Hydrogen and s-Block Elements: questions with solutions
3 free questions from this chapter (the full set has 30). Marking: Multi-correct: +4 all correct, +1 per correct option if none wrong, −2 if any wrong · Numerical: +4 / 0 · Matching: +3 / −1.
Question 1Level 3 · AdvancedOne or more correct
Consider the following statements about hydrogen and its isotopes. Which of them is/are correct? One or more options may be correct.
ATritium is radioactive; it decays by β− emission to 23He.
BHydrogen (in combined form) is the most abundant element by mass in the Earth's crust.
CProtium, deuterium and tritium have essentially the same chemical properties; they differ mainly in their rates of reaction, largely because of their different bond dissociation enthalpies.
DLike the halogens, hydrogen forms a diatomic molecule and a uninegative ion; unlike a halide ion, however, the hydride ion cannot survive in water, reacting as H−+H2O→H2+OH−.
Show answer and solution
Answer: A, C, D
(A) Correct. Tritium is radioactive: 13H→23He+−10e (a β− emission; mass number unchanged, atomic number up by 1).
(B) Incorrect. In combined form hydrogen is a major constituent of the crust and oceans, but by mass oxygen (and silicon) far exceed it in the Earth's crust. Hydrogen is the most abundant element in the universe, not in the crust.
(C) Correct. The isotopes have the same electronic configuration, so their chemistry is essentially the same; they differ mainly in rates of reaction, largely because the bond dissociation enthalpies differ (e.g. H2 435.88, D2 443.35 kJ mol−1).
(D) Correct. Hydrogen resembles the halogens in forming H2 and H− (the H− ion has the He configuration). But H− is a very strong base and reacts with water: H−+H2O→H2+OH−, so it cannot exist in water, unlike Cl−.
Difficulty: Very hard
Question 2Level 2 · ExamSingle correct
Which of the following statements about heavy water, D2O, is correct? Data at 298 K: ionic product Kw(H2O)=1.0×10−14, Kw(D2O)=1.95×10−15; dielectric constant of H2O = 78.39, of D2O = 78.06; melting point of H2O = 273.0 K, of D2O = 276.8 K. Exactly one option is correct.
AD2O is a better solvent for ionic compounds than H2O because its dielectric constant is larger.
BD2O is used as a moderator in nuclear reactors because it absorbs neutrons strongly.
CD2O is obtained by exhaustive electrolysis of water, because H2 is liberated in preference to D2 and D2O accumulates in the residual liquid.
DPure D2O at 298 K is more extensively self-ionised than pure H2O, and it freezes below 273 K.
Show answer and solution
Answer: C
(C) is correct. On prolonged (exhaustive) electrolysis of water, H2 is liberated in preference to D2 (the O-H bond is broken faster than O-D: an isotope effect on rate), so D2O accumulates in the residual liquid.
(A) is wrong: the dielectric constant of D2O (78.06) is slightly smaller, not larger, than that of H2O (78.39).
(B) is wrong: D2O is a moderator because it slows down (thermalises) fast neutrons while absorbing very few of them; strong absorption would stop the chain reaction.
(D) is wrong on both counts: Kw(D2O)=1.95×10−15<1.0×10−14, so D2O is less self-ionised, and it freezes at 276.8 K, above 273 K.
Difficulty: Hard
Question 3Level 2 · ExamOne or more correct
Which of the following statements about the preparation of dihydrogen and its use as a fuel is/are correct? Standard enthalpies of combustion at 298 K (water formed as liquid): H2(g): −286 kJ mol−1; CH4(g): −890 kJ mol−1. Treat the gases as ideal. One or more options may be correct.
AZinc with dilute HNO3 is the standard laboratory method for preparing dihydrogen.
BIn steam reforming, CH4+H2O→CO+3H2 (about 1270 K, Ni catalyst); the yield of H2 is then increased by the water-gas shift reaction CO+H2O→CO2+H2 (about 673 K, iron chromate catalyst).
CAt the same temperature and pressure, burning a given volume of H2 releases more heat than burning the same volume of CH4.
DDihydrogen of very high purity (>99.95%) is obtained by electrolysing warm aqueous barium hydroxide between nickel electrodes.
Show answer and solution
Answer: B, D
(A) Incorrect. Dilute HNO3 is an oxidising acid; with zinc it gives oxides of nitrogen (or NH4+), not a clean supply of H2. The laboratory method uses granulated zinc with dilute HCl (or dilute H2SO4).
(B) Correct. Steam reforming: CH4+H2O1270K,NiCO+3H2. The CO of the syngas is then converted by the water-gas shift reaction CO+H2O673K,ironchromateCO2+H2, increasing the H2 yield (CO2 is removed by scrubbing).
(C) Incorrect. Equal volumes at the same T and P contain equal numbers of moles, so per unit volume the heats are in the ratio 286 : 890; H2 gives only about 0.32 times the heat of CH4. (Per unit mass H2 is better: 286/2 = 143 vs 890/16 = 55.6 kJ g−1; this is why storage volume is the problem in using hydrogen as a fuel.)
(D) Correct. High-purity (>99.95%) H2 is made by electrolysing warm aqueous Ba(OH)2 between nickel electrodes.
Difficulty: Hard
Get the full chapter for ₹9
Timed attempt with real exam marking, your rank among everyone who took it, solutions for every question, and spaced revision of your mistakes.