Chapter 9 Study of the First Element- Hydrogen Frank Chemistry Class 9 Frank Chemistry

Frank Solutions for Chapter 9 Study of the First Element- Hydrogen Class 9 Chemistry ICSE

Exercise 9.1

1. (a) Write the symbol and molecular formula of hydrogen.

(b) What is the difference between 2H and H₂?

(c) Who gave hydrogen its name and why?

Answer

(a) Symbol of hydrogen is ‘H' and its molecular formula is H₂.

(b) 2 H means 2 atoms of hydrogen while H₂ means one molecule of hydrogen.

(c) Lavoisier gave "Hydrogen" its name because it means water product.

2. Name the various sources from which hydrogen can be obtained.

Answer

Hydrogen is found in free state as well as in combined state.

1. In free state, it is found in traces in the earth’s crust and atmosphere. On the sun and bright stars, hydrogen is very abundant. It is suggested that the source of sun's energy is fusion of hydrogen to helium.
2. In combined state it is found in the compounds like water, acids, organic materials and minerals. Natural gas coming out from oil wells, coal mines and volcanoes invariably contains this gas.

3. How does hydrogen resemble the alkali metals (Group 1)?

Answer

Hydrogen resembles the alkali in following manner-

1. Electronic configuration- Hydrogen as well as alkali metals have one electron in their valence shell.
   H (1) = 1; Li (3) = 2,1; Na (11) = 2, 8, 1
2. Ion formation- Hydrogen loses one electron to form H⁺ ion like the alkali metals which form Li⁺, Na⁺, K⁺ etc.
3. Valency electrons- Like alkali metals, hydrogen exhibit valency of one in its compounds, as it has only one electron in its outermost shell.
4. Combination with non-metals-Like alkali metals, hydrogen combines with non-metals such as oxygen, chlorine and sulphur forming their oxides, chlorides and sulphides respectively.
5. Reducing action- Like alkali metals, hydrogen is also a very good reducing agent.

4. What is the justification for hydrogen being placed in the group of halogens in the periodic table.

Answer

Hydrogen resembles the alkali in following manner-

1. Electronic configuration- All the halogens have seven electrons in their outermost shell and need just one more electron to attain stable inert gas configuration. Similarly, hydrogen with one electron in its outermost shell requires one electron to attain a stable inert gas(Helium) configuration.
2. Valency- Hydrogen and halogen both show a valency of one.
3. Non-metals- Hydrogen is non-metallic like halogen.
4. Atomicity- Hydrogen as well as halogens are diatomic gases, e.g.H₂, Cl₂, Br₂ etc.
5. Ion formation- Halogens have a strong tendency to gain an electron to form halide ions.in similar way, hydrogen shows tendency to gain one electron to form hydride ion (H).

5. How hydrogen can be prepared with zinc using acid and alkali ? Give an equation in each case.

Answer

By the action of dilute sulphuric acid or hydrochloric acid with zinc we can prepare hydrogen.

Zn + H₂SO₄ → ZnSO₄ + H₂

Zinc Sulphuric acid Zinc sulphate Hydrogen

Zn + 2HCl → ZnCl₂ + H₂

Zinc Hydrochloric acid Zinc chloride Hydrogen

Zinc liberate hydrogen from hot concentrated solution of alkali.

Zn + 2NaOH → Na₂ZnO₂ + H₂

Zinc Sodium hydroxide Sodium zincate Hydrogen

Zn + 2KOH → K₂ZnO₂ + H₂

Zinc Potassium Hydroxide Potassium Zincate Hydrogen

6. Why following metals are not suitable for the laboratory preparation of hydrogen?

(i) Calcium

(ii) Iron

(iii) Aluminium

(iv) Sodium

Answer

(i) Calcium:

Ca + 2H₂O → Ca(OH)₂ + H₂

This reaction is vigorous but less vigorous as compared as compared to reaction of sodium and potassium. So, not used for preparation of hydrogen. Also, Calcium is expensive.

(ii) Iron:

3Fe + 4H₂O → Fe₃O₄ + 4H₂

This reaction is a reversible reaction. Hence, the hydrogen formed should be continuously drawn out. Otherwise, it will reduce the iron oxide (ferrous-ferric oxide) back to iron. And, also the hydrogen thus produced contains impurities like hydrogen sulphide and sulphur dioxide.

