Chemistry · Chemistry of the elements · Grade 7

Hydrogen — the lightest gas

How many times lighter than air is hydrogen, and why is it tested with a “pop” before being lit?

The situation

In a school laboratory stands a three-bulb glass vessel — a Kipp apparatus: when the tap is opened, zinc and hydrochloric acid inside “boil” and give off hydrogen; when it is closed, the reaction stops by itself. Hydrogen is the lightest gas: a soap bubble filled with it floats up to the ceiling. But before burning it, it must be tested in a test tube for a “pop” — why? Hydrogen turns black copper(II) oxide into red copper, and when it burns it forms only water. Hydrogen obtained by splitting water with an electric current can be turned back into electricity in a fuel cell — this is how hydrogen buses run.

Topic

Hydrogen — the lightest gas. From a Kipp apparatus to a hydrogen bus: hydrogen from zinc and acid, the “squeaky pop” test and the fuel cell

Key ideas

Kipp's apparatus
Explosive (“squeaky pop”) gas
Fuel cell

Formulas

Zn + 2 HCl \overset{}{\to} ZnCl_{2} + H_{2} ↑

Laboratory preparation: zinc and hydrochloric acid (in a Kipp apparatus).

2 H_{2} + O_{2} \overset{}{\to} 2 H_{2} O

Combustion: 286 kJ per mole of H₂ (≈ 142 MJ/kg). An H₂ : O₂ = 2 : 1 mixture is “detonating gas”.

CuO + H_{2} \overset{\overset{}{t}}{\to} Cu + H_{2} O

Hydrogen is a reducing agent: it takes oxygen from copper(II) oxide.

2 H_{2} O \overset{}{\to} 2 H_{2} + O_{2}

Electrolysis of water: hydrogen at the cathode, oxygen at the anode, volume ratio 2 : 1.

V (H_{2}) = \frac{I t}{2 F} V_{m}

Hydrogen released in electrolysis: $F$ = 96 485 C/mol; at 20 °C $V_{m}$ ≈ 24.1 L/mol.

Theory

Hydrogen — the first element

Hydrogen (H) is element 1 of the periodic table: its atom has one proton and one electron. About 3/4 of ordinary matter in the Universe (by mass) is hydrogen; on Earth it occurs mainly in water and organic substances. The simple substance H₂ is a colourless, odourless gas, the lightest substance: at STP its density is 0.0899 g/L, 14.4 times lighter than air (1.293/0.0899; from the molar masses roughly $D_{a i r} = 29 / 2 \approx 14.5$ ). It is almost insoluble in water and liquefies at −253 °C.

Practical: preparation and collection

In the laboratory hydrogen is made by the action of a dilute acid on an active metal: $Zn + 2 HCl \overset{}{\to} ZnCl_{2} + H_{2} ↑$ (or $Zn + H_{2} SO_{4} \overset{}{\to} ZnSO_{4} + H_{2} ↑$ ). In a Kipp apparatus (P. Kipp, Delft, 1844) the zinc sits in the middle bulb: when the tap is opened the gas escapes and the acid rises to the zinc; when the tap is closed, the hydrogen that collects pushes the acid into the lower bulb and the upper funnel — the reaction stops by itself. H₂ is lighter than air, so the test tube is collected mouth downwards; because it is insoluble in water, it can also be collected by displacing water. In industry hydrogen is mainly obtained from natural gas ( $CH_{4} + H_{2} O \overset{}{\to} CO + 3 H_{2}$ , at high temperature and with a catalyst); electrolysis of water gives only a small share for now, but its importance for “green hydrogen” is growing.

Testing purity: “detonating gas”

When hydrogen is mixed with air at 4 % to 75 % by volume, it ignites if a flame is applied — often with an explosion; the strongest is the 29.5 % mixture, which has enough oxygen, while a 2 : 1 mixture with pure oxygen is called detonating gas. That is why, before burning hydrogen coming from the apparatus, its purity is checked: a test tube is filled mouth downwards, closed with a finger and taken to a flame. A sharp pop means the gas contains air — the apparatus has not yet been purged of air. Quiet burning (almost no sound, a pale blue flame at the mouth of the tube) means the hydrogen is pure; a muffled sound means it is almost pure, but a little air is still present.

