3.1.3 · D5Hydrogen and s-Block
Question bank — Preparation, properties, uses of dihydrogen
This bank draws on ideas from Redox Reactions, Electrolysis and Faraday's Laws, Water Gas Shift Reaction, Haber Process, Hydrides - Ionic, Covalent, Metallic, Thermodynamics of Chemical Reactions, Graham's Law of Diffusion and Hydrogen - Introduction and Isotopes.
The symbols you'll need first
Before any trap uses shorthand, here is what each symbol means in plain words, anchored to the figures below.
The seesaw picture above is the key to the metal-reactivity traps: a metal below hydrogen tips the balance so , dragging downhill.
This ladder of values shows why Zn displaces H₂ but Cu does not — read it before the True/False section.
True or false — justify
Zinc reacts with dilute HCl to give H₂, so it should react even faster with concentrated H₂SO₄.
False. Concentrated H₂SO₄ is an oxidiser — it grabs the electrons and is reduced to SO₂, so you get almost no H₂; dilute acid just supplies H⁺ to be reduced.
Any metal placed in dilute acid will liberate hydrogen.
False. Only metals whose lies below the H⁺/H₂ couple (0.00 V) make ; by that gives (spontaneous). Copper (+0.34 V) makes , so and it does not displace hydrogen.
The overall steam-reforming reaction is exothermic because the shift step releases heat.
False. Reforming is kJ, the shift is only kJ, so the sum kJ is still endothermic — external heat is required.
In electrolysis of water the cell EMF is V, which means no reaction is possible.
False. The negative value means the reaction is non-spontaneous (), i.e. you must supply energy; applying ≥1.23 V (in practice ~1.8–2.0 V with overpotential) drives it forward.
Hydrogen has the lowest boiling point of all molecular gases because it has the smallest molar mass.
True. With the lowest molar mass (~2 g mol⁻¹) and no permanent dipole, H₂ has only feeble London dispersion forces (low polarizability), so almost no thermal energy is needed to separate the molecules — hence b.p. ≈ 20 K.
CaH₂ reacting with water is a redox reaction just like Na with water.
Partly true — it is redox. H⁻ (oxidation number −1) and H⁺ (+1) combine to give H₂ (0), so hydrogen is both oxidised and reduced — a comproportionation. It is a redox reaction that happens to look acid–base; unlike Na (where the metal 0→+1 is oxidised), here calcium's oxidation number is unchanged.
Since H₂ is the lightest gas, Graham's law says it effuses faster than any other gas.
True. Rate of effusion ∝ , and H₂ has the smallest molar mass (≈2 g mol⁻¹), so it diffuses/effuses fastest of all gases.
A stronger H–H bond (436 kJ/mol) makes H₂ more chemically reactive at room temperature.
False. The strong bond makes H₂ kinetically inert; it needs high temperature, a catalyst, or a flame to overcome that large activation barrier before it reacts.
Spot the error
"1 mol of Zn gives 2 mol of H₂ because there are two H atoms in ZnCl₂'s acid."
The stoichiometry is 1:1 — gives one H₂ per Zn; the two H atoms combine into a single H₂ molecule.
"In the cathode half-reaction , water is oxidised."
It is reduced — hydrogen goes from +1 to 0 and electrons are gained; reduction always happens at the cathode.
"Faraday's law uses for hydrogen because H₂ carries one charge."
For H₂, ( = electrons per molecule) — two electrons are needed to reduce two H⁺ into one H₂ molecule, so half as many moles form per coulomb of .
"Steam reforming needs 1000 K because higher temperature always speeds up the shift step too."
The high temperature is for the endothermic reforming step (equilibrium favours products when hot); the exothermic shift is deliberately run cooler (~673 K) so equilibrium favours CO₂ + H₂.
"Concentrated H₂SO₄ produces SO₂ and H₂ together, doubling the yield."
It produces essentially SO₂ instead of H₂ — the H is oxidised to water, so your hydrogen yield collapses, it does not double.
"Collecting H₂ over water works because H₂ is denser than water vapour."
