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

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

Question 1

Justify the position of Hydrogen in the periodic table.

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.

Question 2

Why does hydrogen show dual nature?

Solution 2

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

Question 3

Compare hydrogen with alkali metals on basis of:

  1. Ion formation
  2. Reducing power
  3. Reaction with oxygen
  4. Oxide formation 
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

Question 4

In what respect does hydrogen differ from:

  1. alkali metals
  2. halogens? 
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. 
Question 5

Give the general group study of hydrogen with reference to

  1. valence electrons
  2. burning
  3. reducing power 
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 

Question 6

Why hydrogen was called 'inflammable air'?

Solution 6

Hydrogen was called inflammable air because of its combustible nature.

Question 7

State some sources of hydrogen.

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.

Question 8

Compare hydrogen and halogens on the basis of:

  1. physical state
  2. ion formation
  3. valency 
  4. reaction with oxygen 
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. 
Question 9

Which metal is preferred for preparation of hydrogen.

  1. from water?
  2. from acid? 
Solution 9
  1. Reactive metals such as potassium, sodium and calcium.
  2. Magnesium, aluminium, zinc and iron. 
Question 10
  1. Write the reaction of steam with red hot iron.
  2. Why this reaction is considered as reversible reaction?
  3. How the reaction can proceed continuously? 
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. 
Question 11

Explain the unique nature of zinc and aluminium. Give balanced equations to support your explanation.

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

Question 12

Write balanced equations for the following:

  1. Iron reacts with dil. HCl
  2. Zinc reacts with caustic soda solution
  3. Lead reacts with potassium hydroxide
  4. Aluminium reacts with fused sodium hydroxide 
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 
Question 13

Write the balanced equations and give your observations when the following metals react:

 i. Sodium with cold water

 ii. Calcium with cold water

 iii. Magnesium with boiling water

 iv. Magnesium with steam

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 

Question 14
  1. Under what conditions iron reacts with water.
  2. Give the balanced equation of the reaction.
  3. What is noticed if the products are not allowed to escape?
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. 
Question 15

From the knowledge of activity series, name a metal which shows the following properties

  1. It reacts readily with cold water.
  2. It displaces hydrogen from hot water.
  3. It displaces hydrogen from dilute HCl.
  4. It forms a base which is insoluble in water. 
Solution 15
  1. Sodium
  2. Magnesium
  3. Zinc
  4. Calcium 
Question 16

 Complete the following word equations:

  1. Sodium hydroxide + zinc → hydrogen + _________
  2. Calcium + water → calcium hydroxide + _________
Solution 16
  1. Sodium hydroxide + zinc → hydrogen + sodium zincate
  2. Calcium + water → calcium hydroxide + hydrogen

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

Question 1

Hydrogen can be prepared with the metal zinc by using:

  1. acid
  2. alkali
  3. water

Give an equation in each case.

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

For laboratory preparation of hydrogen, give the following:

  1. materials used
  2. method of collection
  3. chemical equation
  4. fully-labelled diagram 
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.  

Selina Solutions Icse Class 9 Chemistry Chapter - Study Of The First Element Hydrogen 

Question 3
  1. Name the impurities present in hydrogen prepared in the laboratory.
  2. How can these impurities be removed? 
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. 

Question 4

Which test should be made before collecting hydrogen in a gas jar?

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.

Question 5

Why nitric acid is not used in the preparation of hydrogen?

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

Question 6

Why hot concentrated sulphuric acid is not used in the preparation of hydrogen?

Solution 6

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

Zn + 2H2SO4 →ZnSO4 + SO2 + 2H2O

Question 7

Hydrogen is manufactured by 'Bosch Process'.

  1. Give the equations with conditions.
  2. How can you obtain hydrogen from a mixture of hydrogen and carbon monoxide? 
Solution 7
  1. C + H2O Selina Solutions Icse Class 9 Chemistry Chapter - Study Of The First Element Hydrogen(CO + H2) - ∆

       (CO + H2) + H2O Selina Solutions Icse Class 9 Chemistry Chapter - Study Of The First Element HydrogenCO2 + 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 

Question 8

Give equations to express the reaction between:

  1. Steam and red hot iron
  2. Calcium and water 
Solution 8
  1. 3Fe + 4H2O Fe3O4 + 4H2
  2. Ca + 2H2O Ca(OH)2 + H2 
Question 9

A small piece of calcium metal is put into a small trough containing water. There is effervescence and white turbidity is formed.

