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Equilibrium — Chemistry STD 11 Science — Question

Gujarat BoardEnglish MediumSTD 11 ScienceChemistryEquilibrium4 Marks
Question
Read the passage given below and answer the following questions from (i) to (v).
When a liquid evaporates in a closed container, molecules with relatively higher kinetic energy escape the liquid surface into the vapour phase and number of liquid molecules from the vapour phase strike the liquid surface and are retained in the liquid phase. It gives rise to a constant vapour pressure because of an equilibrium in which the number of molecules leaving the liquid equals the number returning to liquid from the vapour. We say that the system has reached equilibrium state at this stage. However, this is not static equilibrium and there is a lot of activity at the boundary between the liquid and the vapour. Thus, at equilibrium, the rate of evaporation is equal to the rate of condensation. It may be represented by
$\text{H}_2\text{O}_{(\text{l})}\rightleftharpoons\text{H}_2\text{O}_{(\text{vap})}$
The double half arrows indicate that the processes in both the directions are going on simultaneously. The mixture of reactants and products in the equilibrium state is called an equilibrium mixture.
Equilibrium can be established for both physical processes and chemical reactions. The reaction may be fast or slow depending on the experimental conditions and the nature of the reactants. When the reactants in a closed vessel at a particular temperature react to give products, the concentrations of the reactants keep on decreasing, while those of products keep on increasing for some time after which there is no change in the concentrations of either of the reactants or products. This stage of the system is the dynamic equilibrium
The chemical equilibrium may be classified in three groups.
  1. The reactions that proceed nearly to completion and only negligible concentrations of the reactants are left. In some cases, it may not be even possible to detect these experimentally.
  2. The reactions in which only small amounts of products are formed and most of the reactants remain unchanged at equilibrium stage.
  3. The reactions in which the concentrations of the reactants and products are comparable, when the system is in equilibrium.
The equilibrium involving ions in aqueous solutions which is called as ionic equilibrium.
Solid-Liquid Equilibrium Ice and water kept in a perfectly insulated thermos flask (no exchange of heat between its contents and the surroundings) at 273K and the atmospheric pressure are in equilibrium state and the system shows interesting characteristic features. We observe that the mass of ice and water do not change with time and the temperature remains constant. However, the equilibrium is not static. The intense activity can be noticed at the boundary between ice and water. Molecules from the liquid water collide against ice and adhere to it and some molecules of ice escape into liquid phase. There is no change of mass of ice and water, as the rates of transfer of molecules from ice into water and of reverse transfer from water into ice are equal at atmospheric pressure and 273 K. It is obvious that ice and water are in equilibrium only at particular temperature and pressure. For any pure substance at atmospheric pressure, the temperature at which the solid and liquid phases are at equilibrium is called the normal melting point or normal freezing point of the substance. The system here is in dynamic equilibrium and we can infer the following:
  1. Both the opposing processes occur simultaneously.
  2. Both the processes occur at the same rate so that the amount of ice and water remains constant.
Solid – Vapour Equilibrium Let us now consider the systems where solids sublime to vapour phase. If we place solid iodine in a closed vessel, after sometime the vessel gets filled up with violet vapour and the intensity of colour increases with time. After certain time the intensity of colour becomes constant and at this stage equilibrium is attained. Hence solid iodine sublimes to give iodine vapour and the iodine vapour condenses to give solid iodine. The equilibrium can be represented as,
$\text{l}_2(\text{solid})\rightleftharpoons\text{l}_2(\text{vapour})$
Other examples showing this kind of equilibrium are,
$\text{Camphor}_{(\text{solid})}\rightleftharpoons\text{Camphor}_{(\text{vapour})}$
$\text{NH}_4\text{CI}_{(\text{solid})}\rightleftharpoons\text{NH}_4\text{CI}_{(\text{vapour})}$
The equilibrium Involving Dissolution of Solid in Liquids Only a limited amount of salt or sugar can dissolves in a given amount of water at room temperature. If we make a thick sugar syrup solution by dissolving sugar at a higher temperature, sugar crystals separate out if we cool the syrup to the room temperature. We call it a saturated solution when no more of solute can be dissolved in it at a given temperature. The concentration of the solute in a saturated solution depends upon the temperature. In a saturated solution, a dynamic equilibrium exits between the solute molecules in the solid state and in the solution: Sugar (solution) Sugar (solid), and the rate of dissolution of sugar = rate of crystallisation of sugar. Equality of the two rates and dynamic nature of equilibrium has been confirmed with the help of radioactive sugar. If we drop some radioactive sugar into saturated solution of non-radioactive sugar, then after some time radioactivity is observed both in the solution and in the solid sugar. Initially there were no radioactive sugar molecules in the solution but due to dynamic nature of equilibrium, there is exchange between the radioactive and non-radioactive sugar molecules between the two phases. The ratio of the radioactive to non- radioactive molecules in the solution increases till it attains a constant value.
