Hydrogen - MCQs with Explanations

1. Hydrogen molecule differs from chlorine molecule in the following respect:

• (a) hydrogen molecule can form intermolecular hydrogen bonds but chlorine molecule does not
• (b) hydrogen molecule is polar while chlorine molecule is non-polar
• (c) hydrogen molecule is non-polar but chlorine molecule is polar
• (d) hydrogen molecule cannot participate in coordinate bond formation but chlorine molecule can.

Correct Answer: (d)

Explanation: For a molecule to participate in coordinate bond formation, it must possess a lone pair of electrons. While chlorine has lone pairs, the hydrogen molecule does not.

2. The decomposition of \( H_{2}O_{2} \) can be checked by the addition of:

• (a) acetanilide
• (b) benzene
• (c) alkali metal oxides
• (d) \( MnO_{2} \)

Correct Answer: (a)

Explanation: Acetanilide acts as a stabilizer that prevents the spontaneous decomposition of hydrogen peroxide.

3. At its melting point, ice is lighter than water because:

• (a) on melting of ice the \( H_{2}O \) molecule shrink in size
• (b) ice crystals have hollow hexagonal arrangement of \( H_{2}O \) molecules
• (c) \( H_{2}O \) molecules are more closely packed in solid state
• (d) ice forms mostly heavy water on first melting.

Correct Answer: (b)

Explanation: Ice has an open-cage, hollow hexagonal crystal structure due to hydrogen bonding, which makes it less dense than liquid water.

4. Which compound is formed when calcium carbide reacts with heavy water?

• (a) \( C_{2}D_{2} \)
• (b) \( CaD_{2} \)
• (c) \( CD_{2} \)
• (d) \( Ca_{2}D_{2} \)

Correct Answer: (a)

Explanation: The reaction is \( CaC_{2} + 2D_{2}O \rightarrow C_{2}D_{2} + Ca(OD)_{2} \), producing deuteroacetylene.

5. Hydrogen peroxide is used as:

• (a) an oxidant of rocket fuel
• (b) an oxidising agent
• (c) a bleaching agent
• (d) all of the above.

Correct Answer: (d)

Explanation: Hydrogen peroxide has diverse applications including use as an oxidant in rocket fuel, a general oxidising agent, and a bleaching agent for textiles.

6. Which of the following metals dissolves in NaOH with the evolution of hydrogen?

• (a) Ca
• (b) Mg
• (c) Sr
• (d) Be

Correct Answer: (d)

Explanation: Beryllium reacts with sodium hydroxide to form sodium beryllate and hydrogen gas: \( Be + 2NaOH \rightarrow Na_{2}BeO_{2} + H_{2} \).

7. In laboratory, \( H_{2}O_{2} \) is prepared by the action of:

• (a) \( MnO_{2} \) and cold \( H_{2}SO_{4} \)
• (b) aqueous alkali on \( Na_{2}O_{2} \)
• (c) cold \( H_{2}SO_{4} \) on \( BaO_{2} \)
• (d) dilute HCl and \( MnO_{2} \).

Correct Answer: (c)

Explanation: A standard laboratory preparation involves reacting hydrated barium peroxide with cold dilute sulphuric acid: \( BaO_{2} \cdot 8H_{2}O + H_{2}SO_{4} \rightarrow BaSO_{4} + H_{2}O_{2} + 8H_{2}O \).

8. Which of the following pairs of substances gives same gaseous product on reaction with water?

• (a) Na and \( Na_{2}O_{2} \)
• (b) Ca and \( CaH_{2} \)
• (c) Ca and CaO
• (d) Ba and \( BaO_{2} \)

Correct Answer: (b)

Explanation: Both Calcium and Calcium hydride react with water to liberate hydrogen gas.

9. The oxygen atom of \( H_{2}O_{2} \) used for oxidation is bound by:

• (a) electrovalent bond
• (b) covalent bond
• (c) coordinate bond
• (d) none of these.

