Competency Questions

Sodium plumbite

Reason — Sodium (Na) is a monovalent positive ion. There are 2 sodium ions, giving a total positive charge of +2. The negative radical in sodium plumbite is plumbite (PbO₂)^-2 which is divalent. Since the compound is electrically neutral, the negative radical must have a charge of −2.
Aluminium nitride is trivalent. Potassium permanganate and Iron [III] nitrate are monovalent.

4NH₃ + 5O₂ ⟶ 4NO + 6H₂O

Reason — There are 4 nitrogen atoms, 12 hydrogen atoms and 10 oxygen atoms on both the reactant and product sides. Hence, it is a balanced equation.

Magnesium to magnesium nitride by burning.

Reason — This is a direct combination reaction because two substances combine directly to form a single product.

3Mg + N₂ ⟶ Mg₃N₂

Acidified water to oxygen is an electrolytic decomposition reaction. Copper sulphate to copper is a displacement reaction. Iron sulphate to iron hydroxide is double decomposition.

Calcium sulphate

Reason — Permanent hardness of water is caused by soluble sulphates and chlorides of calcium and magnesium. Since calcium sulphate is a soluble salt, it dissolves in water and releases calcium ions, which cause hardness.

Calcium sulphide is not responsible for hardness of water. Calcium sulphite is sparingly soluble and unstable. Calcium carbonate is almost insoluble in water.

Number of nucleons

Reason — Since, both have the same atomic number = 19
So, they have the same number of protons = 19
and same number of electrons = 19

But their mass numbers are different 39 and 41

Number of nucleons = protons + neutrons

Therefore, number of nucleons is different.

Water

Reason — Water is a polar covalent molecule because the shared electrons are unequally distributed and the molecule has an angular shape. Carbon tetrachloride, oxygen and methane are non-polar covalent molecules.

Period 4

Reason — Period 4 contains bromine and it is a halogen and present in liquid form.
Period 2 contains fluorine, Period 3 contains chlorine, which are gases. Period 1 contains only hydrogen and helium; there is no halogen in Period 1.

Fluorine

Reason — Electronegativity is the ability of an atom to attract electrons towards itself in a chemical bond. Fluorine strongly attracts electrons in a bond. Therefore, it is the most electronegative element in the periodic table.
Sodium is a metal with low electronegativity as it tends to lose electrons. Potassium is even more electropositive than sodium and has very low electronegativity. Neon is a noble gas with a complete outer shell; it does not usually form bonds.

Aluminium

Reason — The metal that can react with steam, acids, and alkalis to produce hydrogen gas is aluminium.

Aluminium reacts with steam to produce hydrogen gas

2Al + 3H₂O ⟶ Al₂O₃ + 3H₂

Aluminium reacts with dilute acids (like HCl) to release hydrogen

2Al + 3H₂SO₄ ⟶ Al₂(SO₄)₃ + 3H₂

Aluminium is amphoteric, so it also reacts with strong alkalis (like NaOH) to produce hydrogen

2Al + 2NaOH + 2H₂O ⟶ 2NaAlO₂ + 3H₂

Unreacted CO₂ is removed from the mixture, by dissolving the mixture in ammoniacal cuprous chloride solution.

Reason — In the Bosch process, CO₂ is removed using water or alkali. Ammoniacal cuprous chloride is used to remove CO.

-223°C

Reason — Kelvin scale starts from absolute zero.

°C = K - 273

Since, K = 50

°C = 50-273

= -223°C

Nitrogen

Reason — Oxides of nitrogen are responsible for acid rain because they form nitric acid in the atmosphere. Also, nitrous oxide is a greenhouse gas that contributes to global warming. So nitrogen is the non-metal whose oxides contribute to both effects.
Carbon forms oxides like carbon dioxide which contributes to global warming but does not cause acid rain. Sulphur forms sulphuric acid which leads to acid rain but not global warming. Phosphorus forms oxides like phosphorus pentoxide which reacts with water to form phosphoric acid. It does not contribute to acid rain or global warming.

O₂ + [O]

Reason — Photolysis means breaking of a molecule by sunlight. In the atmosphere, when UV light acts on ozone, ozone breaks into molecular oxygen and nascent oxygen.

O₃ $\xrightarrow{\text{UV}}$ O₂ + [O]

Sodium sulphide

Reason — When sodium sulphide reacts with a dilute acid, it produces hydrogen sulphide gas.