  1. Which of the following symbol represents equilibrium.
  1. $\rightleftharpoons$
  2. $\leftrightarrows$
  3. $\nLeftrightarrow$
  4. $\uparrow\downarrow$
  1. When there is no change in the concentrations of either of the reactants or products, this stage of the system is the …
  1. Static equilibrium
  2. Dynamic equilibrium
  3. Physical equilibrium
  4. Chemical equilibrium
  1. A … solution means no more of solute can be dissolved in it at a given temperature.
  1. Unsaturated
  2. Supersaturated
  3. Saturated
  4. None of these.
  1. The equilibrium involving ions in aqueous solutions which is called as …
  1. Static equilibrium
  2. Dynamic equilibrium
  3. Physical equilibrium
  4. Ionic equilibrium
  1. The concentration of the solute in a saturated solution depends upon the …
  1. Solvent
  2. Pressure
  3. Temperature
  4. System
✓

Answer

  1. (a) $\rightleftharpoons$
  1. (b) Dynamic equilibrium
  1. (c) Saturated
  1. (d) Ionic equilibrium
  1. (c) Temperature

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Read the passage given below and answer the following questions from (i) to (v).

The attractive force which holds variousconstituents (atoms, ions, etc.) together in differentchemical species is called a chemical bond. In order to explain the formation of chemicalbond in terms of electrons, a number ofattempts were made, but it was only in 1916 when Kössel and Lewis succeededindependently in giving a satisfactoryexplanation. They were the first to providesome logical explanation of valence which wasbased on the inertness of noble gases. Lewis postulated that atoms achieve thestable octet when they are linked bychemical bonds. In the formation of amolecule, only the outer shell electrons takepart in chemical combination and they areknown as valence electrons. The inner shellelectrons are well protected and are generallynot involved in the combination process.G.N. Lewis, an American chemist introducedsimple notations to represent valenceelectrons in an atom. These notations arecalled Lewis symbols. For example, the Lewissymbols for the elements of second period areas under:
The bond formed, as a result of theelectrostatic attraction between thepositive and negative ions was termed as the electrovalent bond. The electrovalenceis thus equal to the number of unitcharge(s) on the ion.
Kössel and Lewis in 1916 developed animportant theory of chemical combinationbetween atoms known as electronic theoryof chemical bonding. According to this,atoms can combine either by transfer ofvalence electrons from one atom to another(gaining or losing) or by sharing of valenceelectrons in order to have an octet in theirvalence shells. This is known as octet rule. when two atoms share oneelectron pair they are said to be joined bya single covalent bond. In many compoundswe have multiple bonds between atoms. Theformation of multiple bonds envisagessharing of more than one electron pairbetween two atoms. If two atoms share twopairs of electrons, the covalent bondbetween them is called a double bond. Forexample, in the carbon dioxide molecule, wehave two double bonds between the carbonand oxygen atoms. Similarly in ethenemolecule the two carbon atoms are joined bya double bond. The Lewis dot structures provide a pictureof bonding in molecules and ions in termsof the shared pairs of electrons and theoctet rule. The Lewis dotstructures can be written by adopting thefollowing steps:
- The total number of electrons required forwriting the structures are obtained byadding the valence electrons of thecombining atoms. For example, in the $CH _4$ molecule there are eight valence electronsavailable for bonding.
