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Class 9 SELINA Solutions Chemistry Chapter 6 - Study of the First Element - Hydrogen

Study of the First Element - Hydrogen Exercise Ex. 6(A)

Solution 1

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

Solution 2

Hydrogen shows dual nature because it resembles the alkali metals of Group IA and the halogens of Group VIIA.

Solution 3

  1. Each of them can form a cation by loss of an electron.

H → H+ + e-

Li →Li+ + e-

  1. Both alkali metals and hydrogen act as reducing agents.

CuO + H2 → Cu + H2O

CuO + Na → Cu + Na2O

  1. Hydrogen burns in oxygen to form its oxide. It burns with a pop sound.

2H2 + O2 → 2H2O

Alkali metals also burn vigorously when heated in oxygen to form their respective oxides.

Lithium forms monoxide.

4Li + O2 → 2Li2O

  1. Hydrogen burns in oxygen to form its oxide. It burns with a pop sound.

2H2 + O2 → 2H2O

Alkali metals also burn vigorously when heated in oxygen to form their respective oxides.

Lithium forms monoxide.

4Li + O2 → 2Li2O

Solution 4

  1. Oxides of alkali metals are basic in nature, whereas the oxide of hydrogen H2O is a neutral oxide.
  2. Hydrogen atom has only one shell, but halogens have two or more shells. 

Solution 5

  1. Hydrogen has one valence electron in its outermost orbit.
  2. Hydrogen burns in oxygen to form its oxide. It burns with a pop sound.

2H2 + O2 → 2H2O

  1. Hydrogen acts as a reducing agent.

CuO + H2 → Cu + H2O 

Solution 6

Hydrogen was called inflammable air because of its combustible nature.

Solution 7

In the free state, hydrogen is found in traces in the earth's crust and atmosphere.

In the combined state, plant and animal tissues are made of compounds of hydrogen with carbon, oxygen and nitrogen.

Solution 8

  1. Like halogens (fluorine and chlorine), hydrogen too is a gas.
  2. Both show a tendency to form anions because they are one electron short of the nearest inert gas configuration.

H + e- → H-

Cl + e-Cl-

  1. Both have valency 1.
  2. Hydrogen reacts with oxygen to form neutral oxide, H2O. Halogens react with oxygen to form acidic oxides like Cl2O and Cl2O7. 

Solution 9

  1. Reactive metals such as potassium, sodium and calcium.
  2. Magnesium, aluminium, zinc and iron. 

Solution 10

  1. 3Fe + 4H2O Fe3O4 + 4H2
  2. The reaction is reversible because if hydrogen formed is not removed, then the iron oxide formed is reduced back to iron.
  3. Because the reaction is a reversible reaction, equilibrium is attained at 700°C. At this stage, the amount of reactants and products does not change. 

Solution 11

They react with acids and can even react with hot concentrated alkalis to form hydrogen and a soluble salt.

Zn + 2NaOH → Na2ZnO2 + H2

2Al + 6NaOH→ 2Na2Al O3 + 3H2

Oxides and hydroxides of zinc and aluminium are amphoteric. They react with both bases and acids to give salt and water.

ZnO + 2HCl → ZnCl2 + H2O

ZnO + 2NaOH →Na2ZnO2 + H2O

Solution 12

  1. Fe +2 HC l → FeCl2 + H2
  2. Zn + 2NaOH → Na2ZnO2 + H2
  3. Pb + 2KOH → K2PbO2 + H2
  4. 2Al + 6NaOH→ 2Na2Al O3 + 3H2 

Solution 13

  1. The reaction is highly exothermic and vigorous with the evolution of hydrogen.

2Na + 2H2O →2NaOH + H2

  1. Calcium sinks in water and the reaction is less vigorous.

Ca + 2H2O → Ca(OH)2 + H2

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

Mg + 2H2O →Mg(OH)2 + H2O

  1. Magnesium burns in steam with an intense white light liberating hydrogen gas and white ash, i.e. magnesium oxide.

Mg + H2O → MgO + H2 

Solution 14

  1. Iron is less reactive than zinc, but red hot iron reacts with steam, forming triferric tetra-oxide and hydrogen gas.
  2. 3Fe + 4H2O Fe3O4 + 4H2
  3. If the product formed, i.e. hydrogen is not removed, then the iron oxide formed is reduced back to iron. 