It works because H₂ is only slightly soluble in water and can push the water down in an inverted tube — density of vapour is irrelevant; the point is low solubility.
"Electrolysis gives impure H₂ because the anode oxygen mixes in."
In a properly divided cell the gases evolve at separate electrodes and are collected apart, so electrolytic H₂ is actually very pure (>99.9 %).
Why questions
Why is granulated zinc preferred over a solid zinc block in lab preparation?
Granules expose far more surface area to the acid, so the heterogeneous reaction proceeds at a usable rate; a smooth block reacts only at its outer face.
Why must the water-gas shift step be added after steam reforming?
It removes poisonous CO (which would poison downstream catalysts) by converting it to CO₂ and it squeezes out an extra mole of H₂ per mole of CO.
Why is a Ni catalyst used in reforming rather than just more heat?
Ni lowers the activation energy for breaking the tough C–H bonds; without it you'd need impractically high temperatures and the reaction would be too slow.
Why does industrial electrolysis run at ~1.8–2.0 V instead of the theoretical 1.23 V?
Real electrodes have overpotential (extra voltage to overcome the sluggish kinetics of gas evolution) plus ohmic resistance losses, so you must overshoot the thermodynamic minimum.
Why does the reduction potential comparison ( V vs V) predict a spontaneous reaction?
The couple with the more negative is oxidised, so Zn gives up electrons to H⁺; this makes V, and , so the reaction proceeds on its own.
Why is CaH₂ (a hydride) able to make H₂ from water at all, when H₂O has no acid added?
Water itself supplies H⁺; the hydride's H⁻ is a powerful base that seizes that H⁺ (and reduces it), so pure water acts as the "acid" partner — this is why CaH₂ is used as a portable dry source of H₂.
Why does H₂ have such a low density (0.089 g/L) compared with air (1.29 g/L)?
Density depends on molar mass at fixed T and P; H₂'s molar mass (~2 g/mol) is roughly 14× smaller than air's (~29 g/mol), giving proportionally lower density.
Edge cases
What happens if you use cold water with sodium instead of dilute acid — is H₂ still evolved?
Yes — Na is reactive enough to reduce water directly (); the reaction is vigorous and even the OH⁻/H₂O couple gets reduced.
If a metal's reduction potential exactly equals 0.00 V (like the H⁺/H₂ reference), does it liberate H₂ from acid?
At exactly 0 V the driving force is essentially nil (, so ), so any evolution is negligibly slow — you effectively get no useful H₂.
For electrolysis, what is the H₂:O₂ volume ratio collected, and why not 1:1?
It is 2:1 — water splits as , so twice as many moles (hence twice the volume at the same T, P) of hydrogen appear as oxygen.
At the limit of pure water with no dissolved ions, why is plain distilled water a poor electrolyte?
With almost no free ions, current can barely flow (very high resistance), so a small amount of acid, base, or salt must be added to carry charge and let electrolysis proceed.
What if steam reforming had no heat supplied — would the overall reaction run?
No — the net process is endothermic (+165 kJ/mol), so without continuous external heating the equilibrium and rate both stall; it cannot self-sustain.
If two different metals (say Zn and Fe) both react with the same acid, does the one with the more negative potential necessarily react faster?
Not necessarily — a more negative means a larger thermodynamic driving force ( more negative), but rate also depends on kinetics (surface area, oxide layers), so speed and spontaneity are separate questions.
Recall One-line traps to re-test yourself
Concentrated H₂SO₄ gives ::: SO₂, not H₂ (it oxidises). Zn + 2HCl mole ratio Zn:H₂ ::: 1:1. for H₂ in Faraday's law ::: 2 (electrons per molecule). What links and ::: ; positive ⇒ negative ⇒ spontaneous. Sign of net ΔH for full steam reforming ::: positive (endothermic, +165 kJ/mol). Reason H₂ is kinetically inert ::: strong 436 kJ/mol H–H bond, high activation energy. Electrolysis H₂:O₂ volume ratio ::: 2:1. Oxidation-number change in CaH₂ + H₂O ::: H⁻ (−1) and H⁺ (+1) → H₂ (0): redox comproportionation.