Study of the First Element — Hydrogen

Question 1

State the electronic configuration of hydrogen [at. no. 1]. Give a reason why hydrogen can be placed in group 1 [1A] and group 17 [VIIA] of the periodic table.

Answer

Hydrogen has the simplest electronic configuration of 1.
As 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.

Question 2

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]

Question 3

How did the name 'hydrogen' originate. How does hydrogen occur in the combined state.

Answer

Hydrogen was initially called 'inflammable gas'. It burnt in air to produce water. Hence, Lavoisier in 1783 established it's name 'hydrogen' meaning water producer. [Greek word Hydro = water, gen = producer]

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.

Question 4

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

Question 5

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]

Question 6

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) As the reaction between calcium and dil. sulphuric acid is highly explosive and practically not feasible, hence, hydrogen is not prepared in the laboratory by the action of calcium with dilute sulphuric acid.

(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.

Question 7

Give balanced equations for the following conversions.

(a) Sodium zincate from zinc

(b) Sodium plumbite from lead

(c) Sodium aluminate from aluminium.

Answer

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

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

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

Question 8

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

(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.

Question 9

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

Answer

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

Question 10

State the conditions and give balanced equations for the conversion of –

(a) coke to water gas,

(b) water gas to hydrogen

in the Bosch process.

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

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

Question 11

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.

Question 12

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.

Question 13

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

Question 14

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 metals oxides of less active metals like zinc, iron, lead and copper reduces the oxides of the metals to free metals.

Question 15

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

Question 16

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

Answer

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

Considering the example of the reaction of hydrogen sulphide with chlorine:

Zn + CuSO₄ ⟶ ZnSO₄ + Cu

Zn + Cu²⁺SO₄^2- ⟶ Zn²⁺SO₄^2- + Cu

Here, Zinc is oxidised and Copper is reduced.

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

Question 17

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 electropositve 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.

Question 1

Answer

Question 2

Answer

Question 3

Answer

Question 1(1984)

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

Answer

Potassium

Question 2(1984)

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

Question 3(1984)

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

calcium + water ⟶ calcium hydroxide + hydrogen

Answer

Ca + 2H₂O ⟶ Ca(OH)₂ + H₂

Question 4(1984)

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.

Question 5(1984)

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

Answer

When powdered zinc reacts with sodium hydroxide it produces hydrogen.

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

Question 1(1985)

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

Answer

Mg + 2HCl (dil.) ⟶ MgCl₂ + H₂

Question 1(1986)

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

Answer

Hydrogen

Question 2(1986)

Write correctly balanced equation for the following :

When steam is passed over red hot iron.

Answer

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

Question 3(1986)

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.

Question 1(1987)

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

Answer

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

Question 2(1987)

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

Question 1(1988)

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

Answer

Sodium hydroxide solution + zinc ⟶ Sodium zincate + hydrogen gas

Question 2(1988)

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.

Question 1(1989)

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.

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

Question 1(1990)

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.

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

Question 2(1990)

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

Question 3(1990)

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.

Question 4(1990)

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

Question 1(1991)

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

Answer

When powdered zinc reacts with sodium hydroxide it produces hydrogen.

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

Question 1(1992)

Complete and balance the following equations :

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

Answer

2Al+ 2NaOH + 2H₂O ⟶ 2NaAlO₂ + 3H₂ [g]

Question 2(1992)

What do the following symbols represent : 2H and H₂.

Answer

• 2H ⟶ 2 atoms of hydrogen
• H₂ ⟶ 1 molecule of hydrogen

Question 1(1993)

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]

Question 2(1993)

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.

Question 3(1993)

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.

Question 1(1994)

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

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

Question 2(1994)

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.

Question 1

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

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

Question 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

Question 3.1

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.

Question 3.2

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.

Question 3.3

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

Question 3.4

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.

Question 3.5

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.

Question 4

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

Question 5

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

Question 6

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.