Solution 15

  1. Sodium
  2. Magnesium
  3. Zinc
  4. Calcium 

Solution 16

  1. Sodium hydroxide + zinc → hydrogen + sodium zincate
  2. Calcium + water → calcium hydroxide + hydrogen

Study of the First Element - Hydrogen Exercise Ex. 6(B)

Solution 1

  1. Zn + HCl → ZnCl2 + H2
  2. Zn + 2NaOH → Na2ZnO2 + H2
  3. Zn + H2O → ZnO + H2 

Solution 2

  1. Granulated zinc, dilute HCl or dil. H2SO4
  2. It is collected by the downward displacement of water.
  3. Zn + HCl → ZnCl2 + H2
  4.  

  

Solution 3

  1. Hydrogen sulphide, sulphur dioxide, oxides of nitrogen, phosphine, arsine, carbon dioxide and water vapour are impurities present in the laboratory.
  2. The impurities can be removed from hydrogen by passing it through

1. Silver nitrate solution to remove arsine and phosphine.

    AsH3 + 6AgNO3 → Ag3As + 3AgNO3 + 3HNO3

    PH3 + 6AgNO3 → Ag3P + 3AgNO3 + 3HNO3

2. Lead nitrate solution to remove hydrogen sulphide.

    Pb(NO3)2 + H2S → PbS + 2HNO3

3. Caustic potash solution to remove sulphur dioxide, carbon dioxide and oxides of nitrogen.

    SO2 + 2KOH → K2SO3 + H2O

    CO2 + 2KOH→ K2CO3+ H2O

    2NO2 + 2KOH →KNO2 + KNO3 + H2O

4. A drying agent used to dry the gas. Common drying agents such as fused calcium chloride, caustic potash stick and phosphorus pentoxide remove water vapour.

So, the gas is purified and dried and then collected over mercury because mercury does not react with it. 

Solution 4

Test: Collect some amount of gas in a test tube and take it to a flame.

If the gas burns quietly, then there is no more air in the flask.

Solution 5

Nitric acid is a powerful oxidising agent, and the oxygen formed due to its decomposition oxidises hydrogen to give water thus defeating the purpose of the reaction.

3Zn + 8HNO3 → 3Zn(NO3)2 + 4H2O + 2NO

Solution 6

Conc. sulphuric acid is not used in the preparation of hydrogen as it will produce sulphur dioxide.

Zn + 2H2SO4 →ZnSO4 + SO2 + 2H2O

Solution 7

  1. C + H2O  (CO + H2) - ∆

       (CO + H2) + H2O  CO2 + 2H2 + ∆

  1. The mixture is passed through ammoniacal cuprous chloride solution in order to dissolve any uncombined carbon monoxide.

        CuCl + CO + 2H2O →CuCl.CO.2H2O 

Solution 8

  1. 3Fe + 4H2O Fe3O4 + 4H2
  2. Ca + 2H2O Ca(OH)2 + H2 

Solution 9

  1. Hydrogen gas. When red litmus is introduced in the solution, it turns blue.
  2. Ca + 2H2O → Ca(OH)2 + H2
  3. The solution turns blue.
  4. If dilute hydrochloric acid is added to the turbid solution, then they react and neutralise each other, forming the soluble salt calcium chloride (CaCl2) and water.

