ICSE Class 9 Chemistry Chapter 09 Hydrogen

Study of The First Element - Hydrogen

Scope of Syllabus

Position of the non-metal (Hydrogen) in the periodic table and general group characteristics with reference to valency electrons, burning, ion formation applied to the above mentioned element.

(i) Hydrogen from water

(ii) Hydrogen from dilute acids

(iii) Hydrogen from alkalies.

Hydrogen from water: Cold water and metals; hot water and metals; steam and metals; steam and non-metals. Application of activity series for the above mentioned preparations. Displacement of hydrogen from dilute sulphuric acid or hydrochloric acid by zinc or iron (no reaction with copper). Displacement of hydrogen from alkalies (NaOH, KOH by Zn, Al) - unique nature of these elements.

(iv) The preparation and collection of hydrogen by a standard laboratory method other than electrolysis.

In the laboratory preparation, the reason for using zinc, the impurities in the gas, their removal and the precautions in the collection of the gas must be mentioned.

Industrial manufacture of hydrogen by Bosch process with main reactions and conditions; separation of CO2 and CO from it.

Important Points to Remember

1. The first element that existed in the universe was hydrogen.

2. Hydrogen in the sun undergoes the process of nuclear fusion to form helium with the liberation of energy in the form of heat and light.

3. The credit of discovery of hydrogen goes to Henry Cavendish.

4. Antoine Lavoisier named the gas as hydrogen, i.e., water producer.

5. Hydrogen is the first element present in the periodic table.

6. Position of hydrogen is controversial as it is placed in group 1 (alkali metals) and group 17 (halogens).

7. Hydrogen resembles the alkali metals in the following ways:

(i) Electronic configuration: Like alkali metals, hydrogen has also got one electron in its valence shell.

(ii) Formation of cations: Like alkali metals, hydrogen loses electron and forms cation.

H - e- → H+

(Hydrogen)

Na - e- → Na+

(Alkali metal)

(iii) Formation of compounds: Hydrogen readily forms stable compounds with oxygen, sulphur and chlorine.

H2O = Water

H2S = Hydrogen sulphide

HCl = Hydrogen chloride

In the similar way the alkali metals form oxides, sulphides and chlorides

Na2O = Sodium oxide

Na2S = Sodium sulphide

NaCl = Sodium chloride

(iv) Hydrogen burns in oxygen to form water (neutral oxide).

2H2 + O2 → 2H2O.

8. Hydrogen resembles halogens in the following ways:

(i) Electronic configuration: Both hydrogen and halogen require one electron to complete their duplet and octet respectively, hence they gain electrons to acquire stable configuration. Therefore, they are electronegative and monovalent.

H + e- → H-

Cl + e- → Cl-

(ii) Atomicity: It is the number of atoms present in one molecule of an element. Both hydrogen and halogens are diatomic, i.e., having two atoms in its one molecule.

Hydrogen - H2 Fluorine - F2

Chlorine - Cl2

Bromine - Br2

Iodine - I2

9. Hydrogen has three isotopes, i.e., these are the atoms of same element having same atomic number but different mass number.

Isotopes differ in number of neutrons.

10. Protium has no neutron.

11. Hydrogen in combined state occurs in the form of water (H2O).

Formation of polar covalent bond in water

12. Water forms polar covalent bond.

13. All plants and animals have hydrogen in the form of carbohydrates, fats and proteins.

14. Organic compounds essentially contain hydrogen in combination with carbon.

15. General methods of preparation of Hydrogen.

(i) By action of metals with cold water: Sodium, potassium and calcium react with cold water to form its respective metallic hydroxides (soluble) with the liberation of hydrogen. It is not a safe method to prepare hydrogen gas in laboratory as the reaction sometimes proceeds with an explosion.

Metal + Cold water → Metallic hydroxide + Hydrogen

2Na + 2H2O → 2NaOH + H2

Sodium Water Sodium hydroxide Hydrogen

2K + 2H2O → 2KOH + H2

Potassium Water Potassium hydroxide Hydrogen

Ca + 2H2O → Ca(OH)2 + H2

Calcium Water Calcium hydroxide Hydrogen

Both sodium and potassium react vigorously with cold water. The solution thus produced as a result of reaction turns red litmus blue showing that the solution formed is basic or alkaline in nature.

(ii) By the reaction of metals with steam:

Heated metal + Steam → Metallic oxide + Hydrogen

Mg + H2O → MgO + H2

Steam

Zn + H2O → ZnO + H2

Steam

3Fe + 4H2O ⇌ Fe3O4 + 4H2

Heated

(iii) By the reaction of dilute acids with active metals:

(a) Activity series: The series in which the metals are arranged in the decreasing order of their reactivity is called activity series.