Na₂S + 2HCl ⟶ 2NaCl + H₂S↑

H₂S gas reacts with lead nitrate solution to form lead sulphide (PbS), which is a black precipitate

Pb(NO₃)₂ + H₂S ⟶ PbS↓ + 2HNO₃

Sodium nitrate does not produce a gas with dilute acid. Sodium carbonate produces CO₂ which turns lime water milky. Sodium sulphite produces SO₂ which bleaches solutions.

A is false but R is true.

Reason — A is false as algae are not a source of water pollution. Excessive algal growth (algal bloom) occurs because of water pollution caused by nutrients like phosphates and nitrates.
R is true as phosphates stimulate algal growth. When the algae die, bacteria decompose them and consume dissolved oxygen. This oxygen depletion can kill fish and other aquatic organisms.

Therefore, A is false but R is true is the correct option.

Four

Reason — Since, one oxygen atom has 2 lone pairs, therefore, 2 oxygen atoms will have 4 lone pairs.

Fe₂S₃

Reason — An acidic radical is the negative ion. A divalent acidic radical is an anion having a charge of -2. In Fe₂S₃, sulphide ion (S^2-) is a divalent acidic radical. OH^-, NO₃^- and Cl^- are monovalent acidic radicals.

8

Reason — Electronic configuration of chlorine: 2, 8, 7
So a neutral chlorine atom has 7 electrons in its valence shell.

Chlorine gains one electron to complete its octet.

Cl + e^- ⟶ Cl^-

Now the electronic configuration becomes: 2, 8, 8

So a chlorine anion has 8 electrons in its valence shell.

Ammonium chloride

Reason — Ammonium chloride produces ammonia gas and hydrogen chloride gas on heating. Therefore, it does not produce a gas that turns blue cobalt chloride paper pink.
Blue vitriol, washing soda and Epsom salt contain water of crystallization and produces water vapour.

Both A & R are true but R is not the correct explanation of A.

Reason — A is true as the conversion of ammonia to nitric oxide is exothermic.
R is true as an endothermic reaction absorbs heat from its surroundings. Therefore, continuous energy input is generally required for it to proceed.
Therefore, Both A & R are true but R is not the correct explanation of A is the correct option.

Both A & R are true and R is the correct explanation of A.

Reason — A is true as Iron [II] chloride contains Fe²⁺, while Iron [III] chloride contains Fe³⁺. R is true because Fe²⁺ loses one electron to form Fe³⁺:

Fe²⁺ ⟶ Fe³⁺ + e^-

Loss of electrons is oxidation. Hence, R correctly explains A.

The molecular weight of compound 'X' is 44. The compound is dinitrogen oxide.

The formation of a compound 'A' from aluminium, containing a trivalent acidic radical, is an example of a direct combination reaction.

Molten lead bromide dissociates into lead & bromide ions by absorption of heat energy.

An example of a pentahydrate salt which changes colour on heating is blue vitriol.

State which of the atoms ⁴⁰₁₈Y has the same number of electrons in the L & M shells.

A cation of calcium Ca²⁺ has 8 electrons in the penultimate shell.

The number of unshared electrons in a molecule of methane is none.

Alkali metals have one valence electron/s, alkaline earth metals have two valence electrons & nickel, a transition element, has two valence electrons.

The metal which reacts with dilute acids, least vigorously to liberate hydrogen is iron while lead has no reaction with dilute acids.

A chemical which destroys the ozone layer and on the other hand, contributes to global warming is CCl₄.

The bluish explosive gas is ozone.

O₂ $\xrightarrow{\text{UV light}}$ [O] + [O]

[O] + O₂ ⟶ O₃

B is nascent oxygen and C is ozone.

$V = K\frac{1}{P}$

Reason — The graph shows Volume (V) on the y-axis and Pressure (P) on the x-axis.
As pressure increases, volume decreases in such a way that the curve is a rectangular hyperbola. This indicates an inverse relationship between pressure and volume.

According to Boyle's Law:

V ∝ $\frac{1}{P}$

$V = K\frac{1}{P}$

A= H₂
It is formed by sharing one pair of electrons (single covalent bond).

B = O₂
It is formed by sharing two pairs of electrons (double covalent bond).

C = HCl
It is formed by sharing one pair of electrons (single covalent bond).

Therefore, B (O₂) is the odd one out because it contains a double covalent bond, whereas A and C contain single covalent bonds.

Anhydrous CaCl₂

Reason — Anhydrous CaCl₂ is hygroscopic and absorbs moisture from the air. It is deliquescent and absorbs so much moisture that it eventually dissolves in the absorbed water. It is commonly used as a drying agent in desiccators.
P₂O₅ and CaO are hygroscopic.

