Study of the First Element — Hydrogen

Answer

Answer

Answer

Name an element which reacts violently with water at room temperature.

Answer

Potassium

State What do the following symbols [or formula] denote : 2H ; H₂ ; H⁺. [two atoms, molecule, ion]

Answer

2H — 2 atoms of hydrogen

H₂ — A molecule of hydrogen

H⁺ — A Hydrogen ion

Name a gas which burns in air or oxygen forming water.

Answer

Hydrogen

Write correctly balanced equation for the following 'word equation' :

calcium + water ⟶ calcium hydroxide + hydrogen

Answer

Ca + 2H₂O ⟶ Ca(OH)₂ + H₂

When steam is passed over red-hot iron, magnetic oxide of iron and hydrogen are obtained. 'The reaction between steam and red-hot iron is a Reversible Reaction'. What is meant by this statement.

Answer

The reaction between steam and red-hot iron is as follows:

$3\text{Fe} + \underset{\text{steam}}{4\text{H}_2\text{O}} \rightleftharpoons \text{Fe}_3\text{O}_4 + 4\text{H}_2$

It is a reversible reaction because oxide of iron and hydrogen recombine to form iron and steam.

State How can you obtain hydrogen from sodium hydroxide [not by electrolysis].

Answer

When powdered zinc reacts with sodium hydroxide it produces hydrogen.

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

Write balanced equation for the following reaction : magnesium + dil. hydrochloric acid ⟶

Answer

Mg + 2HCl (dil.) ⟶ MgCl₂ + H₂

Write correctly balanced equation for the following :

When steam is passed over red hot iron.

Answer

$3\text{Fe} + \underset{\text{[steam]}}{4\text{H}_2\text{O}} \rightleftharpoons \underset{\text{[magnetic oxide of iron]}}{\text{Fe}_3\text{O}_4} + 4\text{H}_2$

Explain the following :

Two jars of H₂ are collected – 'one burns quietly and the other does not'.

Answer

The jar of hydrogen which burns quietly contains pure hydrogen whereas the other jar does not burn quietly because it contains a mixture of hydrogen and air which is explosive in nature and burns with a pop sound.

Write correctly the balanced equation for the following : 'When zinc filings are added to a concentrated solution of sodium hydroxide'.

Answer

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

Describe one chemical test applied to the following gases, which would enable you to distinguish between them : 'carbon monoxide and hydrogen'.

Answer

CO reacts with excess of Oxygen to give CO₂ gas that turns lime water milky.

2CO + O₂ ⟶ 2CO₂

CO₂ + Ca(OH)₂ ⟶ CaCO₃ + H₂O

Hydrogen reacts with Oxygen to form water. It doesn't turn lime water milky.

2H₂ + O₂ ⟶ 2H₂O

Write down the 'word equation' for the following reaction : sodium hydroxide solution + zinc ⟶

Answer

Sodium hydroxide solution + zinc ⟶ Sodium zincate + hydrogen gas

Explain briefly how hydrogen is manufactured on a large scale, from steam.

Answer

Step I

Reaction : 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

Step II

Reaction : Reduction of steam to hydrogen by carbon monoxide

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

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.]

Step III

Reaction : 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 over.

State the products of the reaction 'when steam is passed over red-hot iron'.

Answer

When steam is passed over red hot iron, magnetic oxide of iron and hydrogen are produced.

$3\text{Fe} + \underset{\text{steam}}{4\text{H}_2\text{O}} \rightleftharpoons \underset{\text{magnetic oxide of iron}}{\text{Fe}_3\text{O}_4} + \underset{\text{hydrogen}}{4\text{H}_2}$

How can you obtain hydrogen from a mixture of hydrogen and carbon monoxide.

Answer

When excess steam is mixed with the mixture of hydrogen and carbon monoxide, at elevated temperature of 450°C and using a catalyst iron [III] oxide, CO is converted to CO₂ with a further yield of hydrogen.

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

State what do you observe when a piece of sodium is dropped into cold water ?

Answer

When a piece of sodium is dropped into cold water, sodium being very reactive, reacts with water violently. It melts into a globule and darts about in water liberating hydrogen gas.

2Na + 2H₂O (cold water) ⟶ 2NaOH + H₂

Give reasons for the following :

'Though hydrogen is lighter than air, it is not collected by the downward displacement of air'.

