Study of the First Element — Hydrogen

Hydrogen is the first element of the periodic table. Its atomic number is 1, and it has only one electron in its valence shell. Therefore, it belongs to the first group and the first period of the periodic table.

As hydrogen shows a dual nature, it resembles alkali metals of Group IA and the halogens of Group VII A (17). Thomson suggested a separate position for hydrogen. He puts hydrogen at the top of the periodic table, that does not disturb the symmetry of the periodic table.

Hydrogen resembles the alkali metals of Group IA and the halogens of Group VIIA. This is due to the fact that it has only one electron in its valence shell, hence it behaves like the alkali metals which have one valence electron. At the same, it also behaves like halogens as it has just one electron less than the nearest inert gas.

(i) Hydrogen and Alkali metals both form a cation, by loss of an electron.

H ⟶ H⁺ + e^-

Li ⟶ Li⁺ + e^-

Na ⟶ Na⁺ + e^-

All these elements have electropositive character.

(ii) Both Hydrogen atoms and alkali metals are reducing agents.

CuO + H₂ ⟶ Cu + H₂O

CuO + 2Na ⟶ Cu + Na₂O

(iii) Both hydrogen and alkali metals react with oxygen to form respective oxides.

Hydrogen - forms H₂O

Sodium - forms Na₂O

(iv) Both alkali metals and Hydrogen burns in air to form oxides.

Hydrogen forms its oxide (water)

2H₂ + O₂ ⟶ 2H₂O

Sodium also forms its peroxide

2Na + O₂ ⟶ Na₂O₂

Properties of hydrogen different from Alkali metals & Halogens are:

1. Hydrogen has only one shell but alkali metals and halogens have two or more shells.
2. Oxides of hydrogen, H₂O, is a neutral oxide. Oxides of halogens like Cl₂O, Cl₂O₇, etc., are acidic in nature, while oxides of alkali metals like Na₂O, K₂O etc., are basic in nature.

(i) Hydrogen has 1 valence electron in its orbit.

(ii) Hydrogen burns in Oxygen to form oxides (water).

Hydrogen burns with a pop sound.

2H₂ + O₂ ⟶ 2H₂O

(iii) Hydrogen acts as a reducing agent.

CuO + H₂ ⟶ Cu + H₂O

Hydrogen is called inflammable air, because it is a highly combustible gas.

In free state, hydrogen is found in traces in earth's crust and atmosphere. Volcanic gases contain 0.025% of it, the earth's crust 0.98%, the earth's atmosphere 0.01% and the atmospheres of the Sun and the stars also contain hydrogen in large amounts.

In combined state :

1. Plants and animal tissues are made up of compounds of hydrogen with carbon, oxygen and nitrogen.
2. Hydrogen is the characteristic constituent of acids, alkalis, hydrocarbons and proteins. In addition to these, sugar, starch, petroleum products, proteins, carbohydrates and also fats contain hydrogen.
3. In water, it is 11.1% by weight.

(i) Like halogens (fluorine and chlorine), hydrogen too is a gas.

(ii) Both show a tendency to form anions since they are one electron short of the nearest inert gas configuration.

H + e^- ⟶ H^-

Cl + e^- ⟶ Cl^-

(iii) Both have the valency 1.

(iv) Hydrogen reacts with oxygen to form neutral oxides (H₂O); Halogen reacts with oxygen to form acidic oxides (Cl₂O, Cl₂O₇ etc.)

(i) Reactive metals like sodium react with cold water forming their corresponding oxides and evolving hydrogen.

2Na + 2H₂O ⟶2NaOH + H₂ ↑

Sodium amalgam (alloy of sodium and mercury) react smoothly with water. Therefore, hydrogen can be safely prepared from this amalgam.

(ii) Zinc reacts with acids liberating hydrogen gas and forming their respective salts.

Zn + 2HCl ⟶ ZnCl₂ + H₂ ↑

(i) 3Fe + 4H₂O ⇌ Fe₃O₄ + 4H₂ ↑

(ii) This reaction is reversible because if the hydrogen formed is not removed, then the iron oxide (triferric tetraoxide) formed is reduced back to iron.

(iii) The hydrogen formed should be removed in order for the reaction to proceed continuously.

Zinc and aluminium are considered to have a unique nature because:

(i) They can react with acids and can even react with hot conc. alkalis to form hydrogen and a soluble salt.

Zn + 2NaOH ⟶ Na₂ZnO₂ + H₂

2Al + 6NaOH ⟶ 2Na₃AlO₃ + 3H₂

Zn + 2HCl ⟶ ZnCl₂ + H₂

2Al + 6HCl ⟶ 2AlCl₃ + 3H₂

(ii) Their oxides and hydroxides are amphoteric in nature i.e., they react with both bases and acids to give salt and water.

