HYDROGEN - Questions & Answers

EXERCISE

Introduction

1. State how hydrogen occurs in the free state. Name three compounds containing hydrogen in the combined state.
In the free state, hydrogen occurs in minute traces in the earth's crust, atmosphere, and volcanic gases. It also occurs in the interior of the sun where it is converted into energy. In the combined state, it is found in compounds like water (H₂O), acids (e.g., HCl), and organic compounds (e.g., proteins).

Preparation

2. Starting from zinc how would you obtain hydrogen using - a] Steam b] A dilute acid c] An alkali [Give balanced equations for each & name the product formed in each case other than hydrogen]. Name a metal which will not react with the reactants above to give hydrogen.
a] Using Steam: Zn + H₂O → ZnO + H₂. Product: Zinc oxide.
b] Using a Dilute Acid: Zn + 2HCl → ZnCl₂ + H₂. Product: Zinc chloride.
c] Using an Alkali: Zn + 2NaOH → Na₂ZnO₂ + H₂. Product: Sodium zincate.
A metal that will not react with the above reactants to give hydrogen is Copper (or Silver/Gold/Platinum).

3. 'Hydrogen is obtained by electrolysis of acidified water'. Answer the following pertaining to the preparation of hydrogen by electrolysis. a] The meaning of the term 'electrolysis' & 'electrolyte'. b] Name the electrode - i] through which the current enters the electrolyte. ii] at which hydrogen is liberated.
a] Electrolysis: It is the process of decomposition of a chemical compound (electrolyte) in the fused or solution state by the passage of an electric current, resulting in the dissociation of the compound into ions.
Electrolyte: A chemical compound which, in the molten or solution state, can conduct an electric current and undergo chemical decomposition.
b] i] Anode. ii] Cathode.

4. In the laboratory preparation of hydrogen from zinc & dilute hydrochloric acid - state a reason for a] Addition of traces of copper [II] sulphate to the reaction medium. b] Collecting the hydrogen by downward displacement of water and not air & collecting it after all the air in the apparatus is allowed to escape. c] Having the end of the thistle funnel dip below the level of the acid in the flask.
a] Traces of copper [II] sulphate act as a catalyst to enhance the speed of the reaction.
b] Hydrogen is collected by downward displacement of water because it is very slightly soluble in water. Air is not used because hydrogen forms a highly explosive mixture with air. It is collected after air escapes to ensure safety and purity.
c] The end of the thistle funnel must dip below the level of the acid to prevent the hydrogen gas formed from escaping through the thistle funnel.

5. In the industrial method of preparation of hydrogen by the Bosch process - give a] Balanced equations for the first two main steps in the production of hydrogen b] The reason for use of addition of a promoter to the catalyst in the final step c] The name of the solution which absorbs the unreacted carbon monoxide.
a] Step I: C + H₂O  1000°C → CO + H₂ - Δ (water gas)
Step II: CO + H₂ + H₂O  450°C, Fe₂O₃ → CO₂ + 2H₂ + Δ
b] A promoter (chromic oxide) is used to enhance the efficiency of the catalyst (iron [III] oxide).
c] Ammoniacal cuprous chloride solution.

Physical properties

6. State the following pertaining to the physical properties of hydrogen: a] Colour & odour b] Solubility in water c] Effect on moist blue litmus paper.
a] Colourless and odourless.
b] Very slightly soluble.
c] It has no effect on moist blue litmus paper as it is a neutral gas.

7. Draw labeled diagrams for two experiments to prove that - hydrogen is lighter than air.
Experiment 1: Filling soap bubbles with hydrogen gas shows them rising upwards in the air.
Experiment 2: Placing an empty test tube 'B' over a test tube 'A' filled with hydrogen; hydrogen moves upward into 'B', which then gives a 'pop' sound when tested with a glowing splinter.

Chemical properties

8. Starting from hydrogen gas how would you obtain a] A neutral liquid b] A basic gas c] A metal by reduction of its heated oxide. [the metal formed is above iron in the activity series]
a] By burning hydrogen in oxygen: 2H₂ + O₂ → 2H₂O (Water is a neutral liquid).
b] By reacting hydrogen with nitrogen: N₂ + 3H₂ ⇌ 2NH₃ (Ammonia is a basic gas).
c] By passing hydrogen over heated zinc oxide: ZnO + H₂ → Zn + H₂O (Zinc is above iron in the series).