Answer

As hydrogen forms explosive mixture with air, hence it is not collected by downward displacement of air even though hydrogen is lighter than air.

Complete the following word equations :

(i) Sodium hydroxide + zinc ⟶ hydrogen + ...............
(ii) Calcium + water ⟶ calcium hydroxide + ...............

Answer

Sodium hydroxide + zinc ⟶ hydrogen + sodium zincate
Calcium + water ⟶ calcium hydroxide + hydrogen

How would you obtain 'hydrogen from sodium hydroxide' solution other than by electrolysis ?

Answer

When powdered zinc reacts with sodium hydroxide it produces hydrogen.

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

Complete and balance the following equations :

Al + NaOH + ............... ⟶ ............... + ...............

Answer

2Al + 2NaOH + 2H₂O ⟶ 2NaAlO₂ + 3H₂ [g]

Write balanced equation of the reaction in the preparation of : hydrogen from a solution of potassium hydroxide [other than by electrolysis].

Answer

Preparation of hydrogen from potassium hydroxide :

2Al + 2KOH + 2H₂O ⟶ 2KAlO₂ + 3H₂ [g]

Describe briefly, with equations, the Bosch Process for the large scale production of hydrogen.

Answer

Bosch process for large production of hydrogen:

Step I

Reaction : 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

Step II

Reaction : Reduction of steam to hydrogen - by carbon monoxide

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

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 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.

Account for the following facts :

(i) Though lead is above hydrogen in the activity series, it does not react with dilute hydrochloric acid or dilute sulphuric acid.

(ii) Potassium and sodium are not used to react with dilute hydrochloric acid or dilute sulphuric acid in the laboratory preparation of hydrogen.

Answer

(i) Lead reacts with dilute hydrochloric acid or dilute sulphuric acid forming an insoluble coating of lead chloride (PbCl₂) and lead sulphate (PbSO₄) respectively, hence, stops the further reaction.

(ii) Potassium and sodium are not used to react with dilute hydrochloric acid or dilute sulphuric acid in the laboratory preparation of hydrogen because the reaction is highly explosive and practically not-feasible.

Place the metals calcium, iron, magnesium and sodium in order of their activity with water, placing the most active first. Write the equation for each of the above metals which react with Water.

Answer

These metals arranged in order of their activity with water:

Na > Ca > Mg > Fe

Equations for reaction of metals with water:

Sodium

$2\text{Na} + \underset{\text{cold water}}{2\text{H}_2\text{O}} \longrightarrow 2\text{NaOH} + \text{H}_2$

Calcium

$\text{Ca} + \underset{\text{cold water}}{2\text{H}_2\text{O}} \longrightarrow \text{Ca(OH)}_2 + \text{H}_2$

Magnesium

$\text{Mg} + \underset{\text{boiling water}}{\text{H}_2\text{O}} \longrightarrow \text{MgO} + \text{H}_2$

Iron

$3\text{Fe} + \underset{\text{[steam]}}{4\text{H}_2\text{O}} \rightleftharpoons \underset{\text{[magnetic oxide of iron]}}{\text{Fe}_3\text{O}_4} + 4\text{H}_2$

Why is copper not used to prepare hydrogen by the action of dilute hydrochloric acid or dilute sulphuric acid on the metal.

Answer

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

State the electronic configuration of hydrogen [at. no. 1].

Answer

Hydrogen has the simplest electronic configuration of 1.

State a reason why hydrogen can be placed in group 1 [1A] and group 17 [VIIA] of the periodic table.

Answer

Hydrogen has 1 valence electron, so it either loses one electron and behaves like electropositive alkali metals of group 1 or it gains one electron and behaves like electronegative halogens of group 17. Hence, it shows similarities with alkali metals as well as halogens, therefore, it can be placed in group 1 [1A] or group 17 [VIIA] of the periodic table.

Give the general group characteristics applied to hydrogen with respect to similarity in properties of hydrogen with –

(a) alkali metals of group 1 [IA]
(b) halogens of group 17 [VIIA].

with special reference to valency electrons & ion formation.

