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Bihar Board Class 12 Chemistry Subjective Questions with Answer

Bihar Board Class 12 Chemistry Subjective Questions with Answer Bihar Board Class 12 Chemistry Subjective Questions with Answer

Bihar Board Class 12 Chemistry Subjective Questions with Answer

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ZnO is white when cold, but yellow when hot. Explain.
Zinc oxide is white in colour but on heating, Oxygen leaves as O2 leaving behind Zn2+ and 2 electrons. Zn2+ and the 2 electrons move to the interstitial sites of the crystal . This provides excess electrons in the crystal lattice of ZnO. When light falls on these crystals then these electrons absorb a part of the light in the visible region and hence impart a yellow colour to the ZnO.
ZnO ---heating---> Zn2+ + 1/2O2 + 2e

How many tetrahedral and octahedral voids are there in closed packing of N spheres ?
There are two tetrahedral voids and only one octahedral void for each sphere. If the number of close packed spheres be N.
No. of tetrahedral voids = 2N
No. of octahedral voids = N

What is Henry's law of solubility ?
Henry's law is one of the gas laws formulated by William Henry in 1803 and states as "at constant temperature, partial pressure of a gas in the vapour phase (p) is proportional to the mole fraction of the gas (X) in the solution.
P ∝ X
Where P = partial pressure of gas
X = Mole fraction of gas in solution
or, P = kH.X
Where kH is called Henry's law constant.

Define Osmosis and Osmotic pressure.
Osmosis is a spontaneous process by which the solvent molecules pass from a solution of low concentration to a solution of high concentration through a semi-permeable membrane. It is of two types. They are Endosmosis and Exosmosis.

Osmotic Pressure is the pressure required to stop the process of osmosis. In other words, we can say that the pressure required to stop the spontaneous flow of solvent molecules from low concentrated solution to high concentrated solution through a semi-permeable membrane.
Osmotic pressure can be calculated using the equation-
π= C S T
where π is the osmotic pressure
C is the molar concentration of the solute
S is the Solution constant
T is the temperature

What is specific conductance ?
The resiprocal of specific resistance (ρ) is called specific conductance.
We know that-
R ∝ l/a
where, l is length of the wire and a is cross sectional area of the wire
or, R = ρ. l/a
or, 1/ρ = 1/R . l/a
or, κ = π . l/a
Unit of κ = ohm−1 . cm/cm2
ohm−1 . cm−1
Its S.I. unit is Sm−1
when, l = 1 and a = 1
then, κ = π
So, specific conductance is the conductance of a conductor of unit length and unit cross sectional area. For electrolytic solutions, the conductance of one cc of the solution is called its specific conductance.

What do you understand by molecularity of a reaction ?
The minimum number of reacting particles (molecules, atoms or ions) that come together or collide in a rate determining step to form product is called the molecularity of a reaction. Molecularity greater than three is generally rare because higher molecularity decreases the chance of their coming together and effective collision to product formation.
Decomposition of H2O2 takes place in the following two steps-
H2O2 → H2O + 1/2O2
Step 1: it is a slow step
H2O2 → H2O + O
Step 2: it is a fast step
O + O → O2
We know that the slowest step is rate-determining. So,from step-1 it is a unimolecular.

What do you understand by physical and chemical adsorption ?
If accumulation of gas on the surface of a solid occurs on account of weak van der Waals' forces, the adsorption is termed as physical adsorption or physisorption. When the gas molecules or atoms are held to the solid surface by chemical bonds, the adsorption is termed chemical adsorption or chemisorption. The chemical bonds may be covalent or ionic in nature. Physisorption is not specific in nature while chemisorption is highly specific in nature. Low temperature favour physisorption while high temperature favour chemisorption.

Write the name and chemical formula of two chief ores of Zinc.
The two chief ores of zinc are calamine ore and zinc blende.
The chemical compound present in calamine is zinc carbonate (carbonate ore) and its chemical formula is ZnCO3.
The chemical compound present in zinc blende is zinc sulphide (sulphide ore) and its chemical formula is ZnS.

Write the formula and structure of pyrophosphoric acid.
Pyrophosphoric acid, also known as diphosphoric acid and its molecular formula is H4P2O7 and structure is given below.
Pyrophosphoric acid

Write two uses of Ozone.
Two uses of ozone is given below-
1. Ozone is used for air purification at crowded places like cinema halls, tunnels, railways, etc.
2. In sterilizing drinking water by oxidising all germs and bacteria.

Arrange HF, HCl, HBr and HI in the increasing order of their acidic strength.
Increasing order of acid strength of the given acids is
HF < HCl < HBr < HI

What are preservatives of food ? Give examples.
Food preservatives are Chemical substances that stop or slow down the growth of bacteria in foods to prevent or delay to spoil the food are called food preservatives. Benzoates, sulphates, Propionates, Nitrates, Vegetable oil, Table salt and Sorbates are generally used as food preservative.