        Ca(OH)2 + 2HCl → CaCl2 + 2H2O 

Solution 10

  1.  
    1. On heating thin strips of magnesium, copper and iron, they form oxides.
    2. Magnesium and iron react with HCl liberating hydrogen and forming their respective salts. Hydrogen cannot be prepared from metals which are below it in the activity series of metals (such as copper) because only metals which are more reactive than hydrogen can displace it from acids.
    3. Only magnesium will displace zinc from zinc sulphate solution because magnesium is more reactive than zinc in the activity series of metals. No reaction takes place in case of copper and iron because they are less reactive than zinc.
  1. Mg > Fe > Cu 

Solution 11

  1. C. Mg 
  2. C. Pd 
  3. C. 1 n and 1 e-
  4. C. Al, Zn, Pb  

Solution 12

  1. Zinc granules are preferred over pure zinc in the lab preparation of hydrogen because the impurity present in granulated zinc is copper, whose catalysing effect speeds up the rate of the reaction.
  2. Purified and dried hydrogen is collected over mercury because mercury has no reaction with it.
  3. The end of the thistle funnel should be dipped under acid so as to prevent the gas from escaping from the thistle funnel.
  4. Dilute sulphuric acid cannot be replaced by concentrated acid in the preparation of hydrogen because it is a strong oxidising agent and it will produce sulphur dioxide.

Study of the First Element - Hydrogen Exercise Ex. 6(C)

Solution 1

  1. In the free state, hydrogen is found in traces in the earth's crust and atmosphere. Volcanic gases contain 0.025%, the earth's crust 0.98%, the earth's atmosphere 0.01% and the atmosphere of the Sun and stars 1.1%.
  2. The name 'hydrogen' originated on account of its ability to form water. 

Solution 2

  1. 2K + 2H2O →2KOH + H2
  2. Ca + 2H2O → Ca(OH)2 + H2 

Solution 3

Metal preferred for collecting hydrogen from

  1. Cold water: Sodium
  2. Hot water: Magnesium
  3. Steam: Aluminium

Solution 4

  1. Zinc is the most preferred metal in the laboratory preparation of hydrogen.
  2. Dilute sulphuric acid.

Conc. nitric acid, even in its dilute form, is not used in the preparation of hydrogen from metals because it is a powerful oxidising agent and oxygen formed due to its decomposition oxidises hydrogen to give water, thus defeating the purpose of the reaction.

Conc. sulphuric acid is not used in the preparation of hydrogen as it will produce sulphur dioxide.

  1. The gas is collected by the downward displacement of water.

Common drying agents such as fused calcium chloride, caustic potash stick and phosphorus pentoxide remove water vapour. 

Solution 5

  1. Calcium is expensive.
  2. Iron has to be heated, and hydrogen thus produced contains impurities such as hydrogen sulphide and sulphur dioxide.
  3. Aluminium forms a protective coating of Al2O3 due to its great affinity for oxygen. So, it does not give hydrogen with acid after the reaction has occurred for some time.
  4. Sodium reacts violently with acid. 

Solution 6

Increasing order of reactivity of metals:

Iron < Zinc < Magnesium < Calcium < Sodium

Solution 7

Hydrogen is evolved when dilute HCl reacts with magnesium which is placed above hydrogen in the activity series. However, this does not occur for metals below hydrogen such as mercury and silver. This is because only metals which are more reactive than hydrogen can displace it from HCl.

Solution 8

  1. With metals:

3Fe + 4H2O Fe3O4 + 4H2

  1. With non-metals:

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

C + H2O  (CO + H2) - ∆

 

Water is mixed with excess steam and passed over heated ferric oxide which acts as a catalyst and chromic oxide which acts as a promoter.

(CO + H2) + H2O  CO2 + 2H2 + ∆

 

The above mixture CO2 + H2 is formed through cold water under pressure (30 atm) or through caustic potash solution, which dissolves the more soluble carbon dioxide leaving hydrogen.

2KOH + CO2 →K2CO3 + H2O

 

The mixture is passed through ammoniacal cuprous chloride solution in order to dissolve any uncombined carbon monoxide.