Correct Answer: (c)

Explanation: In its oxidising capacity, the oxygen atom provided by hydrogen peroxide is considered to be bound by a coordinate bond.

10. Which one of the following statements is incorrect?

• (a) The percentage of ortho-hydrogen at 300 K and above is 75%.
• (b) The equilibrium mixture of para and ortho-hydrogen contains almost 100% para-hydrogen at absolute zero.
• (c) It is possible to get 100% ortho-hydrogen at any temperature.
• (d) Para-hydrogen is stable for weeks in the absence of catalysts.

Correct Answer: (c)

Explanation: While almost 100% para-hydrogen can be obtained at absolute zero, it is not possible to obtain 100% pure ortho-hydrogen because the maximum equilibrium concentration at high temperature is 75%.

11. On being placed in water, sodium peroxide produces an alkaline solution and bubbles. If the compound undergoes redox disproportionation, (A) and (B) are:

• (a) \( H_{2}O_{2} \) and NaOH
• (b) \( H_{2}O \) and \( O_{2} \)
• (c) NaOH and \( O_{2} \)
• (d) \( Na_{2}O \) and NaOH

Correct Answer: (c)

Explanation: Sodium peroxide reacts with water to initially form NaOH and \( H_{2}O_{2} \). The \( H_{2}O_{2} \) then disproportionates into water and oxygen gas, making the final products NaOH and \( O_{2} \).

12. Which of the following statements about heavy water are correct?

(i) Heavy water is used as a moderator in nuclear reactors. (ii) Heavy water is more associated than ordinary water. (iii) Heavy water is more effective solvent than ordinary water.

• (a) (i) and (ii)
• (b) (i), (ii) and (iii)
• (c) (ii) and (iii)
• (d) (i) and (iii)

Correct Answer: (a)

Explanation: Heavy water is used as a moderator and is more associated than regular water. However, it is a less effective solvent due to its lower dielectric constant.

13. Which of the following statements is not true?

• (a) \( D_{2}O \) freezes at lower temperature than \( H_{2}O \).
• (b) Reaction between \( H_{2} \) and \( Cl_{2} \) is much faster than \( D_{2} \) and \( Cl_{2} \).
• (c) Ordinary water is electrolysed more rapidly than \( D_{2}O \).
• (d) Bond dissociation energy of \( D_{2} \) is greater than \( H_{2} \).

Correct Answer: (a)

Explanation: The statement is false because \( D_{2}O \) actually has a higher freezing point (\( 3.8^{\circ}C \)) than \( H_{2}O \) (\( 0^{\circ}C \)).

14. The oxidation states of the most electronegative element in the products of the reaction between \( BaO_{2} \) with dilute \( H_{2}SO_{4} \) are:

• (a) -1, -2
• (b) +1, +2
• (c) -2, +1
• (d) 0, -1

Correct Answer: (a)

Explanation: The products are \( BaSO_{4} \) and \( H_{2}O_{2} \). Oxygen (the most electronegative element) has an oxidation state of -2 in \( BaSO_{4} \) and -1 in \( H_{2}O_{2} \).

15. When \( H_{2}O_{2} \) is oxidised by a suitable reductant, one of the products is:

• (a) \( OH^{-} \)
• (b) \( HO_{2}^{-} \)
• (c) \( O^{2-} \)
• (d) \( O_{2} \)

Correct Answer: (d)

Explanation: When hydrogen peroxide acts as a reducing agent (being oxidised), it releases oxygen gas: \( H_{2}O_{2} + [O] \rightarrow H_{2}O + O_{2} \).

16. To prepare \( H_{2} \) from water, you would allow water to react with which metal and oxidising agent or reducing agent?

• (a) Ag, oxidising agent
• (b) Al, reducing agent
• (c) Ca, oxidising agent
• (d) Au, reducing agent

Correct Answer: (b)

Explanation: Active metals like Aluminium act as reducing agents to liberate hydrogen from water.