(iii) Aluminium: It forms a protective coating of AlgO3 due to its great affinity for oxygen. So, it does not give hydrogen with acid after the reaction has produced for some time.

(iv) Sodium:

2Na + 2H₂O— 2NaOH

This reaction is violent. Sodium melts into silvery globules and darts about freely on the surface of water, making collection of hydrogen gas is difficult.

7. For the laboratory preparation of hydrogen, give the following:

(i) Reactants used

(ii) Reaction

(iii) Procedure

(iv) Complete labelled diagram

(v) Method of selection

(vi) Drying agents for gas

Answer

(i) Reactants used: Zinc and sulphuric acid/Hydrochloric acid

(ii) Reaction:

Zn + H₂SO₄ → ZnSO₄ + H₂

Zinc Sulphuric acid Zinc sulphate Hydrogen

Zn + 2HCl → ZnCl₂ + H₂

Zinc Hydrochloric acid Zinc Chloride Hydrogen

(iii) Procedure: Some pieces of granulated zinc are placed in the Woulfe’s bottle and the apparatus is made air tight. Now, dilute sulphuric acid or dilute hydrochloric acid is poured through thistle funnel. Hydrogen gas is evolved.

(iv) Complete labelled diagram:

Fig – Laboratory preparation of hydrogen

(v) Method of collection: Hydrogen is collected by downward displacement of water.

(vi) Drying agent for gas: Hydrogen gas is dried by passing it through anhydrous calcium chloride or P₂O₅.

8. What are the impurities present in the hydrogen gas obtained in the laboratory by dilute H₂SO₄ and granulated zinc? How are they removed?

Answer

Hydrogen gas obtained in the laboratory by dilute H₂SO₄ and granulated zinc have following impurities-

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

They are removed by passing through-

1. Lead nitrate solution-It absorbs H₂S.
2. Silver nitrate solution (AgNO₃) - It absorbs PH₃, AsH₃
3. Caustic potash (solid KOH) – It absorbs CO₂, SO₂, oxides of nitrogen
4. Anhydrous calcium chloride or P₂O₅ - It absorbs moisture.

9. (a) Write two equations for the reactions in which hydrogen is used as a reducing agent.

(b) Which of the following metal oxides cannot be ree7duced to metal by hydrogen ?

Answer

(a) When passed over heated metallic oxides, hydrogen reduces the oxides to the metals by taking away oxygen.

1. CuO + H₂ → Cu + H₂O
2. ZnO + H₂ → Zn + H₂O

(b) MgO and PbO cannot be reduced to metal by hydrogen.

10. (i) Write balanced equation of the reaction in the preparation of hydrogen from a solution of potassium hydroxide.

(ii) Describe briefly, with equations, the Bosch process for the large scale production of hydrogen.

(iii) Account for the following facts:

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

Answer

Potassium hydroxide on reaction with zinc produces potassium zincate and hydrogen.

Zn + 2KOH → K₂ZnO₂ + H₂

Zinc Potassium hydroxide Potassium zincate Hydrogen

Bosch Process -

The various steps involved in the process are as follows:

(1) Super heated steam is passed over heated coke at about 1000°C in special furnaces called converters to form water gas.

This reaction is endothermic.

(2) The water gas is so obtained is then mixed with more steam and passed over heated ferric oxide and chromic oxide at a temperature of about 450°C. Ferric oxide acts as a catalyst and chromic oxide acts as promoter. The CO from water gas takes away the oxygen from steam to produce carbon dioxide and more hydrogen. In this reaction poisonous carbon monoxide is converted to non-poisonous carbon dioxide and more hydrogen is produced.

(3) Separation of hydrogen from a mixture of CO₂ and H₂ is carried out by-

(a) Both the carbon dioxide and hydrogen obtained are then passed through water under high pressure to form carbonic acid. Hydrogen hardly dissolves in carbonic acid, so easily collected.

(b) The mixture of CO₂ and H₂ can also be passed through caustic potash which reacts with CO₂. Hydrogen gas does not react and can be collected.

2KOH + CO₂ → K₂CO₃ + H₂

Caustic potash

(4) Separation of carbon monoxide: Hydrogen gas is generally contaminated with carbon monoxide. In order to remove it hydrogen is dissolved in ammonical cuprous chloride solution. This solution absorbs any carbon monoxide gas present in the mixture, Finally, H₂ gas is dried over by passing through anhydrous CaCl₂ or P₂O₃.