  1. Name the gas formed in the reaction. How would you test the gas?
  2. Write an equation for the reaction.
  3. What do you observe when a few drops of red litmus solution are added to the turbid solution.
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 

Question 10

Thin strips of magnesium,copper and iron are taken.

  1. Write down what happens when these metals are treated as follows:
    1. Heated in presence of air
    2. Heated with dil.HCl
    3. Added to an aqueous solution of zinc sulphate
  1. Arrange these metals in descending order of reactivity. 
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 
Question 11

Choose the correct option:

  1. Hydrogen is evolved by the action of cold dil. HNO3 on
    A. Fe B. Cu C. Mg D. Zn
  1. Which metal absorbs hydrogen?
    A. Al B. Fe C. Pd D. K
  1. The composition of nucleus of deuterium is
    A. 1 e- and 1P B. 1 P and 1 A
    C. 1 n and 1 eD. 2P and 1 e-
  1. Elements which show unique nature in the preparation of hydrogen are:
    A. Na, K, Li B. Mg, Ca, Ba
    C. Al, Zn, Pb D. Fe, Cu, Ag
Solution 11
  1. C. Mg 
  2. C. Pd 
  3. C. 1 n and 1 e-
  4. C. Al, Zn, Pb  
Question 12

Give reasons for the following:

  1. Zinc granules are used in lab preparation of hydrogen.
  2. Purified and dried hydrogen is collected over mercury.
  3. The end of the thistle funnel should be dipped under acid
  4. Dilute sulphuric acid cannot be replaced by concentrated acid in the preparation of hydrogen.

 

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.

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

Question 1
  1. Where does hydrogen occur in free state?
  2. How did the name 'hydrogen' originate? 
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. 
Question 2

Hydrogen can be prepared with the help of cold water. Give a reaction of hydrogen with:

  1. A monovalent metal
  2. A divalent metal 
Solution 2
  1. 2K + 2H2O →2KOH + H2
  2. Ca + 2H2O → Ca(OH)2 + H2 
Question 3

Which metal is preferred for collecting hydrogen from:

  1. Cold water
  2. Hot water
  3. Steam
Solution 3

Metal preferred for collecting hydrogen from

  1. Cold water: Sodium
  2. Hot water: Magnesium
  3. Steam: Aluminium
Question 4

Hydrogen may be prepared in the laboratory by the action of a metal on an acid.

  1. Which of the metals copper, zinc, magnesium or sodium would be the most suitable?
  2. Which of the acids dilute sulphuric, concentrated sulphuric, dilute nitric acid and concentrated nitric acid would you choose? Explain why you would not use the acids you reject.
  3. How would you modify your apparatus to collect dry hydrogen? Which drying agent would you employ for this purpose? 
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. 

Question 5

Why are the following metals not used in the lab. preparation of hydrogen?

  1. calcium
  2. iron
  3. aluminium 
  4. sodium 
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. 
Question 6

Based on the reactions of water on metals, arrange the following metals in increasing order of reactivity: iron, sodium, magnesium, zinc, calcium

Solution 6

Increasing order of reactivity of metals:

Iron < Zinc < Magnesium < Calcium < Sodium

Question 7

Hydrogen is evolved when dilute HCl reacts with magnesium, but nothing happens in the case of mercury and silver. Explain.

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.

Question 8

Steam can react with metal and non-metal to liberate hydrogen. Give necessary conditions and equations for the same.

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 Selina Solutions Icse Class 9 Chemistry Chapter - Study Of The First Element Hydrogen(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 Selina Solutions Icse Class 9 Chemistry Chapter - Study Of The First Element HydrogenCO2 + 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 

 

Question 9

Hydrogen is obtained by displacement from:

  1. dilute sulphuric acid
  2. dilute hydrochloric acid

Write equations using zinc and iron.