17. Calgon is used as a water softening agent because it:

• (a) form soluble complexes with anionic species
• (b) precipitate anionic species
• (c) form soluble complexes with cationic species
• (d) precipitate cationic species.

Correct Answer: (c)

Explanation: Sodium hexametaphosphate (Calgon) softens water by forming soluble complexes with calcium and magnesium cations, preventing them from interfering with soap.

18. Water softening by Clarke's process uses:

• (a) calcium bicarbonate
• (b) sodium bicarbonate
• (c) potash alum
• (d) calcium hydroxide.

Correct Answer: (d)

Explanation: Clarke's process involves adding a calculated amount of lime (calcium hydroxide) to hard water to precipitate insoluble carbonates.

19. \( H^{+} \) ion always get associated with other atoms or molecules because:

• (a) ionisation enthalpy of hydrogen resembles that of alkali metals
• (b) its reactivity is similar to halogens
• (c) it resembles both alkali metals and halogens
• (d) loss of an electron from hydrogen atom results in a nucleus of very small size.

Correct Answer: (d)

Explanation: The removal of the electron from a hydrogen atom leaves a bare nucleus (proton) of extremely small size, which cannot exist independently and always associates with other molecules.

20. Which hydrides are used to remove traces of water from organic solvents due to high reactivity?

• (a) ionic hydrides
• (b) covalent hydrides
• (c) metallic hydrides
• (d) interstitial hydrides.

Correct Answer: (a)

Explanation: Ionic hydrides contain the \( H^{-} \) ion, which is a very strong base and reacts instantly with water to produce hydrogen and hydroxide, making them excellent drying agents.

21. The pale blue syrupy liquid, \( H_{2}O_{2} \), is estimated by using:

• (a) KI
• (b) \( KMnO_{4} \)
• (c) \( Na_{2}CO_{3} \)
• (d) NaOH

Correct Answer: (b)

Explanation: Hydrogen peroxide is quantitatively estimated via permanganometry, where it reacts with acidified \( KMnO_{4} \).

22. Which of the following is not correct regarding the electrolytic preparation of \( H_{2}O_{2} \)?

• (a) Lead is used as cathode.
• (b) 50% \( H_{2}SO_{4} \) is used.
• (c) Hydrogen is liberated at anode.
• (d) Sulphuric acid undergoes oxidation.

Correct Answer: (c)

Explanation: In the electrolytic process, hydrogen is liberated at the cathode, not the anode.

23. Which of the following statements is incorrect?

• (a) Molecular hydrides are electron-deficient.
• (b) Metallic hydrides conduct heat and electricity.
• (c) Ionic hydrides do not conduct electricity in solid state.
• (d) Ionic hydrides are very good conductors of electricity in solid state.

Correct Answer: (d)

Explanation: Ionic hydrides are non-conductors in the solid state; they only conduct electricity in their molten state when ions become mobile.

24. What is the reason for unusual high boiling point of water?

• (a) Presence of \( H^{+} \) and \( OH^{-} \) ions
• (b) Dipole-dipole interactions (hydrogen bonding)
• (c) London forces
• (d) van der Waals forces.

Correct Answer: (b)

Explanation: The high boiling point of water is due to extensive intermolecular hydrogen bonding (a strong dipole-dipole interaction).

25. Metallic hydride is formed by:

• (a) iron
• (b) manganese
• (c) palladium
• (d) molybdenum.

Correct Answer: (c)

Explanation: Transition metals like Palladium form metallic (interstitial) hydrides.

26. In which one of the following reactions is \( H_{2} \) liberated?

• (a) \( NaH + H_{2}O \rightarrow \)
• (b) \( NaCl + H_{2}O \rightarrow \)
• (c) \( Na_{2}O + H_{2}O \rightarrow \)
• (d) \( NaOH + H_{2}O \rightarrow \)

Correct Answer: (a)

Explanation: Alkali metal hydrides like \( NaH \) react vigorously with water to produce sodium hydroxide and hydrogen gas.

