixture of two or more substances. A  solution  may exist in any phase. A  solution  consists of a solute and a solvent. The solute is the substance that is dissolved in the solvent.

4. Types of Solutions

5. Learning Outcomes: Students will be able to Define concentration of a solution Know the ways of expressing concentration of a solution. Define and explain molarity with examples Define and explain molality with examples Define and explain normality with examples Define and explain mole fraction with examples

6. Concentration of Solutions The concentration of a solution is defined as the relative amount of solute present in a solution. On the basis of concentration of solution there are two types of solutions. ( i ) Dilute solution (ii) Concentrated solution

7. The ways of expressing concentration of a solution The amount of solute, solvent and solution may be measured by volume, weight or number of moles. Therefore, concentration of a solution can be measured in many ways. Molarity (M) 2. Molality (m) 3. Normality (N) 4. Mole fraction (x)

8. Molarity (M): The number of moles of solute dissolved in one litre (1dm 3 ) of a solution.

9. Molality (M): The number of moles of solute dissolved per 1000 g (1 Kg) of solvent.

10. Normality (N): Number of gram equivalent weight of solute (substance) dissolved in one litre (1000 mL) of solution. Normality Examples: Molecular weight of NaOH

11. SOLVED PROBLEM. 5 g of NaCl is dissolved in 1000 g of water. If the density of the resulting solution is 0.997 g per ml, calculate the molality, molarity, normality and mole fraction of the solute , assuming volume of the solution is equal to that of solvent.

12. A simple solution is made of two substances : one is the solute and the other solvent. Mole fraction , X , of solute is defined as the ratio of the number of moles of solute and the total number of moles of solute and solvent. Mole fraction (X):

13. Liquid-liquid solutions can be classified into ideal and non-ideal solutions on the basis of Raoult’s law. Ideal Solutions The solutions which obey Raoult’s law over the entire range of concentration are known as ideal solutions .  mix H = (No energy evolved or absorbed)  mix V = (No expansion or contraction on mixing) Those solutions in which solute-solute (B-B) and solvent-solvent ( A-A) interactions are almost similar to solvent solute (A-B) interactions are called ideal solutions. These solutions satisfy the following conditions : It should obey Raoult’s law i.e., P 1 = χ 1 and P 2 = χ 2

14. Some solutions behave like nearly ideal solutions, e.g., benzene + toluene. n-hexane + n-heptane, ethyl iodide + ethyl bromide, chlorobenzene + bromobenzene .

15. The solution which deviate from ideal behaviour are called non ideal solution or real solutions and they do not obey Raoult’s law over entire range of composition. It has been found that on increasing dilution, a non ideal solution tend to be ideal. non ideal solution or real solutions For such solutions , Δ H mix  Δ V mix  (a) Non-ideal solutions showing positive deviation In such a case, the A – B interactions are weaker than A – A or B – B interactions and the observed vapour pressure of each component and the total vapour pressure are greater than that predicted by Raoult‟s law.

16. It forms minimum boiling azeotropes, for example, C 2 H 5 OH + cyclohexane. The Bonding present in pure C 2 H 5 OH is cut off on adding cyclohexane. For such solution, ΔV and ΔH are positive. Examples: Acetone + carbon disulphide , Acetone + benzene Carbon tetrachloride + chloroform or Toluene Methyl alcohol + water Acetone + C 2 H 5 OH

17. (b) Non-ideal solution showing negative deviation In such a case, the A – B interactions are stronger than A – A or B – B interactions and the observed vapour pressure of each component and the total vapour pressure are lesser than that predicted by Raoult‟s law. It forms maximum boiling azeotrope, for example, CHCl 3 + CH 3 COCH 3 .For such solutions, ΔV and ΔH are negative . Examples: Chloroform + benzene or diethyl ether Acetone + aniline Nitric acid (HNO 3 ) + water Acetic acid + pyridine

18. SOLUTIONS OF LIQUIDS IN LIQUIDS The solutions of liquids in liquids may be divided into three classes as follows : SOLUBILITY OF PARTIALLY MISCIBLE LIQUIDS A large number of liquids are known which dissolve in one another only to a limited extent e . g ., ether and water. Ether dissolves about 1.2% water; and water also dissolves about 6.5% ether. Since their mutual solubilities are limited, they are only partially miscible. When equal volumes of ether and water are shaken together, two layers are formed, one of a saturated solution of ether in water and the other of a saturated solution of water in ether.

