Model Question Paper — 2

Two atoms of hydrogen gas

Reason — H represents the element hydrogen and prefix 2 shows 2 atoms of the element hydrogen.

The temperature of surroundings decreases

Reason — Heat is absorbed during an endothermic reaction, hence the temperature of the surrounding will decrease.

$\dfrac{1}{2}$ times

Reason — According to Boyle's law volume of a given mass of a dry gas is inversely proportional to its pressure at constant temperature.

Hence, if pressure is doubled for a fixed mass of a gas, its volume will become $\dfrac{1}{2}$ times

No reaction takes place.

Reason — HCl + AgCl ⟶ no reaction

As the anion of both the reacts is same hence, no reaction is possible to form new products.

Z

Reason — Z (12) = 2, 8, 2 forms a cation. It will lose its 2 electrons to achieve a stable electronic configuration thus forming a cation.

D

Reason — The electronic configuration of D[20] = [2,8,8,2]. Hence, it has 2 electrons in its N shell.

One

Reason — The chloride ion [Cl^-] has one atom.

SO₃^-2

Reason — An Acid radical is the radical that remains after an acidic molecule loses a hydrogen ion (H⁺). Hence, Sulphurous acid H₂SO₃, has SO₃^-2 as the acidic radical.

Oxidation

Reason — When an oxygen ion O^-2 having electronic configuration [2,8] is converted to an oxygen atom O having electronic configuration [2,6], it loses two electrons and hence undergoes oxidation.

Gold

Reason — Gold and Platinum are two elements whose atomic weights were corrected on the basis of their positions in Mendeleev's periodic table.

Global warming

Reason — Carbon dioxide is a greenhouse gas and it traps heat radiated from the Sun. An excessive release of carbon dioxide in the atmosphere leads to rise in temperature, thus causing global warming.

Period III, group 13

Reason — The element having three shells [K, L, M] with three electrons in the valence shell is in group 13 [III A] and period 3. The name of the element is Aluminium.

Their electronic configuration is stable

Reason — Noble gases have completely filled valence shell and so do not react with other elements and are therefore inert or unreactive hence, they are monatomic gases.

Water has a high specific heat capacity and ice has a high latent heat of fusion.

Reason — Rivers and lakes do not freeze easily because the specific latent heat of fusion of ice is sufficiently high (= 336 J g-1). The water in lakes and rivers will have to liberate a large quantity of heat to the surrounding before freezing. The layer of ice formed over the water surface, being a poor conductor of heat, will also prevent the loss of heat from the water of lake, hence the water does not freeze all at once

Single displacement reaction

Reason — A chemical change in which a more active element displaces a less active element from its salt solution is called a single displacement reaction as shown above.

(i) The table is known as Mendeleev's periodic table.

(ii) The Periodic Law for the above periodic table is that physical and chemical properties of elements are a periodic function of their atomic masses.

(iii) (i) SO₃

(ii) SH₂

(iv) One defect of this table was the grouping of chemically dissimilar elements. Elements such as copper and silver bear no resemblance to alkali metals (lithium, sodium, etc.) but they have been placed together in the first group.

(v) Modern periodic table is based on the Modern Periodic Law which states that — Physical and chemical properties of elements are periodic function of their atomic number.

(i) The element below sodium in the Periodic Table is more, reactive than sodium.

(ii) Nitrogen is a gas having a triple bond.

(iii) The average kinetic energy of the molecules of the gas is proportional to the absolute temperature.

(iv) If temperature is reduced to half, volume would also reduce to half provided pressure remains constant.

(v) Ozone depletion is mainly caused by the active chlorine atoms generated from CFC in the presence of UV radiation.

(i) Given,

solute = 20 g

solvent = 200 g

concentration = ?

Concentration of solution = $\dfrac{\text{Mass of solute}}{\text{\text{Mass of solute + Mass of solvent}}}$ x 100

Concentration of solution = $\dfrac{20}{\text{200 + 20}}$ x 100

= 9.09%

Hence, concentration = 9.09%

(ii) Given,

concentration = 25%

solvent = 150 cm³

solute = ?

