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CBSE Class 12th Chemistry Notes: Solutions (Part – I)

Solution is an important chapter of CBSE Class 12 Physical Chemistry. Concepts used in this chapters are further used in other chapters of CBSE Class 12 Chemistry. In this article, we have provided notes (Part - I) on this chapter. The topics covered in these notes are: Solutions, types of solutions, Different Methods of Expressing Concentration of Solutions, Solubility and factors affecting solubility, Henry's law.

Aug 5, 2016 15:00 IST
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Chapters in CBSE Class 12 Physical Chemistry are: Solid State, Solutions, Electrochemistry, Chemical Kinetics and Surface Chemistry.
The weightage of these chapters in CBSE Class 12 Chemistry board exam is 23 Marks (out of 70). Solution is an important chapter of CBSE Class 12 Physical Chemistry. Given below is the Part I of Chapter Notes on Solution, which is chapter number 2nd of NCERT textbook.

Introduction

A Solution is a homogenous mixture of two or more components. It is defined by using the terms solute and solvent.

Solvent: The component that is present in largest quantity is called solvent. It determines the physical state of solution.

Solute:  One or more components present in solution other than solvent is called solute.

Binary solutions: Solution consisting of two components only.

Types of Solutions

According to the phase of solvent, a binary solution can be classified in following types:

Types of Solutions

Solute

Solvent

Examples

Gaseous solutions

Gas

Gas

Mixture of O2 and N2


Liquid

Gas

Chloroform mixed with N2 Gas


Solid

Gas

Camphor in nitrogen gas





Liquid Solutions

Gas

Liquid

Oxygen dissolved in water


Liquid

Liquid

Ethanol dissolved in water


solid

Liquid

Glucose dissolved in water





Solid Solutions

Gas

solid

Solution of H2 and Pd


Liquid

solid

Amalgam of Hg with Na


Solid

solid

Alloy

In this chapter we are mainly focusing on binary solutions (solution made up of two components) of liquid known as liquid solutions.

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Different Methods of Expressing Concentration of Solutions

The composition of solution is defined in terms of concentration. There are several ways to define concentration of solution as follows:

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Intext Questions

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Solubility

Solubility of a substance is the maximum amount that can be dissolved in given amount of solvent at specific temperature. Factors affecting the solubility:

- Nature of solute

- Nature of solvent

- Temperature

- Pressure

Solubility of Solid in a Liquid

Nature of solute and solvent:

According to the nature of solute and solvent the solubility of solid in a liquid follow the principle “ Like dissolves like”

If the nature of solute and solvent is same, the intermolecular force of interaction would be same. That helps in solubility of solute in solvent.

Polar solute dissolves in polar solvent. For example: NaCl and sugar dissolves in water. Non-polar solute dissolves in non-polar solvent. For example: Naphthalene and anthracene dissolves in benzene not in water.

Saturated solution: If the concentration of solute in solution remain constant at given set of temperature and pressure is called saturated solution. If we add more solute in it, it would precipitate out.

Un-Saturated solution: If the concentration of solute in solution can increase at given set of temperature and pressure is called un-saturated solution. If we add more solute in it, it would get dissolve and increase the concentration of solution.

Effect of temperature:

The solubility of solute in solvent always follows the dynamic equilibrium.

Solute + Solvent Solution.

It follows the Le Chateliers principle for the change in temperature at dynamic equilibrium.

If the solution is formed by giving heat means dissolution is endothermic. By increasing the temperature, the reaction will proceed in forward direction and solubility of solute increases.

If the heat is released in formation of solute means dissolution is exothermic. By increasing the temperature, the reaction will proceed in backward direction and solubility of solute decreases.

Effect of pressure:

Pressure has no significant effect on solubility of solid in liquid.

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Solubility of Gas in a Liquid

Nature of solute and solvent:

Solubility of gas in liquid is also somewhat affected by nature of solute and solvent. Oxygen dissolves only a small extent in water but HCl is highly soluble in water because of polar nature of solute and solvent.

Effect of pressure:

Solubility of gas in liquid is highly affected by temperature and pressure. As the pressure of the gas above the surface of the liquid increases, it increases the solubility of gas in liquid. The quantitative relation of this equation is given by Henry’s Law.

Henry’s Law:

It states that, “At constant temperature, the solubility of gas in a liquid is directly proportional to the partial pressure of the gas present above the surface of the liquid or solution.”

If we consider mole fraction of gas in a solution to measure its solubility then it can be said that, “Mole fraction of gas in a liquid is proportional to the partial pressure of gas above the liquid or solution.”

Now the Henry’s Law can be stated as , “ The partial pressure of a gas in vapour phase (p) is proportional to the mole fraction of gas(x) in solution.”

Expression for Henry’s Law:

px

p= KH. x

KH is Henry’s Law constant

Important point regarding Henry’s Law:

  • Different gases have different KH values at the same temperature. That is KH depends on the nature of gas.
  • Higher the value of KH at given pressure, lower is the solubility of gas in given liquid.
  • KH value of particular gas increases with increasing temperature. It indicates that solubility of gas decreases with increasing temperature.

Effect of temperature:

Dissolution of gas in a liquid is an exothermic process. As dissolution process involves dynamic equilibrium, it follows Le Chaterlier’s principle. Hence the solubility of gas in liquid, decreases with increase in temperature.

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Intext Questions

Question: State the formula relating pressure of a gas with its mole fraction in a liquid solution in contact with it. [CBSE 2005]

Sol: Henry’s Law give the relation between pressure of gas with its mole fraction in a liquid solution.

The required formula is

p= KH. x

Where,

p → Partial pressure of the gas in vapour phase

x → Mole fraction of the gas in solution

KH → Henry’s constant

Question: State Henry's Law. What is the effect of temperature on the solubility of a gas in a liquid? [CBSE 2014,2012]

Sol: It states that, “At constant temperature, the solubility of gas in a liquid is directly proportional to the partial pressure of the gas present above the surface of the liquid or solution.”

If we consider mole fraction of gas in a solution to measure its solubility then it can be said that, “Mole fraction of gas in a liquid is proportional to the partial pressure of gas above the liquid or solution.”

Now the Henry’s Law can be stated as , “ The partial pressure of a gas in vapour phase (p) is proportional to the mole fraction of gas(x) in solution.”

Expression for Henry’s Law:

p x

p= KH. x

KH is Henry’s Law constant

The solubility of gases in liquid is dependent on temperature. An increase in temperature results in a decrease the solubility of gas in liquid, while a decrease in temperature results in an increase of solubility of gas in liquid.

Dissolution of gas in a liquid is an exothermic process. As dissolution process involves dynamic equilibrium, it follows Le Chaterlier’s principle. Hence the solubility of gas in liquid, decreases with increase in temperature. Or in other words, when the temperature increases the kinetic energy of the molecules also increase. This will result in more rapid motion of molecules, breaking intermolecular bonds and enable the molecules to escape from the solution. So, the dissolved gas evaporates more readily. That is rise of temperature will cause decrease in the solubility of gases in liquids.

Question: State Henry’s law correlating the pressure of a gas and its solubility in a solvent and mention two applications for the law

Sol: For Henry’s Law follow the text:

Applications of Henry’s Law:

Some of the applications of Henry’s law are given.

(i) To increase the solubility of CO2 in soft drinks and soda water, the bottles are sealed under high pressure.

(ii) The air used for scuba diving is diluted with He to prevent the medical condition known as bends.

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