Class 12 chemistry ch 2

Class 12 chemistry chapter 2

NCERT Notes For Class 12 Chemistry

Chapter 2: solutions

Chapter 2 Solutions

NCERT Notes For Class 12 Chemistry

Chapter 2: Solutions

Solution is a homogeneous mixture of two or more substances in same or different physical phases. The substances forming the solution are called components of the solution. On the basis of number of components a solution of two components is called binary solution.

Solute and Solvent

In a binary solution, solvent is the component which is present in large quantity while the other component is known as solute.

Classification of Solutions

(A) Following types of solutions are seen on the basis of physical state of solute and solvent.

[if water is used as a solvent, the solution is called aqueous solution and if not, the solution is called non-aqueous solution.]

(B) Depending upon the amount of solute dissolved in a solvent we have the following types of solutions:

(i) Unsaturated solution A

solution in which more solute can be dissolved without raising temperature is called an unsaturated solution.

(ii) Saturated solution A

solution in which no solute can be dissolved further at a given temperature is called a saturated solution.

(iii) Supersaturated solution A

solution which contains more solute than that would be necessary to saturate it at a given temperature is called a supersaturated solution.

Solubility

The maximum amount of a solute that can be dissolved in a given amount of solvent (generally 100 g) at a given temperature is termed as its solubility at that temperature.

The solubility of a solute in a liquid depends upon the following factors:

(i) Nature of the solute 

(ii) Nature of the solvent 

(iii) Temperature of the solution 

(iv) Pressure (in case of gases)

Henry’s Law

The most commonly used form of Henry‟s law states “the partial pressure (P) of the gas in vapour phase is proportional to the mole fraction (x) of the gas in the solution” and is expressed as

p = KH . x

Greater the value of KH, higher the solubility of the gas. The value of KH decreases with increase in the temperature. Thus, aquatic species are more comfortable in cold water [more dissolved O2] rather than Warm water.

Applications

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 (bends) accompanying the decompression of deep sea divers. O2diluted 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‟.

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

Methods of Expressing Concentration of Solutions

Various expression for the concentrations of solutions can be summarised as

(i) Percentage by weight

(w / w %) It is defined as the amount of solute present in 100 g of solution.

w / w % = weight of solute / weight of solution * 100

(ii) Percentage by volume

(w / V%) It is defined as the weight 01 solute present in 100 mL of solution.

w / V % = weight of solute / weight of solution * 100

or the volume of solute present in 100 mL of solution.

u / V % = volume of solute / volume of solution * 100

(iii) Mole fraction

(x) It is defined as the ratio of the number of moles of a component to the total number of moles of all the components. For a binary solution, if the number of moles of A and B are nA and nB respectively, the mole fraction of A will be

(iv) Parts per million

(ppm) It is defined as the parts of a component per million parts (106) of the solution. It is widely used when a solute is present in trace quantities.

ppm = number of parts of the component / total number of parts of all the components * 106 (v) Molarity (M) It is the number of moles of solute present in 1L(dm3) of the solution.

M = number of moles of solute / volume of solution (L)

M = mass of solute (in gram) * 1000 / mol. wt. of solute x volume of solution (in mL)

Molarity varies with temperature due to change in volume of solution.

[When molarity of a solution is 1 M, it is called a molar solution. 0.1 M solution is called a decimolar solution while 0.5 M solution is known as semi molar solution]

Molarity = Percent by mass * density * 10 / molecular weight

Dilution law, M1 V1 = M2 V2 (for dilution from volume V1 to V2)

For reaction between two reactants, M1 V1 / n1 = M2 V2 / n2 where, n1 and n2 arc stoichiometric coefficient in balanced equation.

(vi) Molality (m) It is the number of moles of solute per kilogram of the solvent.

Molality = mass of solute in gram * 1000 / mol. wt. of solute * mass of solvent (in g)

Molality is independent of temperature.

[Whcn solvent used is water, a molar (1 M) solution is more concentrated than a molal (1 M) solution.]

(vii) Normality (N) The number of gram equivalents of solute present in 1 L of solution.

Normality = number of grams – equivalent of solute / volume of solution in L

Number of gram-equivalents of solute = mass of solute in gram / equivalent weight

[Relationship between normality and molarity N x Eq. weight = M x mol. weight ]

If two solutions of the same solute having volumes and molarities V1, M1 and V2, M2 are mixed, the molarity of the resulting solution is

To dilute V1 mL of a solution having molarity M1 to molarity M2 up to the final volume V2 mL, the volume of water added is

(viii) Formality (F) This is the number of formula weights of the solute per liter of solution.