Concentration of solution = $\dfrac{\text{Volume of solute}}{\text{\text{Volume of solute + Volume of solvent}}}$ x 100

$\dfrac{25}{100}$ = $\dfrac{\text{Volume of solute (x)}}{\text{\text{Volume of solute (x)+ 150 }}}$

$\dfrac{1}{4}$ = $\dfrac{\text{Volume of solute (x)}}{\text{\text{Volume of solute (x)+ 150 }}}$

4x = x + 150

3x = 150

x = 50 cm³

Hence, alcohol added = 50 cm³

(iii) Solubility of a substance at 45°C is 36.5g means that 36.5g of substance dissolves in 100 g of water at the temperature of 45°C.

(i) Balanced chemical equation :

1. 2Pb(NO₃)₂ $\xrightarrow{\space \Delta \space}$ 2PbO + 4NO₂ + O₂

2. C₂H₅Br + NaOH ⟶ C₂H₅OH + NaBr

3. 6CO₂ + 12H₂O ⟶ C₆H₁₂O₆ + 6H₂O + 6O₂

(ii) K₂Cr₂O₇ + H₂SO₄ + 3SO₂ ⟶ K₂SO₄ + Cr₂(SO₄)₃ + H₂O

(i) Carbon dioxide gas has no effect on acidified potassium permanganate (KMnO₄) and acidified potassium dichromate (K₂Cr₂O₇) solution whereas sulphur dioxide gas turns acidified potassium permanganate from pink to clear colourless and acidified potassium dichromate from orange to clear green.

2KMnO₄ + 2H₂O + 5SO₂ ⟶ K₂SO₄ + 2MnSO₄ + 2H₂SO₄

K₂Cr₂O₇ + H₂SO₄ + 3SO₂ ⟶ K₂SO₄ + Cr₂(SO₄)₃ + H₂O

(ii) Hydrogen gas burns with a 'pop' sound in air whereas hydrogen chloride forms a curdy white ppt. on passage through AgNO₃ solution. The precipitate dissolves in excess of NH₄OH.

Potassium plumbite [K¹⁺PbO₂^2-] has basic radical [K¹⁺] and acidic radical as [PbO₂^2-]

(i) Bosch process

(ii) Reactions of Bosch process (with proper conditions):

Step I

Reaction : Production of water gas

$\underset{\text{hot coke}}{\text{C}} + \underset{\text{steam}}{\text{H}_2\text{O}} \xrightarrow{1000 \degree \text{C}} \underset{\text{water gas}}{[\text{CO} + \text{H}_2 ]} - \Delta$

Reactants : White hot coke and steam
Temperature : Around 1000°C
Process : Passage of steam over white hot coke [carbon]
Chamber : Specially designed convertor

Step II

Reaction : Reduction of steam to hydrogen by carbon monoxide

$\underset{\text{water gas}}{[\text{CO} + \text{H}_2 ]} + \underset{\text{excess steam}}{\text{H}_2\text{O}} \xrightarrow[\text{Fe}_2\text{O}_3]{450 \degree \text{C}} \text{CO}_2 + 2\text{H}_2 + \Delta$

Reactants : Water gas and excess steam
Temperature : Around 450 °C
Catalysts : Iron [III] oxide [Fe₂O₃], promoter chromic oxide [Cr₂O₃]
Process : Excess steam is mixed with water gas, passed over a catalyst at elevated temperature.
[CO is converted to CO₂ with a further yield of hydrogen.]

Step III

Reactions : Separation of carbon dioxide and carbon monoxide from the above mixture

(a) CO₂ is removed by dissolving mixture in water under pressure [30 atmospheres], or caustic potash solution to dissolve CO₂.

2KOH + CO₂ ⟶ K₂CO₃ + H₂O

(b) CO is removed by dissolving mixture in ammoniacal cuprous chloride solution.

