Maharashtra Board Class 11 Chemistry Chapter 10 States of Matter Gaseous and liquid states PDF Download

10. States of Matter: Gaseous and Liquid States

10.1 Introduction

We have learnt that substances exist in one of the three main states of matter. The three distinct physical forms of a substance are Solid, Liquid, and Gas.

Water exists in the three different forms: solid ice, liquid water and gaseous vapours.

Three states of matter are interconvertible by exchange of Heat.

Key points of differentiation between the three states can be understood as given in Table 10.1.

Teacher's Note

Ice is a solid, water is a liquid, and steam is a gas. All three are the same substance in different states. Temperature changes the state of water.

Exam Trick

Remember: Solid = fixed shape and volume. Liquid = fixed volume but takes container shape. Gas = no fixed volume or shape. This is the order: Solid, Liquid, Gas.

Points to Remember

Solids have particles packed tightly in fixed positions.
Liquids have particles loosely packed that can move around.
Gases have particles very far apart and moving randomly in all directions.
Temperature can change a substance from one state to another.
Adding heat changes solid to liquid, and liquid to gas.

10.2 Intermolecular Forces

Intermolecular forces are the attractive forces as well as repulsive forces present between the neighbouring molecules. The attractive force decreases with the increase in distance between the molecules. The intermolecular forces are strong in solids, less strong in liquids and very weak in gases. Thus, the three physical states of matter can be determined as per the strength of intermolecular forces.

The physical properties of matter such as melting point, boiling point, vapor pressure, viscosity, evaporation, surface tension and solubility can be studied with respect to the strength of attractive forces between the molecules. During the melting process intermolecular forces are partially overcome, whereas they are overcome completely during the vapourization process.

10.2.1 Types of Intermolecular Forces

The five types of intermolecular forces are:

i. Dipole-dipole interactions

ii. Ion-dipole interactions

iii. Dipole-Induced dipole interaction

iv. Hydrogen bonding

v. London Dispersion Forces

i. Dipole-Dipole Interactions

Polar molecules experience dipole-dipole forces due to electrostatic interactions between dipoles on neighboring molecules.

Polar covalent molecule is also described as "dipole" meaning that the molecule has two poles. The covalent bond becomes polar due to electronegativity difference between the bonding atoms. Hence polarity is observed in the compounds containing dissimilar atoms. For example, HCl molecule.

One end (pole) of the molecule has partial positive charge on hydrogen atom while at other end chlorine atom has partial negative charge (denoted by Greek letter delta).

Dipole moment (P) is the product of the magnitude of the charge (Q) and the distance between the centres of positive and negative charge (r). It is designated by a Greek Letter (P) (mu). Its unit is debye (D).

Dipole moment is a vector quantity and is depicted by a small arrow with tail in the positive centre and head pointing towards the negative centre.

For example, the dipole moment of HF may be represented as: \(\text{H-F}\)

The crossed arrow above the Lewis structure represents an electron density shift.

Thus polar molecules have permanent dipole moments. When a polar molecule encounters another polar molecule, the positive end of one molecule is attracted to the negative end of another polar molecule. Many such molecules have dipoles and their interaction is termed as dipole-dipole interaction. These forces are generally weak, with energies of the order of 3-4 kJ mol-1 and are significant only when molecules are in close contact, i.e. in a solid or a liquid state.

For example C4H9Cl (butyl chloride), CH3 - O - CH3 (dimethyl ether) ICl (iodine chloride B.P. 27 °C), are dipolar liquids.

In brief, more polar the substance, greater the strength of its dipole-dipole interactions. Table 10.2 enlists several substances with similar molecular masses but different dipole moments. From Table 10.2, it is clear that higher the dipole moment, stronger are the inter molecular forces, generally leading to higher boiling points.

When different substances coexist in single phase, following intermolecular interactions are present.

Teacher's Note

Dipole-dipole forces are like magnets. Positive ends attract negative ends. Water molecules attract each other because they have dipoles.

Exam Trick

Remember: Higher dipole moment = stronger intermolecular forces = higher boiling point. Think of it like how strong magnets attract more than weak magnets.

Points to Remember

Polar molecules have two poles (positive and negative).
Dipole moment measures the strength of polarity.
Dipole-dipole forces occur between polar molecules.
These forces are weak but important in liquids and solids.
Higher dipole moment means stronger attractive forces.

ii. Ion-Dipole Interactions

An ion-dipole force is the result of electrostatic interactions between an ion (cation or anion) and the partial charges on a polar molecule.

The strength of this interaction depends on the charge and size of an ion. It also depends on the magnitude of dipole moment and size of the molecule.

Cations are smaller in size than the isoelectronic anions. The charge density on cation (Na+) is more concentrated than anion (Cl-). This makes the interaction between (Na+) and negative end of the polar H2O molecule stronger than the corresponding interaction between (Cl-) and positive end of the polar H2O molecule.

More the charge on cation, stronger is the ion-dipole interaction. For example, Mg2+ ion has higher charge and smaller ionic radius (78 pm) than Na+ ion (98 pm), hence Mg2+ ion is surrounded (hydrated) more strongly with water molecules and exerts strong ion-dipole interaction.

Thus the strength of interaction increases with increase in charge on cation and with decrease in ionic size or radius. Therefore, ion-dipole forces increase in the order: Na+ < Mg2+ < Al3+.

Ion-dipole forces are particularly important in aqueous solutions of ionic substances such as sodium chloride (NaCl). When an ionic compound, sodium chloride is dissolved in water, the ions get separated and surrounded by water molecules which is called Hydration of sodium ions.

iii. Dipole-Induced Dipole Interaction

When polar molecules (like H2O, NH3) and nonpolar molecules (like benzene) approach each other, the polar molecules induce dipole in the non-polar molecules. Hence Temporary dipoles are formed by shifting of electron clouds in nonpolar molecules. For example, Ammonia (NH3) is polar and has permanent dipole moment while Benzene (C6H6) is non polar and has zero dipole moment. The force of attraction developed between the polar and nonpolar molecules is of the type dipole - induced dipole interaction.

Teacher's Note

Polar molecules can push electrons in nonpolar molecules. This creates temporary dipoles. Like how a magnet can make iron slightly magnetic temporarily.

Exam Trick

Remember: Dipole-induced dipole = one molecule makes the other one become polar. Polar pushes nonpolar to become polar. Think: one teaches the other.

Points to Remember

Polar molecules have a permanent dipole moment.
Nonpolar molecules have no permanent dipole.
When polar and nonpolar molecules meet, temporary dipoles form in nonpolar molecules.
These forces are weaker than regular dipole-dipole forces.
Both molecules must be close together for this to happen.

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