Why does copper not show similar behavior?

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. 

Question 10

Give reason for the following:

  1. Though lead is above hydrogen in the activity series, it does not react with dilute hydrochloric acid or dilute sulphuric acid.
  2. Potassium and sodium are not used for reaction with dilute hydrochloric acid or dilute sulphuric acid in laboratory preparation of hydrogen. 
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. 
Question 11

Name two alkalies that can displace hydrogen. Give balanced equations for the same. Why are the metals you have used considered to have unique nature?

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.

Question 12

Complete and balance the following reactions.

  1. Na + H2O →_____________ +___________
  2. Ca + H2O →_____________ +___________
  3. Mg + H2O →_____________ +___________
  4. Zn + H2O →_____________ +___________
  5. Fe + H2O →_____________ +___________
  6. Zn + HCl →_____________ +___________
  7. Al + H2SO4 →_____________ +___________
  8. Fe + HCl →_____________ +___________
  9. Zn + NaOH →_____________ +___________
  10. Al + KOH + H2O→_____________ +___________ 
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 
Question 13

If the following are kept in closed vessels at over 400°C, what would happen to them?

  1. iron filing and steam
  2. hydrogen and magnetic oxide of iron? 
Solution 13
  1. Iron oxide is formed with the evolution of hydrogen gas.
  2. Hydrogen reduces heated magnetic oxide of iron. 
Question 14
  1. A metal in the powdered from reacts very slowly with boiling water, but it decomposes in steam. Name the metal.
  2. Write a balanced equation for the reaction occurring in (a). 
Solution 14
  1. Magnesium
  2. Mg + 2H2O → Mg(OH)2 + H2

Mg + H2O →MgO + H2 

Question 15

What do you observe when hydrogen gas is passed through a soap solution?

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.

Question 16

Under what conditions can hydrogen be made to combine with?

  1. nitrogen?
  2. chlorine?
  3. sulphur?
  4. oxygen?

Name the products in each case and write the equation for each reaction.

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 

Question 17

When hydrogen is passed over a black solid compound A, the products are a 'colourless liquid' and a 'reddish brown metal B'.

Substance B is divided into two parts each placed in separate test tubes.

Dilute HCl is added to one part of substance B and dilute HNO3 to the other.

  1. Name the substances A and B.
  2. Give two tests for the colourless liquid formed in the experiment.
  3. What happens to substance A when it reacts with hydrogen? Give reasons for your answer.
  4. Write an equation for the reaction between hydrogen and substance A.
  5. Is there any reaction between substance B and dilute hydrochloric acid? Give reasons for your answer.  
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 + H2Selina Solutions Icse Class 9 Chemistry Chapter - Study Of The First Element Hydrogen Cu + H2O
  2. Cu + HCl →No reaction

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

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

Question 1

Describe briefly the ionic concept of oxidation and reduction. Give an equation to illustrate.

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 
Question 2

Is it essential that oxidation and reduction must occur side by side in a chemical reaction? Explain

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. 

Question 3

State, giving reasons, whether the substances printed in bold letters have been oxidized or reduced.

  1. PbO + CO Pb + CO2
  2. Mg + 2HCl MgCl2 + H2
  3. H2S + Cl2 2HCl + S
  4. Cl2 + H2S 2HCl + S 
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. 
Question 4

State whether the following conversions are oxidation or reduction:

  1. PbO2 + SO2PbSO4 
  2. Cu2+ + 2 e- Cu 
  3. K K+ + e- 
  4. 2Cl- - e- Cl2 

 

Solution 4
  1. Oxidation
  2. Reduction
  3. Oxidation
  4. Oxidation 
Question 5

In the following reaction: A+ + B → A + B+. Write half reactions for this reaction and name:

  1. oxidizing agent
  2. substance oxidized
  3. reducing agent 
Solution 5

Half reaction:

A+ + e-A (Reduction)

B B + e- (Oxidation)

a. A

b. B

c. B 

Question 6

Divide the following reactions into oxidation and reduction half reactions:

