Hydrogen

In Free state:

• In traces : Hydrogen occurs in minute traces in the earth's crust, atmosphere & in volcanic gases
• In & around the sun : Hydrogen occurs in the interior of the sun which consists mainly of hydrogen, which is converted to energy released as heat & light.

The compounds containing hydrogen are: ammonia (NH₃), hydrochloric acid (HCl), water(H₂O)

(a) Steam : Zn reacts with steam to form zinc oxide and liberate hydrogen gas.

Zn + H₂O ⟶ ZnO + H₂ ↑

Lead (Pb) will not react with steam to give hydrogen.

(b) A dilute acid : Zn reacts with dil. sulphuric acid to form zinc sulphate and liberate hydrogen gas.

Zn + H₂SO₄ ⟶ ZnSO₄ + H₂ ↑

Copper (Cu) will not react with dilute acid to give hydrogen.

(c) An alkali : Zn reacts with sodium hydroxide to form sodium zincate and liberate hydrogen gas.

Zn + 2NaOH ⟶ Na₂ZnO₂ + H₂ ↑

Silver (Hg) will not react with alkali to give hydrogen.

(a) Electrolysis — It is the process of decomposition of a chemical compound [electrolyte] in the fused [molten] or solution state by passage of an electric current resulting in the dissociation of the chemical compound into ions which are discharged as neutral atoms at the respective electrodes.

Electrolyte — A chemical compound which in the molten or solution state can conduct electric current & undergo chemical decomposition due to flow of current.

(b) The electrodes are:

1. Anode
2. Cathode

(a) Addition of traces of copper [II] sulphate to the reaction medium enhances the speed of the reaction.

(b) Hydrogen forms a highly explosive mixture with air. Hence, it is not collected by the downward displacement of air and collected only after all the air in the apparatus is allowed to escape. As it is almost insoluble in water, so it is collected by downward displacement of water.

(c) The lower end of the thistle funnel should dip below the level of the acid in the flask, otherwise hydrogen gas produced will escape through thistle funnel.

(a) Balanced equations for first two main steps of Bosch process:

Step I — Production of water gas

$\underset{\text{coke}}{\text{C}} + \underset{\text{steam}}{\text{H}_2\text{O}} \xrightarrow{1000 ^{\circ} \text{C}} \underset{\text{water gas}}{[\text{CO} + \text{H}_2 ]} - \Delta$

Step II — Reduction of steam to hydrogen by carbon monoxide

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

(b) Promoter increases the efficiency of the catalyst.

(c) Ammoniacal cuprous chloride solution [CuCl].

(a) Colour & odour — colourless and odourless

(b) Solubility in water — Very slightly soluble in water

(c) Effect on moist blue litmus paper — neutral to litmus

Experiment 1

Procedure : Bubble hydrogen gas by means of a small pipe into a dish containing soap solution.

Observation : Soap bubbles filled with H₂ rise upwards.

Conclusion : Hydrogen is lighter than air hence soap bubbles filled with hydrogen rise up.

Experiment 2

Procedure : Take two dry test tubes.

Test tube 'A' - Filled with hydrogen.

Test tube 'B' - Filled with air.

Place test tube 'B' over test tube 'A' and test the gas in the upper test tube 'B' with a glowing splinter.

Observation : A 'pop' sound is heard in test tube 'B'. [hydrogen burns with a 'pop' sound].

Conclusion : Hydrogen is lighter than air and has moved upwards displacing the air.

(a) Hydrogen burns quietly in oxygen with a pale blue flame and water is formed.

2H₂ + O₂ ⟶ 2H₂O

(b) Three volumes of hydrogen, reacts with one volume of nitrogen to form ammonia which is a basic gas.

$\text{N}_2 + 3\text{H}_2 \xrightleftharpoons[\text{450-500 ^{\circ} C}]{\text{Fe}} \text{2NH}_3 + \Delta$

(c) Hydrogen reduces oxides of less active metals e.g. zinc to form the reduced metal & water.

