1st PUC Chemistry Model Question Paper 4 with Answers

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Karnataka 1st PUC Chemistry Model Question Paper 4 with Answers

Time: 3.15 Hours
Max Marks: 100

Instructions:

1. The question paper has four parts A, B, C and D, All parts are compulsory.
2. Write balanced chemical equations and draw labelled diagram wherever required.
3. Use log tables and simple calculators if necessary (Use of scientific calculators is not allowed].

Part -A 

I. Answer ALL of the following (each question carries one mark): ( 10 × 1 = 10 )

Question 1.
Express 0.00025 in scientific notation.
Answer:
2.5 x 10^-4

Question 2.
How does viscosity varies with temperature?
Answer:
Viscosity’ decrease in the temperature because intermolecular force of attraction decreases.

Question 3.
Write the solubility product expression for BaSO₄.
Answer:
Let the solubility of BaSO₄ be S mol dm^-3
BaSO_4(s) ⇌ Ba_(aq)²⁺ +SO_4(aq)^2- ; [Ba²⁺ ] = Smol dm^-3 and [SO₄^2-] = S mol dm^-3
∴ K_sp =[Ba²⁺].[SO₄^2-] = S.S = S²

Question 4.
What would be the IUPAC name for the element with atomic number 104?
Answer:
Unnilquadium.

Question 5.
Define oxidation in terms of electron transfer.
Answer:
Loss of electron(s) by a species.

Question 6.
Which alkali metal gives golden yellow colour to the flame?
Answer:
Sodium → yellow or golden yellow.

Question 7.
Write the molecular formula of Borax.
Answer:
Na₂B₄O₇.10H₂O or boric acid / diborane.

Question 8.
What is dry ice?
Answer:
Solid CO₂ is called dry ice. It is called dry ice because it directly changes into gaseous state without becoming liquid.

Question 9.
Suggest a method to separate a mixture of chloroform and aniline.
Answer:
Distillation is a process of conversion of a liquid into vapours by heating followed by condensation of vapours produced by cooling. The method is used for the purification of liquids which boil without decomposition and are associated with non-volatile impurities. The impure liquid is boiled and the vapours, this formed are condensed to get the pure liquid. Ex: A mixture of chloroform (BP: 334 K) and aniline (BP: 456 K) can easily separated by the techniques.

Question 10.
Name the catalyst used in Friedel-crafts reaction.
Answer:
Anhydrous AlCl₃

Part – B

II. Answer any FIVE of the following questions carrying TWO marks: ( 5 x 2 = 10 )

Question 11.
Mention any two postulates of Daltons atomic theory.
Answer:
The postulates are as follows:

1. Matter is made up of small, indivisible particles called atoms.
2. Atoms of given element are identical in all respects, i.e. having same size, shape, mass and properties.

Question 12.
(a) State Gay-Lussac’s law.
Answer:
It states that “pressure of a fixed mass of a gas at constant
volume is directly proportional to the temperature on kelvin Pressure scale”. Mathematically, p ∝ T (At constant). P = K₃T or \(\frac{\mathrm{P}}{\mathrm{T}}\) = K₃ a constant, if volume is constant.

The equation, P = K₃T Ig of the type y = mx. Hence a graph of pressure along y-axis and temperature (kelvin) along x-axis is a straight line passing through the origin.
It slope represents K₃. The law can also be written as \(\frac{P_{1}}{T_{1}}=\frac{P_{2}}{T_{2}}\) If volume is constant.

(b) Write Vander Waal’s equation for ‘n’ mole of a gas.
Answer:
\(\left(P+\frac{a n^{2}}{V^{2}}\right)\) (V-nb) = nRT

Question 13.
Give any two differences between Sigma bond and Pi bond.
Answer:
σ bond :

1. Overlapping is along the axis
2. Overlapping is maximum
3. Due to maximum overlap the bond is strong
4. s-orbital can participate

π bond :

1. Overlapping is on side wise
2. Overlapping is minimum
3. Due to minimum overlap, the band is weak
4. s-orbital cannot participate

Question 14.
Complete the following reactions:
(a) 2(CaSO₄.2H₂O) → ………. + 3H₂
Answer:

(b)

Answer:

Question 15.
How to obtain diborane from brorontrifluoride? Write the equation.
Answer:
Diborane is prepared in the laboratory by treating borontrifluoride with LiAlH₄ in diethyl ether.
4BF_3(g) + 3LiAIH_4(s) → B₂H₆, + 3L1F_(s)

Question 16.
Give the IUPAC name of the major product formed when HBr is added to propene. Write its equation.
Answer:
The mechanism of addition of hydrogen bromide to propene takes place in three steps.

