Chemistry Class 11 Notes 2021 States of Matter-III Solids Chapter 6
Chemistry Class 11 Notes 2021 States of Matter-III Solids cha 6 for kpk board short question, long question, and pdf format download for free now 2021 notes.
Chemistry Class 11 Notes 2021 States of Matter-III Solids Chapter 6 for kpk
Table of Contents
Q.2 i) Define and describe the terms with reference to NaCl.
a. Space lattice b. Unit cell
Answer: a) Space lattice A space lattice (also known as crystal lattice), is an array of points, representing atoms, ions or molecules of a crystal, arranged at different sites in three dimensional spaces. NaCl has cubic crystalline structure that constitute of all edges with equal length and interfacial angles are at 90º. Space lattice of NaCl structure is shown in the figure below.
b) Unit cell The smallest unit by volume of the crystal lattice having all the characteristic features of the entire crystal is called a unit cell. Unit cell of NaCl is shown in the figure below.
A unit cell is the building block of the crystal from which the entire crystal lattice can be constituted by repeating it in three dimensions. A unit cell being the structural unit, carries complete information about the given crystal.
Q.2 ii) Define the following.
a. Plane of symmetry b. Centre of symmetry c. Point of symmetry
Answer: a) Plane of Symmetry If a crystal can be divided by an imaginary plane into two equal halves such that one half is the exact mirror image of the other, it is said to have a plane of symmetry.
b) Centre of Symmetry Centre of symmetry is a point at the centre of the crystal which is equidistant from two opposite faces off a crystal. A crystal may have a number of planes of symmetry or axis of symmetry but it can have only one centre of symmetry.
c) Point of Symmetry A special center point for certain kinds of symmetric figures or graphs. If a figure or graph can be rotated 180° about a point P and end up looking identical to the original, then P is a point of symmetry.
Q.2 iii) Define and differentiate between crystalline and amorphous solids.
Solids in which the structural unit is arranged in an orderly manner which repeats itself in three dimensions, are called crystalline solids.
Solids in which the constituent particles (atoms, molecules or ions) are packed together almost at random and lack the ordered and regular arrangement.
They have rigid and definite geometrical shapes.
They do not have any specific shape and geometry.
Crystalline solids have sharp melting points.
Amorphous solids melt in a range of temperature.
Sodium chloride (NaCl), calcium carbonate (CaCO3) and diamond are examples of crystalline solids.
Glasses, plastics, rubber, coal tar are examples of amorphous solids.
Why metallic crystals are good conductors of electricity but ionic crystals are not?
Metallic crystals have free electrons present in the crystal due to the low ionization energies of metals. These free electrons make the metals good conductors of electricity.
On the other hand, ionic crystals do not have free electrons and the ions are fixed at their position because of strong electrostatic forces. The absence of any free charges (electrons or ions) makes the ionic crystals non-conductors of electricity.
Why do crystalline solids have sharp melting points while amorphous solids don’t?
Crystalline solids have sharp melting points because of orderly arrangement of its structural units. When heated, all the particles, eventually, leave their fixed positions at the same temperature and start mobility. The temperature remains constant, until all the particles become mobile. On the other hand due to random arrangement of particles in amorphous solids, particles leave their positions in a range of temperature and become mobile upon heating. So they do not have sharp melting points.
Q.2 vi) Cleavage is an anisotropic behaviour. Discuss
Answer: Anisotropic behavior is the phenomena in which a crystal has different magnitude of physical properties in different directions. The cleavage is the breakage of a crystal along definite plane (cleavage plane). Since cleavage can take place only in particular direction, so it is an anisotropic behavior.
Q.2 vii) Isomorphic substances have usually the same atomic ratio.
Answer: Two or more different substances which exist in the same crystalline form are called isomorphic substances. Structure of a substance is dependent upon the way of a combination of atoms. If two substances have the same atomic ratio, they follow the same pattern of a combination of atoms which results in the same crystalline structure. Hence, isomorphic substances have usually the same atomic ratio. For example, NaCl and MgO have same atomic ratio and both have a cubic crystal structure.
Q.2 viii) What is axis of symmetry? Describe with a simple diagram.
Answer: It is an imaginary line drawn through the crystal, such that rotating the crystal through 360o, the crystal presents exactly the same appearances more than once. The following figure describes the axis of symmetry.
Q.2 ix) Define allotropy, giving examples.
Answer: Allotropy When an element exists in more than one crystalline forms, the phenomenon is called allotropy and the various crystalline forms are called allotropes or allotropic forms. Examples Sulphur (S8) exists in two allotropic forms (Rhombohedral and monoclinic). Tin exists in two allotropic forms, grey tin (cubic) and white tin (tetragonal).
Q.2 x) Describe why covalent crystals are hard while molecular crystals are soft?