ZnO + H₂ ⟶ Zn + H₂O

Experiment : Take a jar filled with hydrogen with inverted mouth downwards and introduce a burning candle near the mouth of jar.

Observation : The gas burns at the mouth of the jar while the burning candle is extinguished when pushed inside the jar.

Conclusion :

• Hydrogen is combustible & hence burns at the mouth of the jar.
• It does not support combustion and extinguishes a burning candle when pushed inside.

As hydrogen is a strong reducing agent hence it reduces heated copper (II) oxide to copper. Therefore, the original black copper (II) oxide changes to reddish brown copper.

$\underset{\text{[black]}}{\text{CuO}} + \text{H}_2 \longrightarrow \underset{\text{[reddish brown]}}{\text{Cu}} + \text{H}_2\text{O}$

(a) Hydrogen is lighter than air but it is a highly combustible gas. It forms an explosive mixture with air (due to the oxygen present in it). Hence, it is not used in air balloons.

(b) A mixture of hydrogen & oxygen on burning produces heat [temperatures upto 2800°C]. Such high temperature flames are used for welding & cutting metals.

(c) Hydrogen combines with nitrogen at 450°C and in the presence of catalyst iron to give ammonia. Ammonia is used in the manufacture of urea, fertilizers, nitric acid, explosives, etc.

When a burning splinter is brought near the mouth of the gas jar containing pure hydrogen, it burns quietly with a pale blue flame whereas the other jar containing hydrogen-air mixture burns with a pop sound.

'Oxidation is a chemical reaction involving addition of oxygen to a substance or removal of hydrogen from a substance. Reduction on the other hand involves addition of hydrogen to a substance or removal of oxygen from a substance.

(a) Chlorine is reduced to HCl.

(b) Hydrogen sulphide is oxidised to sulphur since the reaction involves removal of hydrogen.

(c) Chlorine acts as an oxidising agent.

1. Zn + 2NaOH ⟶ Na₂ZnO₂ + H₂ ↑

2. 2H₂O ⇌ 2H₂ + O₂

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

4. Fe₂O₃ + 3H₂ ⟶ 2Fe + 3H₂O

5. N₂ + 3H₂ ⇌ 2NH₃

1. In the metal reactivity series, copper (Cu) is below hydrogen but zinc (Zn) is above hydrogen. Hence, copper does not displace hydrogen from dilute hydrochloric acid, but zinc does.

2. Pure water is almost a non-electrolyte and will not normally conduct electricity. It consists almost entirely of molecules. It can be electrolytically decomposed by addition of traces of dil. H₂SO₄ which dissociates as: H⁺¹ and SO₄^-2 and help in dissociating water into H⁺ and OH^- ions.

3. Granulated zinc contains traces of impurities, which has a slight catalyzing effect on the reaction and speeds it up.

4. CO₂ is removed by dissolving mixture in caustic potash solution.
   2KOH + CO₂ ⟶ K₂CO₃ + H₂O

5. Hydrogen sulphide reacts with chlorine as per the below equation:
   H₂S + Cl₂ ⟶ 2HCl + S
   Here, H₂S is oxidised to S and Cl₂ is reduced to HCl, at the same time. Hence, it is a redox reaction.

1. A metal which reacts with water to give a metallic hydroxide & liberate hydrogen — B: Sodium

2. The metallic compound used as a catalyst in Bosch process — D: Iron [III] oxide

3. The metal used as a catalyst in hydrogenation of oils — A: Nickel

4. The metal which reacts with steam liberating hydrogen & the reaction is reversible — C: Iron

5. The metallic compound formed when a metal above aluminium in the activity series reacts with steam — E: Magnesium oxide

1. The acid dil. HNO₃ is not used in the laboratory preparation of hydrogen, using zinc and an acid.

2. In Bosch process the catalytic reduction of steam to hydrogen is carried out by CO .

3. A foul smelling gas formed when hydrogen reacts with a molten non-metal is hydrogen sulphide.

4. The product formed on combustion of hydrogen in air is water.

5. The gas which has now replaced hydrogen in air balloons is helium.