- For anions, each negative charge wouldmean addition of one electron. Forcations, each positive charge would result in subtraction of one electron from the totalnumber of valence electrons. For example,for the $CO _3{ }^{2-}$ ion, the two negative chargesindicate that there are two additionalelectrons than those provided by theneutral atoms.
- Knowing the chemical symbols of thecombining atoms and having knowledgeof the skeletal structure of the compound, it is easyto distribute the total number of electronsas bonding shared pairs between theatoms in proportion to the total bonds.
- In general the least electronegative atomoccupies the central position in themolecule/ion. For example in the $NF _3$ andCO ${ }_3{ }^{2-}$, nitrogen and carbon are the centralatoms whereas fluorine and oxygenoccupy the terminal positions.
- After accounting for the shared pairs ofelectrons for single bonds, the remainingelectron pairs are either utilized for multiplebonding or remain as the lone pairs. Thebasic requirement being that each bondedatom gets an octet of electrons.
i. ... postulated that atoms achieve the stable octet when they are linked by chemical bonds.
  1. … postulated that atoms achieve the stable octet when they are linked by chemical bonds.
  1. Lewis
  2. Debye
  3. Charles
  4. Sidgwick
  1. … in 1916 developed an important theory of chemical combination between atoms known as electronic theory of chemical bonding.
  1. Kössel
  2. Lewis
  3. Both a) & b)
  4. Sidgwick
  1. In the formation of a molecule, only the outer shell electrons take part in chemical combination and they are known as …
  1. Backscattered electrons
  2. Valence electrons
  3. Primary electrons
  4. Secondary electrons
  1. In the $CH_4$​​​​​​​ molecule there are … valence electrons available for bonding.
  1. 4
  2. 6
  3. 8
  4. 10
  1. The type of bond between atoms in a molecule of CO2 is:
  1. Ionic bond
  2. Metallic bond
  3. Hydrogen bond
  4. Covalent bond.
→
The existing large number of organic compounds and their ever-increasing numbers has made it necessary to classify them on the basis of their structures. Organic compounds are broadly classified as open-chain compounds which are also called aliphatic compounds. Aliphatic compounds further classified as homocyclic and heterocyclic compounds. Aromatic compounds are special types of compounds. Alicyclic compounds, aromatic compounds may also have heteroatom in the ring. Such compounds are called heterocyclic aromatic compounds. Organic compounds can also be classified on the basis of functional groups, into families or homologous series. The members of a homologous series can be represented by general molecular formula and the successive members differ from each other in a molecular formula by a $- CH _2$ unit.

1. The successive members of a homologous series differ by which mass of amu?
OR
Is tetrahydrofuran is aromatic compounds?
2. Does Pyridine, pyrrole, thiophene are all heteroaromatic compounds
3. Difference between heterocyclic and homocyclic compound.
→
Read the passage given below and answer the following questions from (i) to (v).
The presence of positive charge on thenucleus is due to the protons in the nucleus.As established earlier, the charge on the proton is equal but opposite to that of electron.Atomic number $(Z)=$ number of protons inthe nucleus of an atom = number of electrons in a nuetral atom. protons and neutrons present in thenucleus are collectively known as nucleons. The total number of nucleons is termed asmass number $(A)$ of the atom.
mass number $(A)=$ number of protons $(Z)+$ number of neutrons $( n )$.
Isobars are the atoms with same massnumber but different atomic number forexample, ${ }_6^4 C$ and ${ }_7^{14} N$. On the other hand, atomswith identical atomic number but differentatomic mass number are known as Isotopes. For example, considering of hydrogen atom again, $99.985 \%$ of hydrogen atoms contain only one proton.This isotope is called protium $\left(1^1 H \right)$. Rest of thepercentage of hydrogen atom contains two otherisotopes, the one containing 1 proton and 1 neutron is called deuterium ( ${ }^2{ }_1 D , 0.015 \%$ )and the other one possessing 1 proton and 2 neutrons is called tritium ( ${ }^3 T$ )..the studies of interactions of radiations with matter haveprovided immense information regarding thestructure of atoms and molecules. Neils Bohrutilised these results to improve upon themodel proposed by Rutherford. Twodevelopments played a major role in theformulation of Bohr's model of atom. Thesewere:
1. Dual character of the electromagneticradiation which means that radiations possess both wave like and particle likeproperties, and