Particles in 'A' have uniform composition.

Reason — A compound always has a fixed ratio of its elements. Every water molecule contains 2 hydrogen atoms and 1 oxygen atom. Therefore, water has a uniform composition throughout.
In a compound, the constituent elements do not retain their original properties. Hydrogen is a combustible gas and oxygen supports combustion, but water has properties entirely different from both.
The elements in a compound are chemically combined. They cannot be separated by physical methods such as filtration, evaporation, or distillation.

Oxygen

Reason — Methane (CH₄) contains carbon and hydrogen. Methanol (CH₃OH) contains carbon, hydrogen, and oxygen. Therefore, the element present in methanol but not in methane is oxygen.

A positive radical which is basic in nature is Na⁺.

A neutral gas which reduces iron [II] oxide to iron is hydrogen.

A solid drying & dehydrating agent which is an acidic oxide is phosphorus pentoxide.

The element from ¹²₆X & ³²₁₆Y whose acidic oxide has no effect on acidified KMnO₄ solution is carbon.

The number of electrons in the M-shell of Mg²⁺ is 0.

Since, electronic configuration is 2, 8.
K shell = 2 electrons
L shell = 8 electrons

The element with stable electronic configuration, in the period of the periodic table having the most reactive halogen is neon.

The reduced product formed when a gas which turns moist lead acetate paper silvery black, is reacted with a gas which turns moist starch iodide paper blue-black is hydrogen chloride.

Gas that turns moist lead acetate paper silvery black is hydrogen sulphide.

Gas that turns moist starch iodide paper blue-black is chlorine.

When H₂S reacts with Cl₂

H₂S + Cl₂ ⟶ 2HCl + S

The value of the standard pressure in atmosphere, when the standard temperature is 273 K is 1 atm.

The gaseous organic compound containing a halogen, which contributes to global warming & destruction of the ozone layer is chlorofluorocarbon (CFC).

An example of a colloidal solution, in which the dispersed phase is carbon and the dispersed medium is air is smoke.

Copper can show two valencies, +1 and +2, because it is a transition element. Therefore, it forms two oxides : Cu₂O and CuO.
Sodium has only one valency, +1, since it has one electron in its outermost shell and loses only that electron. Hence it forms only one oxide: Na₂O.

2AgNO₃ ⟶ 2Ag + 2NO₂ + O₂ is thermal decomposition.

2AgCl ⟶ 2Ag + Cl₂ is photochemical decomposition.

Temporary hardness can be removed by boiling because on boiling the calcium & magnesium bicarbonates in water decompose to give insoluble carbonates, carbon dioxide & water, whereas permanent hardness cannot be removed by boiling because chlorides and sulphates are not decomposed on heating.

³⁹₁₉X & ⁴¹₁₉X are isotopes of the same element. They have same atomic number but differ in the number of neutrons. Therefore, have different mass number.
Two atoms of ³⁹₁₉X have identical numbers of protons and neutrons and therefore the same atomic mass.

Yes, a molecule of nitrogen (N₂) has lone pairs of electrons.
Each nitrogen atom has 5 valence electrons. In an N₂ molecule, the two nitrogen atoms share three pairs of electrons, forming a triple covalent bond. After forming the triple bond, each nitrogen atom still has one pair of unshared (lone) electrons left.

As we move from left to right across a period, the nuclear charge increases but the number of shells remains the same. So, elements gradually lose their tendency to lose electrons and gain a tendency to gain electrons. Hence, there is a gradual change from metallic character to non-metallic character across a period.
As we move down a group, the number of electron shells increases. All the elements in a group have the same number of valence electrons, so they show similar chemical properties. Therefore, the metallic or non-metallic nature generally remains the same down a group (metals remain metals and non-metals remain non-metals), and no transition from metallic to non-metallic character is observed down a group.

Conversion of coke to water gas

C + H₂O ⟶ CO + H₂

Steam is converted to hydrogen. The oxidation state of hydrogen decreases from +1 to 0, water is reduced.

CO + H₂ + H₂O ⟶ CO₂ + 2H₂

Steam is converted to hydrogen gas. The oxidation state of hydrogen decreases from +1 to 0. Therefore, water undergoes reduction.

The ideal gas equation:
PV = nRT

At extremely high pressures, these assumptions are no longer valid because gas molecules are forced very close together, so their actual volume becomes significant compared to the container volume. The molecules come close enough for intermolecular forces of attraction and repulsion to affect their behaviour.