Answer: In covalent crystals, atoms are held through covalent bonds. A covalent bond in these crystals is very strong and that is why covalent crystals are hard. On the other hand in molecular solids, the neighbouring structural units are held together by weak van der Waal’s forces. Weak forces can easily be overcome and thus molecular solids appear to be soft in nature.
Q.3 a) What is a solid state of matter. How does it differ from the gaseous and the liquid states?
Answer: Solid State
The physical state of matter which is characterized by its rigidity, hardness, definite shape, definite volume and mechanical strength is called solid state.
Difference from Gases & Liquids Solids differ in their properties from gases and liquids in the following respects.
The structural units (atoms, ions, molecules) in solids cannot move from their positions because of strong attractive forces. While in gases and liquids they move randomly because of very weak attractive forces.
Solids have definite shape while gases and liquids take the shape of the container in which they are kept.
Solids do not have the property of diffusion while liquids and gases can diffuse easily because of random motion of their particles.
Solids and liquids have definite volume while the volume of a gas is the volume of container in which it is filled.
Q.3 b) Explain with reason, why amorphous solids are also termed as “Super cooled” liquids?
Answer: The type of solids in which the constituent particles (atoms, molecules or ions) are packed together almost at random and lack the ordered and regular arrangement. Amorphous means without any specific shape or geometry. Amorphous solids have many properties in common with liquids. Amorphous solids have disorderly arrangement of particles, they look more like liquids. But as the kinetic energies of particles in amorphous solids are much lower than liquids, they are sometimes referred as super cooled liquids.
Q.4) How kinetic molecular theory (KMT) explains the behaviour of solid substances? Discuss in detail.
Answer: Kinetic molecular theory (KMT) explains the behavior of solid substances in terms of vibration of molecules, intermolecular forces and kinetic energy. Important features of KMT regarding solid substances are discussed below.
Particles (atoms, molecules or ions) in solids are closely packed. They are close enough and are tightly bound to their rigid places. In fact, they are separated by a distance of only a few pico meters (1 pm = 10-12 m).
In solids, the particles are arranged in three dimensional pattern which is called lattice. Each point in the lattice is occupied by a particle of the substance.
Forces of attraction between the constituent particles in solids are very strong as compared to those in liquids and gases.
Solid particles cannot move from their positions because of strong forces of attractions. They can only vibrate about their mean positions.
Q.5) Define and explain the following properties of crystalline solids.
i. Geometrical shape ii. Cleavage plane iii. Habit of a crystal iv. Anisotropy v. Crystal growth vi. Transition temperature
Answer: i) Geometrical Shape Crystalline solids have a definite and orderly arrangement of the constituent particles (atoms, molecules or ions), it gives the crystalline solids a definite geometrical shape. The interfacial angles are always the same for a given crystal, irrespective of the method and conditions of preparations. Similarly, upon grinding to a very fine powder, the crystalline solids still retain their specific geometrical shapes.
ii) Cleavage plane “The surfaces formed when crystalline solid breaks or splits, are called the cleavage planes”. Two adjacent cleavage planes intersect always at a definite angle. The magnitude of the interfacial angle, after cleavage has taken place, may differ from solid to solid and is a characteristic of a given solid.
iii) Habit of a crystal “The shape in which a crystal usually grows is called ‘Habit of a crystal”. For example, sodium chloride (NaCl) has a cubic habit. It means NaCl will always have cubical geometrical shape whenever its crystals are formed.
iv) Anisotropy “Anisotropic behaviour is the phenomena in which a crystal has a different magnitude of physical properties in different directions”. Crystalline solids are anisotropic because of the regular and well-ordered arrangement of the constituent particles. For example, electrical conductivity of graphite is greater in one direction than in other. Actually, electrons in graphite are mobile for electrical conduction parallel to the layers only. Therefore, its conductivity in this direction is far better than perpendicular to the layers. Similarly, cleavage itself is an anisotropic property. Some other physical properties like refractive index, thermal conductivity, coefficient of thermal expansion are also anisotropic properties for some of the crystals.
v) Crystal Growth Crystals are grown (prepared) by slow cooling of a substance in liquid state or cooling a hot saturated solution of the substance concerned. The apparent shape of the crystal depends on the method and conditions of preparation. For example, sodium chloride (NaCl) with cubic crystal habit, may grow into a cubic (three dimensional), a flat shaped (two dimensional) or a long needle like structure (one dimensional), depending upon the conditions under which it is prepared. A 10% urea present in its solution, as an impurity, compels it to grow in one dimension (needle-like). Growth of crystals in different shapes is shown in the figure below.
vi) Transition Temperature “The temperature at which one crystalline form of a substance changes to another form is called transition temperature”. At this temperature the two crystalline forms of the same substance coexist in equilibrium with each other. For example, the transition temperatures of some substances are given below.
Q.6) Explain lattice energy by giving an example.