CuCl + CO + 2H2O →CuCl.CO.2H2O 

 

Solution 9

  1. Zn + H2SO4 → ZnSO4 + H2
  2. Fe + H2SO4 → FeSO4 + H2

Hydrogen cannot be prepared from metals which are below it in the activity series of metals such as copper because only metals which are more reactive than hydrogen can displace it from acids. 

Solution 10

  1. It forms an insoluble coating of lead sulphate or lead chloride. So, further reaction is prevented.
  2. Potassium and sodium react violently with acid. Hence, potassium and sodium are not used for reaction with dilute hydrochloric acid or dilute sulphuric acid in the laboratory preparation of hydrogen. 

Solution 11

NaOH and KOH

Zn + 2NaOH → Na2ZnO2 + H2

Zn + 2KOH → K2ZnO2 + H2

Metals such as zinc, lead and aluminium have a unique nature. They react with acids and can even react with hot alkalis to form hydrogen and a soluble salt.

Solution 12

  1. 2Na + 2H2O → 2NaOH + H2
  2. Ca + 2H2O → Ca(OH)2 + H2
  3. Mg + 2H2O → Mg(OH)2 + H2
  4. Zn + H2O → ZnO + H2
  5. 3Fe + 4H2O Fe3O4 + 4H2
  6. Zn + 2HCl → ZnCl2 + H2
  7. 2Al + 3H2SO4 →Al2(SO4)3 + 3H2
  8. Fe +2HCl →FeCl2 + H2
  9. Zn + 2NaOH → Na2ZnO2 + H2
  10. 2Al + 2KOH + 2H2O →2KAlO2 + 3H2 

Solution 13

  1. Iron oxide is formed with the evolution of hydrogen gas.
  2. Hydrogen reduces heated magnetic oxide of iron. 

Solution 14

  1. Magnesium
  2. Mg + 2H2O → Mg(OH)2 + H2

Mg + H2O →MgO + H2 

Solution 15

On passing hydrogen gas through soap solution, soap bubbles filled with hydrogen fly high and burst. This behavior proves that hydrogen is lighter than air.

Solution 16

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

N2 + 3H2 2NH3 

  1. Equal volumes of hydrogen and chlorine react slowly in diffused sunlight to form hydrogen chloride.

H2 + Cl2 →2HCl

  1. Hydrogen gas on passing through molten sulphur reacts to give hydrogen sulphide.

H2 + S → H2S

  1. Hydrogen burns in the presence of electric spark with a 'pop' sound in oxygen and with a blue flame forming water.

2H2 + O2 →2H2O 

Solution 17

  1. A = CuO, B = Cu
  2. Blue and red litmus paper when dipped in the colourless liquid do not change colour. This confirms the liquid formed is neutral and is water.

It changes white anhydrous copper sulphate to blue salt.

  1. Black copper oxide (A) on heating with hydrogen reduces copper oxide to reddish brown copper and itself gets oxidised to water.

Hydrogen is a strong reducing agent and removes oxygen from less active metals, i.e. it removes oxygen from heated metal oxides when passed over them and itself gets oxidised to water.

  1. CuO + H2  Cu + H2O
  2. Cu + HCl →No reaction

Copper is less reactive than hydrogen and hence cannot displace it from HCl. 

Study of the First Element - Hydrogen Exercise Ex. 6(D)

Solution 1

In the electronic concept, oxidation is a process in which an atom or ion loses electron(s).

Zn Zn2+ + 2e-

Oxidation is also defined as a chemical process which involves

  1. Addition of oxygen
  2. Addition of electronegative ion
  3. Removal of hydrogen
  4. Removal of electropositive ion (element)

In the electronic concept, reduction is a process in which an atom or ion gains electrons.