Answer

Similarity in properties of hydrogen with

(a) Alkali metals of group 1 [IA] :

1. Electronic Configuration — Valency electron - one valence electron e.g., H = 1; Li = 2, 1; Na = 2, 8, 1; K = 2, 8, 8, 1
2. Ion Formation — Electropositive character exhibited.
   H – 1e^– ⟶ H¹⁺
   Li – 1e^– ⟶ Li¹⁺
   Na – 1e^– ⟶ Na¹⁺
3. Valency — Electrovalency of 'one' exhibited. H¹⁺, Li¹⁺, Na¹⁺

(b) Halogens of group 17 [VIIA] :

1. Electronic Configuration — One electron less than the noble gas e.g., H = 1 [He = 2]; F = 2,7; [Ne = 2,8]; Cl = 2,8,7 [Ar = 2,8,8]
2. Ion Formation — Electronegative character exhibited.
   H + 1e^– ⟶ H^1-
   F + 1e^– ⟶ F^1-
   Cl + 1e^– ⟶ Cl^1-
3. Valency — Electrovalency and covalency exhibited
   Hydrogen - forms NaH [electrovalent]; CH₄ [covalent]
   Chlorine - forms NaCl [electrovalent]; CCl₄ [covalent]

State how does hydrogen occurs in the combined state.

Answer

Hydrogen occurs in the combined state:

1. In plant and animal tissues - which are made of compounds of hydrogen along with carbon, oxygen and nitrogen.
2. In water - about one ninth by mass of water is hydrogen.
3. As a constituent of different substances i.e., acids, alkalis, petroleum products and organic substances.

Give balanced equations for obtaining hydrogen from cold water using –

(a) A monovalent active metal

(b) A divalent active metal

Answer

(a) Monovalent active metal Na :

$2\text{Na} + \underset{\text{cold water}}{2\text{H}_2\text{O}} \longrightarrow 2\text{NaOH} + \text{H}_2$

(b) Divalent active metal Ca :
$\text{Ca} + \underset{\text{cold water}}{2\text{H}_2\text{O}} \longrightarrow \text{Ca(OH)}_2 + \text{H}_2$

Give balanced equations for obtaining hydrogen from ?

(a) Boiling water using a divalent metal

(b) Steam using a trivalent metal

(c) Steam using a metal – and the reaction is reversible.

Answer

(a) Mg + H₂O ⟶ MgO + H₂ [g]

(b) 2Al + 3H₂O ⟶ Al₂O₃ + 3H₂ [g]

(c) 3Fe + 4H₂O ⇌ Fe₃O₄ + 4H₂ [g]

State why hydrogen is not prepared in the laboratory by the action of –

(a) Sodium with cold water.

(b) Calcium with dilute sulphuric acid.

(c) Lead with dilute hydrochloric acid.

Answer

(a) Sodium is very reactive and reacts with water violently. It melts into a globule and darts about in water. Hence, the collection of hydrogen is difficult using sodium and cold water.

(b) Calcium is not used with dilute sulphuric acid because insoluble calcium sulphate forms a coating on the metal surface and stops further reaction. Hence, hydrogen is not prepared in the laboratory by this method.

(c) Lead reacts with dilute hydrochloric acid forming an insoluble coating of lead chloride (PbCl₂) and hence, further reaction comes to a stop. Therefore, hydrogen is not prepared in the laboratory by the action of lead with dilute hydrochloric acid.

Give balanced equations for the following conversions.

(a) Sodium zincate from zinc

(b) Sodium plumbite from lead

(c) Sodium aluminate from aluminium.

Answer

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

$\text{(b) } \text{Pb} + 2\text{NaOH} \longrightarrow \underset{\text{[sodium plumbite]}}{\text{Na}_2\text{PbO}_2} + \text{H}_2 \text{[g]}$

$\text{(c) } 2\text{Al} + 2\text{NaOH} + 2\text{H}_2\text{O} \longrightarrow \underset{\text{[sodium aluminate]}}{2\text{NaAlO}_2} + 3\text{H}_2 \text{[g]}$

Give reasons for the following : In the laboratory preparation of hydrogen from zinc and dil. acid.

(a) The complete apparatus is air-tight.

(b) Dilute nitric acid is not preferred as the reactant acid.

(c) The lower end of the thistle funnel should dip below the level of the acid in the flask.

(d) Hydrogen is not collected over air.

Answer

(a) Hydrogen forms an explosive mixture with air hence the complete apparatus is air tight preventing any leakage of the gas.

(b) Nitric acid is a strong oxidizing agent, it oxidizes the hydrogen formed to water and hence is not preferred as the reactant acid.