2. Experimental results regarding atomicspectra.

James Maxwell (1870) was the first to givea comprehensive explanation about theinteraction between the charged bodies andthe behaviour of electrical and magnetic fieldson macroscopic level. He suggested that whenelectrically charged particle moves underaccelaration, alternating electrical and magnetic fields are produced and transmitted.These fields are transmitted in the forms ofwaves called electromagnetic waves orelectromagnetic radiation.radiations are characterised by theproperties, namely, frequency $(v)$ and wavelength $(\lambda)$.The SI unit for frequency $(v)$ is hertz $\left( Hz , s ^{-1}\right)$, after Heinrich Hertz. It is defined asthe number of waves that pass a given pointin one second. Wavelength should have the units of lengthand as you know that the SI units of length ismeter ( m ). Since electromagnetic radiationconsists of different kinds of waves of muchsmaller wavelengths, smaller units are used.In vaccum all types of electromagneticradiations, regardless of wavelength, travel atthe same speed, i.e., $3.0 \times 10^8 m s ^{-1}$ ( $2.997925 \times 10^8 ms^{-1}$, to be precise). This is called speedof light and is given the symbol ' c '. Thefrequency $( V )$, wavelength $(\lambda)$ and velocity of light(c) are related by the following equation.
$c=v \lambda$
The other commonly used quantityspecially in spectroscopy, is the wavenumber.It is defined as the number of wavelengthsper unit length. Its units are reciprocal ofwavelength unit, i.e., $m^{–1}$​​​​​​​. However commonlyused unit is $cm^{–1}​​​​​​​$​​​​​​​
  1. The presence of positive charge on the nucleus is due to the …. in the nucleus.
  1. Protons
  2. Neutrons
  3. Electron
  4. Nucleons
  1. Atomic Number is denoted by:
  1. $A$
  2. $Z$
  3. $N$
  4. $M$
  1. Atomic Mass number is denoted by:
  1. $M$
  2. $Z$
  3. $N$
  4. $A$
  1. … are the atoms with same mass number but different atomic number.
  1. Isotopes
  2. Allotropes
  3. Isobars
  4. None of above
  1. Atoms with identical atomic number but different atomic mass number are known as ..
  1. Isotopes
  2. Allotropes
  3. Isobars
  4. None of above
→
The molecular orbital theory is based on the principle of a linear combination of atomic orbitals. According to this approach when atomic orbitals of the atoms come closer, they undergo constructive interference as well as destructive interference giving molecular orbitals, i.e., two atomic orbitals overlap to form two molecular orbitals, one of which lies at a lower energy level (bonding molecular orbital). Each molecular orbital can hold one or two electrons in accordance with Pauli's exclusion principle and Hund's rule of maximum multiplicity. For molecules up to $N _2$, the order of filling of orbitals is:
Image
Bond order $=\frac{1}{2}$ [bonding electrons - antibonding electrons]
Bond order gives the following information:
i. If bond order is greater than zero, the molecule/ion exists otherwise not.
ii. Higher the bond order, higher is the bond dissociation energy.
iii. Higher the bond order, greater is the bond stability.
iv. Higher the bond order, shorter is the bond length.

1. Arrange the following negative stabilities of $CN , CN ^{+}$and $CN ^{-}$in increasing order of bond.
2. The molecular orbital theory is preferred over valence bond theory. Why?
3. Ethyne is acidic in nature in comparison to ethene and ethane. Why is it so?
OR
Bonding molecular orbital is lowered by a greater amount of energy than the amount by which antibonding molecular orbital is raised. Is this statement correct?
→
Covalent molecules formed by heteroatoms bound to have some ionic character. The ionic character is due to shifting of the electron pair towards A or B in the molecule AB . Hence, atoms acquire small and equal charge but opposite in sign. Such a bond which has some ionic character is described as a polar covalent bond. Polar covalent molecules can exhibit a dipole moment. The dipole moment is equal to the product of charge separation, q and the bond length, d for the bond. The unit of dipole moment is Debye. One Debye is equal to $10^{-18}$ esu cm.