19. These two solutions are referred to as conjugate solutions . The effect of temperature on the mutual solubility of these mixtures of conjugate solutions is of special interest. We will study the effect of temperature on the composition of such mixtures with reference to three typical systems : (1) Phenol-Water system (2) Triethylamine -Water system (3) Nicotine-Water system The temperature at which the two liquids which are otherwise partially miscible at ordinary temperature. Become completely miscible is called critical solution temperature (CST). Type 1. System with an upper critical solution temperature Phenol-water system. If phenol and water are mixed in roughly equal proportions, two layers are produced, one of a solution of phenol in water and the other of a solution of water in phenol.

20. At any given temperature, the composition of these layers is fixed and can be determined by suitable analytical methods. If the system is warmed, the amount of water in phenol layer increases as well as the amount of phenol in water layer increases. Ultimately, at a particular temperature, the composition of both the layers become the same and thus the two become completely miscible. Experiments have shown that the minimum temperature at which phenol and water become completely miscible is 66 o C and the composition is 34% of phenol by weight. Thus phenol-water system has an upper critical solution temperature of 66 o C. the results on being plotted give a parabolic curve as shown in Fig.1. Alternatively, the same curve is obtained if solutions of phenol of different suitable compositions are prepared and the temperature at which the two liquids become completely miscible is determined in each case (from the disappearance of turbidity) and then plotted against the composition.

21. Fig.1. Phenol-water system

22. Type II. System with a lower critical solution temperature Triethylamine-water system . The temperature-composition curve of mutual solubilities of triethylamine and water is given in Fig . 1. The left hand side of the curve indicates the solubility curve of triethylamine in water and the right hand side of that of water in triethylamine. Unlike phenol-water system, the solubilities decrease with the increase in temperature in this system. The two conjugate solutions mix up completely at or below 18.5°C. This temperature is also called the critical solution temperature or the lower consolate temperature. As in the above case, any point above the horizontal line corresponds to heterogeneity of the system (two layers) while below it is complete homogeneity (one layer). Thus an equi -component mixture (50–50) will be completely miscible at 10°C but at 50°C there will be separating out two layers having compositions corresponding to the points C and D . Triethylamine-water system

23. Type III. System with an upper as well as a lower critical solution temperature Nicotine-water system. This is the system in which the mutual solubility of the two liquids increases with increase of temperature as well as with decrease of temperature. The two liquids become completely miscible above a particular temperature as well as below a particular temperature. Such systems have an upper critical solution temperature as well as a lower critical solution temperature which are 208 o C and 60.8 o C respectively for nicotine-water system. The plot obtained between temperature and composition in such cases is a closed curve as shown in Fig.1 Nicotine-water system.

24. EXAMINATION QUESTIONS Define or explain the following terms : (a) Mole fraction ( b) Molarity ( c) Molality ( d) Normality Write note on Theory of fractional distillation State and explain Henry’s Law . Define and explain the term “Solution”. What are the different ways of expressing the concentration of a solution ? Write briefly on Raoult’s law and azeotropes . Calculate the normality of a solution containing 6.3 g of oxalic acid crystals (Mol. wt. 126) dissolved in 500 ml of solution . Answer . 0.2 N Define an ideal solution . Write short notes on : ( i ) Steam distillation ( ii) Azeotropic mixture Give illustration of maximum and minimum azeotropic mixtures . Give brief accounts of ( i ) Raoult’s law ( ii) Henry’s law Draw vapour pressure-composition curves for non-ideal solutions What is critical solution temperature or consolute temperature? Describe different types of partially miscible liquids. Briefly explain critical solution tempearature of nicotine-water system.

25. THANK YOU