(c) The lower end of the thistle funnel should dip below the level of the acid in the flask, otherwise hydrogen gas produced will escape through thistle funnel.

(d) Hydrogen is collected over water and not air as it forms an explosive mixture with air.

In the Bosch process, state the conditions and give balanced equations for the conversion of –

(a) coke to water gas,

(b) water gas to hydrogen

Answer

(a) coke to water gas

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

(b) water gas to hydrogen

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

State how are the unreacted gases separated out in 'Bosch process' in the manufacture of hydrogen.

Answer

(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.

Compare the combustibility of –

(a) pure hydrogen

(b) hydrogen-air mixture.

Answer

Pure hydrogen burns quietly in air with a pale blue flame forming water:
2H₂ + O₂ ⟶ 2H₂O.

Hydrogen air mixture explodes on burning.

State the reactant added to hydrogen to obtain the respective product in each case.

(a) Ammonia

(b) Hydrogen chloride

(c) Water

(d) Hydrogen sulphide

Answer

Reactant added to obtain :

(a) Ammonia — Nitrogen and hydrogen.
     N₂ + 3H₂ ⇌ 2NH₃

(b) Hydrogen chloride — Hydrogen and chlorine.
     H₂ + Cl₂ ⟶ 2HCl

(c) Water — Hydrogen and oxygen.
     2H₂ + O₂ ⟶ 2H₂O

(d) Hydrogen sulphide — Hydrogen and sulphur.
     H₂ + S ⟶ H₂S

State the use of hydrogen –

(a) As a fuel

(b) In hydrogenation of oil & coal

(c) In extraction of metals

Answer

(a) As a fuel — Hydrogen is used in the form of coal gas, water gas and liquid hydrogen.

(b) In hydrogenation of oil & coal — Vegetable oil [palm oil] turns to semi solid fats [vegetable ghee] in the presence of platinum or nickel catalyst under high pressure at about 200°C by hydrogenation.
Passage of hydrogen under high pressure over powdered coal in presence of catalyst and at a suitable temperature produces a product similar to petroleum.

(c) In extraction of metals — Hydrogen when passed over heated metal oxides of less active metals like zinc, iron, lead and copper reduces the oxides of the metals to free metals.

Explain the terms oxidation and reduction in terms of addition and removal of oxygen/hydrogen with suitable examples.

Answer

Oxidation is addition of oxygen to a substance or removal of hydrogen from a substance. For example,

Addition of oxygen — 2Cu + O₂ ⟶ 2CuO

Removal of hydrogen — H₂S + Cl₂ ⟶ 2HCl + S

Reduction is addition of hydrogen to a compound or removal of oxygen from a compound.

Removal of oxygen — CuO + H₂ ⟶ Cu + H₂O

Addition of hydrogen — Cl₂ + H₂S ⟶ 2HCl + S

Explain the term redox reaction with an example involving the reaction of hydrogen sulphide with chlorine.

Answer

A chemical reaction in which oxidation of one substance and reduction of another substance take place simultaneously is called a redox reaction.

For example, in the reaction of hydrogen sulphide with chlorine:

H₂S + Cl₂ ⟶ 2HCl + S

H₂S loses hydrogen and is oxidised to sulphur. Chlorine gains hydrogen and is reduced to hydrogen chloride. Hence, this is a redox reaction.

Reducing agent — H₂S
Oxidising agent — Cl₂

State what are oxidising and reducing agents. Give examples of oxidising and reducing agents in the gaseous, liquid and solid form. Give two tests each generally answered by oxidising and reducing agents respectively.

Answer

Oxidizing Agents :

• Loses oxygen or electronegative radical
• Gains hydrogen or electropositive radical
• Accepts electrons.

Examples:

• Gases - O₂, O₃, Cl₂, F₂
• Liquids - H₂O₂, K₂Cr₂O₇ [acidified], conc. HNO₃, conc. H₂SO₄
• Solids - MnO₂, HgCl₂, PbO₂, KNO₃

Reducing Agents :

• Gains oxygen or electronegative radical
• Loses hydrogen or electropositive radical
• Donates electrons.