Step 1: Hydrogen bromide dissociates into H⁺ and Br^–
H – Br → H⁺ + Br^–

Step 2: The electrophile H+ attacks propene to form carbocation.

of the two carbocations (I) and (II), carbocations (I) is more stable and is formed more readily.

Step 3: The nucleophile Br attacks the carbocation (I) to give 2 – bromopropane.

Question 17.
Explain Wurtz reaction with an example.
Answer:
When alkyl halides are heated with sodium metal in ether medium higher alkanes are formed. This reaction is known as Wurtz reaction and employed for the synthesis of higher alkanes containing even number of carbon atoms.

Question 18.
Write any two common chemicals of photochemical smog.
Answer:
It is of two types: (i) Classical smog: It occurs in cool humid climate and is the result of build-up of sulphur dioxides and particulate matter from fuel combustion. It is reducing in nature, (ii) Photochemical smog: It occurs in warm, dry and sunny climate and results from the action of sunlight on the nitrogen oxides and hydrocarbons produced by automobiles and factories; it is oxidizing in nature.

Part – C

III. Answer any FIVE of the following questions carrying THREE marks: ( 5 x 3 = 15 )

Question 19.
(a) Define ionisation enthalpy. How does it vary down the group in the periodic table?
Answer:
The amount of energy required to remove the most loosely bound electron from an isolated gaseous atom. Ionization energy increases along the period and decreases down the group,

(b) Among Al⁺³ and Na⁺ ion, which one has smaller ionic radius?
Answer:
Al³⁺ is smallest because it has highest number of protons (13) among Na⁺ Mg^2- and Al³⁺ ions, due to which effective nuclear charge is maximum.

Question 20.
(a) Mention any two factors which influence the formation of ionic bond.
Answer:

1. Lower ionisation energy of metals.
2. Higher electron affinity of non-metal

(b) Name the type of hydrogen bond present in water.
Answer:
Intermodular hydrogen bond: Electrostatic force of attraction that exists between hydrogen atom of one molecule and electronegative atom of another molecule is called intermodular hydrogen atom.
H₂O:

Question 21.
(a) With respect to the formation of ethene molecule mention:
(i) hybridisation of carbon (ii) number of sigma bonds in the molecule.
Answer:
The molecular formula of ethene is C₂H₄
Electronic configuration of C is ground state – 1s²2s²2p²
Electronic configuration of C is excited state – 1s²2s¹2p³
Valence orbital representation

sp² hybridization involves 2 carbon atoms combines with 4 atoms of hydrogen.
In ethene molecule, there is 5 sigma and 1 pi bond. The angle of sp² hybridized ethane is 120°.

(b) Write Lewis dot structure of nitrogen molecule.
Answer:

Question 22.
Write the molecular orbital electronic configuration for oxygen molecule. Calculate its bond order and comment on its magnetic property.
Answer:
Atomic number of oxygen is 8: 1s² 2s², 2P⁴

(a) Bond order = \(\frac{8-4}{2}\) = 2
(b) Electronic configuration of oxygen molecule is
KKσ2s² σ*2s² σ2p_z² π2p_x¹ π2p²_y π*2p¹_x π*2p¹_y
(c) Magnetic property: Paramagnetic.

Question 23.
Balance the following redox reaction by using half reaction method.
Fe²⁺_(aq) + Cr₂O₇^2-_(aq) + Fe³⁺_(aq) + Cr³⁺_(aq)
Step 1.
(LEO = Loss of Electron Oxidation )

Step 2.
Fe²⁺ → Fe³⁺ …..(1) (2 → 3 = 1 unit O-LN)
Cr₂O₇^2- → 2Cr³⁺… (2) (+6 → +3 = 3 units 2 sets ⇒ 6units ON)

Step 3: Cross multiplication i.e., (1) x 6 and (2) x 1
(1) x 6 → 6Fe²⁺ → 6Fe³⁺ ….. (3)
(2) x 1 → Cr₂O₇^2- → 2Cr³⁺…… (4)

Step 4: Add (3) + (4) → 6Fe²⁺ + CrO₇^2- → 6Fe³⁺ + 2Cr³⁺

Step 5: Balance oxygen and then hydrogen
6Fe²⁺ + CrO₇^2- + 14H⁺ → 6Fe³⁺ + 2Cr³⁺ + 7H₂O.