ZnO + 2HCl ⟶ ZnCl₂ + H₂O

ZnO + 2NaOH ⟶ Na₂ZnO₂ + H₂O

Al₂O₃ + 6HCl ⟶ 2AlCl₃ + 3H₂O

Al₂O₃ + 2NaOH ⟶ 2NaAlO₂ + H₂O

(i) Fe + 2HCl ⟶ FeCl₂ + H₂

(ii) Zn + 2NaOH ⟶ Na₂ZnO₂ + H₂

(iii) Pb + 2KOH ⟶ K₂PbO₂ + H₂

(iv) 2Al + 6NaOH ⟶ 2Na₃AlO₃ + 3H₂

(i) When Sodium reacts with cold water, it melts, forming a silvery globule, which darts about on the surface of water. It catches fire and burns with a golden yellow flame. Bubbles of hydrogen gas are liberated. The solution formed is colourless, soapy, slightly warm and alkaline.

2Na + 2H₂O ⟶2NaOH + H₂ ↑

(ii) Calcium sinks in water and the reaction is less vigorous than Sodium. Bubbles of hydrogen gas are liberated and the solution turns milky, turbid and alkaline. If red litmus is introduced in solution, it turns blue.

Ca + 2H₂O ⟶ Ca(OH)₂ + H₂ ↑

(iii) Magnesium reacts slowly with boiling water and forms a base, magnesium hydroxide, liberating hydrogen gas.

Mg + 2H₂O ⟶ Mg(OH)₂ + H₂ ↑

(iv) Magnesium burns in steam with an intense white light, liberating hydrogen gas and white ash which is magnesium oxide.

Mg + H₂O ⟶ MgO + H₂ ↑

Magnesium oxide crumbles down due to heating. Further exposing magnesium to steam results in the liberation of hydrogen gas.

(i) Red hot iron reacts with steam, forming triferric tetra-oxide and hydrogen gas.

(ii) 3Fe + 4H₂O ⇌ Fe₃O₄ + 4H₂ ↑

(iii) If the hydrogen formed is not removed the iron oxide formed is reduced back to iron, as the reaction is reversible.
In the beginning, the forward reaction is fast and the backward reaction is slow. As the products are formed and the reactants get consumed the backward reaction becomes faster. There comes a time when the forward and the backward reactions acquire the same speed. An equilibrium stage is reached at 700°C and the amounts of the reactants and the products do not change.

(i) Sodium

(ii) Magnesium

(iii) Zinc

(iv) Calcium

(a) Sodium hydroxide + zinc ⟶ hydrogen + sodium zincate

(b) Calcium + water ⟶ calcium hydroxide + hydrogen

Mg

Reason — Magnesium (Mg) and manganese (Mn) are the only metals that react with very dil. nitric acid to liberate hydrogen. The oxidizing action of the acid is much reduced due to its over dilution.

Mg + 2HNO₃ ⟶ Mg(NO₃)₂ + H₂ ↑

Na

Reason — Reactive metals like potassium, sodium and calcium react with cold water forming their corresponding oxides and evolving hydrogen.

2Na + 2H₂O ⟶ 2NaOH + H₂ ↑

1P and 1n

Reason — Hydrogen has three isotopes i.e., namely protium (¹₁H), deuterium (²₁H) and tritium (³₁H). Deuterium (²₁H) has 1 proton and 1 neutron in its nucleus.

Al, Zn, Pb

Reason — Metals like zinc, aluminium and lead are considered to have a unique nature in the preparation of hydrogen. They react with acids and can even react with hot concentrated alkalis to form hydrogen and a soluble salt.

(i) Zn + 2HCl ⟶ ZnCl₂ (dil.) + H₂ ↑

(ii) Zn + 2NaOH ⟶ Na₂ZnO₂ + H₂ ↑

(iii) Zn + H₂O ⟶ ZnO + H₂ ↑

(a) Granulated zinc, dilute hydrochloric acid or dil. sulphuric acid.

(b) It is collected by the downward displacement of water.

(c) Zn + 2HCl ⟶ ZnCl₂ + H₂ ↑

(d) Labelled diagram of laboratory preparation of hydrogen:

(a) Impurities present are:

1. Hydrogen sulphide (H₂S)
2. Sulphur dioxide (SO₂)
3. Oxides of nitrogen
4. Phosphine (PH₃)
5. Arsine (AsH₃)
6. Carbon dioxide and
7. Water vapour

(b) The impurities can be removed from hydrogen by passing it through:

1. Silver nitrate solution [to remove arsine and phosphine].
   AsH₃ + 6AgNO₃ ⟶ Ag₃As + 3AgNO₃ + 3HNO₃
   PH₃ + 6AgNO₃ ⟶ Ag₃P + 3AgNO₃ + 3HNO₃
2. Lead nitrate solution [to remove hydrogen sulphide].
   Pb(NO₃)₂ + H₂S ⟶ PbS + 2HNO₃
3. Caustic potash solution [to remove sulphur dioxide, carbon dioxide and oxides of nitrogen].
   SO₂ + 2KOH ⟶ K₂SO₃ + H₂O
   CO₂ + 2KOH ⟶ K₂CO₃ + H₂O
   2NO₂ + 2KOH ⟶ KNO₂ + KNO₃ + H₂O
4. A drying agent used to dry the gas. Common drying agents such as fused calcium chloride, caustic potash stick and phosphorus pentoxide remove water vapour.