  1. Zn + Pb2+ Pb + Zn 2+
  2. Zn + Cu2+ Cu + Zn 2+
  3. Cl2 + 2Br-  Br2 + 2Cl-
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
Question 7
  1. Write the equation in the ionic form

CuSO4(aq)  + Fe(s) FeSO4(aq) + Cu(s)

  1. Divide the above equation into oxidation and reduction half reactions.

 

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 

Question 8

Give reasons:

  1. Hydrogen is collected by the downward displacement of water and not of air, even though it is lighter than air.
  2. A candle brought near the mouth of a jar containing hydrogen gas starts burning but is extinguished when pushed inside the jar.
  3. Apparatus for laboratory preparation of hydrogen should be air tight and away from a naked flame.
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.
Question 9

Select the odd one out and justify your answer.

  1. Zn, Fe, Mg and Na
  2. SO2, H2S, NH3 and CO3
  3. Fe, Zn, Cu and Mg
  4. Fe, Pb, Al and Zn 
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. 

Question 10
  1. Helium is preferred to hydrogen for filling balloons because it is:
    1. lighter than air
    2. almost as light as hydrogen
    3. non-combustible
    4. inflammable
  1. Reacting with water, an active metal produces
    1. oxygen
    2. nitric acid
    3. a base
    4. none of these

c. A metal oxide that is reduced by hydrogen is

 i. Al2O3

 ii. CuO

 iii. CaO

 iv. Na2O

  1. Which of the following statements about hydrogen is incorrect?
    1. It is an inflammable gas
    2. It is the lightest gas.
    3. It is not easily liquefied
    4. It is a strong oxidizing agent.
  1. For the reaction PbO + H2 Pb + H2O, which of the following statements is wrong?
    1. H2 is the reducing agent.
    2. PbO is the oxidizing agent.
    3. PbO is oxidized to Pb.
    4. H2 is oxidized to H2O.
  1. Which metal gives hydrogen with all of the following: water, acids, alkalis?
    1. Fe
    2. Zn
    3. Mg
    4. Pb
  1. Which of the following metals does not give hydrogen with acids?
    1. Iron
    2. Copper
    3. Lead
    4. Zinc
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

Question 11

Choose terms from the options given in brackets to complete these sentences.

  1. When CuO reacts with hydrogen,………………… is reduced and ……………….is oxidized to ………………… .

(CuO, H2,Cu,H2O)

  1. Hydrogen is ………………… soluble in water.

(sparingly, highly, moderately)

  1. Metals like …………….. , ……………… and ……………… give H2 with steam.

(iron, magnesium, aluminium, sodium , calcium)

  1. Sodium ………………. reacts smoothly with cold water.

(metal, amalgam, in the molten state)

  1. A metal …………….. hydrogen in the activity series gives hydrogen with …………… acid or …………… acid.

(above, below, dilute hydrochloric, concentrated hydrochloric, dilute sulphuric). 

Solution 11
  1. CuO, H2, H2O
  2. sparingly
  3. magnesium, iron and aluminium
  4. amalgam
  5. above, dilute hydrochloric, dilute sulphuric acid 
Question 12

Correct the following statements:

  1. Hydrogen is separated from CO by passing the mixture through caustic potash solution.
  2. All metals react with acids to give hydrogen.
  3. Hydrogen is dried by passing it through conc. H2SO4.
  4. Very dilute nitric acid reacts with iron to produce hydrogen.
  5. Conc. H2SO4 reacts with zinc to liberate hydrogen. 
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.
Question 13

Name:

  1. an oxidizing agent that does not contain oxygen.
  2. a substance that oxidizes concentrated HCl to chlorine.
  3. a substance that will reduce aqueous Iron(III)ions to Iron(II)ions.
  4. a liquid that is an oxidizing agent as well as a reducing agent.
  5. a gas that is an oxidizing as well as a reducing agent.
  6. a solid that is an oxidizing agent. 
Solution 13
  1. Chlorine
  2. MnO2
  3. H2S
  4. Hydrogen peroxide
  5. MnO2