Write one use of the following:
(A) Paracetamol
(B) Tincture of iodine

Paracetamol is used to bring down the body temperature during high fever.
Tincture of iodine (2-3% solution of iodine in alcohol and water) is used as an antiseptic.

Amino acids are amphoteric. Explain.
Amino acids have both acidic (-COOH) and basic (-NH2) group in the same molecule. In the aqueous solution the acidic group losses proton while basic group accepts that proton consequently zwitter ion formed.
Zwitter ion
This zwitter ionic form of α amino acid shows amphoteric behaviour, i.e. act both as acid and base.

Write structural formulae of the following compounds :
(A) N-Ethylethanamine
(B) N, N- Dimethylmethanamine

Structural formulae of N-Ethylethanamine and N, N- Dimethylmethanamine are given below-
N-Ethylethanamine and N, N- Dimethylmethanamine

What is Hell Volhard Zelinsky (HVZ) reaction ?
Carboxylic acids react with chlorine or bromine in presence of small amount of red phosphorous to give alpha halo carboxylic acids. For Hell Volhard Zelinsky reaction molecule must have at least one α hydrogen atom.
Example- R-CH2-COOH ---P/Br2---> R-CHBr-COOH

Hell Volhard Zelinsky (HVZ) reaction

Write the name of two reagents which is capable of converting – C=O group to >CH2 group.
Clemmensen reduction and Wolf-Kishner Reduction are used to convert a carbonyl group(C = O) into -CH2 group.
In Clemmensen reduction, carbonyl group(C = O) converted into -CH2 group by Zinc amalgam in acidic medium while in Wolf-Kishner Reduction carbonyl group(C = O) converted into -CH2 group by Hydrazine in basic medium.

Write the names and formulae of two compounds which are obtained by the polymerisation of Acetaldehyde.
Acetaldehyde on polymerization gives paraldehyde which is trimer of acetaldehyde and metaldehyde which is tetramer of acetaldehyde. The molecular formula of paraldehyde is C6H12O3 and that of metaldehyde is C8H16O4.

What is power alcohol ? Write its uses ?
Power alcohol is a mixture of 20% absolute alcohol, 80% petrol and a small amount of benzene. It is used as a fuel in automobiles.

What is Markownikoff's rule ?
In unsymmetrical alkenes or alkynes, the positive part of the reagent goes to that multiple bonded carbon atom which has largest number of hydrogen atom(s).
CH3-CH = CH2 + HX → CH3-CH(X)-CH3

What happens when –
A. Chlorine gas reacts with NaOH ?
B. Nitric acid reacts with Zn or Cu at different concentrations ?

A. Cl2 gas react with hot and cold NaOH
a. With hot concentrated NaOH, Sodium chlorate and Sodium chloride forms.
Cl2 + NaOH → NaClO3 + 5NaCl + 3H2
b.With cold NaOH, Sodium hypochlorite and Sodium chloride forms.
Cl2 + 2NaOH → NaCl + NaOCl + H2O

B. Nitric acid reacts with Zn or Cu at different concentrations
Reaction of zinc with Nitric Acid-
Zinc reacts with dilute HNO3 to form Zinc nitrate and Nitrous oxide.
Zn + 10HNO3 → 4Zn(NO3)2 + N2O + 5H2O
Zinc reacts with concentration HNO3 to form Zinc nitrate and Nitrogen dioxide.
Zn+4HNO3 → Zn(NO3)2 + 2NO2 + 2H2O

Reaction of Copper with Nitric Acid-
Copper reacts with concentrated HNO3 to form copper nitrate and nitrogen dioxide.
Cu + 4HNO3 = Cu(NO3)2 + NO2 + H2O
Copper reacts with dilute HNO3 to form copper nitrate and nitric oxide.
3Cu + 8HNO3 = 3Cu(NO3)2 + 2NO + 4H2O

How does Zn react with H2SO4 ?
Zn reacts with both the dilute and concentrated sulphuric.
Zinc liberate hydrogen gas at room temperature when it reacts with dilute sulphuric acid
Zn + H2SO4 → ZnSO4 + H2
Sulphur dioxide along with zinc sulphate formed when Zn is heated with concentrated sulphuric acid.
Zn + 2H2SO4 → ZnSO4 + SO2 + 2H2O

How Acetic acid is obtained from Methyl cyanide ?
Acetic acid is obtained from methyl cyanide by hydrolysis.
CH3CN ---H+---> CH3COOH
Methyl Cyanide     Acetic Acid