CuCl + CO + 2H₂O ⟶ CuCl.CO.2H₂O.

Thus, hydrogen gas is left over.

1. H and P are nobles gases with fully filled valence shells.
2. G and O are halogens having 7 electrons in their outermost orbit.
3. D and L

Charles law states that pressure remaining constant, the volume of a given mass of dry gas increases or decreases by $\dfrac{1}{273}$ of its volume at 0°C for each 1°C increase or decrease in temperature, respectively.

Mathematical expression of Charles' law :

Suppose, a gas occupies V₁ cm³ at T₁ temperature and V₂ cm³ at T₂ temperature, then by Charles' law:

    V₁ α T₁

or V₁ = kT₁ (k is constant)

or $\dfrac{\text{V}_1}{\text{T}_1}$ = k and

    V₂ α T₂

or $\dfrac{\text{V}_2}{\text{T}_2}$ = k

∴ $\dfrac{\text{V}_1}{\text{T}_1}$ = $\dfrac{\text{V}_2}{\text{T}_2}$ = k (at constant pressure)

(i) Hygroscopy

(ii) Efflorescence

(i) When carbon burns in oxygen to form carbon dioxide, lot of heat is produced. It is also a combination reaction.

C + O₂ ↑ ⟶ CO₂ ↑ + Heat

(ii) Zinc carbonate decomposes to form zinc oxide and releases carbon dioxide gas.

ZnCO₃ $\xrightarrow{\space \Delta \space}$ ZnO + CO₂ ↑

(iii) Formation of white precipitate of silver chloride on mixing sodium chloride solution and silver nitrate solution is a double decomposition reaction.

AgNO₃ + NaCl ⟶ AgCl ↓ + NaNO₃

A chemical reaction which involves, oxidation of one substance and reduction of other substance is called a Redox Reaction.

Considering the reaction of Zinc with copper sulphate:

Zn + CuSO₄ ⟶ ZnSO₄ + Cu

Zn + Cu²⁺SO₄^2- ⟶ Zn²⁺SO₄^2- + Cu

Here, Zinc is oxidised and Copper is reduced.

Oxidation — Zn - 2e^- ⟶ Zn²⁺
Reduction — Cu²⁺ + 2e^- ⟶ Cu

(i) Neon

(ii) Lithium, Sodium and Potassium

(i) The ozone layer acts as a blanket in the atmosphere above 16 km from the earth’s surface. Ozone absorbs the harmful ultra-violet rays coming from the sun and prevents them from reaching the earth’s surface. Thus, it protects the life on earth from harmful effects of ultra-violet rays that can cause skin cancer and destroy many organisms necessary for life in general.

(ii) Hydrated Calcium Sulphate [CaSO₄.2H₂O]

Initial conditions [S.T.P.] :

P₁ = Initial pressure of the gas = P
V₁ = Initial volume of the gas = V
T₁ = Initial temperature of the gas = -15°C = -15 + 273 = 258 K

Final conditions :

P₂ (Final pressure) = pressure decreases to 60% of its original value

= $\dfrac{60}{100}$ of P

= $\dfrac{3}{5}$P

V₂ (Final volume) = volume increases by 50% of its original value

= 1 + $\dfrac{50}{100}$ of V

= $\dfrac{100 + 50}{100}$ of V

= $\dfrac{150}{100}$ of V

= $\dfrac{3}{2}$V

T₂ (Final temperature) = ?

By Gas Law:

$\dfrac{\text{P}_1\times\text{V}_1}{\text{T}_1} = \dfrac{\text{P}_2\times\text{V}_2}{\text{T}_2}$

Substituting the values :

$\dfrac{\text{PV }}{258} = \dfrac{\dfrac{3}{5} \text{P}\times\dfrac{3}{2}\text{V}}{\text{T}_2} \\[1em] \text{T}_2 = \dfrac{3\times 3\times 258}{5 \times 2} = \dfrac{2322}{10} \\[1em] \text{T}_2 = 232.2 \text{K} \\[1em]$

∴ Final temperature of the gas = 232.2 - 273 = -40.8°C

(i) The sudden release of latent heat of condensation causes the violence associated with torrential rain.