Cu2+ + 2e-→ Cu

Reduction is also defined as a chemical process which involves

  1. Removal of oxygen
  2. Addition of electropositive ion
  3. Addition of hydrogen
  4. Removal of electronegative ion 

Solution 2

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

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

CuSO4 + Zn → ZnSO4 + Cu

Cu + 2SO42- + Zn →Zn + 2SO42- + Cu

Writing the half reaction,

Zn Zn2+ + 2e- (Oxidation)

Cu2+ + 2e-→ Cu (Reduction)

They occur simultaneously as

Cu2+ + Zn Zn2++ Cu

Thus, oxidation and reduction always occur simultaneously. 

Solution 3

  1. PbO in the given reaction is reduced to Pb by losing oxygen.
  2. Magnesium undergoes oxidation by loss of electrons (Mg - 2e- Mg2+).
  3. H2S undergoes oxidation by loss of hydrogen to give sulphur.
  4. Chlorine undergoes reduction by the addition of hydrogen to form HCl. 

Solution 4

  1. Oxidation
  2. Reduction
  3. Oxidation
  4. Oxidation 

Solution 5

Half reaction:

A+ + e-A (Reduction)

B B + e- (Oxidation)

a. A

b. B

c. B 

Solution 6

  1. Zn + Pb2+ → Pb + Zn 2+
    Zn → Zn 2+ + 2 e- ---- Oxidation
    Pb2+ + 2 e Pb ---- Reduction
  1. Zn + Cu2+ Cu + Zn 2+
    Zn Zn 2+ + 2 e- ---- Oxidation
    Cu2+ + 2 e Cu ---- Reduction
  1. Cl2 + 2Br-  Br2 + 2Cl
    Cl2 2Cl- + 2 e- ---- Oxidation
    2Br + 2 e Br2---- Reduction

Solution 7

  1. Equation in the ionic form:

Cu2+ SO42-   + Fe Fe2+  SO42- + Cu

  1. Fe Fe2+  2 e- ---- Oxidation  

Cu2+ + 2 e Cu ---- Reduction 

Solution 8

  1. Hydrogen is collected by the downward displacement of air because
    i. It is insoluble in water.
    ii. It forms an explosive mixture with air and therefore cannot be collected by the downward displacement of air even though it is lighter than it.
  1. Hydrogen is combustible, but it does not support combustion. So, the candle burns in air or oxygen when brought near the mouth of a jar containing hydrogen but is extinguished when pushed inside the jar as the supply of oxygen is cut off.
  2. Apparatus for laboratory preparation of hydrogen should be airtight and away from a naked flame because a mixture of hydrogen and air explodes violently when brought near a flame.

Solution 9

  1. Na

The other metals react with dil. HCl liberating hydrogen gas, while sodium reacts violently with acid.

  1. NH3 is basic in nature.
  2. Cu

Metals more reactive than hydrogen can displace it from acids.

  1. Pb

Lead reacts with dilute sulphuric acid or HCl and forms an insoluble coating of lead sulphate or lead chloride.

The others react with dilute sulphuric acid or HCl to liberate hydrogen. 

Solution 10

a. non-combustible

b. base

c. CuO

d. It is a strong oxidising agent.

e. (iii) PbO is oxidised to Pb.

f. (ii) Zn

g. (ii) Cu

Solution 11

  1. CuO, H2, H2O
  2. sparingly
  3. magnesium, iron and aluminium
  4. amalgam
  5. above, dilute hydrochloric, dilute sulphuric acid 

Solution 12

  1. Hydrogen is separated from CO by passing the mixture through caustic potash solution.
  2. All metals above hydrogen in the activity series react with acids to give hydrogen. 
  3. Hydrogen is dried by passing it through calcium chloride, caustic potash and phosphorous pentoxide. 
  4. Very dilute nitric acid reacts with magnesium and manganese to produce hydrogen.
  5. Dil. H2SO4 reacts with zinc to liberate hydrogen.

Solution 13

  1. Chlorine
  2. MnO2
  3. H2S
  4. Hydrogen peroxide
  5. MnO2 
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