Differentiate between Methanoic acid and Ethanoic acid
Methanoic acid and Ethanoic acid can be differentiate by these chemical tests-
A. Methanoic acid gives silver mirror test with Tollen's reagent , whereas ethanoic acid does not give that test.
HCOOH + 2[Ag(NH3)2]+ + 2OH → 2Ag↓ + 2H2O + CO2 + 4NH3
B. Methanoic acid gives white precipitate but not Ethanoic acid with mercuric chloride solution.
HCOOH + HgCl2 → Hg2Cl2 + CO2 + 2HCl

How will you separate 1°, 2° and 3° alcohol by Lucas test ?
Lucas reagent is the mixture of conc. HCl and ZnCl2.
R – OH + HCl ---ZnCl2---> R – Cl + H2O
The solution of 3° alcohol turns turbid and forms an oily layer immediately when Lucas reagent is added.
The solution of 2° alcohol turns turbid and forms an oily layer after five minutes when Lucas reagent is added.
The solution of 1° alcohol remains colourless unless it is subjected to heat. The solution forms an oily layer when heated.

What are the homogeneous and heterogeneous catalysts ? Explain with examples.
Homogenous Catalysis: Those catalysis in which both reactants and catalysts are in same state, are called homogeneous catalysis.
SO2(g) + O2(g) ---NO(g)---> 2SO3(g)
Heterogeneous Catalysis: Those catalysis in which of reactants and catalysts are in different states, are called heterogeneous catalysis.
N2(g) + 3H2(g) ---Fe(S)---> 2NH3(g)

Potassium permaganate is oxidising agent. Explain it with reactions.
Potassium permaganate is strong oxidising agent. It is acting as oxidising agent in acidic, basic and neutral medium.
Acidic medium-
2KMnO4 + 8H2SO4 + 10KI → 6K2SO4 + 2MnSO4 + 8H2O +5I2
Alkaline medium-
2KMnO4 + H2O + KI → KIO3 + 2KOH + 2MnO2
Neutral medium-
2KMnO4 + H2O → 2KOH + 2MnO2 + 3[O]

What is meant by peptization.
The process of converting a freshly prepared precipitate into colloidal form by the addition of a suitable electrolyte in small amount is called peptization. The electrolyte used in the process is called a peptizing agent.
When the ferric chloride is added to the precipitate of ferric hydroxide, the hydroxide precipitate transfer to the sol by absorbing ferric ions. In other words, Fe3+ is absorbed on Fe(OH)3 and splits into colloids.
Fe(OH)3 + Fe3+ → [Fe(OH)3]Fe+3

Nitrogen forms only NCl3 but phosphorus forms PCl3 and PCl5 both why ?
There is no vacant d-orbital in the outermost orbit of Nitrogen. Thus nitrogen show valency only three. There are valent d-orbitals in the outer most orbit of phosphorus and hence it shows variable valency, 3 in ground state and 5 in excited state. Hence nitrogen forms only NCl3 but phosphorus forms PCl3 and PCl5both.

How are vitamins classified ? Name the vitamin responsible for coagulation of blood.
Vitamins are classified into two groups depending upon their solubility in water and fat.
1. Water soluble vitamins – Vitamin B complex and vitamin C.
2. Fat soluble vitamins – Vitamin A, D, E and K.
Vitamin K is responsible for the coagulation of blood.

SO2 is an oxidising and reducing agent both. Explain.
Valence electron of sulphur is six. In SO2, sulphur is in +4 oxidation state, so it can lose its two more electrons to attain +6 oxidation state. It can also gain four electrons. Therefore it can lose and gain electron and acts as oxidising as well as reducing agent.

How can you separate O-Nitrophenol and P-Nitrophenol from the mixture ?
Boiling point of para-nitrophenol is greater than that of ortho-nitrophenol due to intermolecular and intramolecular hydrogen bond respectively. Hence ortho-nitrophenol and para-nitrophenol are separated by steam distillation process.

What are reducing sugars ?
Carbohydrates which can reduce Tollen’s reagent and give red precipitate with Fehling’s solution are called reducing sugar. All monosaccharides (both aldoses and ketoses) and disaccharides (except sucrose) are reducing sugars.

What is Salt Bridge ?
A salt bridge is a device used in an electrochemical cell for connecting its oxidation and reduction half cells.
The salt bridge usually consists of a strong electrolyte which is further made up of ions and maintains electrical neutrality within the internal circuit, preventing the cell from rapidly running its reaction to equilibrium.
There are two types of salt bridges that are used in electrochemical cells. They are- Glass Tube Bridge and Filter Paper Bridge.

What are primary cell ?
Write the chemical reaction taking place at anode and cathode in a zinc-carbon dry cell.
Or What are primary cells ? Describe dry cell in short.