Examples:

• Gases - H₂, CO, SO₂, NH₃
• Liquids - HCl, HBr, H₂SO₃, HNO₂
• Solids - Carbon [coke], metals [K, Na, Ca], Sugar [C₁₂H₂₂O₁₁], SnCl₂

Test for an Oxidizing Agent :

• Decomposes to give a colourless gas which rekindles a glowing splint.
• Liberates chlorine on heating with conc. HCl

Test for a Reducing Agent :

• On reaction with iron[III] salts - the colour changes from yellowish brown to light green [iron [II] ions are formed].
• Liberates nitrogen dioxide on heating with conc. HNO₃.

The element with which hydrogen does not show similarity.

1. Li
2. K
3. F
4. C

Answer

C

Reason — Hydrogen shows similarity with alkali metals like lithium and potassium because it has one valence electron. It also shows similarity with halogens like fluorine because it can gain one electron to form H^1-. It does not show similarity with carbon.

A metal which reacts with hot conc. alkali to give hydrogen, but not with dil. acid to liberate the same.

1. Zn
2. Pb
3. Fe
4. Al

Answer

Pb

Reason — Pb reacts with hot concentrated alkali & gives hydrogen. It does not react with dilute acids easily because an insoluble coating of lead chloride or lead sulphate is formed with dilute HCl and dilute H₂SO₄ .
Zinc, Iron, Aluminium react with dilute acids & give hydrogen gas.

A metal which reacts with steam and with an alkali, to liberate hydrogen.

1. Al
2. Fe
3. Pb
4. Mg

Answer

Al

Reason — Aluminium reacts with steam to form aluminium oxide & hydrogen. It reacts with alkali to form aluminate & hydrogen.
Iron, Magnesium react with steam to give hydrogen. They do not react with alkali. Lead reacts with alkali but does not react with steam.

State which of the statements pertaining to the laboratory preparation of hydrogen, is incorrect.

1. Granulated zinc is placed in a flat bottom flask & dilute HCl added through the thistle funnel.
2. The apparatus is airtight, since hydrogen forms an explosive mixture with air.
3. Hydrogen like ammonia gas, is lighter than air & hence like ammonia is collected by the downward displacement of air.
4. Dil. sulphuric acid or dil. hydrochloric acid, are preferred to dilute nitric acid in the laboratory preparation.

Answer

Hydrogen like ammonia gas, is lighter than air & hence like ammonia is collected by the downward displacement of air.

Reason — Hydrogen is not collected by downward displacement of air in labs because hydrogen forms an explosive mixture with air. Hydrogen is safely collected by downward displacement of water.
Granulated zinc and dilute HCl in a flask is the standard lab method. The apparatus must be airtight because hydrogen is highly explosive with air. Dilute HCl or sulphuric acid are used. Dilute nitric acid is not preferred because it is an oxidising agent and does not liberate hydrogen properly.

In Bosch process [industrial preparation], pick the incorrect statement in each step of the production.

1. Step 1 — involves production of, water gas by passage of steam over white hot coke, at temperatures around 1000°C.
2. Step 2 — involves production of hydrogen by reaction of water gas with excess steam, in presence of a catalyst iron [III] oxide.
3. The conversion of water gas to hydrogen in the above reaction, is an oxidation process.
4. In the above process — both potassium hydroxide solution & ammoniacal cuprous chloride solution can be used, for removal of unreacted CO & CO₂ in the above mixture.

Answer

The conversion of water gas to hydrogen in the above reaction, is an oxidation process.

Reason — In the Bosch process, steam is reduced to hydrogen by carbon monoxide. Carbon monoxide itself is oxidised to carbon dioxide, but the production of hydrogen from steam is a reduction process. Hence, statement 3 is incorrect.

Assertion: Hydrogen is now replaced by helium, in meteorological balloons.

Reason: Hydrogen is lighter than air.

1. Both A and R are true and R is the correct explanation of A.
2. Both A and R are true but R is not the correct explanation of A.
3. A is true and R is false.
4. A is false and R is true.

Answer

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

Reason — A is true as Helium is preferred because it is non-flammable and safe, unlike hydrogen which is explosive.
R is true as hydrogen is the lightest gas, even lighter than helium.

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

Vegetable oils turn to semi solid fats, by absorption of hydrogen on the surface of a certain metals like palladium. The phenomenon takes place due to, or by -

1. Hydrogenation
2. Oxidation
3. Occlusion
4. Reduction

Answer

Occlusion

Reason — The absorption or adsorption of large volumes of hydrogen on the surface of metals like platinum, nickel, gold and palladium is called occlusion. This property is useful in the hydrogenation of vegetable oils.