(iii) Account for the following facts:

(a) Lead reacts with dilute sulphuric or hydrochloric acid and forms an insoluble coating of lead sulphate or lead chloride .Therefore further reaction is prevented.

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

11. (a) Place the metals calcium, iron, magnesium and sodium in order of their reactivity with water, placing the most active first. Write the equation for each of the above metals which react with water.

(b) Why is copper not used to prepare hydrogen by the action of dilute HCI or dilute H₂SO₄ on the metal?

Answer

(a) Sodium > Calcium > Magnesium > Iron

(i) Sodium

2Na + 2H₂O → 2NaOH + H₂

(ii) Calcium

Ca + 2H₂O → Ca(OH)₂ + H₂

(iii) Magnesium

Mg + H₂O → MgO + H₂

(iv) Iron

(b) Copper is not used to prepare hydrogen by the action of dilute HCI or dilute H₂SO₄ on the metal since it is less reactive than hydrogen so cannot displace hydrogen from acid. Only the metals that are more reactive than the hydrogen itself can displace hydrogen from acids.

12. Thin strips of three different metals, A, B and C are known to be magnesium, copper and iron respectively.

(a) Write down what happens when the metals are treated as follows:

(i) When each metal is heated.

(ii) When each metal is heated with dilute HCl.

(iii) When each metal is added to aqueous solution of zinc sulphate.

(b) Arrange these metals in the descending order of their activities.

Answer

(a) (i) When magnesium is heated in air, it reacts with oxygen. During this oxidation reaction, magnesium oxide is produced.

2Mg + O₂ →  2MgO (s)

When copper is heated, copper oxide is formed.

Cu + O₂ → CuO

When iron is heated in presence of air, it will also form iron oxide.

4Fe + 3O₂ → Fe₂O₃

(ii) Magnesium and iron will form chlorides whereas copper does not show any reaction

Mg + 2HCI → MgCl₂  + H₂

Cu + HCl → No reaction

Fe + 2HCI → FeCl₂ + H₂

(iii) Magnesium being at the higher position than zinc in the reactivity series will displace Zinc from zinc sulphate.

Mg + ZnSO₄ → MgSO₄ + Zn

Copper and iron being at lower position than zinc will not displace zinc from zinc sulphate.

Cu + ZnSO₄ → No reaction

Fe + ZnSO₄ → No reaction

(b) Magnesium > Iron > Copper

13. Fill in the blanks:

(a) Vegetable oil can be changed into a solid by the use of a___ catalyst and the ____ element.

(b) A metal ____ hydrogen in activity series gives hydrogen with ___ acids.

(c) Hydrogen form ___ bonds with strongly ___ elements.

(d) When CuO reacts with hydrogen, ___ is reduced and ___is oxidised to __.

(e) Water gas is a mixture of ____ and __

(f) Hydrogen gas is liberated by the action of aluminium with concentrated solution of ___.

(g) The hydrogen at the moment of its formation is called ___.

(h) When hydrogen burns ___ is produced.

Answer

(a) Nickel, hydrogen

(b) above, dilute mineral

(c) covalent, electronegative

(d) CuO, hydrogen, water

(e) CO, H₂

(f) alkali

(g) nascent hydrogen

(h) water

14. Indicate which of the following statements are true and which are false:

(a) Hydrogen gas is easily liquefiable.

(b) Hydrogen is combustible as well as supporter of combustion.

(c) Anhydrous calcium chloride is used to dry hydrogen gas.

(d) The reaction between hydrogen and oxygen is exothermic.

(e) Zinc gives hydrogen when reacted with dilute HCI and conc. NaOH.

(f) The position of hydrogen is not fixed in periodic table.

(g) Magnesium with very dilute HNO₃ produces hydrogen.

(h) Occluded hydrogen is less active than ordinary hydrogen.

Answer

(a) False

(b) False

(c) True

(d) True

(e) True

(f) True

(g) True

(h) False

15. Give reasons:

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

(b) Apparatus for the laboratory preparation of hydrogen should be air tight and away from the flame.

(c) Nitric acid and hot concentrated H₂SO₄ are not used in the preparation of hydrogen.

(d) A large piece of sodium should not be placed in water.

(e) Na, K and Ca cannot be used to prepare hydrogen from acids.

(f) In the laboratory preparation of hydrogen zinc granules are preffered over pure zinc metal.

(g) In Bosch process, steam should not be passed over white coke for a long time.

