Hydrogen and Its Compounds
The simplest atom and the most versatile element — its split personality on the periodic table, the hydrides it forms with almost everything, and the chemistry of water and hydrogen peroxide
- Why hydrogen resembles both the alkali metals and the halogens, yet belongs to neither.
- The three isotopes — protium, deuterium, tritium — and how they differ.
- Laboratory and industrial preparation of dihydrogen, including water gas and steam reforming.
- The three families of hydrides — ionic, covalent and metallic.
- The structure of water, the chemistry of hardness, and how to soften it.
- Hydrogen peroxide — its structure, dual redox role, volume strength, and uses.
The Position of Hydrogen
Hydrogen has the configuration \(1s^1\) — a single electron, like an alkali metal, yet just one electron short of helium's closed shell, like a halogen. This dual character is why no single position on the table fits it perfectly. It is usually placed atop Group 1, but its resemblance to Group 17 is just as strong.
| Like alkali metals (Group 1) | Like halogens (Group 17) |
|---|---|
| one valence electron (\(ns^1\)) | one electron short of a noble gas |
| forms \(\ce{H+}\), electropositive | forms \(\ce{H-}\) (hydride ion) |
| combines with non-metals | diatomic \(\ce{H2}\), like \(\ce{X2}\) |
| — | high ionization enthalpy |
Isotopes of Hydrogen
Hydrogen has three isotopes, identical in chemistry but differing in the number of neutrons — and therefore in mass and nuclear stability.
| Isotope | Symbol | Neutrons | Abundance | Nature |
|---|---|---|---|---|
| Protium | \(\ce{^1_1H}\) | 0 | 99.98 % | stable |
| Deuterium | \(\ce{^2_1H}\) (D) | 1 | 0.0156 % | stable |
| Tritium | \(\ce{^3_1H}\) (T) | 2 | trace | radioactive (β-emitter) |
Preparation of Dihydrogen
In the laboratory, \(\ce{H2}\) comes from the action of a dilute acid — or a strong base — on a suitable metal. Industrially, the cheapest routes start from water or hydrocarbons.
Granulated zinc with dilute sulphuric acid is the standard bench reaction. Zinc also liberates \(\ce{H2}\) from hot alkali: \(\ce{Zn + 2NaOH -> Na2ZnO2 + H2 ^}\).
| Industrial route | Reaction |
|---|---|
| Water gas (syngas) | \(\ce{C + H2O ->[1270\,K] CO + H2}\) |
| Water-gas shift | \(\ce{CO + H2O ->[catalyst] CO2 + H2}\) |
| Steam reforming | \(\ce{CH4 + H2O ->[Ni] CO + 3H2}\) |
| Electrolysis of water | \(\ce{2H2O ->[electricity] 2H2 + O2}\) |
Properties of Dihydrogen
\(\ce{H2}\) is a colourless, odourless, tasteless gas — the lightest known — only sparingly soluble in water. Its strong \(\ce{H-H}\) bond (\(436\ \text{kJ mol}^{-1}\)) makes it fairly unreactive at room temperature, but at high temperature or with a catalyst it combines readily.
| Reaction with | Product | Equation |
|---|---|---|
| Oxygen | water | \(\ce{2H2 + O2 -> 2H2O}\) |
| Nitrogen | ammonia | \(\ce{N2 + 3H2 ->[Fe] 2NH3}\) |
| Halogen | hydrogen halide | \(\ce{H2 + Cl2 -> 2HCl}\) |
| Active metal | ionic hydride | \(\ce{2Na + H2 -> 2NaH}\) |
| Unsaturated oil | saturated fat | \(\ce{C=C + H2 ->[Ni] C-C}\) |
Hydrides — Three Families
Hydrogen combines with most elements to form hydrides, sorted by bonding into three families. Which forms depends on the partner's electronegativity and position in the table.
| Type | Formed with | Bonding | Examples | Behaviour |
|---|---|---|---|---|
| Ionic (saline) | s-block metals | contains \(\ce{H-}\) | \(\ce{NaH},\ \ce{CaH2}\) | react with water → \(\ce{H2}\) |
| Covalent (molecular) | p-block non-metals | shared pairs | \(\ce{CH4},\ \ce{NH3},\ \ce{H2O},\ \ce{HF}\) | volatile, low mp |
| Metallic (interstitial) | many d/f-block metals | H in lattice holes | \(\ce{LaH2.87},\ \ce{TiH_{1.7}}\) | often non-stoichiometric |
The hydride ion \(\ce{H-}\) is a powerful base and reducing agent: it snatches a proton from water, liberating dihydrogen. \(\ce{CaH2}\) ("hydrolith") is used as a portable, dry source of \(\ce{H2}\) in the field.
Water: Structure & Properties
The water molecule is bent, with an \(\ce{H-O-H}\) angle of about \(104.5^\circ\) and two lone pairs on oxygen. Its polarity and extensive hydrogen bonding give water a strikingly high boiling point, high specific heat, and the rare property that its solid floats on its liquid.
| Property | Value / behaviour | Why it matters |
|---|---|---|
| Boiling point | \(100\,^\circ\text{C}\) (anomalously high) | hydrogen bonding |
| Max. density | at \(4\,^\circ\text{C}\) | ice floats, protects aquatic life |
| Specific heat | very high | moderates climate |
| Solvent power | "universal solvent" | dissolves ionic & polar solutes |
Hard Water & Its Removal
Hard water contains dissolved \(\ce{Ca^2+}\) and \(\ce{Mg^2+}\) ions, which form a scum with soap and scale in boilers. Temporary hardness (from bicarbonates) is removed simply by boiling or by Clark's method; permanent hardness (from chlorides and sulphates) needs a chemical or ion-exchange treatment.