Question 24.
(a) How does dihydrogen prepared in the laboratory? Write the equation.
Answer:
In the laboratory, dihydrogen is prepared by the treatment of zinc granules with dil. H₂SO₄.
Zn + H₂SO₄(dil) → ZnSO₄ + H ₂↑

(b) Give an example for saline hydride.
Answer:
These contain positive metal ions and negative hydride ions. These are also called saline hydrides. Ex: (Na⁺H^–) sodium hydride, [Ca²⁺(H^–)₂] calcium hydride.

Question 25.
Write the equations during the preparation of sodium carbonate by “Solvay’s Process”.
Answer:
Solvay’s process to prepare Na₂CO₃ is as follows:

Step 1: Ammonical brine is prepared: Ammonia is passed through saturated solution of sodium chloride in water and cooled to 30°C.

Step 2: Carbonation: Carbon dioxide is passed through ammonical brine solution to give ammonium hydrogen carbonate later to give a milky solution of sodium hydrogen carbonate.

Sodium bicarbonate is removed away by filtration to get solid sodium bicarbonate.

Step 3: Calcination: Solid NaHCO₃ on heating gives sodium carbonate and CO₂

Again after recrystallisation iron water produces sodium carbonate.

Step 4: Recovery of ammonia: Quick lime is heated with NH₄Cl in above step to give ammonia gas which is used again to prepare ammonical brine.

Question 26.
(a) Give an example of element of group-14.
(i) Which reacts with water.
Answer:
With water: Tin reacts with steam to form stannic oxide liberating hydrogen gas.

(ii) Shows maximum catenation property.
Answer:
Carbon.

(b) Give one example for man made silicates.
Answer:
Aluminium silicates.

Part – D

IV. Answer any FIVE of the following questions carrying FIVE marks: ( 5 x 5 = 25 )

Question 27.
(a) Define Molarity. How many grams of water is produced by the complete combustion of methane?
Answer:
It is defined as the number of moles of the solute present in 1000 ml or 1L of the solution.

CH_4(g)+ 2O_2(g) → CO_2(g) + 2H₂O₍₁₎
1 mole = 16 g of CH₄ gives 2 moles of H₂O i.e., 2 x 18 = 36g of H₂O .

(b) A hydrocarbon contains 75% of carbon and 25% of hydrogen. What is its empirical formula?
Answer:

∴ Empirical formula = CH₄, Molecu ar Mass = n * Empirical formula
⇒ n = Molecular Mass/Empirical Formula Mass
Molecular mass = 2 x 39 = 78
Molecular formula = (CH)_n = (CH)₆ = C₆ H₆ .

(c) State law of definite proportion.
Answer:
This law states that “A chemical compound always contains same elements combined together in same proportion by mass.”
Example: Pure water is obtained from different sources like river, well, sea, etc…. but always contains H and O combined together in the ratio 1:8 by mass.

Question 28.
(a) Write any three postulates of Bohr’s model of an atom.
Answer:
Bohr’s model of an atom, the postulates are:

1. Electrons revolve around the nucleus of an atom in a certain definite path called orbit or stationary state of shell.
2. The shells are having different energy levels denoted as K, L, M, N…
3. As long as the electron remains in an orbit, they neither absorb nor emit energy.
4. The electron can move only in that orbit in which angular momentum is quantized,
   i.e., the angular momentum of the electron is an integral multiple of \(\frac{\mathrm{h}}{2 \pi}\)

Write the de-Broglie equation.
Answer:
λ = \(\frac{\mathrm{h}}{\mathrm{mV}}\) where = wavelength of wave, h = Plank’s constant, m = mass of the electron,
V = velocity of the electron.