27. The best explanation for not placing hydrogen with alkali metals or halogens is:

• (a) hydrogen can form compounds with all other elements
• (b) it is much lighter than both groups
• (c) its ionization energy is too high for alkali metals but too low for halogens
• (d) none of the above.

Correct Answer: (c)

Explanation: Hydrogen's unique properties, specifically its ionization energy being significantly different from both groups, justify its separate placement.

28. In acidic medium, \( H_{2}O_{2} \) acts as a reducing agent in its reaction with:

• (a) \( FeSO_{4} \)
• (b) \( KMnO_{4} \)
• (c) \( K_{2}MnO_{4} \)
• (d) \( K_{4}[Fe(CN)_{6}] \)

Correct Answer: (b)

Explanation: Hydrogen peroxide reduces \( KMnO_{4} \) in an acidic medium, evolving oxygen gas.

29. The bleaching properties of \( H_{2}O_{2} \) are due to its:

• (a) acidic nature
• (b) basic nature
• (c) oxidising nature
• (d) reducing nature.

Correct Answer: (c)

Explanation: Bleaching action occurs because \( H_{2}O_{2} \) provides nascent oxygen which oxidises coloured substances into colourless ones.

30. In reactions: (i) \( H_{2}O_{2} + 2HI \rightarrow I_{2} + 2H_{2}O \) and (ii) \( HOCl + H_{2}O_{2} \rightarrow H_{3}O^{+} + Cl^{-} + O_{2} \), \( H_{2}O_{2} \) acts as:

• (a) an oxidising agent in both
• (b) oxidising in (i) and reducing in (ii)
• (c) reducing in (i) and oxidising in (ii)
• (d) a reducing agent in both.

Correct Answer: (b)

Explanation: In the first reaction, it oxidises iodide to iodine. In the second, it reduces hypochlorous acid to chloride.

31. The ratio of ortho to para hydrogen in ordinary hydrogen at 300 K is:

• (a) 1:3
• (b) 2:3
• (c) 3:3
• (d) 3:1

Correct Answer: (d)

Explanation: At room temperature (300 K), the equilibrium mixture contains approximately 75% ortho-hydrogen and 25% para-hydrogen, a 3:1 ratio.

32. Reaction between following pairs will produce hydrogen except:

• (a) Cu + HCl
• (b) \( Fe + H_{2}SO_{4} \)
• (c) Mg + steam
• (d) Na + alcohol

Correct Answer: (a)

Explanation: Copper is below hydrogen in the reactivity series and cannot displace it from dilute acids like HCl.

33. The reaction \( 2H_{2}O_{2} \rightarrow 2H_{2}O + O_{2} \) shows that:

• (a) \( H_{2}O_{2} \) is decomposed
• (b) it acts as oxidising agent
• (c) it acts as reducing agent
• (d) none of the above.

Correct Answer: (a)

Explanation: This reaction represents the spontaneous or catalytic decomposition of hydrogen peroxide into water and oxygen.

34. Lead pipes are not safe for carrying drinking water because:

• (a) they are covered with lead carbonate
• (b) they are corroded by air and moisture
• (c) water containing dissolved air attacks lead forming soluble hydroxide
• (d) none of the above.

Correct Answer: (c)

Explanation: Lead dissolves in water containing dissolved oxygen to form toxic lead hydroxide, a phenomenon known as plumbosolvency.

35. Heavy hydrogen is used:

• (a) in filling the balloons
• (b) in studying reaction mechanism
• (c) in calculating heat of formation
• (d) as oxidant.

Correct Answer: (b)

Explanation: Deuterium (heavy hydrogen) is extensively used as a tracer in the study of organic and biochemical reaction mechanisms.