(ii) Due to it's high specific heat capacity, the presence of a large amount of water is able to modify the climate of the nearby land areas, making them warmer in winter and cooler in summer. Land and sea breezes also take place because of this great moderating property of water.

(iii) Pure water is tasteless. The taste in water is due to the gases and solids dissolved in it i.e., impurities present in it. As boiled and distilled water are pure containing no impurities hence they taste flat i.e., are tasteless.

(i) Laboratory preparation of hydrogen

Zn + 2HCl ⟶ ZnCl₂ + H₂ ↑

(ii) Hydrogen from alkali

$\text{Zn} + 2\text{NaOH} \longrightarrow \underset{\text{[sodium zincate]}}{\text{Na}_2\text{ZnO}_2} + \text{H}_2 \text{[g]}$

The solubility curves for KNO₃ and CaSO₄ are shown below:

Two impurities present in the hydrogen gas prepared in the laboratory are :

1. Hydrogen sulphide (H₂S)
2. Sulphur dioxide (SO₂)

Lead nitrate solution is used to remove hydrogen sulphide.

Hydrogen and Nitrogen combine to form ammonia [NH₃].

Atomic orbit structure diagram for formation of ammonia molecule is shown below:

One importance of dissolved salts in water is that the dissolved salts supply the essential minerals needed for the growth and development of plants.

One importance of dissolved gases in water is that aquatic plants make use of dissolved carbon dioxide for photosynthesis, i.e., to prepare their food.

When potassium is added to water, the metal melts and moves very quickly on the surface of the water. The metal ignites along with hydrogen gas, hence, sparks along with lilac flame is visible.

2K + 2H₂O ⟶ 2KOH + H₂

(i) Both alkali metals and hydrogen act as reducing agents.

CuO + H₂ ⟶ Cu + H₂O

CuO + 2Na ⟶ Cu + Na₂O

Both halogens and hydrogen are non-metals. They show electronegative character.

H + e^- ⟶ H^-

F + e^- ⟶ F^-

(ii) Hydrogen atom has only one shell but alkali metal and halogens have two or more shells.

(i) The Newland's law of Octaves did not work with heavier elements i.e., those lying beyond Calcium. As more and more elements were discovered, they could not be fitted into Newland's Octaves. Hence, this law was discarded.

(ii) The phenomenon of occurrence of characteristic properties of elements at definite intervals in the modern periodic table when elements are arranged in increasing order of their atomic numbers is called periodicity.

Reasons for Periodicity is — after definite intervals of atomic number, similar valence shell electronic configuration occurs and as properties of elements depend upon the arrangement of electrons in various shells including valence shells, hence periodicity in properties occur.

The lightest gas, hydrogen is collected by downward displacement of water.

To dry the gas common drying agents like fused calcium chloride, caustic potash stick and phosphorus pentoxide can be used.

After purifying and drying, the gas is collected over mercury because mercury has no reaction with it.

Below diagram shows the bond formation between an alkali metal Sodium and a halogen Chlorine to form Sodium chloride [NaCl]:

(i) Ordinary air contains 78% nitrogen, 21% oxygen and 0.01% carbon dioxide. But oxygen is more soluble in water as compared to nitrogen. Hence, the composition of air dissolved is different from ordinary air.

The composition of air dissolved in water is 33% oxygen, 66% nitrogen and 1% carbon dioxide.

(ii) Steam has a higher heat content on account of high specific latent heat of condensation that is 2268 J g-1. Hence, steam at 100°C carries more heat than water. Therefore, steam at 100°C burns more severely than water at 100°C.

(iii) Since volume of a gas changes remarkably with change in temperature and pressure, it becomes necessary to choose standard values of temperature and pressure to which gas volumes can be referred.