The dipole moment is a vector quantity. It has both magnitude and direction. Hence, the dipole moment of molecules depends upon the relative orientation of the bond dipole, but not the polarity of bonds alone. The symmetrical structure shows a zero dipole moment. Thus, a dipole moment help to predict the geometry of the molecules. Dipole moment values can be used to distinguish between cis- and trans-isomers; ortho-, meta- and para-forms of a substance, etc. The percentage of ionic character of a bond can be calculated by the application of the following formula:
$
\% \text { ionic character }=\frac{\text { Experimental value dipole moment }}{\text { Theoretical value of dipole moment }} \times 100
$
Image
ii. A diatomic molecule has a dipole moment of 1.2 D . If the bond length is $1.0 \times 10^{-8} cm$, what fraction of charge does exist on each atom? (1)
iii. The dipole moment of $NF _3$ is very much less that of $NH _3$. Why? (2)
OR
A covalent molecule, $x-y$, is found to have a dipole moment of $1.5 \times 10^{-29} cm$ and a bond length 150 pm . What will be the percentage of ionic character of the bond? (2)
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Read the passage given below and answer the following questions from $1$ to $5$. Lithium metal is used to make useful alloys, for example with lead to make ‘white metal’ Bearings for motor engines, with aluminium to make aircraft parts, and with magnesium to make armour plates. It is used in Thermonuclear reactions. Lithium is also used to make electrochemical cells. Sodium is used To make a Na/Pb alloy needed to make $PbEt_4$​​​​​​​ and $PbMe_4​​​​​​​$​​​​​​​. These organolead compounds were Earlier used as anti-knock additives to petrol, But nowadays vehicles use lead-free petrol. Liquid sodium metal is used as a coolant in Fast breeder nuclear reactors. Potassium has a vital role in biological systems. Potassium Chloride is used as a fertilizer. Potassium Hydroxide is used in the manufacture of soft Soap. It is also used as an excellent absorbent of carbon dioxide. Caesium is used in devising Photoelectric cells. Points of Difference between Lithium and other Alkali Metals –
i) Lithium is much harder. Its m.p. and b.p. are higher than the other alkali metals.
ii) Lithium is least reactive but the strongest Reducing agent among all the alkali metals. On combustion in air it forms mainly Monoxide, $Li_2O$ and the nitride, $Li_3N$ unlike Other alkali metals.
 iii) LiCl is deliquescent and crystallises as a Hydrate, LiCl.$2H_2O$ whereas other alkali Metal chlorides do not form hydrates.
iv) Lithium hydrogencarbonate is not Obtained in the solid form while all other Elements form solid hydrogencarbonate.
v) Lithium unlike other alkali metals forms No ethynide on reaction with ethyne.
vi) Lithium nitrate when heated gives lithium Oxide, $Li_2O$, whereas other alkali metal Nitrates decompose to give the Corresponding nitrite. $4\text{LiNO}_3\rightarrow2\text{L}\text{i}_2\text{O}+4\text{NO}_2+\text{O}_2$ $2\text{NaNO}_3\rightarrow2\text{NaNO}_2+\text{O}_2$ vii) LiF and $Li_2O$ are comparatively much less Soluble in water than the corresponding Compounds of other alkali metals. Sodium carbonate is generally prepared by Solvay Process. In this process, advantage is Taken of the low solubility of sodium Hydrogencarbonate whereby it gets Precipitated in the reaction of sodium chloride with ammonium hydrogencarbonate. The Latter is prepared by passing $CO_2​​​​​​​$​​​​​​​ to a Concentrated solution of sodium chloride Saturated with ammonia, where ammonium Carbonate followed by ammonium Hydrogencarbonate are formed. The equations For the complete process may be written as $2\text{NH}_3+\text{H}_2\text{O}+\text{CO}_2\rightarrow{\text{(NH}_4)_2}\text{CO}_3$ $(\text{NH}_4)_2\text{CO}_3+\text{H}_2\text{O}+\text{CO}_2\rightarrow2\text{NH}_4\text{HCO}_3$ $\text{NH}_4\text{HCO}_3+\text{NaCl}\rightarrow\text{NH}_4\text{Cl}+\text{NaHCO}_3$ Sodium hydrogencarbonate crystal Separates. these are heated to give sodium Carbonate. The most abundant source of sodium chloride is sea water which contains $2.7$ to $2.9 \%$ by Mass of the salt. In tropical countries like India, Common salt is generally obtained by Evaporation of sea water. Approximately $50$ Lakh tons of salt are produced annually in India by solar evaporation. Crude sodium Chloride, generally obtained by crystallisation Of brine solution, contains sodium sulphate, Calcium sulphate, calcium chloride and Magnesium chloride as impurities. Calcium Chloride, $CaCl_2$​​​​​​​, and magnesium chloride, $MgCl_2$​​​​​​​ are impurities because they are Deliquescent (absorb moisture easily from the Atmosphere). To obtain pure sodium chloride, The crude salt is dissolved in minimum amount Of water and filtered to remove insoluble Impurities. The solution is then saturated with Hydrogen chloride gas. Crystals of pure Sodium chloride separate out. Calcium and Magnesium chloride, being more soluble than Sodium chloride, remain in solution. Sodium Hydroxide (Caustic Soda), NaOH is generally prepared Commercially by the electrolysis of sodium Chloride in Castner-Kellner cell. A brine Solution is electrolysed using a mercury Cathode and a carbon anode. Sodium metal Discharged at the cathode combines with Mercury to form sodium amalgam. Chlorine Gas is evolved at the anode. Cathod: $\text{Na}^++\bar{\text{e}}\xrightarrow{\text{Hg}}\text{Na}-\text{amalgam}$ Anode: $\text{Cl}^-\rightarrow\frac{1}{2}\text{Cl}_2+\text{e}^-$ The amalgam is treated with water to give Sodium hydroxxide and hydrogen gas. $2$ Na - amalgam $+ 2H_2O \rightarrow 2NaOH + 2Hg + H_2$​​​​​​​
  1. NaOH Sodium hydroxide is generally prepared Commercially by the electrolysis of … in Castner-Kellner cell.
  1. $NaCl$
  2. $Na_2CO_3$
  3. $NaHCO_3$
  4. $NaNH_2$
  1. … is used in the manufacture of soft Soap.
  1. Sodium Hydroxide
  2. Potassium Hydroxide
  3. Aluminium hydroxide
  4. Beryllium hydroxide
  1. … is used in devising Photoelectric cells.
  1. Hydrogen
  2. Lithium
  3. Caesium
  4. Helium
  1. … compounds were Earlier used as anti-knock additives to petrol.
  1. Organomagnesium
  2. Organosilicon
  3. Organochloride
  4. Organolead
  1. The sodium amalgam is treated with water to gives ….
  1. $NaOH$
  2. $Na_2CO_3$
  3. $NaHCO_3$
  4. $NaNH_2$​​​​​​​
→
Read the passage given below and answer the following questions from (i) to (vi).
The atomic theory of matter was first proposed on afirm scientific basis by JohnDalton, a British schoolteacher in 1808. His theory, called Dalton’s atomictheory, regarded the atom as the ultimate particle ofmatter Dalton’s atomic theory was able to explainthe law of conservation of mass, law of constantcomposition and law of multiple proportion verysuccessfully. However, it failed to explain the results ofmany experiments.In mid 1850s many scientists mainlyFaraday began to study electrical dischargein partially evacuated tubes, known ascathode ray discharge tubes.Electrical discharge carried out in the modifiedcathode ray tube led to the discovery of canalrays carrying positively charged particles. Thecharacteristics of these positively chargedparticles are listed below.
  1. Unlike cathode rays, mass of positivelycharged particles depends upon thenature of gas present in the cathode raytube. These are simply the positivelycharged gaseous ions.
  2. The charge to mass ratio of the particlesdepends on the gas from which theseoriginate.
  3. Some of the positively charged particlescarry a multiple of the fundamental unitof electrical charge.
  4. The behaviour of these particles in themagnetic or electrical field is opposite tothat observed for electron or cathoderays.