 Formality = moles of substance added to the solution / volume solution (in L))

 (ix) Mass fraction The mass fraction of any component in the solution is the mass of that component divided by the total mass of the solution.

 Molality, molar fraction and mass fraction are preferred over molarity, normality, etc., since the former involves weights which do not change with temperature.

 (x) Demal (D) This represents one mole of solute present in 1 L solution at 0 ° C.

 Raoult's law

 Raoult's law states: "For a solution of two volatile liquids, the vapor pressure of each liquid in the solution is lower than the respective vapor pressure of pure liquids and the partial equilibrium vapor pressure of the liquid is directly proportional to its molar fraction.

 For a solution containing two liquids A and B, the partial vapor pressure of liquid A is

 The proportionality constant is obtained by taking the pure liquid into account when χA = 1 then k = P ° A, the vapor pressure of the pure liquid, so

 Konowaloff rule

 At any fixed temperature, the vapor phase is always richer with the most volatile component compared to the solution phase.  In other words, the molar fraction of the most volatile constituent is always higher in the vapor phase than in the solution phase.

 The composition of the vapor phase in equilibrium with the solution is determined by the partial pressure of the components.  If Y1 and Y2 die

 components 1 and 2 respectively in the vapor phase thereafter.  use of Dalton's partial pressure law,

 p1 = y1 * Punctual

 p2 = y2 * Punctual

 Ideal solutions

 Solutions in which interaction between solute (B-B) and solvent-solvent (A-A) are almost the same as the interaction with solvent-solute (A-B) are called ideal solutions.  These solutions meet the following conditions:

 (i) The solution must comply with Raoult's law, that is:

 (ii) ΔHmix = 0 (no energy released or absorbed)

 (iii) mVix = 0 (no expansion or contraction during mixing)

 Some solutions are like almost ideal solutions, for example benzene + toluene.  n-hexane + nheptane, ethyl iodide + ethyl bromide, chlorobenzene + bromobenzene.

 Non-ideal solutions

 These solutions that deviate from Raoult's law are called non-ideal solutions.

 For such solutions,

 ΔHmix ≠ 0

 MVix ≠ 0

 a) Non-ideal solutions that show a positive deviation

 In such a case, the interactions A - B are weaker than the interactions A - A or B - B and the observed vapor pressure of each component and the total vapor pressure are higher than those predicted by Raoult's law.

 For such solutions

 (b) Non-ideal solution showing a negative deviation

 In such a case, the interactions A - B are stronger than the interactions A - A or B - B and are the observed vapor pressure of each component and the total vapor pressure is lower than predicted by Raoult's law.

 Azeotropic mixture

 A mixture of two liquids boiling at a certain temperature as a pure liquid and distilling in the same composition is known as constant boiling mixtures.  It is formed by non-ideal solutions.

 (i) Azeotropes at minimum boiling point

 are formed by these fluid pairs which show a positive deviation from ideal behavior.  These azeotropes have boiling points lower than one of the components, for example C2H5OH (95.57%) + H2O (4.43%) (by mass).

 (ii) Azeotropes at maximum boiling

 are formed by these fluid pains;  which shows a negative deviation from ideal behavior.  These azeotropes have higher boiling points than any component.  for example H2O (20.22%) + HCl (79.78%) by mass.

 Colligative properties

 [Colligatil1e: from Latin.  = Co means "together";  ligars means "to bind".]

 Colligative properties are those properties that depend only on the number of dissolved particles in a solution, regardless of their nature.

 Relative lowering of vapor pressure

 Relative lowering of vapor pressure

 This is the ratio of the drop in the vapor pressure to the vapor pressure of the pure solvent.  The relative lowering of the vapor pressure of a solution containing a non-volatile solute is equal to the molar fraction of the solute.

 The above expression is used to find the molecular weight of an unknown solute dissolved in a given solvent.  True, WB and WA = mass of the solute and solvent, respectively.  MB and MA = molecular weight of the solute and the solvent.

 The Ostwald and Walker method is used to determine the relative reduction of vapor pressure.

 Boiling point rise (ΔTb)

 The boiling point of a liquid is the temperature at which the vapor pressure equals the atmospheric pressure.  Since the vapor pressure of a solution containing a non-volatile solvent is lower than that of the pure solvent, the boiling point will be higher than that of the pure solvent, as shown in the figure.  The increase of the boiling point is called the increase of the boiling point, ΔTb

 ΔTb = Tb - T ° b ΔTb = Kb m (true; m = molality)

 Kb is the molar elevation constant or ebullioscopic constant.  The molecular weight of the solute can be calculated as follows:

 where, WB and WA = mass respectively of the solute and solvent.