Thus, the gas is purified and dried and then collected over mercury because mercury has no reaction with it.

The gas should be collected only after all the air in the apparatus has escaped. This can be ascertained by collecting some amount of gas in a test tube and taking it to a flame. If the gas burns quietly, then there is no more air in the flask.

As nitric acid is a powerful oxidizing agent, and the oxygen formed due to its decomposition oxidizes the hydrogen to give water thus defeating the purpose of the reaction. Hence, Nitric acid is not used in the preparation of hydrogen.

3Zn + 8HNO₃ ⟶ 3Zn(NO₃)₂ + 4H₂O + 2NO ↑

Hot concentrated sulphuric acid is not used in the preparation of hydrogen as it is a strong oxidizer and will produce sulphur dioxide.

Zn + 2H₂SO₄ ⟶ ZnSO₄ + SO₂ + 2H₂O

(a) Steam is passed over hot coke (1000°C) in furnaces of a special design, called converters, giving water gas.

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

The reaction is endothermic.

(b) Water gas i.e., hydrogen and carbon monoxide is mixed with excess steam and passed over heated ferric oxide, which acts as a catalyst and chromic oxide Cr₂O₃ which acts as a promoter.

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

(a) 3Fe + 4H₂O ⇌ Fe₃O₄ + 4H₂ ↑

(b) Ca + 2H₂O ⟶ Ca(OH)₂ + H₂ ↑

(a) Hydrogen gas.

It burns silently in air or oxygen with a pale blue flame, forming water.

2H₂ + O₂ ⟶ 2H₂O

When it is premixed with air or oxygen, it explodes with a pop sound.

(b) Ca + 2H₂O ⟶ Ca(OH)₂ + H₂ ↑

(c) The solution turns blue.

(a) (i) When we heat magnesium, copper and iron in presence of air they form respective oxides.

(ii) Magnesium and iron react with HCl liberating hydrogen and forming their respective salts.

Mg + 2HCl ⟶ MgCl₂ + H₂ ↑

Fe + 2HCl ⟶ FeCl₂ + H₂ ↑

As Cu is below hydrogen in metal reactivity series, hence, it cannot displace hydrogen from acid and so no reaction takes place.

Cu + HCl (dil) ⟶ no reaction

(iii) Magnesium will displace zinc from zinc sulphate solution as magnesium is more reactive than zinc and is placed above zinc in the reactivity series of metals.

Mg + ZnSO₄ ⟶ MgSO₄ + Zn

Copper and iron are less reactive and are placed below zinc in the reactivity series, hence will not displace it and no reaction will take place.

(b) Mg > Fe > Cu

(a) Granulated zinc contains traces of impurities like copper, which has a slight catalyzing effect on the reaction and speeds it up. Hence, granulated zinc is preferred in the lab preparation of hydrogen over pure zinc.

(b) Hydrogen gas is purified and dried and then collected over mercury because mercury has no reaction with it.

(c) The end of the thistle funnel should be dipped under acid so as to prevent gas from escaping the thistle funnel.

(d) Dilute Sulphuric acid cannot be replaced by concentrated acid in the preparation of hydrogen as it is a strong oxidiser and it will produce sulphur dioxide.

Zn + 2H₂SO₄ ⟶ ZnSO₄ + SO₂ + 2H₂O

(a) In free state, hydrogen is found in traces in earth's crust and atmosphere. Volcanic gases contain 0.025% of it, the earth's crust 0.98%, the earth's atmosphere 0.01% and the atmospheres of the Sun and the stars also contain hydrogen in large amounts.

(b) It is on account of its ability to form water that Lavoisier, in 1783, named it hydrogen (Greek word meaning water former).

(a) 2K + H₂ $\xrightarrow{\Delta}$ 2KH

(b) Ca + H₂ $\xrightarrow{\Delta}$ CaH₂

(a) Calcium is preferred for preparing hydrogen from cold water as the reaction is less vigorous than Sodium.
Ca + 2H₂O ⟶ Ca(OH)₂ + H₂ ↑

(b) Magnesium reacts with boiling water and forms a base, magnesium hydroxide, liberating hydrogen gas.
Mg + 2H₂O ⟶ Mg(OH)₂ + H₂ ↑

(c) Zinc is preferred for preparing hydrogen from steam. When steam is passed over heated zinc, hydrogen is liberated and zinc is converted to zinc oxide.
Zn + H₂O ⟶ ZnO + H₂ ↑

(a) Zinc, because sodium reacts violently with acid, magnesium is expensive and copper does not displace hydrogen when reacted with acids as it lies below hydrogen in the metal activity series.