In the reaction,

2H₂S + SO₂ ⟶ 2H₂O + 3S

the reaction takes place by-

1. Removal of oxygen from a compound
2. Addition of hydrogen to a compound
3. Removal of electronegative element
4. Removal of hydrogen from a compound

Answer

Removal of oxygen from a compound

Reason — Reduction can be defined as removal of oxygen. In SO₂, oxygen is removed, forming elemental sulphur.
Removal of electronegative element is also reduction but here specifically oxygen is removed. Addition of hydrogen is not happening here. Removal of hydrogen from a compound indicates oxidation.

Redox reactions involve :

1. Loss or gain of electrons, in the substance involved in the reaction.
2. Oxidation of one substance, only in the reaction.
3. Reduction of one substance, only in the reaction.
4. Gain of electropositive radical, by one substance only in the reaction.

Answer

Loss or gain of electrons, in the substance involved in the reaction.

Reason — A redox reaction always involves both oxidation and reduction happening together.
Oxidation cannot occur alone; reduction must also occur simultaneously.

Hydrogen is placed above Lithium [Li] in group 1 or above Fluorine [F] in group 17 but not above beryllium [Be] in group 2 or oxygen [O] in group 16. Give reasons.

Answer

Hydrogen shows dual nature. Like Lithium, hydrogen has one electron in its outermost shell. Both can lose one electron to form Li⁺ and H⁺. Like Fluorine, hydrogen needs one electron to complete its shell. It can gain one electron to form H^- (hydride ion). Beryllium has two valence electrons. Hydrogen has only one electron, so it cannot behave like Group 2 elements. Oxygen has six valence electrons. It needs two electrons to complete its octet. Hydrogen needs only one electron, so it does not match Group 16 behavior.

The reaction of iron with steam is reversible, but of zinc with steam is not. Give reasons.

Answer

The difference is due to the reactivity of the metals and stability of their oxides. Iron is moderately reactive. So, the oxide formed is not extremely stable. Hence, hydrogen formed can reduce the oxide back to iron, making the reaction reversible. Zinc is more reactive than iron. So, the oxide formed is very stable. Hydrogen cannot easily reduce ZnO back to zinc. Therefore, the reaction is irreversible.

In the laboratory preparation of hydrogen by the action of dilute acids on metals reaction 'A' is used but reaction 'B' is not. Give reasons.

A: Zn + HCl [dil.] ⟶

B: Pb + HNO₃ [dil.] ⟶

Answer

Reaction A is used because zinc reacts with dilute hydrochloric acid to liberate hydrogen gas.

Zn + 2HCl ⟶ ZnCl₂ + H₂ [g]

Reaction B is not used because dilute nitric acid is an oxidising agent. The nascent oxygen formed on its decomposition oxidises the hydrogen to water.

Hence, Zn + dil. HCl is used for laboratory preparation of hydrogen, but Pb + dil. HNO₃ is not used.

Magnesium reacts with very dil. nitric acid at low temperatures liberating hydrogen. Give reasons.

Answer

Magnesium reacts with dil. HNO₃ at low temperature liberating H₂, since oxidising action of the acid is much reduced due to dilution.

In the laboratory preparation of hydrogen from Zn & dil. HCl, the hydrogen is collected & then tested only after all the air in the apparatus is allowed to escape. Give reasons.

Answer

Hydrogen is collected and tested only after all the air in the apparatus has escaped because the first jar may contain a mixture of hydrogen and air. Such a mixture is explosive and burns with a pop sound. Pure hydrogen burns quietly in air; hence its purity is tested only after air has been displaced.

In the reaction in the Bosch process, for the manufacture of hydrogen the reaction is -

CO + H₂ + H₂O $\xrightarrow[\text{Fe}_2\text{O}_3]{450 \degree \text{C}}$ CO₂ + 2H₂ + $\Delta$

(a) State which substance is oxidised & which is reduced.

(b) To separate CO₂ from unreacted CO, state why the mixture is not dissolved in water under normal atmospheric pressure, but in water subjected to high pressure.

(c) The reason for adding chromic oxide to iron [III] oxide, in the reaction.

Answer

(a) Carbon monoxide is oxidised.
Water is reduced.

(b) The mixture is dissolved in water under high pressure because carbon dioxide is much more soluble in water at high pressure. At normal atmospheric pressure, CO₂ does not dissolve sufficiently and hence cannot be separated efficiently from unreacted CO.