(h) When a burning splinter is introduced into a jar of hydrogen, the splinter gets extinguished.

(i) Hydrogenated vegetable fats are not advised to be used in preparation of foodstuffs.

(j) Though hydrogen is the lightest element it is not used in weather ballons.

(k) Lead is not used to prepare hydrogen from acids.

Answer

(a) Although hydrogen is lighter than air yet it cannot be collected by the downward displacement of air as it forms an explosive mixture with air. So, it is collected by downward displacement of water not aw.

(b) Hydrogen is highly inflammable and forms explosive mixture with air so apparatus for the laboratory preparation of hydrogen should be air tight and away from the flame.

(c) Nitric acid is not used in preparation of hydrogen because it is strong oxidizing agent and oxdises H₂ to H₂O.

Hot concentrated H₂SO₄  is not used in preparation of hydrogen as a part of it is reduced to SO₂

(d) As sodium reacts violently with water.

2Na + 2 H₂O → 2NaOH + H₂

This reaction is very violent .Sodium melts into silvery globules and darts about freely on the surface of water ,making collection of hydrogen gas is difficult so large piece of sodium should not be placed in water.

(e) Na, K and Ca cannot be used to prepare hydrogen from acids because these displace hydrogen from dilute acids with explosive violence.

(f) In the laboratory preparation of hydrogen zinc granules are preffered over pure zinc metal because copper is present as impurity in granulated zinc, which catalyse the reaction and increase its rate.

(g) This process requires heat (endothermic). So the steam should not be passed over hat coke as it may cool down the coke. So after some time steam is alternatively replaced by air stream.

2nd-

This reaction is – C (white hot) + HaO (steam) → CO + H₂

if steam is passed for a long time, it may react with carbon mono oxide (CO) and make carbon dioxide which is not a required product.

(h) Since hydrogen is a non-supporter of combustion so it does not allow substances to bum in it and hence when a burming splinter is introduced into a jar of hydrogen, the splinter gets extinguished.

(i) In nature, most dietary fats and oils exist in a structural form which is called the “cis” form. When these natural cis form fats are processed by bubbling hydrogen gas through them at high temperatures, they become partially hydrogenated which changes their structure to the “trans” form. The natural cis fat has a bend and the processed trans fat is a straight molecule. This difference in cis and trans shapes is of major significance. When eaten, fats and oils are incorporated into cell membranes altering the composition of these delicate structures. When they interact with normal fat metabolism, they disturb function in a most deleterious manner. Hence, these substances meet the definition of a poison.” Trans fats interfere with important, normal functions by inhibiting enzymes which are necessary for the body's normal metabolism of fats and they keep doing it for a long time.

(j) Since hydrogen is highly inflammable in nature so inspite of being the lightest element it is not used in weather balloons.

(k) Lead reacts with dilute sulphuric acid or hydrochloric acid and forms an insoluble coating of lead sulphate or lead chloride which prevents further reaction.

Exercise 9.2

1. (a) Write the symbol and molecular formula of hydrogen.

(b) What is the difference between 2H and H₂?

(c) Who gave hydrogen its name and why?

Answer

(a) Symbol of hydrogen is ‘H' and its molecular formula is H₂.

(b) 2 H means 2 atoms of hydrogen while H₂ means one molecule of hydrogen.

(c) Lavoisier gave "Hydrogen" its name because it means water product.

2. Name the various sources from which hydrogen can be obtained.

Answer

Hydrogen is found in free state as well as in combined state.

1. In free state, it is found in traces in the earth’s crust and atmosphere. On the sun and bright stars, hydrogen is very abundant. It is suggested that the source of sun's energy is fusion of hydrogen to helium.
2. In combined state it is found in the compounds like water, acids, organic materials and minerals. Natural gas coming out from oil wells, coal mines and volcanoes invariably contains this gas.

3. How does hydrogen resemble the alkali metals (Group 1)?

Answer

Hydrogen resembles the alkali in following manner-

(i) Electronic configuration-Hydrogen as well as alkali metals have one electron in their valence shell.

H (1) = 1; Li (3) = 2,1; Na (11) = 2, 8, 1

(ii) lon formation - Hydrogen loses one electron to form H⁺ ion like the alkali metals which form Li⁺, Na⁺, K⁺ etc.

(iii) Valency electrons-Like alkali metals, hydrogen exhibit valency of one in its compounds, as it has only one electron in its outermost shell.