The outermost shell has one electron and the penultimate shell contains 18 electrons. However the penultimate shell has not attained stability and one or more electrons sometimes jump to the outermost shell thereby increasing the valency electron. Therefore, copper has +1 valency in Cu₂O & +2 valency in CuO.

Law of conservation of matter states :
Matter can neither be created nor destroyed in a chemical reaction. Therefore, the total mass of the reactants is always equal to the total mass of the products.

H₂S + 2FeCl₃ ⟶ 2FeCl₂ + 2HCl + S

This equation is balanced. There are 2 atoms of iron, 2 atoms of hydrogen, 1 atom of sulphur and 6 atoms of chlorine on both sides of the reaction.

²⁴₁₂X has electronic configuration 2, 8, 2 and tends to lose 2 electrons to attain a stable octet.
³⁵₁₇Y has electronic configuration 2, 8, 7 and tends to gain 1 electron to complete its octet.
X becomes positively charged and Y becomes negatively charged. One X atom transfers two electrons to two Y atoms.

Each ³⁵₁₇Y atom has 7 valence electrons and needs 1 more electron to complete its octet. Since both ³⁵₁₇Y atoms need electrons, neither can donate electrons to the other. Therefore, they share one pair of electrons.

A photochemical reaction is a chemical reaction that is initiated or carried out by the absorption of light energy. In the atmosphere, ultraviolet rays from the Sun split oxygen molecules. The oxygen atoms then combine with oxygen molecules to form ozone. Since sunlight (UV radiation) is essential for the reaction, the formation of ozone is a photochemical reaction.

O₂ $\xrightarrow{\text{UV light}}$ [O] + [O]

Hydrogen is produced from brine (aqueous sodium chloride) by electrolysis. Here, the reaction is brought about by electrical energy, not by light energy.

2NaCl + 2H₂O $\xrightarrow{\text{electricity}}$ 2NaOH + H₂ + Cl₂

Equation for A:

2Al + 3H₂SO₄ ⟶ Al₂(SO₄)₃ + 3H₂

Equation for B:

2Al + 2NaOH + 2H₂O ⟶ 2NaAlO₂ + 3H₂

Equation for C:

2Al + 3H₂O ⟶ Al₂O₃ + 3H₂

Differences between 'photolysis' and 'ozone depletion' are:

(a) The greenish-yellow oxidizing agent is chlorine gas and the foul-smelling gas is hydrogen sulphide, which smells like rotten eggs. Hydrogen sulphide is oxidized by chlorine to sulphur, a yellow non-metal.

H₂S + Cl₂ ⟶ 2HCl + S

The yellow non-metal formed is sulphur.

(b) Equation A :

O₂ + [O] ⟶ O₃

Equation B :

O₃ + [O] ⟶ 2O₂

(a) B & C cannot be distinguished using lime water only.

CO₂ turns lime water milky due to formation of calcium carbonate. SO₂ also turns lime water milky due to formation of calcium sulphite. In both cases, the milkiness disappears on passing excess gas due to formation of soluble bicarbonate or bisulphite.

(b) B & C can be distinguished using acidified K₂Cr₂O₇ solution as SO₂ is a reducing agent and changes orange acidified potassium dichromate to green.
CO₂ has no effect.

(c) A & B cannot be distinguished using acidified KMnO₄ solution because H₂S reduces acidified potassium permanganate, decolourising the purple solution. SO₂ also reduces acidified potassium permanganate, decolourising the purple solution.

(d) A & B can be distinguished using lead nitrate solution because H₂S gives a black precipitate of lead sulphide. SO₂ does not produce a black precipitate with lead nitrate solution.

(a) Pure hydrogen burns slowly in air to form water, releasing heat gradually. Since the reaction is not very rapid, it burns with a pale blue flame without producing any sound.

(b) When hydrogen is mixed with air, oxygen is already intimately mixed with the hydrogen. On ignition, the reaction occurs very rapidly, causing sudden expansion of gases and producing a 'pop' sound.

(c) Direct sunlight initiates a photochemical reaction between hydrogen and chlorine. The reaction proceeds extremely rapidly (chain reaction), releasing a large amount of energy in a very short time, resulting in an explosion.

(a) Increasing order of valency of the elements: Li, Be, B, C

(b) Increasing order of electronegativity of the elements: C, N, O, F

(c) Decreasing order of electronegativity of the elements: F, Cl, Br, I

(d) Decreasing order of non-metallic character of the elements: F, O, N, C