(c) Chromic oxide is added as a promoter to increase the efficiency and life of the catalyst.

In extraction of metals, hydrogen reduces oxides of metals, e.g. Zn, Fe & Cu but not oxides of K, Na & Ca to free metals.

Answer

Hydrogen reduces oxides of less active metals such as zinc, iron and copper to their free metals. However, oxides of highly reactive metals such as potassium, sodium and calcium are very stable. Hydrogen is not a strong enough reducing agent to remove oxygen from these stable oxides. Hence, K, Na and Ca cannot be extracted from their oxides by reduction with hydrogen.

Is the conversion of potassium iodide to iodine, a reduction or an oxidation reaction. Give reasons.

Answer

Potassium iodide loses electropositive radical potassium & is therefore oxidised to iodine.

2KI + H₂O₂ ⟶ 2KOH + I₂

Iodide ions lose electrons and loss of electrons is called oxidation. Therefore, the conversion of potassium iodide to iodine is an oxidation reaction.

In the reaction

2FeCl₃ + H₂S ⟶ 2FeCl₂ + 2HCl + S

H₂S acts as a reducing agent. Give reasons.

Answer

Hydrogen sulphide acts as a reducing agent because it reduces iron [III] chloride to iron [II] chloride. In this reaction, Fe³⁺ gains electrons and is reduced to Fe²⁺, while H₂S is oxidised to sulphur. Since H₂S causes the reduction of Fe³⁺, it acts as a reducing agent.

Select from A to G the reactant added, to give the products 1 to 5, in the preparation of hydrogen gas.

A : dilute acid
B : dilute alkali
C : cold water
D : conc. alkali
E : boiling water
F : conc. acid
G : steam

1. Ca(OH)₂ + H₂
2. MgO + H₂
3. Fe₃O₄ + H₂
4. Al₂(SO₄)₃ + H₂
5. NaAlO₂ + H₂

Answer

1. C : cold water

$\text{Ca} + \underset{\text{cold water}}{2\text{H}_2\text{O}} \longrightarrow \text{Ca(OH)}_2 + \text{H}_2$

2. E : boiling water

$\text{Mg} + \underset{\text{boiling water}}{\text{H}_2\text{O}} \longrightarrow \text{MgO} + \text{H}_2$

3. G : steam

$3\text{Fe} + \underset{\text{steam}}{4\text{H}_2\text{O}} \rightleftharpoons \text{Fe}_3\text{O}_4 + 4\text{H}_2$

4. A : dilute acid

$2\text{Al} + \underset{\text{dil. acid}}{3\text{H}_2\text{SO}_4} \longrightarrow \text{Al}_2(\text{SO}_4)_3 + 3\text{H}_2$

5. D : conc. alkali

$2\text{Al} + \underset{\text{conc. alkali}}{2\text{NaOH}} + 2\text{H}_2\text{O} \longrightarrow 2\text{NaAlO}_2 + 3\text{H}_2$

Give balanced equations for the following conversions, 1 to 5.

1. MgCl₂ ⟵ HCl ⟶ FeCl₂
2. KAlO₂ ⟵ KOH ⟶ K₂ZnO₂
3. ZnO ⟵ H₂O ⟶ Fe₃O₄
4. CO + H₂ ⟵ H₂O ⟶ CO₂ + H₂
5. NH₃ ⟵ H₂ ⟶ H₂S

Answer

1. MgCl₂ ⟵ HCl ⟶ FeCl₂
   Mg + 2HCl ⟶ MgCl₂ + H₂
   Fe + 2HCl ⟶ FeCl₂ + H₂

2. KAlO₂ ⟵ KOH ⟶ K₂ZnO₂
   2Al + 2KOH + 2H₂O⟶ 2KAlO₂ + 3H₂ [g]
   Zn + 2KOH ⟶ K₂ZnO₂ + H₂[g]

3. ZnO ⟵ H₂O ⟶ Fe₃O₄
   Zn + H₂O ⟶ ZnO + H₂ [g]
   3Fe + 4H₂O ⇌ Fe₃O₄ + 4H₂ [g]

4. CO + H₂ ⟵ H₂O ⟶ CO₂ + H₂
   $\underset{\text{coke}}{\text{C}} + \underset{\text{steam}}{\text{H}_2\text{O}} \xrightarrow{1000 \degree \text{C}} \underset{\text{water gas}}{[\text{CO} + \text{H}_2 ]} - \Delta$
   $\underset{\text{water gas}}{[\text{CO} + \text{H}_2 ]} + \underset{\text{excess steam}}{\text{H}_2\text{O}} \underset{\text{Fe}_2\text{O}_3} {\xrightarrow{450 \degree \text{C}}} \text{CO}_2 + 2\text{H}_2 + \Delta$