| Type | Cause | Removal | Reaction |
|---|---|---|---|
| Temporary | \(\ce{Ca(HCO3)2},\ \ce{Mg(HCO3)2}\) | boiling / Clark's method | \(\ce{Ca(HCO3)2 ->[\Delta] CaCO3 v + H2O + CO2}\) |
| Temporary | (slaked lime) | Clark's method | \(\ce{Ca(HCO3)2 + Ca(OH)2 -> 2CaCO3 v + 2H2O}\) |
| Permanent | \(\ce{CaCl2},\ \ce{MgSO4}\) | washing soda | \(\ce{CaCl2 + Na2CO3 -> CaCO3 v + 2NaCl}\) |
| Permanent | (ion exchange) | zeolite / resin | \(\ce{Na2Z + Ca^2+ -> CaZ + 2Na+}\) |
Hydrogen Peroxide
Hydrogen peroxide (\(\ce{H2O2}\)) is a pale-blue, syrupy liquid with a non-planar "open-book" structure: an \(\ce{O-O}\) single bond with the two \(\ce{O-H}\) bonds on different planes. Its real interest is its dual redox character — it can both oxidise and reduce, in acidic or basic media.
When peroxide is the oxidant, its oxygen goes from \(-1\) to \(-2\); when it is the reductant, oxygen goes from \(-1\) to \(0\), releasing \(\ce{O2}\). It decomposes slowly, \(\ce{2H2O2 -> 2H2O + O2}\), so it is stored cold in dark, wax-lined bottles.
| Preparation | Reaction / method |
|---|---|
| Acidifying a peroxide | \(\ce{BaO2 + H2SO4 -> BaSO4 v + H2O2}\) |
| Industrial (anthraquinone) | auto-oxidation of 2-ethylanthraquinol |
| Electrolytic | oxidation of acidified sulphate, then hydrolysis |
Heavy Water & Hydrogen as a Fuel
Heavy water (\(\ce{D2O}\)) is water in which both hydrogens are deuterium. It is obtained by the exhaustive electrolysis of ordinary water (the lighter \(\ce{H2O}\) decomposes faster, concentrating \(\ce{D2O}\)) and serves as a moderator in nuclear reactors, slowing neutrons without absorbing them.
Hydrogen is also a clean fuel. It has the highest energy per unit mass of any chemical fuel and burns to give only water. In a hydrogen–oxygen fuel cell the energy of \(\ce{2H2 + O2 -> 2H2O}\) is converted directly to electricity, and the hydrogen economy envisions storing and transporting energy as \(\ce{H2}\) rather than as fossil fuels.
Putting It to Work
Problem. Classify \(\ce{CaH2},\ \ce{NH3}\) and \(\ce{TiH_{1.7}}\) by hydride type.
Solution. Partner element fixes the family — s-metal, p-non-metal, d-metal:
Problem. Write the balanced reaction when calcium hydride reacts with water.
Solution. Each \(\ce{H-}\) takes a proton from water to give \(\ce{H2}\):
Problem. Write the reaction by which boiling removes temporary hardness due to \(\ce{Ca(HCO3)2}\).
Solution. Heat decomposes the bicarbonate to insoluble carbonate:
Problem. Express the strength of a "10 volume" \(\ce{H2O2}\) solution in grams per litre.
Solution. Apply \(\text{strength}=\dfrac{17}{5.6}\times\text{vol. strength}\):
Problem. In \(\ce{2KMnO4 + 5H2O2 + 3H2SO4 -> 2MnSO4 + K2SO4 + 5O2 + 8H2O}\), is \(\ce{H2O2}\) the oxidant or reductant?
Solution. Oxygen in \(\ce{H2O2}\) rises from \(-1\) to \(0\) (loses electrons):
Problem. An isotope of hydrogen has mass number 3 and is radioactive. Name it and give its symbol and neutron count.
Solution. Mass 3 with \(Z=1\) means 2 neutrons:
Chapter Summary
Resembles both alkali metals and halogens; best treated as a unique element.
Protium, deuterium, tritium (radioactive) — same chemistry, very different masses.
Lab: \(\ce{Zn + acid}\). Industrial: water gas, shift reaction, steam reforming, electrolysis.
Ionic (s-block), covalent (p-block), metallic/interstitial (d/f-block).
Bent, 104.5°, H-bonded; hardness from \(\ce{Ca^2+}/\ce{Mg^2+}\) — temporary vs permanent.
Open-book structure; both oxidant and reductant; volume strength \(=\tfrac{17}{5.6}\times\) (g/L).
Problems
For each item, first decide which idea it tests — position, preparation, hydrides, water or peroxide — then apply the relevant rule. Difficulty rises down the list.
- Give two ways hydrogen resembles alkali metals and two ways it resembles halogens.
- Name the three isotopes of hydrogen and state which is radioactive.
- Write the laboratory preparation of dihydrogen from zinc and dilute sulphuric acid.
- What is water gas? Write the reaction and the water-gas shift that follows.
- Classify as ionic, covalent or metallic: \(\ce{NaH},\ \ce{HCl},\ \ce{PdH_{0.6}}\).
- Write the reaction of sodium hydride with water and explain why \(\ce{H-}\) acts as a base.
- Why does ice float on water? Relate your answer to hydrogen bonding.
- Distinguish temporary and permanent hardness, giving the salt responsible for each.
- Describe Clark's method and write the reaction it relies on.
- Draw the structure of \(\ce{H2O2}\) and explain why it is called "open-book".
- A sample of \(\ce{H2O2}\) is labelled "30 volume". Calculate its strength in g/L.
- Explain how heavy water is obtained and why it is used in nuclear reactors.