(b) State Paulie’s Exclusion Principle. Among 4s and 3d orbital which one has lower energy?
Answer:
Pauli’s Exclusion principle: No two electrons in the same atom can have all the four quantum numbers same.

Illustration : In 1 s orbital there are two electrons, the set of quantum number of one electron differ with another in spin quantum number.

4s orbital.

Question 29.
(a) Name the quantum number that specifies.
(i) Shape of an orbital
(ii) Size of an orbital
Answer:
(i) Azimuthal quantum number.
(ii) Principal quantum number.

(b) State Heisenberg’s uncertainty principle. Write its mathematical form.
Answer:
It is impossible to determine both the momentum (particle nature) and position (wave nature) of a moving sub atomic particle simultaneously with absolute accuracy.
Mathematically ∆x x ∆p = h / π4 where ∆x = uncertainty in position:
∆p = uncertainty in momentum; h = Plank’s constant = 6.626 x 10^-34Js

(c) How many orientations are possible for ‘p’ orbital?
Answer:
p orbital has 3 orientations p_x, p_y and p_z

Question 30.
(a) Mention any three assumptions of Kinetic theory of gases.
Answer:

1. Gases are made up of large number of the minute particles.
2. Pressure is exerted by a gas.
3. There is no loss of kinetic theory.
4. Molecules of gas attract on one another.

(b) On ship sailing in pacific ocean, where temperature is 23.4°C a balloon is filled with 2 L air. What will be the volume of the balloon when ship reaches Indian Ocean where temperature is 23.1°C.
Answer:

Question 31.
(a) Calculate the enthalpy formation of methane from the following data.
C_(s)+ C_2(g) → CO_2(g) ∆H° = -393.5 kJ
H_2(g) + \(\frac{1}{2}\)O_2(g) → H₂O_(l) ∆H° = -285.9 kJ
CH_4(g) + 2O_2(g) → CO_2(g) + H₂ O_(l) ∆H° = -890 kJ
Answer:
C(graphite) + O₂(g) → CO₂(g);   ∆_fH° = -393.5 kJ …… (1)
H₂(g) + \(\frac{1}{2}\)O₂(g) → H₂O(l);   ∆_fH° = -285.8 kJ …… (2)
CO₂(g) + 2H₂O(l) → CH₄(g) + 2O₂(g);   ∆_fH° = +890 kJ …… (3)

Here as we want one mole of C(graphite) as reactant, we write down equation (1) as it is as. We want two moles of H₂(g) as reactant, we multiply equation (2) by 2. As we want one mole of CH₄ as product, we write it down as it is.

C(graphite) + O₂(g) → CO₂(g);   ∆_fH° = -393.5 kJ …… (1)
2H₂(g) + O₂(g) → 2H₂O(l);   ∆_fH° = 2(-285.8 kJ/mol) =-571.6kJ/mol …… (2)
CO₂(g) + 2H₂O(l) → CH₄(g) + 2O₂(g);   ∆_fH° = +890 kJ …… (3)

Adding 1,2 and 3 we obtain:
C(graphite) + 2H₂(g) → CH₄(g);   ∆_fH° = – 74.8 kJ/mol

(b) State Hess’s law of constant heat summation.
Answer:
Hess’s law : Whether a chemical reaction takes place in a single step or in several steps, the ‘ total change in enthalpy remains the same.
Consider the formation of CO₂ from C.

I Method (Single step): C(s) + O₂ (g) → CO₂(g); ∆H = -xkJ
Heat liberated = xkJ

II Method (Two step) : C(s) + \(\frac{1}{2}\) O₂ (g) → CO(g);∆H = -x,kJ
CO(g) +\(\frac{1}{2}\) O₂(g) → CO₂(g) ; ∆H = -x₂kJ .
Total heat liberated = (x₁ + x₂ )kJ
According to Hess’s law, x = x₁ + x₂

Question 32.
(a) Define (i) Isolated system (ii) Molar heat capacity.
Answer:
(i) An isolated system is one in which there is exchange of neither matter nor energy with its surroundings. Example: A sealed bottle completely insulated.
(ii) It is the amount of heat required to rise the temperature by 1°C of one mole of a substance.

(b) What is entropy? How does it vary in a spontaneous process?
Answer:
A measure of degree of disorder of a system. Increases.

