Hydrogen

Copper

Reason — Copper (Cu) is less reactive and lies below hydrogen in the reactivity series, so it cannot displace hydrogen from dilute acids and therefore does not produce hydrogen gas. Also, copper does not react with water (cold or hot) to give hydrogen.
Magnesium reacts with hot water and acids to produce hydrogen gas. Iron reacts slowly with water (steam) and with acids to release hydrogen.

MgO

Reason — When magnesium reacts with steam (water vapour), it forms magnesium oxide (MgO) along with hydrogen gas (H₂).

Mg(s) + H₂O(g) ⟶ MgO(s) + H₂(g)

Mg(OH)₂ is formed when magnesium reacts with cold water, not steam. MgH₂ is not formed in this reaction. MgCl₂ is formed when magnesium reacts with hydrochloric acid, not steam.

O₂

Reason — During water electrolysis, hydrogen gas (H₂) is produced at the cathode (negative electrode), and oxygen gas (O₂) is produced at the anode (positive electrode).

hydrogen chloride

Reason — In the presence of diffused sunlight, the hydrogen and chlorine molecules undergo a combination reaction to form hydrogen chloride.

H₂(g) + Cl₂(g) $\xrightarrow{\text{sunlight}}$ 2HCl(g)

calcium

Reason — Calcium (Ca) reacts readily with cold water to form calcium hydroxide (Ca(OH)₂) and hydrogen gas (H₂).

Ca(s) + 2H₂O(l) ⟶ Ca(OH)₂(aq) + H₂(g)

Magnesium reacts very slowly with cold water, but reacts readily with hot water/steam. Aluminium does not react with cold water due to a protective oxide layer. Iron reacts only with steam, not with cold water.

top

Reason — The reactivity series is a list of metals arranged in order of their ability to displace other metals from solutions or react with substances like water and acids. The most reactive metals, such as potassium and sodium, are at the top, while less reactive metals like gold and platinum are at the bottom.

all of the above

Reason — Hydrogen is used as a fuel to produce heat and light, in the hydrogenation of vegetable oils to produce hydrogenated oil, and in the manufacture of ammonia used for fertilisers.

combustible

Reason — Hydrogen is combustible in nature, but is itself a non-supporter of combustion.

carbon monoxide and hydrogen

Reason — Water gas is a mixture of carbon monoxide (CO) and hydrogen (H₂).

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

Explanation — A is true as hydrogen readily reacts with other elements, especially oxygen, to form compounds like water, and with metals as well.
R is true as this reactivity is one of the reasons it is mostly found in compounds.

Therefore, Both A and R are true and R is the correct explanation of A is the correct option.

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

Explanation — In the laboratory, hydrogen is commonly prepared by reacting granulated zinc with a dilute acid (dil. HCl / dil. H₂SO₄). So, Assertion (A) is true.
Granulated zinc may contain small impurities such as copper, which can help the reaction start faster. So, Reason (R) is true.
However, Reason (R) is not the correct explanation of Assertion (A) as granulated zinc is mainly used because its rough, uneven surface provides larger effective surface area, so hydrogen is evolved at a steady and controllable rate.

Therefore, Both A and R are true but R is not the correct explanation of A is the correct option.

A is false but R is true.

Explanation — Pure hydrogen burns quietly (pale blue flame). The pop sound is heard only when hydrogen is mixed with air/oxygen. So, Assertion (A) is false.
Hydrogen forms an explosive mixture with air, which produces the pop on ignition. So, Reason (R) is true.

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

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

Explanation — A is true, when hydrogen gas is passed over hot metallic oxides of copper, lead, iron, etc., it removes oxygen from them and thus reduces them to their corresponding metals.
R is true as hydrogen acts as a good reducing agent.

Therefore, Both A and R are true and R is the correct explanation of A is the correct option.

A is true but R is false.

Explanation — Oxyhydrogen flame is used for cutting and welding metals. So, Assertion (A) is true.
For cutting and welding metals, a high temperature is required. Oxyhydrogen flame has a temperature of about 2800°C–3000°C, which is sufficient to melt the metals. So, Reason (R) is false.

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

(a) lighter

(b) sparingly

(c) pale blue, pop

(d) above, dilute acids

(e) metal, water

(f) hydrogen, oxygen

(g) simultaneously

(a) False
Corrected statement — Hydrogen atom is monovalent; hydrogen molecule is diatomic.

(b) False
Corrected statement — The removal of hydrogen from a substance is called oxidation.

(c) True

(d) True

(e) True

(f) False
Corrected statement — Hydrogen is combustible but it is a non-supporter of combustion.

(g) False
Corrected statement — Hydrogen gas cannot be easily liquefied.

(a) Oxidation

(b) Bosch process

(c) Electrode

(d) Ions

(e) Reducing agent

(a) Sodium (Na) and Potassium (K)

(b) Iron (Fe)

(c) Copper(II) oxide (CuO)

(d) Copper(II) sulphate solution (CuSO₄)

(e) Copper

(a) H₂ + Cl₂ ⟶ 2HCl

(b) H₂ + S ⟶ H₂S

(c) Zn + 2HCl ⟶ ZnCl₂ + H₂

(d) CuO + H₂ ⟶ Cu + H₂O

(e) 3Fe + 4H₂O (steam) ⟶ Fe₃O₄ + 4H₂

(f) 2K + 2H₂O ⟶ 2KOH + H₂

(a) Catalytic hydrogenation is a process by which hydrogen gas is passed through vegetable oils in the presence of a catalyst like Ni, Pt or Pd to convert the oils into vanaspati ghee.

(b) Oxidation is a chemical process that involves addition of oxygen to a substance or removal of hydrogen from a substance.