Answer: Lattice Energy
“The amount of energy released when one mole of an ionic crystal is formed from gaseous ions of opposite charges”.
The energy given in this case is in the negative value. It may also be defined as:
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“The energy required to break one mole of an ionic crystal into its constituent ions in the gas phase is called lattice energy”.
The sign of energy, in this case, is positive. It is expressed in kJmol-1. Example: The magnitude of lattice energy of NaCl is 787 kJmol-1. Na+ + Cl– → NaCl ΔHL.E = -787 kJmol-1 or, NaCl → Na+ + Cl– ΔHL.E = +787 kJmol-1
Q.7) How crystalline solids are classified on the basis of forces present between their particles. Discuss their properties briefly.
Answer: Types of Crystalline Solids Crystalline solids are classified into the following four types on basis of forces present between their particles.
Properties of Ionic crystals Ionic crystals have the following properties.
In ionic crystals, the cations and anions are closely packed into a compact specific geometrical shape and have strong ionic bond.
Ionic crystals are solid and usually very hard at room temperature. They never exist in liquid or gaseous form at ordinary temperature and pressure.
They have high melting points and heats of fusion (ΔHf).
They are usually non-conductors of heat and electricity.
They are brittle and are soluble in polar solvent like water.
Properties of Covalent crystals Covalent crystals have the following general properties.
Covalent crystals are very hard and cannot be cleaved easily.
They have high melting points and heats of fusion.
They are poor conductor of heat and electricity.
They are usually insoluble in polar solvents.
Properties of Metallic crystals General properties of metallic crystals are given below.
Metallic crystals have usually compact structure and the atoms are closely packed together.
They are hard and have high melting points. (Few of them are very soft and can be cut with a knife, e.g. Sodium).
They have metallic luster due to the presence of free electrons.
They are good conductor of heat and electricity.
They are malleable (can be transformed into sheets) and ductile (can be drawn into wires).
Properties of Molecular crystals Molecular crystals have the following general properties.
They are soft in nature because the forces of attractions present in these crystals are very weak van der Waals forces.
They have low melting points.
They are usually soluble in non-polar solvents.
Q.8) Compare the following pairs.
i. Polymorphism and Isomorphism. ii. Covalent Crystals and molecular Crystals. iii. Melting temperature and transition temperature. iv. Density of ice and density of liquid water.
Answer: i) Polymorphism and Isomorphism
The phenomena in which a certain crystalline compound exists in more than one crystalline form under different conditions is called polymorphism and the different crystalline forms are called polymorphs of each other.
The phenomena in which two or more different compounds exist in the same crystalline form, is called isomorphism and the compounds are called isomorphs of each other.
Polymorphs have the same chemical properties but different physical properties due to different arrangement of particles in their structure.
Physical and chemical properties of isomorphs are quite different from each other as they are totally different compounds of different nature.
For example KNO3 exists in two crystalline forms Rhombohedral and Orthorhombic which are polymorphs of KNO3.
For example, NaCl and MgO both have cubic crystalline structure due to same atomic ratio and are isomorphs of each other.
ii. Covalent Crystals and molecular Crystals.
Those crystals in which atoms are held together through covalent bonds are called covalent crystals.
In molecular crystals, the structural units (atoms or molecules) are held together by weak van der Waals forces.
Covalent crystals are hard and cannot be cleaved easily.
Molecular crystals are soft and break easily.
Covalent crystals have high melting points.
Molecular solids have low melting points.
Covalent crystals are usually poor conductor of heat and electricity.
Molecular crystals are also poor conductor of heat and electricity.
Covalent crystals are insoluble in polar solvents.
Molecular crystals are also insoluble in polar solvents are are usually soluble in non-polar solvents.
Some examples of covalent crystals are diamond, carborundum and quartz.
Examples of molecular crystals are ice, sugar and iodine.
iii. Melting temperature and transition temperature.
The melting temperature of a solid is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium.
The temperature at which one crystalline form of a substance changes to another form is called transition temperature. At this temperature the two crystalline forms of the same substance coexist in equilibrium with each other.
For example, NaCl changes its state from solid to liquid at 801oC which is its melting temperature.
Sulphur S8 changes its crystalline form from rhombic to monoclinic at 95.5oC which is its transition temperature.
iv. Density of ice and density of liquid water
Density of liquid water
Density of ice
In liquid water, the molecules are always in motion and hydrogen bonding cannot bound the atoms on their places.
In ice, the molecules have low kinetic energies and thus hydrogen bonding bounds the molecules on certain positions.
There are no empty spaces present because of the continuous random motion of the water molecules.
Many empty spaces are created because of hydrogen bonding and bond angles of the molecules.
Liquid water has high density because empty spaces are not present.
Ice has low density than water due to the presence of many empty spaces in it and it floats on liquid water.