(c) Write Gibb’s energy equation.
Answer:
∆G = ∆H – T∆S

Question 33.
(a) Define acid and base according to Bronsted-Lowry concept.
Answer:
A substance or molecule which donates a proton is called acid. [Protogenic]
A substance or molecule which accepts a proton is called abase. [Protophilic],

(b) Calculate the P^H of 0.002 M CH₃COOH. (Assuming complete dissociation)
Answer:
[H⁺] = 0.002M = 2 x 10^-3
P^H = -log₁₀ [H+] = -log (2 x 10^-3)
= -Iog2 – log10^-3
= -0.3010 + 3 = 2.6990

(c) Write Henderson’s-Hesselblach equation for acidic buffer.
Answer:
Consider the acidic buffer, a mixture of acetic acid and sodium acetate.
pH = pK_a + log \frac{[\mathrm{Salt}]}{[\mathrm{Acid}]}
∴ pH = pKa + log\frac{[\mathrm{S}]}{[\mathrm{A}]}

Question 34.
(a) Write any three applications of equilibrium constant Kp (or) K..
Answer:

1. Predicting the extent of a reaction.
2. Predicting the direction of a reaction.
3. Calculating the equilibrium concentrations.

(b) What is homogeneous eqilibrium? Give an example for solid-vapour equilibrium.
Answer:
A system in which different substances in equilibrium remain in a single phase is called a homogeneous equilibrium.
Example: I_2(S) ⇌ I_2(V)

Part – E

V. Answer any TWO of the follow ing questions carrying FIVE marks: ( 2 x 5 = 10 )

Question 35.
(a) Explain chain isomerism with an example.
Answer:
Two or more compounds having same molecular formula but differ in the carbon chain are called chain isomers. This phenomenon is called chain isomerism. Ex: C₄H₁₀

(b) Given any two differences between Inductive effect and Mesomeric effect.
Answer:
Inductive Effect :

1. It operates in saturated compounds especially,
2. It involves electrons of a-bonds.
3. The electron pair is slightly displace from its position and hence partial charges are developed,
4. It is transmitted over a short distance it becomes negligible after second carbon atom in the chain.

Mesomeric Effect

1. It operates in unsaturated compounds having conjugated systems.
2. It involves electrons of it bonds or lone pair of electrons.
3. The electron pair is completely transferred and hence unit positive and negative charges are developed.
4. It is transmitted through the entire chain provided conjugation is present.

(c) Write the IUPAC name of

Answer:
2-methyl-propan-2-ol

Question 36.
(a) Write the principle and formula involved in the estimation of sulphur by Carius method.
Answer:
Principle: An organic compound containing sulphur to heated in carius tube with fuming nitric acid or sodium peroxide. Sulphur present is oxidised to sulphuric acid. It is then precipitated as burium sulphate by adding excess of barium chloride solution. The precipitate formed is filtred, washed, dried and weighed. The % of S is calculated from the mass of barium sulphate.

Calculations: Mass of organic substance = ‘m’ g
Mass of barium sulphate formed = ‘x’ g
Molecular mass of barium sulphate = 233 g
∴ Mass of sulphur present = \(\frac{32 \times x}{233}\) = ‘Y’g
∴ % of sulphur = \(=\frac{\mathrm{y}}{\mathrm{m}} \times 100\).

(b) What are nucleophiles? Give an example.
Answer:
The molecules or negatively charged ions which are capable of one donating an electron pair to electron deficient centre of the substrate are called nucleophiles.

Question 37.
(a) Write the steps involved in the mechanism of nitration of benzene.
Answer:
Nitration of benzene reacts with a mixture of concentrated nitric acid and concentrated sulphuric acid at 50°C to form nitrobenzene.

Mechanism: This involves the following steps:

Step 1: Generation of electrophile nitronium ion NO₂⁺
HNO₃ + 2H₂SO₄ → NO₂⁺ + H₃O⁺ + 2HSO₄^–

Step 2: The electrophile NO₂⁺ attacks the benzene ring to form a carbocation which is resonance stabilized.

Step 3: Loss of a proton to give nitrobenzene. The proton is removed by HSO₄^–.

(b) How do you prepare ethyne from calcium carbide? Write the equation.
Answer:
When calcium carbide is treated with water, ethyne is formed.