5. NH₃ ⟵ H₂ ⟶ H₂S
   N₂ + 3H₂ ⇌ 2NH₃
   H₂ + S ⟶ H₂S

Give reasons for the following:

Nitric acid in the dilute form is not used in the laboratory preparation of hydrogen from metals.

Answer

Nitric acid is a strong Oxidizing agent and nascent oxygen formed on it's decomposition oxidizes the hydrogen to water. Hence, nitric acid in the dilute form is not used in the laboratory preparation of hydrogen from metals.

Give reasons for the following:

Granulated zinc is preferred to metallic zinc in the preparation of hydrogen using dilute acid.

Answer

Granulated zinc contains traces of impurities, which has a slight catalyzing effect on the reaction and speeds it up. Hence, granulated zinc is preferred to metallic zinc in the preparation of hydrogen using dilute acid.

Give reasons for the following:

Hydrogen and alkali metals of group 1 [IA] react with copper [II] oxide to give copper.

Answer

Hydrogen and alkali metals of group 1 [IA] act as reducing agents and thus, reduce CuO to Cu.

CuO + H₂ ⟶ Cu + H₂O

Give reasons for the following:

Hydrogen is collected by the downward displacement of water and not air even though it is lighter than air.

Answer

Hydrogen is almost insoluble in water. Even though hydrogen is lighter than air, it is not collected by downward displacement of air since it forms an explosive mixture with air.

Give reasons for the following:

A mixture of hydrogen and chlorine can be separated by passage through a porous pot.

Answer

As hydrogen diffuses more than chlorine in the porous pot, therefore, most of chlorine remains outside the pot. Hence, the mixture of hydrogen and chlorine can be separated by passage through a porous pot.

Name the following

1. A metal below iron but above copper in the activity series of metals which has no reaction with water.
2. A metal which cannot be used for the preparation of hydrogen using dilute acids.
3. The salt formed when aluminium reacts with potassium hydroxide, during the preparation of hydrogen from alkalis.
4. A gaseous reducing agent which is basic in nature.
5. A compound formed between hydrogen and an element from group 17 [VIIA] – period 3.

Answer

1. Lead [Pb]
2. Lead [Pb]
3. Potassium aluminate [KAlO₂]
4. Ammonia [NH₃]
5. Element from group 17 [VIIA] – period 3 is Cl and compound formed is - HCl

Select the correct answer from the symbols in bracket.

1. The element placed below hydrogen in group 1 [IA]. [Na, Li, K, F],
2. The element other than hydrogen, which forms a molecule containing a single covalent bond. [Cl, N, O]
3. The element, which like hydrogen has one valence electron. [He, Na, F, O]
4. The element, which like hydrogen is a strong reducing agent. [Pb, Na, S, Cl]
5. The element which forms a diatomic molecule. [C, Br, S, P]

Answer

1. Li
2. Cl
3. Na
4. Na
5. Br

The diagram represents the preparation & collection of hydrogen by a standard laboratory method.

1. State what is added through the thistle funnel 'Y'.
2. State what difference will be seen if pure zinc is added in the distillation flask 'X' instead of granulated zinc.
3. Name a solution which absorbs the impurity – H₂S.
4. State why hydrogen is collected after all the air in the apparatus is allowed to escape.
5. Name a gas other than hydrogen collected by the same method.

Answer

1. Dilute hydrochloric acid [HCl]
2. When pure zinc is added in the distillation flask 'X' instead of granulated zinc, the rate of production of H₂ is slow. Granulated zinc is commercial zinc obtained from molten zinc, it contains traces of impurities which has a slight catalyzing effect on the reaction.
3. Lead nitrate Pb(NO₃)₂ solution absorbs H₂S
4. Hydrogen is collected after all the air in the apparatus is allowed to escape as hydrogen forms an explosive mixture with air.
5. Oxygen gas.