(b) Dilute sulphuric acid is preferred to prepare hydrogen in the laboratory because:

1. Nitric acid, even in its dilute form, is not used in the preparation of hydrogen from metals because it is a powerful oxidizing agent, and the oxygen formed due to its decomposition oxidizes the hydrogen to give water, thus defeating the purpose of the reaction.
   3Zn + 8HNO₃ ⟶ 3Zn(NO₃)₂ + 4H₂O + 2NO↑
2. The reason for not using concentrated sulphuric acid is that it is a strong oxidiser and will produce sulphur dioxide.
   Zn + 2H₂SO₄ ⟶ ZnSO₄ + SO₂ + 2H₂O

(c) The purified and dry hydrogen gas is collected over mercury because mercury has no reaction with it.
Common drying agents like fused calcium chloride, caustic potash stick and phosphorous pentoxide can be used to dry hydrogen gas.

The following metals are not used in the lab. preparation of hydrogen because:

(a) Calcium is expensive.

(b) Iron has to be heated, but then the hydrogen thus produced contains impurities like hydrogen sulphide and sulphur dioxide.

(c) Aluminium forms a protective coating of Al₂O₃ due to its great affinity for oxygen. Due to the coating of Al₂O₃, aluminium does not give hydrogen with acid.

(d) Sodium reacts violently with acid.

Iron < Zinc < Magnesium < Calcium < Sodium

As magnesium is more reactive than hydrogen and is placed above it in the metal reactivity series, hence hydrogen is displaced by magnesium but mercury and silver are less reactive than hydrogen and are placed below it in the series, hence they can't displace hydrogen.

The conditions necessary for the reaction of steam with a metal and a non-metal to liberate hydrogen are:

1. Presence of steam — Steam (water vapor) is required for this reaction to occur.
2. Heat — The reaction is typically carried out at high temperatures to facilitate the reaction between the metal or non-metal and steam.

Reaction of Zinc (metal) with steam to liberate hydrogen:

$\underset{\text{heated}}{\text{Zn}} + \underset{\text{steam}}{\text{H}_2\text{O}} \longrightarrow \text{ZnO} + \text{H}_2 \uparrow$

Reaction of Carbon (non-metal) with steam to liberate hydrogen:

$\underset{\text{Red Hot Coke}}{\text{C}} + \underset{\text{steam}}{\text{H}_2\text{O}} \longrightarrow \underbrace{\text{CO} + \text{H}_2}_{\text{Water gas}}$

(a) dilute sulphuric acid

Zn + H₂SO₄ (dil.) ⟶ ZnSO₄ + H₂ ↑

Fe + H₂SO₄ (dil.) ⟶ FeSO₄ + H₂ ↑

(b) dilute hydrochloric acid

Zn + 2HCl (dil.) ⟶ ZnCl₂ + H₂ ↑

Fe + 2HCl (dil.) ⟶ FeCl₂ + H₂ ↑

Zinc and iron are above hydrogen in activity series of metals whereas copper is below hydrogen in the series. As metals below hydrogen cannot displace it from dilute acids hence, copper does not show similar behaviour as zinc and iron.

(a) Lead reacts with dilute sulphuric acid or hydrochloric acid and forms an insoluble coating of lead sulphate or lead chloride. Therefore, further reaction is prevented. Hence, it is not used to prepare hydrogen.

(b) Potassium and sodium react violently with acid, hence they are not used for reaction with dilute hydrochloric acid or dilute sulphuric acid in the laboratory preparation of hydrogen.

NaOH and KOH

Zn + 2NaOH ⟶ Na₂ZnO₂ + H₂ ↑

Zn + 2KOH ⟶ K₂ZnO₂ + H₂ ↑

Zinc and aluminium are considered to have a unique nature because:

(i) They can react with acids and can even react with hot conc. alkalis to form hydrogen and a soluble salt.

Zn + 2HCl ⟶ ZnCl₂ + H₂

(ii) Their oxides and hydroxides are amphoteric in nature i.e., they react with both bases and acids to give salt and water.

ZnO + 2HCl ⟶ ZnCl₂ + H₂O

ZnO + 2NaOH ⟶ Na₂ZnO₂ + H₂O

(a) 2Na + 2H₂O ⟶ 2NaOH + H₂

(b) Ca + 2H₂O ⟶ Ca(OH)₂ + H₂

(c) Mg + 2H₂O ⟶ Mg(OH)₂ + H₂

(d) Zn + H₂O ⟶ ZnO + H₂

(e) 3Fe + 4H₂O ⇌ Fe₃O₄ + 4H₂

(f) Zn + 2HCl ⟶ ZnCl₂ + H₂

(g) 2Al + 3H₂SO₄ ⟶ Al₂(SO₄)₃ + 3H₂

(h) Fe + 2HCl ⟶ FeCl₂ + H₂

(i) Zn + 2NaOH ⟶ Na₂ZnO₂ + H₂

(j) 2Al + 2KOH + 2H₂O ⟶ 2KAlO₂ + 3H₂

(a) Iron oxide is formed with liberation of hydrogen gas.