 Kb has units of K / m or K kg mol-1, for water, Kb = 0.52 K kg mol-1

 The rise in the boiling point of a solution is determined by

 (i) Landsberger method

 (ii) Cottrell method

 Freezing Depression (ΔTf)

 The freezing point of a liquid is the temperature at which the vapor pressure of the solvent in the liquid and solid phase is equal.  Since we know that the vapor pressure of a solution containing a non-volatile solvent is lower than that of the pure solvent, the solid forms at a lower temperature, as shown in the figure.

 This decrease in the freezing point of a liquid is called freezing depression.

 Freezing point depression (ΔTf) = T ° f - Tf

 To find the molecular weight of the solute,

 where, Kf is a molar depression constant or a cryoscopic constant.

 Kf has units of K / m or K kg mol-1.

 Ethylene glycol is usually added to the water in the radiator to lower the freezing point.  This is the antifreeze solution.

 [Common salt (NaCI) and anhydrous CaC12 are used to remove snow from roads as it lowers the freezing point of water.  Freezing depression is determined by the Beckmann method or the Rast method.]

 Calculations of the molar elevation constant (Kb) and the molar depression constant (Kf)

 Osmotic pressure (π)

 Osmosis is the phenomenon of spontaneous flow of solvent molecules through a semi-permeable membrane from the pure solvent to the solution or from a dilute solution to a concentrated solution.  It was first observed by Father Nollet.

 Some natural semi-permeable membranes are the animal bladder, the cell membrane, etc.

 CU2 [Fe (CN) 6] is an artificial semi-permeable membrane that does not work in non-aqueous solutions because it dissolves there.

 Osmosis can be

 (i) Exosmosis

 It is a flow of water or solvent to the outside of a cell through a semi-permeable membrane.

 (ii) Endosmosis

 It is an incoming flow of water or solvent from a cell through a semi-permeable membrane.

 The hydrostatic pressure developed on the solution and which prevents osmosis of the pure solvent in the solution through a semi-permeable membrane is called osmotic pressure.

 where, d = density, R = solution constant, T = temperature, MB = molar mass of the solute

 Osmotic pressure can be determined by any of the methods below

 (i) Pfeffer's method

 (ii) Berkeley and Hartley method (very good method)

 (iii) Morse and Frazer method

 Based on the osmotic pressure may be the solution

 (i) Hypertonic solution

 It is claimed that a solution is hypertonic if the osmotic pressure is higher than that of the solution from which it is separated by a semi-permeable membrane.

 When a plant cell is placed in a hypertonic solution, the liquid comes out of the plant cell and the cell shrinks, this phenomenon is called plasmolysis.

 (ii) Hypotonic solution

 (iii) Isotonic solution

Two solutions are called isotonic if they exert the same osmotic pressure. These solutions have same molar concentration. 0.91% solution of pure NaCl is isotonic with human RBC‟s.

Two solutions are isotonic if they have the same molar concentration, e.g., if x % solution of X is isotonic with y % solution of Y, this means molar concentration of X = Molar concentration of Y

Osmotic pressure method is the best method for determining the molecular masses of polymers since observed value of any other colligative property is too small to be measured with reasonable accuracy.

Reverse osmosis

When the external pressure applied on the solution is more than osmotic pressure, the solvent flows from the solution to the pure solvent, I which is called reverse osmosis. Desalination of sea water is done by reverse Osmosis.

Abnormal Molecular Masses

In some cases, observed colligative properties deviate from their normal calculated values due to association or dissociation of molecules. As we know,

Colligative property ∝ 1 / MB

lienee, higher and lower values of molar mass is observed in case of association and dissociation respectively, e.g., in benzene, acetic acid gets associated, so, its observed

molecular mass is 120. Similarly KCI undergoes dissociation in aqueous solution, so its observed molecular mass is 37.25.

These observed values are corrected by multiplying with van‟t Hoff factor (i).

van’t Hoff Factor (i)

It is the ratio of observed value of colligative property to the calculated value of colligative property.

i = observed value of colligative property / calculated value of colligative property

or i = normal molecular mass / observed molecular mass

or i = number of particles after association or dissociation / number of particles initially

So to correct the observed value of molar mass, van‟t Hoff factor (i) must be included in different expressions for colligative properties.

Degree of Dissociation (α) and van’t Hoff Factor (i)

(i) If one molecule of a substance gets dissociated into n particles or molecules and α is the degree of dissociation then

Degree of Association (α) and van’t Hoff Factor (i)

If n molecules of a substance A associate to form An and α is the degree of association then

van‟t Hoff factor (i) > 1 for solutes undergoing dissociation and it is < 1 for solutes undergoing association.