(iv) Combination with non-metals-Like alkali metals, hydrogen combines with non-metals such as oxygen, chlorine and sulphur forming their oxides, chlorides and sulphides respectively.

(v) Reducing action-Like alkali metals, hydrogen is also a very good reducing agent.

4. What is the justification for hydrogen being placed in the group of halogens in the periodic table.

Answer

Hydrogen resembles the alkali in following manner-

1. Electronic configuration-All the halogens have seven electrons in their outermost shell and need just one more electron to attain stable inert gas configuration. Similarly, hydrogen with one electron in its outermost shell requires one electron to attain a stable inert gas(Helium) configuration.
2. Valency-Hydrogen and halogen both show a valency of one.
3. Non-metals-Hydrogen is non-metallic like halogen.
4. Atomicity-Hydrogen as well as halogens are diatomic gases, e.g.H₂, Cl₂, Br₂ etc.
5. Ion formation-Halogens have a strong tendency to gain an electron to form halide ions.in similar way, hydrogen shows tendency to gain one electron to form hydride ion (H).

5. How hydrogen can be prepared with zinc using acid and alkali ? Give an equation in each case.

Answer

By the action of dilute sulphuric acid or hydrochloric acid with zinc we can prepare hydrogen.

Zn + H₂SO₄ → ZnSO₄ + H₂

Zinc Sulphuric acid Zinc sulphate Hydrogen

Zn + 2HCl → ZnCl₂ + H₂

Zinc Hydrochloric acid Zinc chloride Hydrogen

Zinc liberate hydrogen from hot concentrated solution of alkali.

Zn + 2NaOH → Na₂ZnO₂ + H₂

Zinc Sodium hydroxide Sodium zincate Hydrogen

Zn + 2KOH → K₂ZnO₂ + H₂

Zinc Potassium Hydroxide Potassium Zincate Hydrogen

6. Why following metals are not suitable for the laboratory preparation of hydrogen?

(i) Calcium

(ii) Iron

(iii) Aluminium

(iv) Sodium

Answer

(i) Calcium –

Ca + 2H₂O → Ca(OH)₂ + H₂

This reaction is vigorous but less vigorous as compared as compared to reaction of sodium and potassium. So, not used for preparation of hydrogen. Also, Calcium is expensive.

(ii) Iron –

3Fe + 4H₂O → Fe₃O₄ + 4H₂

This reaction is a reversible reaction. Hence, the hydrogen formed should be continuously drawn out. Otherwise, it will reduce the iron oxide (ferrous-ferric oxide) back to iron. And, also the hydrogen thus produced contains impurities like hydrogen sulphide and sulphur dioxide.

(iii) Aluminium - It forms a protective coating of AlgO3 due to its great affinity for oxygen. So, it does not give hydrogen with acid after the reaction has produced for some time.

(iv) Sodium -

2Na + 2H₂O— 2NaOH

This reaction is violent. Sodium melts into silvery globules and darts about freely on the surface of water, making collection of hydrogen gas is difficult.

7. For the laboratory preparation of hydrogen, give the following:

(i) Reactants used

(ii) Reaction

(iii) Procedure

(iv) Complete labelled diagram

(v) Method of selection

(vi) Drying agents for gas

Answer

(i) Reactants used - Zinc and sulphuric acid/Hydrochloric acid

(ii) Reaction –

Zn + H₂SO₄ → ZnSO₄ + H₂

Zinc Sulphuric acid Zinc sulphate Hydrogen

Zn + 2HCl → ZnCl₂ + H₂

Zinc Hydrochloric acid Zinc Chloride Hydrogen

(iii) Procedure – Some pieces of granulated zinc are placed in the Woulfe’s bottle and the apparatus is made air tight. Now, dilute sulphuric acid or dilute hydrochloric acid is poured through thistle funnel. Hydrogen gas is evolved.

(iv) Complete labelled diagram –

 Fig – Laboratory preparation of hydrogen

(v) Method of collection – Hydrogen is collected by downward displacement of water.

(vi) Drying agent for gas-hydrogen gas is dried by passing it through anhydrous calcium chloride or P₂O₅.

8. What are the impurities present in the hydrogen gas obtained in the laboratory by dilute H₂SO₄ and granulated zinc? How are they removed?

