Properties of hydrogen gas
- colourless
- odourless
- tasteless
- slightly soluble in water
- neutral to litmus
- lightest known gas
- 14.4 times less denser than air
- highly flammable
Test for hydrogen gas
Ignite the gas with a burning splint and it burns with loud pop.
The position of hydrogen on the Periodic Table
On most Periodic Tables, hydrogen is often placed by itself because of its unique properties. Hydrogen behaves like a Group I element because it has 1 electron in the outer shell and also like a Group IV element because its electronic configuration is 1 electron short of a noble gas.
On other Periodic Tables it is shown at the top of either Group I or Group VII.
The table below shows the comparisons of the properties of hydrogen, a Group I element and a Group VII element.
| Lithium (Group 1) | Hydrogen | Fluorine (Group VII) |
|---|---|---|
| Solid | Gas | Gas |
| Forms a positive ion | Forms either positive or negative ions | Forms a negative ion |
| 1 electron in outer shell | 1 electron in outer and 1 electron short of a full outer shell | 1 electron short of a full outer shell |
| Loses 1 electron to form a noble gas configuration | Needs 1 electron to form a noble gas configuration | Needs 1 electron to form a noble gas configuration |
Hydrogen gas as a reducing agent for metal oxides
Hydrogen is a reducing agent and it removes oxygen from oxides of less reactive on the electrochemical series. Oxides of iron, lead and copper can be reduced by hydrogen as shown by the equations below.
- Fe3O4(s) + 4H2(g) → 3Fe(s) + 4H2O(l)
- CuO(s) + H2(g) → Cu(s) + H2O(l)
- PbO(s) + H2(g) → Pb(s) + H2O(l)
Hydrogen itself is oxidised to water when it reduces a metal oxide to a metal.
Reactions of hydrogen gas with electronegative elements
Hydrogen reacts with electronegative elements as follows:
- H2(g) + Cl2(g) → 2HCl(g) (hydrogen chloride)
- H2(g) + Br2(g) → 2HBr(g) (hydrogen bromide)
- H2(g) + I2(g) → 2HI(g) (hydrogen iodide)
- 3H2(g) + N2(g) → 2NH3(g) (ammonia)
- H2(g) + S(s) → H2S(g) (hydrogen sulphide)
- 2H2(g) + O2(g) → 2H2O(l) (water)
Uses of hydrogen
- Welding metals – the oxyhydrogen flame uses oxygen and hydrogen to form a hot flame of over 2000°C.
- Manufacture of margarine – hydrogen is used in the hardening of oils to make solid fat such as margarine.
- Making petrol – in countries with large reserves of coal, synthetic petrol is manufacture by hydrogenation of coal.
- Manufacture of ammonia – hydrogen and nitrogen are used to manufacture ammonia in the haber process.
- As rocket fuel – some rocket engines use liquid hydrogen as a propellant. An oxygen-hydrogen liquid mixture is most powerful known propellant.