3Fe + 4H₂O ⇌ Fe₃O₄ + 4H₂ ↑

(b) Hydrogen when passed over heated magnetic oxide of iron, it reduces the oxide to free metal.

Fe₂O₃ + 3H₂ ⟶ 2Fe + 3H₂O

(a) Magnesium

(b) The reaction of Magnesium with:

1. boiling water
   Mg + 2H₂O ⟶ Mg(OH)₂ + H₂ ↑
2. steam
   Mg + H₂O [steam] ⟶ MgO + H₂ ↑

On passing hydrogen gas through the soap solution, soap bubbles filled with hydrogen start coming out of the solution and rise up in the air. This behaviour proves that hydrogen is lighter than air.

(a) Three volumes of hydrogen and one volume of nitrogen react at temperature 450-500°C, pressure 200-900 atm in the presence of a catalyst, finely divided iron, with molybdenum as a promoter to give ammonia.

$\text{N}_2 + 3\text{H}_2 \xrightleftharpoons[\text{450-500 \degree C}]{\text{Fe}} \text{2NH}_3 + \Delta$

(b) Equal volumes of hydrogen and chlorine react slowly in diffused sunlight but explosively in direct sunlight, to form hydrogen chloride.

$\text{H}_2 + \text{Cl}_2 \xrightarrow{\text{sunlight}} 2\text{HCl}$

(c) Hydrogen gas on passing through molten sulphur reacts to give hydrogen sulphide.

$\text{H}_2 + \underset{\text{molten sulphur}}{\text{S}} \longrightarrow \text{H}_2\text{S}$

(d) Hydrogen burns (in presence of electric spark) with a 'pop' sound in oxygen and with a blue flame forming water.

$2\text{H}_2 + \text{O}_2 \xrightarrow{\text{electric spark}} 2\text{H}_2\text{O}$

(a) A = copper oxide [CuO],
B = copper [Cu]

(b) The liquid does not change the colour of blue or red litmus paper, showing neutral nature.

It changes the colour of cobalt chloride paper from blue to pink colour. Hence, the liquid produced is water.

(c) Black copper oxide (A) is reduced to copper, by losing oxygen. This happens because, hydrogen, the reducing agent reduces copper oxide to copper.

(d) CuO + H₂ $\xrightarrow{\Delta}$ Cu + H₂O

(e) As copper is less reactive than hydrogen and lies below it in the metal reactivity series, hence it cannot displace it from HCl. Hence, there is no reaction between them.

Cu + HCl ⟶ No reaction

In a chemical reaction, if one substance is oxidized, the other substance must necessarily be reduced. This is because the electrons lost during oxidation are simultaneously gained during reduction and vice versa.

For example, Zinc reacts with copper sulphate to form zinc sulphate and copper.

CuSO₄ + Zn ⟶ ZnSO₄ + Cu

Cu²⁺ SO₄^2- + Zn ⟶ Zn²⁺ SO₄^2- + Cu

Writing half-reaction,

Zn ⟶ Zn²⁺ + 2e^- (Oxidation)

Cu²⁺ + 2e^- ⟶ Cu (Reduction)

They occur simultaneously as

Cu²⁺ + Zn ⟶ Zn²⁺ + Cu

Thus, oxidation and reduction always occur simultaneously in a chemical reaction.

Oxidation is a process in which an atom or ion loses an electron(s).

Example: Zn ⟶ Zn²⁺ + 2e^-

Reduction is a process in which an atom or ion gains electron(s).

Example: Cu²⁺ + 2e^- ⟶ Cu (Reduction)

Similarity of hydrogen with group 1 elements:

1. Electronic configuration — Both hydrogen and alkali metals have only one electron in their outermost orbits.
2. Ion formation — Both form a cation by loss of an electron.
   H ⟶ H⁺ + e^-
   Na ⟶ Na⁺ + e^-
3. Valency — Both have the valency 1.
4. Reducing power — Both act as reducing agents.
   CuO + H₂ ⟶ Cu + H₂O
   CuO + 2Na ⟶ Cu + Na₂O
5. Reaction with Oxygen — Both react with oxygen to form respective oxides.
   Hydrogen forms H₂O
   Sodium forms Na₂O

Similarity of hydrogen with group 17 elements:

1. Electronic configuration — Hydrogen and halogens have one electron less than the nearest inert gas.
2. Physical state — Like halogens (fluorine and chlorine), hydrogen too is a gas.
3. Ion formation — Both form an anion by accepting an electron.
   H + e^- ⟶ H^-
   Cl + e^- ⟶ Cl^-
4. Valency — Both have the valency 1.
5. Atomicity — Hydrogen as well as halogens exist in the form of diatomic molecules (H₂, F₂, Cl₂, Br₂, I₂).