(b) The metal lying at the top is the most reactive metal and the metal present at the bottom is the least reactive metal.

(c) The metals placed above hydrogen are called active metals as they can displace hydrogen readily from water and dilute acids.

(d) Highly reactive metals like sodium, potassium and calcium react vigorously at ordinary temperature with dilute acids liberating hydrogen.

2Na + 2HCl → 2NaCl + H2

dil.

2K + 2HCl → 2KCl + H2

dil.

Ca + 2HCl → CaCl2 + H2

dil.

(e) Metals like magnesium, zinc, aluminium, iron, etc. react moderately at ordinary temperature with dilute acids to liberate hydrogen.

Metal + Dil. acid → Salt + Hydrogen

Mg + 2HCl → MgCl2 + H2

dil.

Mg + H2SO4 → MgSO4 + H2

dil.

Zn + 2HCl → ZnCl2 + H2

dil.

Zn + H2SO4 → ZnSO4 + H2

dil.

2Al + 6HCl → 2AlCl3 + 3H2

dil.

2Al + 3H2SO4 → Al2(SO4)3 + 3H2

dil.

Fe + 2HCl → FeCl2 + H2

dil.

Fe + H2SO4 → FeSO4 + H2

dil.

(f) Nitric acid reacts with only magnesium and manganese to liberate hydrogen. With rest of the metals it produces oxides of nitrogen or ammonium nitrate and not hydrogen as it is an oxidising agent.

Mg + 2HNO3 → Mg(NO3)2 + H2

v. dil.

Mn + 2HNO3 → Mn(NO3)2 + H2

v. dil.

(g) Lead is not used for the preparation of hydrogen by using dilute hydrochloric acid and dilute sulphuric acid because the products are insoluble lead chloride and lead sulphate which settle on fresh lead metal and thus, prevents the reaction of metal with acids.

(iv) By the action of alkalies with metals: Metals like zinc, aluminium and lead in powder or concentrated potassium hydroxide or concentrated sodium hydroxide form dissolve when boiled with concentrated sodium hydroxide or concentrated potassium hydroxide to form their respective soluble double complex salts with the liberation of hydrogen.

Zn + 2NaOH → Na2ZnO3 + H2

boiled

Sodium zincate

Zn + 2KOH → K2ZnO2 + H2

boiled

Potassium zincate

2Al + 2NaOH + 2H2O → 2NaAlO2 + 3H2

boiled

Sodium aluminate

2Al + 2KOH + 2H2O → 2KAlO2 + 3H2

boiled

Potassium aluminate

Laboratory Preparation of Hydrogen.

In laboratory, hydrogen is prepared by the reaction of dilute sulphuric acid with granulated zinc.

Zn + H2SO4 → ZnSO4 + H2

dil.

(i) Zinc is preferred to other metals because sodium and potassium react explosively or violently with cold water or dilute acids. In calcium and magnesium, the liberation of hydrogen is very rapid that it cannot be collected. Aluminium gets coated with the thin but tough layer of oxide which prevents the reaction of metal with water and dilute acids.

In iron, the liberation of hydrogen is very slow and as copper lies below hydrogen in activity series so, it cannot displace hydrogen from dilute acids.

(ii) The apparatus used for the preparation of hydrogen gas should be airtight. The lower end of the thistle funnel should dip in the acid as otherwise the gas will escape from the thistle funnel. The apparatus should be kept away from the flame.

(iii) Hydrogen obtained by the reaction of granulated zinc with dilute sulphuric acid is not pure, it contains the impurities of the following gases:

(a) Sulphur dioxide - SO2

(e) Arsine - AsH3

(b) Carbon dioxide - CO2

(f) Phosphine - PH3

(c) Water vapour - H2O

(g) Nitrogen dioxide - NO2

(d) Hydrogen sulphide - H2S

Although pure hydrogen is odourless but because of the presence of the above named gases as impurities it possesses a characteristic peculiar odour.

(iv) For the purification of hydrogen, the impure gas is passed through

(a) Silver nitrate solution: To absorb arsine and phosphine.

(b) Lead nitrate solution: To absorb hydrogen sulphide.

(c) Potassium hydroxide solution: To absorb carbon dioxide, sulphur dioxide and nitrogen dioxide.

(d) Anhydrous calcium chloride: To absorb moisture.

Teacher's Note

Hydrogen preparation is a foundational chemistry experiment that demonstrates metal reactivity and gas generation - concepts students will use in industrial applications like ammonia production and petroleum refining.

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