Answer

Hydrogen gas obtained in the laboratory by dilute H₂SO₄ and granulated zinc have following impurities-

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

They are removed by passing through-

1. Lead nitrate solution-It absorbs H₂S.
2. Silver nitrate solution (AgNO₃) - It absorbs PH₃, AsH₃
3. Caustic potash (solid KOH) – It absorbs CO₂, SO₂, oxides of nitrogen
4. Anhydrous calcium chloride or P₂O₅ - It absorbs moisture.

9. (a) Write two equations for the reactions in which hydrogen is used as a reducing agent.

(b) Which of the following metal oxides cannot be ree7duced to metal by hydrogen ?

Answer

(a) When passed over heated metallic oxides, hydrogen reduces the oxides to the metals by taking away oxygen.

1. CuO + H₂ → Cu + H₂O
2. ZnO + H₂ → Zn + H₂O

(b) MgO and PbO cannot be reduced to metal by hydrogen.

10. (i) Write balanced equation of the reaction in the preparation of hydrogen from a solution of potassium hydroxide.

(ii) Describe briefly, with equations, the Bosch process for the large scale production of hydrogen.

(iii) Account for the following facts:

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

Answer

Potassium hydroxide on reaction with zinc produces potassium zincate and hydrogen.

Zn + 2KOH → K₂ZnO₂ + H₂

Zinc Potassium hydroxide Potassium zincate Hydrogen

Bosch Process -

The various steps involved in the process are as follows:

(1) Super heated steam is passed over heated coke at about 1000°C in special furnaces called converters to form water gas.

This reaction is endothermic.

(2) The water gas is so obtained is then mixed with more steam and passed over heated ferric oxide and chromic oxide at a temperature of about 450°C. Ferric oxide acts as a catalyst and chromic oxide acts as promoter. The CO from water gas takes away the oxygen from steam to produce carbon dioxide and more hydrogen. In this reaction poisonous carbon monoxide is converted to non-poisonous carbon dioxide and more hydrogen is produced.

(3) Separation of hydrogen from a mixture of CO₂ and H₂ is carried out by-

(a) Both the carbon dioxide and hydrogen obtained are then passed through water under high pressure to form carbonic acid. Hydrogen hardly dissolves in carbonic acid, so easily collected.

(b) The mixture of CO₂ and H₂ can also be passed through caustic potash which reacts with CO₂. Hydrogen gas does not react and can be collected.

2KOH + CO₂ → K₂CO₃ + H₂

Caustic potash

(4) Separation of carbon monoxide: Hydrogen gas is generally contaminated with carbon monoxide. In order to remove it hydrogen is dissolved in ammonical cuprous chloride solution. This solution absorbs any carbon monoxide gas present in the mixture, Finally, H₂ gas is dried over by passing through anhydrous CaCl₂ or P₂O₃.

(iii) Account for the following facts:

(a) Lead reacts with dilute sulphuric or hydrochloric acid and forms an insoluble coating of lead sulphate or lead chloride .Therefore further reaction is prevented.

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

11. (a) Place the metals calcium, iron, magnesium and sodium in order of their reactivity with water, placing the most active first. Write the equation for each of the above metals which react with water.

(b) Why is copper not used to prepare hydrogen by the action of dilute HCI or dilute H₂SO₄ on the metal?

Answer

(a) Sodium > Calcium > Magnesium > Iron

(i) Sodium

2Na + 2H₂O → 2NaOH + H₂

(ii) Calcium

Ca + 2H₂O → Ca(OH)₂ + H₂

(iii) Magnesium

Mg + H₂O → MgO + H₂

(iv) Iron

(b) Copper is not used to prepare hydrogen by the action of dilute HCI or dilute H₂SO₄ on the metal since it is less reactive than hydrogen so cannot displace hydrogen from acid. Only the metals that are more reactive than the hydrogen itself can displace hydrogen from acids.

12. Thin strips of three different metals, A, B and C are known to be magnesium, copper and iron respectively.

(a) Write down what happens when the metals are treated as follows:

(i) When each metal is heated.

(ii) When each metal is heated with dilute HCl.

(iii) When each metal is added to aqueous solution of zinc sulphate.

(b) Arrange these metals in the descending order of their activities.

Answer

(a) (i) When magnesium is heated in air, it reacts with oxygen. During this oxidation reaction, magnesium oxide is produced.

2Mg + O₂ → 2MgO (s)

When copper is heated, copper oxide is formed.

Cu + O₂ → CuO

When iron is heated in presence of air, it will also form iron oxide.