The smallest and lightest positive ion wasobtained from hydrogen and was called
proton. This positively charged particle wascharacterised in 1919. Later, a need was feltfor the presence of electrically neutral particleas one of the constituent of atom. Theseparticles were discovered by Chadwick (1932)by bombarding a thin sheet of beryllium byα-particles. When electrically neutral particleshaving a mass slightly greater than that ofprotons were emitted. He named theseparticles as neutrons.J. J. Thomson, in 1898, proposed that an atom possesses a spherical shape (radiusapproximately 10–10 m) in which the positivecharge is uniformly distributed. The electronsare embedded into it in such a manner as togive the most stable electrostatic arrangementMany different names are given tothis model, for example, plum pudding, raisinpudding or watermelon. This model can be visualised as a pudding or watermelon ofpositive charge with plums or seeds (electrons)embedded into it. An important feature of thismodel is that the mass of the atom is assumed to be uniformly distributed over theatom.Rutherford and his students (Hans Geiger andErnest Marsden) bombarded very thin gold foilwith α–particles. Rutherford’s famous α–particle scattering experiment.The observations of Scattering experiment are as follows-:
  1. most of the α–particles passed throughthe gold foil undeflected.
  2. a small fraction of the α–particles wasdeflected by small angles.
  3. a very few α–particles (∼1 in 20,000)bounced back, that is, were deflected bynearly 180°.
On the basis of observations andconclusions from this experiment, Rutherford proposed the nuclearmodel of atom. According to this model:
  1. The positive charge and most of the massof the atom was densely concentrated inextremely small region. This very smallportion of the atom was called nucleusby Rutherford.
  2. The nucleus is surrounded by electronsthat move around the nucleus with a veryhigh speed in circular paths called orbits.Thus, Rutherford’s model of atomresembles the solar system in which thenucleus plays the role of sun and theelectrons that of revolving planets.
  3. Electrons and the nucleus are held together by electrostatic forces of attraction.
  1. The atomic theory of matter was first proposed on afirm scientific basis by:
  1. John Dalton
  2. Ernest Rutherford
  3. J.Thomson
  4. Henry Moseley
  1. The cathode rays start from … and move towards the….
  1. Anode, Cathode
  2. Centre, Anode
  3. Cathod, Anode
  4. Cathod, Centre
  1. Negativelycharged particles in atoms, called…
  1. Protons
  2. Electrons
  3. Neutron
  4. Positron
  1. The smallest and lightest positive ion wasobtained from …. and was called proton.
  1. Oxygen
  2. Nitrogen
  3. Carbon
  4. Hydrogen
  1. Electrically neutral particles having a mass slightly greater than that of protons, these particles termed as:
  1. Protons
  2. Electrons
  3. Neutron
  4. Positron
  1. J.J. Thomson’s atomic model is also named as:
  1. Plum pudding
  2. Raisin pudding
  3. Watermelon
  4. All the above
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Read the passage given below and answer the following questions from (i) to (v).
Chemical properties of a substance do not change withthe change of its physical state; but rate of chemicalreactions do depend upon the physical state. Many timesin calculations while dealing with data of experiments werequire knowledge of the state of matter. Therefore, itbecomes necessary for a chemist to know the physical laws which govern the behaviour of matter indifferent states. Intermolecular forces are the forces ofattraction and repulsion between interactingparticles (atoms and molecules). This termdoes not include the electrostatic forces thatexist between the two oppositely charged ionsand the forces that hold atoms of a moleculetogether i.e., covalent bonds.Attractive intermolecular forces are knownas van der Waals forces, in honour of Dutchscientist Johannes van der Waals (1837-1923) . van der Waals forces vary considerablyin magnitude and include dispersion forcesor London forces, dipole-dipole forces, anddipole-induced dipole forces. A particularlystrong type of dipole-dipole interaction ishydrogen bonding. Only a few elements canparticipate in hydrogen bond formation, therefore it is treated as a separatecategory.
Atoms and nonpolar molecules are electricallysymmetrical and have no dipole momentbecause their electronic charge cloud issymmetrically distributed. But a dipole maydevelop momentarily even in such atoms andmolecules. The temporary dipoles of two different atomattract each other. Similarly temporary dipolesare induced in molecules also. This force ofattraction was first proposed by the Germanphysicist Fritz London, and for this reasonforce of attraction between two temporary dipoles is known as London force. dispersion force forces are always attractive and interactionenergy is inversely proportional to the sixthpower of the distance between two interactingparticles (i.e.,$1/r ^6$ where r is the distancebetween two particles). These forces areimportant only at short distances (~500 pm)and their magnitude depends on thepolarisability of the particle.