36. Nascent hydrogen consists of:

• (a) hydrogen atoms with excess energy
• (b) hydrogen molecules with excess energy
• (c) hydrogen ions in excited state
• (d) solvated protons.

Correct Answer: (a)

Explanation: Nascent hydrogen refers to hydrogen at the moment of its generation, existing as atoms with higher chemical reactivity than molecular hydrogen.

37. What is false about \( H_{2}O_{2} \)?

• (a) Acts both as oxidising and reducing agent.
• (b) Two OH bonds lie in the same plane.
• (c) Pale blue liquid.
• (d) Can be oxidised by \( O_{3} \).

Correct Answer: (b)

Explanation: The statement is false; the two O-H bonds in hydrogen peroxide lie in different planes, resulting in a non-planar open-book structure.

38. Dielectric constant of \( H_{2}O_{2} \):

• (a) increases with dilution
• (b) decreases with dilution
• (c) is unaffected on dilution
• (d) none of the above.

Correct Answer: (a)

Explanation: The dielectric constant of hydrogen peroxide solution increases as it is diluted with water.

39. In reactions: (i) \( H_{2}O_{2} + 2KI \rightarrow I_{2} + 2KOH \) and (ii) \( H_{2}O_{2} + O_{3} \rightarrow 2O_{2} + H_{2}O \), \( H_{2}O_{2} \) behaves as:

• (a) an oxidising agent in both
• (b) oxidising in (i) and reducing in (ii)
• (c) reducing in (i) and oxidising in (ii)
• (d) a reducing agent in both.

Correct Answer: (b)

Explanation: It oxidises iodide to iodine in the first case and reduces ozone to oxygen in the second.

40. Ordinary water is not used as a moderator in nuclear reactors because:

• (a) it cannot slow down fast moving neutrons
• (b) it cannot remove the heat
• (c) it absorbs the fast moving neutrons
• (d) of its corrosive action.

Correct Answer: (c)

Explanation: Ordinary water has a high neutron-capture cross-section and tends to absorb fast-moving neutrons rather than just slowing them down.

41. Hydrogen gas is used on industrial scale in the manufacture of:

• (a) \( H_{2}SO_{4} \)
• (b) \( C_{2}H_{2} \)
• (c) margarine
• (d) water.

Correct Answer: (c)

Explanation: Hydrogen is used for the catalytic hydrogenation of vegetable oils into solid fats like margarine (ghee).

42. The structure of \( H_{2}O_{2} \) is:

(Diagrams show various models)

• (a) Linear
• (b) Open book (non-planar)
• (c) Planar
• (d) Triangular

Correct Answer: (b)

Explanation: Hydrogen peroxide has a non-planar "open-book" structure where the O-H bonds are in different planes.

43. Heavy water was discovered by:

• (a) Nernst
• (b) Haber
• (c) Urey and Washburn
• (d) Aston.

Correct Answer: (c)

Explanation: Heavy water (\( D_{2}O \)) was discovered through the work of Urey and Washburn.

44. \( H_{2}O_{2} \) cannot oxidise:

• (a) PbS
• (b) \( O_{3} \)
• (c) \( Na_{2}SO_{3} \)
• (d) KI

Correct Answer: (b)

Explanation: Ozone is a more powerful oxidiser than hydrogen peroxide, so \( H_{2}O_{2} \) cannot oxidise ozone; instead, it reduces it to oxygen.

45. Match the following correctly:

1. Heavy water; 2. Temporary hard water; 3. Soft water; 4. Permanent hard water

• (a) 1-c, 2-d, 3-b, 4-a
• (b) 1-b, 2-a, 3-c, 4-d
• (c) 1-b, 2-d, 3-c, 4-a
• (d) 1-c, 2-a, 3-b, 4-d

Correct Answer: (d)

Explanation: Heavy water is \( D_{2}O \). Temporary hardness is caused by bicarbonates. Soft water has no foreign ions. Permanent hardness is caused by sulphates and chlorides.