4Fe + 3O₂ → Fe₂O₃

(ii) Magnesium and iron will form chlorides whereas copper does not show any reaction

Mg + 2HCI → MgCl₂  + H₂

Cu + HCl → No reaction

Fe + 2HCI → FeCl₂ + H₂

(iii) Magnesium being at the higher position than zinc in the reactivity series will displace Zinc from zinc sulphate.

Mg + ZnSO₄ → MgSO₄ + Zn

Copper and iron being at lower position than zinc will not displace zinc from zinc sulphate.

Cu + ZnSO₄ → No reaction

Fe + ZnSO₄ → No reaction

(b) Magnesium > Iron > Copper

13. Fill in the blanks:

(a) Vegetable oil can be changed into a solid by the use of a___ catalyst and the ____ element.

(b) A metal ____ hydrogen in activity series gives hydrogen with ___ acids.

(c) Hydrogen form ___ bonds with strongly ___ elements.

(d) When CuO reacts with hydrogen, ___ is reduced and ___is oxidised to __.

(e) Water gas is a mixture of ____ and __

(f) Hydrogen gas is liberated by the action of aluminium with concentrated solution of ___.

(g) The hydrogen at the moment of its formation is called ___.

(h) When hydrogen burns ___ is produced.

Answer

(a) Nickel, hydrogen

(b) above, dilute mineral

(c) covalent, electronegative

(d) CuO, hydrogen, water

(e) CO, H₂

(f) alkali

(g) nascent hydrogen

(h) water

14. Indicate which of the following statements are true and which are false:

(a) Hydrogen gas is easily liquefiable.

(b) Hydrogen is combustible as well as supporter of combustion.

(c) Anhydrous calcium chloride is used to dry hydrogen gas.

(d) The reaction between hydrogen and oxygen is exothermic.

(e) Zinc gives hydrogen when reacted with dilute HCI and conc. NaOH.

(f) The position of hydrogen is not fixed in periodic table.

(g) Magnesium with very dilute HNO₃ produces hydrogen.

(h) Occluded hydrogen is less active than ordinary hydrogen.

Answer

(a) False

(b) False

(c) True

(d) True

(e) True

(f) True

(g) True

(h) False

15. Give reasons:

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

(b) Apparatus for the laboratory preparation of hydrogen should be air tight and away from the flame.

(c) Nitric acid and hot concentrated H₂SO₄ are not used in the preparation of hydrogen.

(d) A large piece of sodium should not be placed in water.

(e) Na, K and Ca cannot be used to prepare hydrogen from acids.

(f) In the laboratory preparation of hydrogen zinc granules are preffered over pure zinc metal.

(g) In Bosch process, steam should not be passed over white coke for a long time.

(h) When a burning splinter is introduced into a jar of hydrogen, the splinter gets extinguished.

(i) Hydrogenated vegetable fats are not advised to be used in preparation of foodstuffs.

(j) Though hydrogen is the lightest element it is not used in weather ballons.

(k) Lead is not used to prepare hydrogen from acids.

Answer

(a) Although hydrogen is lighter than air yet it cannot be collected by the downward displacement of air as it forms an explosive mixture with air. So, it is collected by downward displacement of water not aw.

(b) Hydrogen is highly inflammable and forms explosive mixture with air so apparatus for the laboratory preparation of hydrogen should be air tight and away from the flame.

(c) Nitric acid is not used in preparation of hydrogen because it is strong oxidizing agent and oxdises H₂ to H₂O.

Hot concentrated H₂SO₄  is not used in preparation of hydrogen as a part of it is reduced to SO₂

(d) As sodium reacts violently with water.

2Na + 2 H₂O → 2NaOH + H₂

This reaction is very violent .Sodium melts into silvery globules and darts about freely on the surface of water ,making collection of hydrogen gas is difficult so large piece of sodium should not be placed in water.

(e) Na, K and Ca cannot be used to prepare hydrogen from acids because these displace hydrogen from dilute acids with explosive violence.

(f) In the laboratory preparation of hydrogen zinc granules are preffered over pure zinc metal because copper is present as impurity in granulated zinc, which catalyse the reaction and increase its rate.

(g) This process requires heat (endothermic). So the steam should not be passed over hat coke as it may cool down the coke. So after some time steam is alternatively replaced by air stream.

2nd-

This reaction is – C (white hot) + HaO (steam) → CO + H₂

if steam is passed for a long time, it may react with carbon mono oxide (CO) and make carbon dioxide which is not a required product.