Chemical properties

Hydrogen burns with a pale blue flame that is almost invisible: $2 H_{2} + O_{2} \overset{}{\to} 2 H_{2} O$ , 286 kJ of heat per mole of H₂ — ≈ 142 MJ per kg, about 2.5 times more than methane. The only product of combustion is water: a cold beaker “sweats” over the flame. Hydrogen is a reducing agent: it takes oxygen from heated copper(II) oxide: $CuO + H_{2} \overset{}{\to} Cu + H_{2} O$ — the black powder turns into red copper, and drops of water appear in the cold part of the tube. It also combines with non-metals: with nitrogen it forms ammonia, with chlorine hydrogen chloride. Its second compound with oxygen is hydrogen peroxide H₂O₂: a strong oxidising agent (the 3 % solution is the antiseptic from the pharmacy), which decomposes into water and oxygen in the presence of MnO₂ (in the oxygen experiment). Order of reduction: first the hydrogen stream drives out the air (otherwise there is an explosion on heating); at the end, first the burner is put out, and hydrogen is passed over the copper until it cools — otherwise hot copper is oxidised in air again.

Uses and the hydrogen economy

Most hydrogen goes into the synthesis of ammonia ( $N_{2} + 3 H_{2} \overset{}{⇌} 2 NH_{3}$ ) — for nitrogen fertilisers. Hydrogenation of vegetable oils gives margarine; liquid hydrogen and liquid oxygen are rocket fuel. In a fuel cell hydrogen and oxygen, without burning, directly produce electric current and water (one cell gives ≈ 0.7 V) — this is how hydrogen buses and cars work. The law of electrolysis: $V (H_{2}) = \frac{I t}{2 F} V_{m}$ — a current of 1 A gives ≈ 449 mL of hydrogen in 1 hour at 20 °C. In Uzbekistan a water-electrolysis shop was started at the Chirchik Electrochemical Combine in 1940: nitrogen fertiliser was made from hydrogen obtained from water with electricity from the Chirchik hydroelectric power stations; later the plant switched to natural gas.

Safety and the model

Safety: wear safety goggles; if hydrochloric acid touches your skin, wash it with plenty of water; do not light a flame near an apparatus giving off hydrogen, and never ignite the gas without checking its purity; in electrolysis use only a low-voltage supply. Model: in the Kipp apparatus the reaction rate is proportional to the fraction of zinc wetted by the acid and to the acid concentration, and the flow through the tap is proportional to the pressure difference (hydrostatics — from the exact volumes of the profiles); the gas in the apparatus and in the test tube mixes uniformly; the loudness of the sound in the test is schematic (the limits of 4 %, 29.5 % and 75 % are from the literature); the reduction kinetics of CuO is schematic (starting from ≈ 150–200 °C); in electrolysis all the current goes into liberating gas; the fuel cell delivers a current of 0.8 A.

Did you know?

In 1766 the English scientist Henry Cavendish separated “inflammable air” by the action of acids on metals and measured that it is much lighter than air. The name “hydrogène” (“water-forming”) was given by Antoine Lavoisier in 1783.
The 1932 decision to build the Chirchik Electrochemical Combine provided for producing nitrogen fertiliser from hydrogen obtained by the electrolysis of water; the water-electrolysis shop started in 1940, and the first ammonium nitrate was obtained in October of that year.

Inside the experiment

The experiment starts from a real-life situation: you predict the result, change the controls, run the experiment and draw a conclusion. Inside are missions, theory, a quiz and a guided tour in which the camera moves and explains the key points.

More experiments in this section

Air: composition and pollution — Grade 7
Oxygen and ozone — Grade 7
Water: composition and properties — Grade 7
Mineral resources — Grade 7
Lime and classes of compounds — Grade 8
Non-metals and halogens — Grade 8
Chlorine and hydrochloric acid — Grade 8
The oxygen group and sulfur — Grade 8
Sulfuric acid and the contact process — Grade 8
Nitrogen, ammonia and nitric acid — Grade 8