Title 2.SOLUTIONS.pdf ✅

ChemContent | Solutions ChemContent 1 Introduction: In this chapter, we will discuss about liquid solutions and their formation. This will be followed by studying the properties of solutions, like vapour pressure and colligative properties. We will begin with types of solutions and expressions for concentration of solutions in different units. Thereafter, we will state and explain Henry’s law and Raoult’s law, distinguish between ideal and non-ideal solution and deviation of real solutions from Raoult’s law. We will also discuss abnormal colligative properties alongwith association and dissociation of solute. Types of Solutions All the three states of matter (solid, liquid and gas) may behave either as solvent or solute. When a solution is composed of only two chemical substances, it is termed as binary solution. Depending upon the state of solute or solvent, binary solutions can be classified as Some Important Definitions • Mixture - When two or more chemically non-reacting substances are mixed, they form mixture. • Heterogeneous Mixture - It consists of distinct phases, and the observed properties are just the sum of the properties of individual phases. • Homogeneous Mixture - It consists of a single phase which has properties that may differ from one of the individual components. • Solution - The homogeneous mixture of two or more components such that at least one component is a liquid is called solution. • Solvent - It is the constituent of solution which has same physical state as that of solution and generally present in greater amount than all the other components. • Solute - The component of a solution other than solvent is called solute, may or may not have same physical state as that of solution. Generally it is in smaller amount.
ChemContent | Solutions ChemContent 2 Example - In a sugar syrup (liquid solution) containing 60% sugar (solid) and 40% water (liquid), water is termed as solvent, due to same physical state as that of solution. Expressing the Strength of Solution For a given solution the amount of solute dissolved per unit volume of solution is called concentration of solute. Strength of solution is the amount of solute in grams dissolved in one litre of solution. It is generally expressed in g/litre. Other methods of expressing the strength of solution are: 1. Mass percentage – Mass % of solute = Mass of solute Total mass of solution× 100 Mass % of solvent = Mass of solvent Total mass of solution× 100 2. Volume percentage – Volume % of solute = Volume of solute Total volume of solution× 100 Volume % of solvent = Volume of solvent Total volume of solution× 100 3. Molality (m) - It is no. of moles of solute dissolved in 1 kg of the solvent. m = (Number of moles of solute) (Mass of solvent {in kg}) 4. Molarity (M) - It is no. of moles of solute dissolved in 1 litre of solution. M = (Number of moles of solute) (Mass of soution {in litre}) 5. Normality (N) - It is no. of gram-equivalents of solute dissolved in 1 litre of solution N = (Number of gram equivalents of solute) (Volume of solution {in litre) 6. Formality - Ionic solutes do not exist in the form of molecules. These molecular mass is expressed as Gram- formula mass. Molarity for ionic compounds is actually called as formality. 7. Mole fraction –
ChemContent | Solutions ChemContent 3 Mole fraction of solute = (Number of moles of solute) (Total moles of solution) Mole fraction of solvent = (Number of moles of solvent) (Total moles of solution) For a binary solution, mole fraction of solute + mole fraction of solvent = 1. 8. Parts per million (ppm) – It is defined in two ways ppm = mass fraction × 106 ppm = mole fraction × 106 Solubility Solubility of a substance is its maximum amount that can be dissolved in a specified amount of solvent at a specified temperature. It depends upon the nature of solute and solvent as well as temperature and pressure. Let us consider the effect of these factors in solution of a solid or a gas in a liquid. 1. Solubility of Solid in Liquid A solute dissolves in a solvent if the intermolecular interactions are similar in them, i.e., like dissolves like. Polar solute dissolves in polar solvent and non-polar solute in non-polar solvent. For e.g., sodium chloride and sugar dissolves readily in water and napthalene and anthracene dissolves readily in benzene. Solute + Solvent ⟶ Solution i. Dissolution: When a solid solute is added to the solvent, some solute dissolves and its concentration increases in solution. This process is called dissolution. ii. Crystallization: Some solute particles collide with solute particles in solution and get separated out. This process is called crystallization. iii. Saturated solution: Such a solution in which no more solute can be dissolved at the same temperature and pressure is called a saturated solution. iv. Unsaturated solution: An unsaturated solution is one in which more solute can be dissolved at the same temperature. v. Effect of temperature: In general, if in a nearly saturated solution, the dissolution process is endothermic, the solubility should increase with rise in temperature, if it is exothermic, the solubility should decrease with rise in temperature. vi. Effect of pressure: Solids and liquids are highly incompressible, so pressure does not have any significant effect on solubility of solids and liquids. vii. Supersaturated solution: When more solute can be dissolved at higher temperature in a saturated solution, then the solution becomes supersaturated. 2. Solubility of Gas in Liquid All gases are soluble in water as well as in other liquids to a greater or lesser extent. The solubility of a gas in liquid depends upon the following factors Nature of the gas, Nature of solvent, Temperature and Pressure. Generally, the gases which can be easily liquified are more soluble in common solvents. For e.g., CO2 is more soluble than hydrogen or oxygen in water. The gases which are capable of forming ions in aqueous solutions
ChemContent | Solutions ChemContent 4 are much more soluble in water than other solvents. For e.g., HCl and NH3 are highly soluble in water but not in organic solvents (like benzene) in which they do not ionize. i. Effect of temperature: Solubility of most of the gases in liquids decreases with rise in temperature. In dissolution of a gas in liquid, heat is evolved and thus this is an exothermic process. The dissolution process involves dynamic equilibrium and thus follows Le Chatelier’s principle. As dissolution is exothermic the solubility of gas should decrease with rise in temperature. ii. Effect of pressure: Henry’s law: At constant temperature, the solubility of a gas in a liquid is directly proportional to the pressure of the gas. p = KH x, KH = Henry’s law constant. Applications of Henry’s law 1. In manufacture of soft drinks and soda water, CO2 is passed at high pressure to increase its solubility. 2. To minimise the painful effects accompanying the decompression of deep sea divers. O2 diluted with less soluble. He gas is used as breathing gas. 3. At high altitudes, the partial pressure of O2 is less then that at the ground level. This leads to low concentrations of O2 in the blood of climbers which causes ‘anoxia’. Vapour Pressure of Solution It is the pressure exerted by vapour on the surface layer of liquid at equilibrium between vapour and liquid. Factors affecting Vapour Pressure i. Nature of liquid - Liquid with higher intermolecular attraction forces form less amount of vapour and hence lower vapour pressure and vice-versa. ii. Temperature - Vapour pressure increases with temperature of liquid. This is because, as temperature increases, kinetic energy of the molecules increases, hence, more molecules leave the surface of the liquid and come into vapour phase. Raoult’s Law According to Raoult’s law, for a solution of volatile liquids, the relative lowering of vapour pressure of solution is directly proportional to its mole fraction of dissolved solvent in solute. Ideal Solution The solution which obeys Raoult’s law at all compositions of solute and solvent and at all temperature is called an ideal solution. Ex- Benzene and Toluene, n-hexane and n-heptane. Characteristics of an ideal Solution 1. Raoult’s law is obeyed by it. 2. ΔHmixing = 0 i.e., no heat should be absorbed or evolved during mixing. 3. ΔVmixing = 0, i.e., no change in volume (expansion or contraction) on mixing.