(h) Since hydrogen is a non-supporter of combustion so it does not allow substances to bum in it and hence when a burning splinter is introduced into a jar of hydrogen, the splinter gets extinguished.

(i) In nature, most dietary fats and oils exist in a structural form which is called the “cis” form. When these natural cis form fats are processed by bubbling hydrogen gas through them at high temperatures, they become partially hydrogenated which changes their structure to the “trans” form. The natural cis fat has a bend and the processed trans fat is a straight molecule. This difference in cis and trans shapes is of major significance. When eaten, fats and oils are incorporated into cell membranes altering the composition of these delicate structures. When they interact with normal fat metabolism, they disturb function in a most deleterious manner. Hence, these substances meet the definition of a poison.” Trans fats interfere with important, normal functions by inhibiting enzymes which are necessary for the body's normal metabolism of fats and they keep doing it for a long time.

(j) Since hydrogen is highly inflammable in nature so inspite of being the lightest element it is not used in weather balloons.

(k) Lead reacts with dilute sulphuric acid or hydrochloric acid and forms an insoluble coating of lead sulphate or lead chloride which prevents further reaction.

16. Name the products obtained when hydrogen is

(a) passed over heated copper oxide.

(b) passed over heated sodium.

Answer

(a) When hydrogen is passed over heated copper oxide reddish brown copper and water is obtained.

(b) When hydrogen is passed over heated sodium, sodium hydride is obtained.

17. How combination of hydrogen with chlorine is affected by sunlight ?

Answer

H₂ + Cl₂ → 2HCl

In diffused sunlight, the reaction takes place smoothly and forms HCl gas. While, in direct sunlight, the reaction occurs with an explosion.

18. Explain the following terms:

(i) Occlusion

(b) Hydrogen

(c) Nascent hydrogen

Answer

(a) Occlusion- Adsorption of gases at the surface of finely divided metal is called occlusion. Hydrogen gas is readily adsorbed or occluded by the metals like palladium, platinum or nickel at ordinary temperature.

(b) Hydrogenation- Unsaturated compounds combine with hydrogen in presence of catalyst like palladium, platinum or nickel to form saturated compounds. These reactions are known as hydrogenation reactions.

Acetylene

(c) Nascent hydrogen- It means newly born hydrogen. Hydrogen which is just formed at the time of its generation is called nascent hydrogen It is more powerful reducing agent than molecular hydrogen.

19. How is hydrogen prepared in laboratory? List five precautions to ensure safe preparation and collection of the gas? How would you confirm that the gas collected is hydrogen?

Answer

Some pieces of zinc granules are placed in Woulfe’s bottle and the apparatus is made in air tight. Now dilute sulphuric acid is poured through the thistle funnel.

Observation: The reaction takes place at room temperature even without heating. A brisk effervescence with the evolution of gas is seen inside the bottle.

If burning candle is brought near the bubbles of hydrogen gas, they explode. This proves that bubbles were full of hydrogen gas and they move up in the air because they are lighter than air.

20. State the reason of use of hydrogen in

(a) meteorological balloons

(b) in metallurgy

(c) in fuel

(d) in making fertilizers

Answer

(a) Meteorological balloons - The low density and high lifting power of hydrogen made it useful in meteorological balloons used for studying air currents and weather conditions. However, due to its highly inflammable nature it has been replaced by helium which has a lifting only slightly less than that of hydrogen.

(b) In metallurgy - Hydrogen acts as a very good reducing agent. It is used to obtain metals by reducing their oxides.

(c) In fuel - Hydrogen has very high heat of combustions, therefore it is used as fuel in the form of coal gas, water gas and liquid hydrogen(for rocket propulsion).

(d) In making fertilizers - A large quantity of hydrogen is used in the manufacture of ammonia by Haber process. Ammonia is used in manufacture of HCI and methyl alcohol.

21. Give one example of a reaction in which hydrogen acts (a) as an oxidizing agent (b) as a reducing agent.

Answer

(a) As an oxidizing agent - With metals hydrogen acts as an oxidizing agent. When dry hydrogen is passed over heated reactive metals (K, Na and Ca) the corresponding metal hydrides, are formed.

2Na + 2H₂ → 2NaH

(b) As a reducing agent -

When passed over heated metallic oxides, hydrogen reduces the oxides to the metals by taking away oxygen.

CuO + H₂ → Cu + H₂O