Zinc is the most preferred metal in the laboratory preparation of hydrogen.

Other metals are not used because of the following reasons:

1. Sodium and potassium react violently with acid.
2. Calcium and magnesium are expensive.
3. Aluminium forms a protective coating of Al₂O₃ due to its great affinity for oxygen. Due to the coating of Al₂O₃, aluminium does not give hydrogen with acid.
4. Iron has to be heated, but then the hydrogen thus produced contains impurities like hydrogen sulphide and sulphur dioxide.
5. Lead reacts with dil. sulphuric acid or hydrochloric acid and forms an insoluble coating of lead sulphate or lead chloride. Therefore, further reaction is prevented.
6. Hydrogen cannot be prepared from metals that are below in the metal reactivity series such as copper and mercury, since only metals that are more reactive than hydrogen can displace it from acids.

Reactants : Granulated zinc, dil. hydrochloric acid or dil. sulphuric acid.

Procedure : Place some pieces of granulated zinc in a flat bottom flask fitted with an air tight cork with two holes. Through one hole, pass a thistle funnel with a long stem provided with a stopper, and through the other, a long delivery tube.

Pour dilute hydrochloric acid (or dilute sulphuric acid) through the funnel.

Zn + 2HCl (dil.) ⟶ ZnCl₂ + H₂ ↑

Zn + H₂SO₄ (dil.) ⟶ ZnSO₄ + H₂ ↑

Observation : Reaction will gradually start in the form of effervescence and evolution of gas. When all the air from the apparatus has been expelled, collect the gas over water by downward displacement of water.

Impurities can be removed from hydrogen by passing it through :

1. Silver nitrate solution [to remove arsine and phosphine].
   AsH₃ + 6AgNO₃ ⟶ Ag₃As + 3AgNO₃ + 3HNO₃
   PH₃ + 6AgNO₃ ⟶ Ag₃P + 3AgNO₃ + 3HNO₃
2. Lead nitrate solution [to remove hydrogen sulphide].
   Pb(NO₃)₂ + H₂S ⟶ PbS + 2HNO₃
3. Caustic potash solution [to remove sulphur dioxide, carbon dioxide and oxides of nitrogen].
   SO₂ + 2KOH ⟶ K₂SO₃ + H₂O
   CO₂ + 2KOH ⟶ K₂CO₃ + H₂O
   2NO₂ + 2KOH ⟶ KNO₂ + KNO₃ + H₂O
4. A drying agent used to dry the gas. Common drying agents such as fused calcium chloride, caustic potash stick and phosphorus pentoxide remove water vapour.

Thus, the gas is purified and dried and then collected over mercury because mercury has no reaction with it.

Hydrogen is manufactured by Bosch process. It consists of following steps:

1. Production of water gas

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

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

2. Reduction of steam to hydrogen by carbon monoxide

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

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 temperatures.
[CO is converted to CO₂ with a further yield of hydrogen.]

3. Separation of carbon dioxide [CO₂] 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 behind.

non-combustible

Reason — Hydrogen is also lighter than air but it is a combustible gas. It forms an explosive mixture with air (due to the oxygen present in it) and to avoid this risk helium gas is preferred.

a base

Reason — Active metals like potassium, sodium and calcium react with cold water forming their corresponding hydroxides (i.e., bases) and evolving hydrogen.

CuO

Reason — Copper oxide loses oxygen, thus reduction of CuO to Cu take place.

CuO + H₂ ⟶ Cu + H₂O

It is a strong oxidizing agent.

Reason — Hydrogen is not a strong oxidizing agent. It is a reducing agent. It reduces the oxides of the less active metals i.e., it removes oxygen from strongly heated metal oxides when passed over them and itself gets oxidized to water.

PbO is oxidized to Pb

Reason — PbO is reduced to Pb, as oxygen has been removed from it.

Zn

Reason — Zn has a unique nature.

Zinc is considered to have a unique nature because it can react with acids and can even react with hot conc. alkalis to form hydrogen and a soluble salt.

Zn + 2HCl ⟶ ZnCl₂ + H₂

Zn + 2NaOH ⟶ Na₂ZnO₂ + H₂ ↑

Zn with water also gives hydrogen

Zn + H₂O ⟶ ZnO + H₂

Copper

Reason — Hydrogen cannot be prepared from metals that are below in the metal reactivity series such as copper, since only metals that are more reactive than hydrogen can displace it from acids.

P, Q and S

Reason — In the above reaction, steam is passed over red hot iron. Oxygen has been added to iron, iron has been oxidised by water vapour. Therefore, water is oxidising agent and water is getting reduced by iron, which is reducing agent.

