What is hydrogen?
Hydrogen is the first element in the periodic table. Its atom has 1 proton and 1 electron. Most hydrogen atoms have no neutron at all.
It is the most common element in the universe. Stars like the Sun are mostly hydrogen. On Earth it is mostly locked in water (H₂O) and in living things.
Where does it sit in the periodic table?
Hydrogen has 1 outer electron, like lithium and sodium, so it is often placed at the top of Group 1. But it is a non-metal gas, and it can also gain 1 electron to make H⁻, like the halogens. So it is a special element that fits neither group perfectly.
Isotopes
- Protium ¹H: 0 neutrons (almost all hydrogen).
- Deuterium ²H or D: 1 neutron. D₂O is "heavy water".
- Tritium ³H or T: 2 neutrons, radioactive.
Isotopes have the same chemistry because they have the same number of electrons.
Physical properties of hydrogen
- Colourless, odourless and tasteless gas.
- Least dense of all gases: about 0.09 g per litre, about 14 times lighter than air. A balloon filled with it rises.
- Hardly dissolves in water.
- Boils at −253 °C, so it is a gas at room temperature.
- Made of H₂ molecules held by one covalent bond (H–H).
Making hydrogen and testing it
In the laboratory
Add granulated zinc to dilute hydrochloric acid or dilute sulfuric acid:
Zn + 2HCl → ZnCl₂ + H₂ or Zn + H₂SO₄ → ZnSO₄ + H₂
The gas is collected over water (it does not dissolve) or by downward displacement of air in an upside-down jar (it is lighter than air).
Other ways
- Very reactive metals with water: 2Na + 2H₂O → 2NaOH + H₂.
- Electrolysis of water: 2H₂O → 2H₂ + O₂ (hydrogen at the negative electrode). With power from the Sun or wind this gives "green hydrogen".
- In industry most hydrogen comes from natural gas and steam: CH₄ + H₂O → CO + 3H₂.
The pop test
Hold a burning splint at the mouth of a test tube of gas. Hydrogen burns with a squeaky pop. Never test a big jar of hydrogen mixed with air: the mixture can explode.
Chemical properties: reactions of hydrogen
With oxygen
2H₂ + O₂ → 2H₂O. A lot of heat is given out. This is why hydrogen is a fuel.
With other non-metals (making hydrides)
A hydride is a compound of hydrogen with one other element.
- Hydrogen chloride: H₂ + Cl₂ → 2HCl. A colourless gas that dissolves very well in water to give hydrochloric acid. Used to clean steel and make PVC.
- Ammonia: N₂ + 3H₂ ⇌ 2NH₃ (Haber process: iron catalyst, about 450 °C, about 200 atm). A colourless gas with a sharp smell, very soluble, makes an alkaline solution. Used for fertilisers, nitric acid and cleaning liquids.
- Hydrogen sulfide: H₂ + S → H₂S. A poisonous gas with a rotten-egg smell. It forms when things rot without air and in volcanic gas; it turns lead acetate paper black.
With metal oxides (reduction)
Hot hydrogen takes oxygen away from some metal oxides: CuO + H₂ → Cu + H₂O. The black powder turns pink-brown copper. Hydrogen is the reducing agent.
With very reactive metals
Sodium or calcium react with hot hydrogen to make ionic hydrides like NaH, in which hydrogen is H⁻.
Uses of hydrogen and hydrogen as a fuel
- Making ammonia for fertilisers (the biggest use).
- Turning vegetable oils into solid fats (hydrogenation, used for vanaspati and margarine).
- Liquid hydrogen as rocket fuel, with liquid oxygen.
- Fuel cells that change hydrogen and oxygen straight into electricity for buses and cars.
- Getting metals like tungsten from their oxides.
Good and bad points of hydrogen fuel
Good: about 142 kJ per gram, about three times petrol; the only product is water. Hard parts: it is hard to store (very light gas, needs high pressure or very cold tanks), it leaks easily and burns explosively with air, and it is "clean" only if the energy used to make it is clean.
Try it
In the 3D free play, set 4 g of hydrogen. Predict the water made before you look (hint: 2 g H₂ gives 18 g H₂O).
Key formulas and definitions
- Zn + 2HCl → ZnCl₂ + H₂ (lab preparation)
- 2H₂ + O₂ → 2H₂O (burning; pop test)
- H₂ + Cl₂ → 2HCl
- N₂ + 3H₂ ⇌ 2NH₃ (Haber process)
- H₂ + S → H₂S
- CuO + H₂ → Cu + H₂O (reduction)
- 2H₂O → 2H₂ + O₂ (electrolysis)
Worked examples
1. Name the gas and write the equation when magnesium ribbon is put in dilute sulfuric acid.
Hydrogen gas forms. Mg + H₂SO₄ → MgSO₄ + H₂. Test: a burning splint gives a squeaky pop.
2. How many grams of water form when 4 g of hydrogen burns completely?
2H₂ + O₂ → 2H₂O. 2 mol H₂ (4 g) gives 2 mol H₂O (2 × 18 = 36 g). So 36 g of water.
3. Why can hydrogen be collected over water but ammonia cannot?
Hydrogen hardly dissolves in water, so it stays as gas in the jar. Ammonia dissolves very well in water, so it would disappear into the water. Ammonia is collected by upward delivery (downward displacement of air) instead.
4. In CuO + H₂ → Cu + H₂O, which substance is reduced and which is the reducing agent?
Copper(II) oxide loses oxygen, so it is reduced. Hydrogen gains oxygen (it is oxidised to water), so hydrogen is the reducing agent.
5. How many hydrogen molecules are needed to make 2 molecules of ammonia?
N₂ + 3H₂ ⇌ 2NH₃. So 3 H₂ molecules (and 1 N₂ molecule) make 2 NH₃.
6. Estimate the energy released by burning 5 g of hydrogen.
About 142 kJ per gram, so 5 × 142 = 710 kJ.
Common mistakes
- Saying hydrogen gas is H. Free hydrogen is a molecule, H₂; always balance with H₂.
- Mixing the tests: a squeaky pop is hydrogen; a glowing splint relighting is oxygen; limewater turning milky is carbon dioxide.
- Thinking isotopes have different chemistry. Protium, deuterium and tritium all have 1 proton and 1 electron, so they react the same way (only mass differs).
- Calling hydrogen fuel always clean. Burning it gives only water, but making it from natural gas releases carbon dioxide.