Concept 01 / 04Hydrogen
Hydrogen: Position, Isotopes, Preparation and Properties
The unique position of hydrogen in the periodic table, its isotopes protium, deuterium and tritium, laboratory and commercial preparation (water-gas, steam reforming, water-gas shift, electrolysis), physical and chemical properties, and uses including the hydrogen economy.
Hydrogen () is the lightest and most abundant element in the universe (about of its mass), but it is rare as free on Earth.
Position in the periodic table. Hydrogen resembles both alkali metals and halogens, so its position is anomalous.
Like alkali metals: one valence electron (), forms (really in water), combines with electronegative elements (oxides, halides), acts as a reducing agent.
Like halogens: needs one electron to complete its shell, forms (hydride) with active metals, exists as a diatomic molecule, has a fairly high ionisation enthalpy (, much higher than Li's ).
Differences: (a bare proton, radius about ) cannot exist freely; hydrogen's electron gain enthalpy () is far less negative than chlorine's (). It is usually placed separately at the top of group 1.
Isotopes (see figure). Protium (), deuterium ( or D) and tritium ( or T). They have the same electronic configuration, so almost identical chemical properties, but differ in rates of reaction and physical properties because of mass (the largest relative mass difference among isotopes of any element). Heavy water (b.p. , density ) is used as a moderator in nuclear reactors and for studying reaction mechanisms (kinetic isotope effect, tracer studies). Tritium is a radioactive emitter.
Preparation.
Laboratory: granulated zinc with dilute acid, , or zinc with aqueous alkali, .
Electrolysis of acidified water (Pt electrodes) gives very pure at the cathode; also a by-product of brine electrolysis (chlor-alkali).
Steam reforming of hydrocarbons: (, Ni catalyst). Coal gasification: (). The mixture is water gas or synthesis gas (syngas).
Water-gas shift: (, iron chromate catalyst) increases the hydrogen yield; is removed by scrubbing with sodium arsenite solution.
Properties. Colourless, odourless, tasteless, very sparingly soluble in water. The H-H bond enthalpy () is the highest of any single bond between two atoms of the same element, so is fairly unreactive at room temperature. Reactions: with halogens (, reactivity ), with oxygen (, highly exothermic), with nitrogen (Haber process), with active metals (hydrides), reduction of metal oxides and ions (), and hydrogenation of vegetable oils with Ni catalyst (vanaspati).
Uses and hydrogen economy. Ammonia synthesis (largest use), methanol (), hydrogenation, rocket fuel (liquid ), fuel cells. The hydrogen economy envisions storing and transporting energy as hydrogen: its combustion gives only water, and it releases more energy per gram than any fuel.
Key Takeaways:
- Hydrogen resembles group 1 and group 17 but fits neither fully.
- Isotopes: H, D, T with , , neutrons; only T is radioactive.
- Water gas: ; shift reaction converts CO to with more .
- Highest single-bond enthalpy between identical atoms: .
- Pitfall: pure zinc reacts slowly with dilute acid; granulated (impure) zinc is used because impurities set up local cells.
Brain-Teaser 1: What volume of at and is produced when of zinc reacts with excess dilute HCl?
Solution: , giving . Volume .
Brain-Teaser 2: How many neutrons are present in of (with )?
Solution: , so . Each molecule has neutrons. Total .
Brain-Teaser 3: Why is the C-D bond broken more slowly than the C-H bond, and how is this useful?
Solution: The bond strengths are nearly the same, but the heavier deuterium lowers the zero-point vibrational energy of the C-D bond, so more energy is needed to break it. A reaction whose rate falls markedly when H is replaced by D (primary kinetic isotope effect) must break that C-H bond in its rate-determining step, which reveals the mechanism.
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