P and R

Reason — The above reaction is an example of reaction between oxide and element, which gives metal. Here zinc (II) oxide is reduced to zinc by losing oxygen. Whereas carbon is getting oxidised by the gain of oxygen. Hence P and Q are incorrect.

All of these

Reason

(i) Zinc reacts with steam.

Zn + H₂O ⟶ ZnO + H₂

(ii) Zinc reacts with dilute acid.

Zn + 2HCl ⟶ ZnCl₂ + H₂

(iii) Zinc reacts with strong alkali.

Zn + 2NaOH ⟶ Na₂ZnO₂ + H₂

B and D

Reason — In the above table, elements (B) and (D), when reacted with both acid and base, evolve hydrogen gas. So, they will form amphoteric oxides.

A is true but R is false.

Explanation — Hydrogen is collected by the downward displacement of water because it forms an explosive mixture with air and therefore cannot be collected by downward displacement of air even though it is lighter than air. Hence the assertion (A) is true.

Hydrogen is virtually insoluble in water (does not dissolve in water). Hence the reason (R) is false.

A is true but R is false.

Explanation — Hydrogen and alkali metals react with copper oxide to give red/ brown copper.

CuO + H₂ ⟶ Cu + H₂O

CuO + 2Na ⟶ Cu + Na₂O
Hence assertion (A) is true.

Both hydrogen and alkali metals acts as reducing agents. Hence reason (R) is False.

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

Explanation — Zinc granules are preferred for preparation of hydrogen over pure zinc because the impurity present in granulated zinc is copper, whose catalysing effect speeds up the rate of reaction. Hence both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

A is true but R is false.

Explanation — In the second step of Bosch process, water gas is mixed with excess steam and passed over heated ferric oxide, which acts as a catalyst and chromic oxide Cr₂O₃, which acts as a promoter to produce hydrogen and carbon dioxide.

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

Hence assertion (A) is true.

The mixture Hydrogen and carbon dioxide(CO₂ + H₂) are separated through caustic potash solution, which removes carbon dioxide by reacting with it, leaving hydrogen. Hence reason (R) is false.

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

Explanation — While preparing hydrogen in laboratory, no leakage of gas should take place and no flame must be present near the apparatus because the mixture of hydrogen and air explodes violently when it is brought near a flame. Hence assertion (A) is true.
When hydrogen is pre-mixed with air or oxygen, it explodes with a pop sound. Hence reason (R) is true.
But reason (R) does not explain why no leakage of gas and no flame must be maintained in laboratory. Hence reason (R) is not correct explanation of assertion (A).

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

Explanation — Hydrogen shows a dual nature. Hydrogen resembles the alkali metals of Group IA and the halogens of Group VIIA. This is due to the fact that it has only one electron in its valence shell, hence it behaves like the alkali metals which have one valence electron. At the same, it also behaves like halogens as it has just one electron less than the nearest inert gas. Hence both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

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

Explanation — Metals which are more reactive than hydrogen, when reacted with both dilute sulphuric acid and dilute hydrochloric acid displace hydrogen. The extent to which these reaction occur for a given metal is also based on the activity series of metal where the more reactive element displaces the less reactive element from its salt solution. Hence both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).

A is true but R is false.

Explanation — Metals like zinc, lead and aluminium have a unique nature because they react with acids and can even react with hot concentrated alkalis to form hydrogen and a soluble salt. Hence, the assertion (A) is true.

Zinc, lead and aluminium have different valencies. Zinc and lead have +2 valency and aluminium has +3 valency. Hence, the reason (R) is false.

A is true but R is false.

Explanation — Zinc granules are preferred for laboratory preparation of hydrogen over pure zinc because the impurity present in granulated zinc is copper, whose catalysing effect speeds up the rate of reaction. Hence the assertion (A) is true.
Zinc is unique in nature and does contain impurity like copper. Hence reason (R) is false.

A is true but R is false.

Explanation — Reduction is also defined as the process that involves removal of oxygen from a substance or the addition of hydrogen to a substance. Hence, the assertion (A) is true.
Hydrogen is a reducing agent not oxidising agent. Hence, the reason (R) is false.

(i) Zn + Pb²⁺ ⟶ Zn²⁺ + Pb

Oxidation : Zn ⟶ Zn²⁺ + 2e^-

Reduction : Pb²⁺ + 2e^- ⟶ Pb

(ii) Zn + Cu²⁺ ⟶ Zn²⁺ + Cu

Oxidation : Zn ⟶ Zn²⁺ + 2e^-

Reduction : Cu²⁺ + 2e^- ⟶ Cu

(iii) Cl₂ + 2Br^- ⟶ Br₂ + 2Cl^-

Oxidation : Cl₂ ⟶ 2Cl^- + 2e^-

Reduction : 2Br^- + 2e^- ⟶ Br₂

(a) The equation in the ionic form:

Cu²⁺ SO₄^2- + Fe ⟶ Fe²⁺ SO₄^2- + Cu

(b) Oxidation : Fe ⟶ Fe²⁺ + 2e^-

Reduction : Cu²⁺ + 2e^- ⟶ Cu

(a) Na, as it reacts with cold water to produce hydrogen gas while others react with hot water/steam to produce hydrogen gas.