(c) Reduction is a chemical process that involves addition of hydrogen to a substance or removal of oxygen from a substance.

(d) Redox reactions are those in which reduction and oxidation both take place simultaneously i.e., one substance is reduced while the other gets oxidised.

(e) Electrolysis is a process in which an electric current is passed through an aqueous solution or a molten state of a compound to bring about a chemical change.

(f) An electrolyte is a compound which, in aqueous solution or molten state, conducts electricity due to the presence of free and mobile ions.

Hydrogen gas is neutral to litmus, i.e. no change is observed in the colour of either moist blue or red litmus paper when it is introduced into a jar containing hydrogen. This proves that hydrogen gas is neutral in nature.

(a) Hydrogen has high heat of combustion and is pollution free so it is used as a fuel. Liquid hydrogen is used as a rocket fuel.

Though hydrogen is lighter than air it cannot be collected by downward displacement of air because a mixture of hydrogen and air can lead to an explosion so it is not safe to collect hydrogen gas by this method.

A pop sound is produced when hydrogen gas is mixed with air/oxygen and burnt because hydrogen forms an explosive mixture with air/oxygen, which burns suddenly on ignition.

Hydrogen gas is highly inflammable in nature, so it is replaced by helium for filling in weather forecasting balloons.

Nitric acid is a strong oxidising agent (even when it is dilute). It oxidises hydrogen gas converting it into water which defeats the purpose. Hence, nitric acid is not used for the preparation of hydrogen gas.

(a)
(i) The reactant which gets reduced is PbO (Lead oxide).

(ii) The reactant which gets oxidised is H₂ (Hydrogen gas).

(b) PbO (Lead monoxide) acts as the oxidising agent.

(a) Bosch process

(b) Equations for the reactions of Bosch process are:

Production of water gas:

$\underset{\text{hot coke}}{\text{C}} + \underset{\text{steam}}{\text{H}_2 \text{O}} \xrightarrow {1000^{\circ}\text{C}} \underset{\text{water gas}}{(\text{CO} + \text{H}_2)} - \text{Heat}$

Reduction of steam by water gas:

$\underset{\text{water gas}}{(\text{CO}+\text{H}_2)} + \underset{\text{steam}}{\text{H}_2 \text{O}} \xrightarrow [\text{450^{\circ} C}] {\text{Fe}_2\text{O}_3/ \text{Cr}_2\text{O}_3} \text{CO}_2 + 2\text{H}_2 + \text{Heat}$

(c) Hydrogen is separated from carbon dioxide by passing the mixture through water under pressure, where carbon dioxide gets dissolved leaving behind hydrogen. Carbon dioxide can also be separated by passing it through caustic potash (KOH) solution.

2KOH + CO₂ ⟶ K₂CO₃ + H₂O

Carbon monoxide is separated by passing through ammoniacal cuprous chloride in which carbon monoxide dissolves leaving behind hydrogen.

CuCl + CO + 2H₂O ⟶ CuCl.CO.2H₂O

When hydrogen is passed over hot metallic oxides of copper, lead, iron, etc. it removes oxygen from them and thus reduces them to their corresponding metals. Hence, hydrogen acts as a good reducing agent.

When hydrogen gas is passed over heated copper oxide, copper oxide gets reduced to metallic copper by the removal of oxygen from it. The equation for the reaction is given below:

CuO + H₂ ⟶ Cu + H₂O

The four uses of hydrogen are:

1. The oxy-hydrogen flame obtained by burning hydrogen and oxygen together has a very high temperature of about 2800°C–3000°C. Hence it is used for cutting and welding of metals.
2. It is used as a fuel as hydrogen has high heat of combustion.
3. It is used for hydrogenation of vegetable oil.
4. It is used as a reducing agent in extraction of metals.

(a) Granulated zinc and dilute hydrochloric acid are the chemicals required to prepare hydrogen gas in the laboratory.

(b) Zn + 2HCl ⟶ ZnCl₂ + H₂ (g)

(c) The diagram below shows the laboratory preparation of hydrogen gas:

(d) Hydrogen gas is collected by downward displacement of water as it is sparingly soluble in water. Though hydrogen gas is lighter than air, it is not collected by downward displacement of air as a mixture of hydrogen and air can lead to an explosion.

(e) Hydrogen gas burns in air with a pop sound. This is because of the presence of impurities in it. This method is widely used as a test to identify hydrogen.

(a)
A → Hydrogen (H₂)
B → Oxygen (O₂)
C → Electrolytic Cell
D → Acidulated Water (Electrolyte)
E → Cathode
F → Anode

(b) Acidulated water is used as pure water is a bad conductor of electricity. When a small amount of acid, alkali or salt is added to it, only then it conducts electricity.

(c) Ratio by volume of the gases collected
Hydrogen : Oxygen = 2 : 1

(a) Hydrogen is called a clean fuel because on burning it produces only water. It does not produce smoke, carbon dioxide, or harmful gases, so it does not pollute the environment.

(b) The process used to prepare vanaspati ghee from vegetable oils is called catalytic hydrogenation. In this process, hydrogen gas is passed through vegetable oil in the presence of a catalyst such as nickel to convert it into vanaspati ghee.

(c) Hydrogen is used for cutting and welding metals because it burns in oxygen to produce an oxy-hydrogen flame of very high temperature. This high heat is useful for cutting and welding metals.

(d) Hydrogen is not used in weather forecasting balloons anymore because it is highly inflammable and can catch fire easily. It may cause explosions, making it unsafe.

(e) Helium has replaced hydrogen in these balloons because it is non-flammable and lighter than air, so it can lift balloons safely.