Dipole-dipole forces act between the moleculespossessing permanent dipole. Ends of thedipoles possess “partial charges” and thesecharges are shown by Greek letter delta (δ).Partial charges are always less than the unitelectronic charge $(1.6\times 10^{–19} C)$. The polarmolecules interact with neighbouringmolecules. This interactionis stronger than the London forces but isweaker than ion-ion interaction because onlypartial charges are involved. The attractiveforce decreases with the increase of distancebetween the dipoles. As in the above case herealso, the interaction energy is inverselyproportional to distance between polarmolecules. Dipole-dipole interaction energybetween stationary polar molecules is proportional to $1/r^3$ and thatbetween rotating polar molecules is proportional to $1/r ^6$​​​​​​​, where r is the distancebetween polar molecules.
Dipole–Induced Dipole Forcesare type of attractive forces operate betweenthe polar molecules having permanent dipoleand the molecules lacking permanent dipole.Permanent dipole of the polar moleculeinduces dipole on the electrically neutralmolecule by deforming its electronic cloud. Thus an induced dipole is developedin the other molecule. In this case alsointeraction energy is proportional to $1/r ^6$​​​​​​​​​​​​​​ where r is the distance between twomolecules. Induced dipole moment dependsupon the dipole moment present in thepermanent dipole and the polarisability of theelectrically neutral molecule.
  1. Partial charges are always less than the unit electronic charge:
  1. $(1.6\times 10^{–19} C)$
  2. $(1.6\times 10^{–18} C)$
  3. $(1.6\times 10^{–17}C)$
  4. $(1.6\times 10^{–16} C)$
  1. Temporary dipoles are induced in molecules also. ,this force of attraction was first proposed by:
  1. Johannes van der Waals
  2. Fritz London
  3. Robert Boyle
  4. Joseph Lewis Gay Lussac
  1. Atoms and nonpolar molecules are electrically:
  1. Compositional
  2. Unsymmetrical
  3. Symmetrical
  4. All the above
  1. Partial Charges denoted by greek letter ….
  1. $\in$
  2. $\zeta$
  3. $\delta$
  4. $\eta$
  1. The attractive force … with the … of distance between the dipoles.
  1. Increase, increase
  2. Decrease, decrease
  3. Increase, decrease
  4. Decreases, increase
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Once an organic compound is extracted from a natural source or synthesised in the laboratory, it is essential to purify it. Various methods used for the purification of organic compounds are based on the nature of the compound and the impurity present in it. Finally, the purity of a compound is ascertained by determining its melting or boiling point. This is one of the most commonly used techniques for the purification of solid organic compounds. In crystallisation Impurities, which impart colour to the solution are removed by adsorbing over activated charcoal. In distillation Liquids having different boiling points vaporise at different temperatures. The vapours are cooled and the liquids so formed are collected separately. Steam Distillation is applied to separate substances which are steam volatile and are immiscible with water. Distillation under reduced pressure: This method is used to purify liquids having very high boiling points.

1. Which method can be used to separate two compounds with different solubilities in a solvent?
OR
Why chloroform and aniline are easily separated by the technique of distillation?
2. Distillation method is used to separate which type of substance?
3. Which technique is used to separate aniline from aniline water mixture?
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When anions and cations approach each other, the valence shell of anions are pulled towards the cation nucleus and thus, the shape of the anion is deformed. The phenomenon of deformation of anion by a cation is known as polarization and the ability of the cation to polarize the anion is called as polarizing power of cation. Due to polarization, sharing of electrons occurs between two ions to some extent and the bond shows some covalent character.
The magnitude of polarization depends upon a number of factors.

1. Out of $AlCl _3$ and $AlI _3$ which halides show maximum polarization?
2. Out of $AlCl _3$ and $CaCl _2$ which one is more covalent in nature?
3. The non-aqueous solvent like ether is added to the mixture of $LiCl , NaCl$ and KCl . Which will be extracted into the ether?
OR
Out of $CaF _2$ and $CaI _2$ which one has a minimum melting point?
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