(b) NH₃, as it is basic in nature but rest are acidic.

(c) Cu, as it is below hydrogen in the metal activity series, hence it cannot displace hydrogen from dilute acids whereas others are above hydrogen and can displace it from dilute acids.

(d) Fe as rest of the elements Pb, Al and Zn have unique nature. They can react with acids as well as hot concentrated alkalis to form hydrogen and a soluble salt.

(a) Hydrogen is collected by the downward displacement of water as it forms an explosive mixture with air and therefore, cannot be collected by downward displacement of air even though it is lighter than air.

(b) Hydrogen is a combustible gas but it does not support the combustion process, hence, when a candle is brought near the mouth of a jar containing hydrogen gas, it starts burning but is extinguished when pushed inside the jar.

(c) Apparatus for laboratory preparation of hydrogen should be air tight becasue hydrogen forms an explosive mixture with air. Hydrogen is a highly flammable gas, if a naked flame is present near the apparatus, it can lead to an ignition of the hydrogen, resulting in an explosion.

Half reaction:

A⁺ + e^- ⟶ A (Reduction)

B ⟶ B + e^- (Oxidation)

a. A, as it is oxidizing B by accepting electrons.

b. B, as it is losing electrons.

c. B, a it is reducing A by providing electrons.

(a) Oxidation, gain of oxygen

(b) Reduction, gain of electron

(c) Oxidation; loss of electron

(d) Oxidation, loss of electron

(a) PbO is reduced, due to removal of oxygen.

(b) Mg is oxidised, due to loss of electrons.

(c) H₂S is oxidised, as hydrogen is lost to give sulphur.

(d) Chlorine is reduced, due to addition of hydrogen to form HCl.

(a) Hydrogen

(b) Downward displacement of water.

This method is used as:

1. Hydrogen is virtually insoluble in water (20 ml of hydrogen dissolves in 1 litre of water under normal conditions)
2. Hydrogen forms an explosive mixture with air and therefore, cannot be collected by downward displacement of air even though it is lighter than air.

(c) Nitric acid, even in its dilute form, is not used in the preparation of hydrogen from metals because it is a powerful oxidizing agent, and the oxygen formed due to its decomposition oxidizes the hydrogen to give water, thus defeating the purpose of the reaction.
3Zn + 8HNO₃ ⟶ 3Zn(NO₃)₂ + 4H₂O + 2NO↑

(d) Conc. H₂SO₄ is a good drying agent. However, it is not used to dry hydrogen as it reacts with hydrogen, thus defeating the purpose of the reaction.
H₂SO₄ + H₂ ⟶ 2H₂O + SO₂

Gas evolved is Hydrogen.

(a) The reaction and the evolution of hydrogen gas will proceed more quickly if the same quantity of zinc dust is placed in the test tube. This is because the surface area of zinc dust is greater than that of zinc granules.

(b) When dilute hydrochloric acid is used in place of sulphuric acid, same amount of gas will evolve.

Zn + 2HCl (dil.) ⟶ ZnCl₂ + H₂ ↑

(c) Zinc reacts with hot concentrated alkalis like sodium hydroxide to form hydrogen and a soluble salt.

Zn + 2NaOH ⟶ Na₂ZnO₂ + H₂ ↑

(a) Hydrogen.

(b) Downward displacement of water.

(c) Hydrogen gas is virtually insoluble in water.

(d) Hydrogen gas is lighter than air.

(e) Hydrogen gas is combustible but does not support combustion.

(a) When CuO reacts with hydrogen, CuO is reduced to Cu and H₂ is oxidized to H₂O

(b) Hydrogen is sparingly soluble in water.

(c) Metals like magnesium, iron and aluminium give H₂ with steam.

(d) Sodium amalgam reacts smoothly with cold water.

(e) A metal above hydrogen in the activity series gives hydrogen with dilute hydrochloric acid or dilute sulphuric acid acid.

a) Chlorine [Cl₂]

b) Manganese dioxide [MnO₂]

c) Hydrogen sulphide [H₂S]

(d) Hydrogen peroxide [H₂O₂]

(e) Hydrogen [H₂]

(f) Potassium permanganate [KMnO₄]

(a) Hydrogen is separated from CO₂ by passing the mixture through caustic potash solution.

(b) All the metals above hydrogen in the metal activity series react with acids to give hydrogen.

(c) Hydrogen is dried by passing it through fused calcium chloride.

(d) Very dilute nitric acid reacts with magnesium [Mg] and manganese [Mn] to produce hydrogen.

(e) Dilute H₂SO₄ reacts with zinc to liberate hydrogen.