In primary cell electrode reaction occurs only once and after some time cell becomes dead, that cell can not reused, or recharged, e.g. mercury cell, nickel cadmium cell, etc.
Dry cell consists of a zinc container which acts as the anode. The cathode is a carbon rod surrounded by a mixture of powdered manganese dioxide and carbon. The reaction occurring at anode and cathode are:
Anode reaction -
Zn(s) → Zn2+(aq) + 2e
Cathode reaction -
2MnO(s) + 2NH4+(aq) + 2e ⇌ Mn2O3 (s) + 2NH3(aq) + H2O(l)

What are secondary a cells ?
Or Why these are called storage cells ?
Or Write a short note on secondary batteries.

A secondary cells is capable of being charged after discharge again and again. A good secondary cell can undergo a large number of discharging and charging cycles. So we can say that a secondary cell is a rechargeable or reusable.
In charging cycle, the reaction that take place during discharge reversed. In this way, it can store the charge for a long period, hence is also called a storage cell. The most common example of a secondary cell is the lead storage battery.

Explain the following phenomenon with the help of Henry's law.
1. Painful condition known as bends.
2. Feeling of weakness and discomfort in breathing at high altitude.
3. Why soda water bottle kept at room temperature fizzes on opening ?

1. Henry's law represents a relation between solubility of gases in liquid and pressure. Scuba divers when comes towards surface, the pressure gradually decreases. This reduce pressure releases the dissolve gas present in blood and leads to formation of bubbles of nitrogen in the blood. This creates a painful condition by blocking capillaries known as blends.
2. At high altitude atmospheric pressure is low as compared to surface which causes difficulty in breathing. On that condition we feel weakness and discomfort.
3. Soda water bottle kept at room temperature fizzes on opening due to different pressure inside and outside the bottle. When the bottle is opened to air, the partial pressure of CO2 above the solution decreases. As a result, solubility decreases and hence CO, bubbles out.

Explain HF is a liquid, while HCl is a gas.
Intermolecular hydrogen bond is present between HF molecules, due to this they form bigger molecule by adding together, therefore it is in liquid state.

On mixing arsenous sulphide sol and ferric hydroxide sol both get precipitated. Why explain ?
On mixing two oppositely charged lyophobic sol in correct ratio both neutralise each other and get precipitated. This is known as natural coagulation. Thus, on mixing negatively charged arsenous sulphide and positively charged ferric hydroxide both collide get precipitated or coagulated.

Explain coagulation with an example.
The process of aggregation of colloidal particles to from insolube precipitate by addition of suitable electrolyte is called coagultion, e.g. Sulphur sol is negatively charged sol. When NaCl (an electrolyte) added to sulphur sol, then in carrying opposite charge present in sol, i.e. Na+ interact with sulphur sol cause neutralisation leading to their coagulation

What is Tyndall effect ? Why it occurs ?
When a strong converging beam of light is passed through a colloidal solution placed in a dark room, the path of beam gets illuminated with a bluish light, this phenomenon is known as Tyndall effect. This is due to reflection of light by big size particles.

How does the precipitation of colloidal smoke take place in cottrell precipitator ? In cottrell precipitator, smoke particles (charged) are passed through a chamber containing plates with charge opposite to the smoke particles. Smoke particles lose their charge on the plates and get precipitated.

Write down for the separation of mixture of helium and neon.
When the mixture of helium and neon comes in contact with charcoal at 93K, neon gets completely adsorbed, but helium gets free.

Electron affinity of noble or inert gases is zero. Why ?
Inert or noble gases are highly stable due to their completely filled valence shell and due to this, they do not have capacity to accept electron. Therefore, their electron affinity is zero.

Why do noble gases have comparatively large atomic sizes ?
Noble gases have comparatively large atomic sizes because they have van der Waals' radii only which are expected to have larger magnitude whereas other members of a period have either covalent or metallic radii which are less in magnitude.

Noble gases have higher values of ionisation potential. Explain ?
Ionisation potential of noble gases is higher because noble gases have stable and complete electronic configuration. Therefore, higher amount of energy is needed in remove an electron.

Why is N2 is less reactive at room temperature ?
In the nitrogen molecule (N2), two atoms of nitrogen are linked by triple bord (N = N). Due to small atomic size of the element (atomic radius = 70 pm), the bond dissociation enthalpy is very high (946 KJ per mol). This means that it is quite difficult to cleave or break the triple bond at room temperature. As a result, N2 is less reactive at room temperature. Oxygen forms O2 but sulphur forms S8, Explain.
Atomic size of oxygen is smaller so it can form σ– bond as well as π– bond between O and O atoms and exists as O2.
Atomic size of sulphur is bigger so it form σ– bond only and can not form π – bond together with s – bond between S and S atoms. To satisfied its valency, it forms S8 molecule in which all sulphur atoms bonded with only σ– bond and exist in crown shape.

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