9. Using a burning candle and a jar of hydrogen - how would you prove experimentally that a] Hydrogen is a combustible gas b] Hydrogen does not support combustion.
a] When a burning candle is brought to the mouth of a jar filled with hydrogen, the gas itself begins to burn with a pale blue flame, proving it is combustible.
b] When the burning candle is pushed inside the jar, it is immediately extinguished, proving that hydrogen does not support combustion.

10. State a reason why, when hydrogen is passed over heated copper oxide, the resultant product formed, differs in colour from the original reactant.
The original reactant, copper oxide, is black. Hydrogen reduces it to metallic copper, which is reddish-brown. This change in colour is due to the chemical reduction of the oxide to the pure metal.

Uses

11. With reference to the uses of hydrogen, give reasons for the following - a] Hydrogen is not used in air balloons b] A mixture of hydrogen & oxygen on burning, find application in welding & cutting metals c] Reaction of hydrogen with nitrogen under specific conditions finds industrial utility.
a] Hydrogen is not used because it is highly inflammable and forms an explosive mixture with air. Helium is preferred instead.
b] The mixture (oxy-hydrogen flame) produces intense heat (up to 2800°C) upon burning, which is sufficient to melt metals for welding and cutting.
c] The reaction produces ammonia, which is a vital starting material for the manufacture of fertilizers, explosives, and nitric acid.

Tests

12. Give a test to differentiate between - jar containing pure hydrogen & the other hydrogen-air mixture.
Pure hydrogen burns quietly at the mouth of the jar with a pale blue flame. A hydrogen-air mixture burns with a characteristic 'pop' sound.

Oxidation & reduction

13. With reference to oxidation & reduction reactions - complete the statement given by filling in the blanks with only the words a] Addition b] Removal.
'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.
14. With reference to the equation: Cl₂ + H₂S → 2HCl + S pertaining to a redox reaction - select the correct answer in each case - 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.

CRITICAL THINKING OBJECTIVE TYPE QUESTIONS

Q.1 Give balanced equations for the following conversions:
1. Zinc to sodium zincate: Zn + 2NaOH → Na₂ZnO₂ + H₂
2. Acidified water to hydrogen: 2H₂O → 2H₂ + O₂
3. Water gas to hydrogen: CO + H₂ + H₂O  450°C, Fe₂O₃ → CO₂ + 2H₂
4. Iron [III] oxide to iron: Fe₂O₃ + 3H₂ → 2Fe + 3H₂O
5. Nitrogen to a basic gas: N₂ + 3H₂ ⇌ 2NH₃

Q.2 Give reasons for the following:
1. Copper is placed below hydrogen in the activity series and cannot displace it, whereas Zinc is above hydrogen and can displace it from acids.
2. Distilled water is a non-conductor of electricity. Adding a few drops of acid allows it to dissociate into ions, enabling it to conduct current.
3. Granulated zinc contains traces of impurities like copper that act as a catalyst, speeding up the production of hydrogen.
4. This is done to remove carbon dioxide (absorbed by water/KOH) and unreacted carbon monoxide (absorbed by ammoniacal cuprous chloride) to obtain pure hydrogen.
5. It is a redox reaction because chlorine is reduced (gains hydrogen) and hydrogen sulphide is oxidised (loses hydrogen) simultaneously.

Q.3 Select the correct answer from A, B, C, D or E for each statement given below:
A: Nickel   B: Sodium   C: Iron   D: Iron [III] oxide   E: Magnesium oxide.
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.

Q.4 Select the correct answer from the choice in bracket to complete each sentence.
1. The acid dil. HNO₃ [dil.H₂SO₄, dil. HNO₃, dil. HCl] 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 [CO₂, CO, C].
3. A foul smelling gas formed when hydrogen reacts with a molten non-metal, is hydrogen sulphide [hydrogen chloride, hydrogen sulphide, ammonia].
4. The product formed on combustion of hydrogen in air is water [water gas, water, producer gas].
5. The gas which has now replaced hydrogen in air balloons is helium [argon, helium, neon].

Q.5 Match the statements in List I with the appropriate answer in List II.
1. An atom of hydrogen: D: One electron
2. A strong oxidising agent: E: Dilute nitric acid
3. A promoter used in Bosch process: A: Chromic oxide
4. A chemical used in the manufacture of fertilizers: B: Ammonia
5. The catalyst used in production of a basic gas from nitrogen: C: Iron