Hydrogen: preparation, properties and uses: WASSCE Chemistry

Hydrogen is the lightest and most abundant element in the universe: the sun is mostly hydrogen. On earth it is found mainly combined, in water and in fuels. This lesson covers how hydrogen is prepared and tested in the laboratory, its properties and its many uses, from making fertilisers to turning palm oil into margarine.

Learning objectives

By the end of this lesson you should be able to:

describe the laboratory preparation of hydrogen, with an equation and a labelled diagram
explain the choice of method of collection and drying
describe the test for hydrogen
state the physical and chemical properties of hydrogen
explain why hydrogen is a reducing agent
state the uses of hydrogen

Occurrence

Free: very little in the atmosphere, because it is so light that it escapes; found in volcanic gases.
Combined: in water (11% hydrogen by mass), in acids, in hydrocarbons (natural gas, petroleum) and in all living things.

Laboratory preparation

Granulated zinc and dilute hydrochloric acid in a conical flask with a thistle funnel; hydrogen is led through a delivery tube and collected over water in an inverted gas jar

Figure 1. Preparing hydrogen from zinc and dilute acid. WASSCE Tutor diagram.

Hydrogen is prepared by the action of a dilute acid on a moderately reactive metal, usually granulated zinc with dilute hydrochloric or sulphuric acid:

\text{Zn(s)} + 2\text{HCl(aq)} \rightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)}

\begin{gathered}\text{Zn(s)} + 2\text{HCl(aq)} \\[6pt] \rightarrow \text{ZnCl}_2\text{(aq)} + \text{H}_2\text{(g)}\end{gathered}

\text{Zn(s)} + \text{H}_2\text{SO}_4\text{(aq)} \rightarrow \text{ZnSO}_4\text{(aq)} + \text{H}_2\text{(g)}

\begin{gathered}\text{Zn(s)} + \text{H}_2\text{SO}_4\text{(aq)} \\[6pt] \rightarrow \text{ZnSO}_4\text{(aq)} + \text{H}_2\text{(g)}\end{gathered}

Notes on the method:

Granulated (impure) zinc reacts faster than pure zinc. A few drops of copper(II) sulphate solution speed it up further (copper acts as a catalyst).
Nitric acid is not used: it is an oxidising agent and gives oxides of nitrogen instead of hydrogen.
Sodium and potassium are not used: they react explosively. Copper does not react at all, because it is below hydrogen in the reactivity series.
Let the first portions of gas escape: they are mixed with air, and a hydrogen–air mixture explodes when lit.

Collection:

Over water, because hydrogen is only slightly soluble in water.
Or by upward delivery (downward displacement of air), because it is much lighter than air.

Drying: pass the gas through concentrated sulphuric acid or fused calcium chloride, then collect by upward delivery (not over water, which would make it wet again).

Three collection methods: over water for gases insoluble in water; downward delivery into an upright jar for gases denser than air; upward delivery into an inverted jar for gases lighter than air

Figure 2. Choosing how to collect a gas. WASSCE Tutor diagram.

Method	Use for	Examples
Over water	Gases insoluble or slightly soluble in water	H₂, O₂, N₂, CO
Downward delivery (upward displacement of air)	Gases denser than air	Cl₂, CO₂, SO₂, HCl
Upward delivery (downward displacement of air)	Gases lighter than air	NH₃, H₂
Gas syringe	Measuring volumes accurately	Any gas

Test for hydrogen

Bring a lighted splint to the mouth of a test tube of the gas: hydrogen burns with a squeaky "pop". The pop is a small explosion of hydrogen with the air in the tube.

Physical properties

Property	Hydrogen
Colour, smell, taste	Colourless, odourless, tasteless
Density	Lightest of all gases (about 1/14 the density of air)
Solubility in water	Only slightly soluble
Boiling point	−253 °C
Effect on litmus	Neutral

Chemical properties

1. Burns in air or oxygen with a pale blue, almost invisible flame, forming water. Mixtures with air explode when lit.

2\text{H}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{H}_2\text{O(l)} \qquad \Delta H = -572 \text{ kJ}

\begin{gathered}2\text{H}_2\text{(g)} + \text{O}_2\text{(g)} \\[6pt] \rightarrow 2\text{H}_2\text{O(l)} \\[6pt] \Delta H = -572 \text{ kJ}\end{gathered}

Hydrogen does not support the combustion of other substances: a burning splint is put out inside the jar.

2. Reducing agent. Hydrogen removes oxygen from the oxides of metals below zinc in the reactivity series:

\text{CuO(s)} + \text{H}_2\text{(g)} \rightarrow \text{Cu(s)} + \text{H}_2\text{O(l)}

\begin{gathered}\text{CuO(s)} + \text{H}_2\text{(g)} \\[6pt] \rightarrow \text{Cu(s)} + \text{H}_2\text{O(l)}\end{gathered}

The black copper(II) oxide turns reddish-brown (copper), and droplets of water form. Hydrogen is oxidised; copper(II) oxide is reduced. Oxides of more reactive metals (Al₂O₃, MgO, ZnO) are not reduced by hydrogen.

3. With non-metals:

Nitrogen (Haber process): N₂ + 3H₂ ⇌ 2NH₃
Chlorine: H₂ + Cl₂ → 2HCl (explosive in sunlight)
Sulphur (heated): H₂ + S → H₂S

4. With very reactive metals, forming ionic hydrides: 2Na + H₂ → 2NaH. Here hydrogen forms the H⁻ ion, like a halogen.

Uses of hydrogen

Use	Reason
Haber process (ammonia for fertilisers)	Combines with nitrogen
Hydrogenation of vegetable oils to make margarine	Adds across C=C bonds (nickel catalyst), turning liquid oils into solid fats
Manufacture of hydrochloric acid	Burns in chlorine
Rocket fuel (liquid hydrogen)	Very high energy per kilogram; product is only water
Fuel cells	Produce electricity cleanly from hydrogen and oxygen
Oxy-hydrogen flame	Very hot flame for welding and cutting
Extraction of metals	Reduces some metal oxides (e.g. tungsten)
Weather balloons (in the past)	Very light (now mostly helium, which does not burn)

In Sierra Leone

Some margarine and shortening sold in Sierra Leone is made from palm oil that has been partly hydrogenated: hydrogen gas is passed through the hot oil with a nickel catalyst. The hydrogen adds to some C=C double bonds, raising the melting point so the fat is solid at room temperature.

Common mistake

Hydrogen burns but does not support combustion. A lighted splint gives a "pop" at the mouth of the tube (the gas burning), but a burning splint pushed right into a jar of hydrogen goes out.

Key points

Prepared from granulated zinc and dilute HCl or H₂SO₄: Zn + 2HCl → ZnCl₂ + H₂. Not with HNO₃, Na, K or Cu.
Collected over water (slightly soluble) or by upward delivery (very light); dried with concentrated H₂SO₄ or CaCl₂.
Test: burns with a squeaky pop.
Lightest gas; burns to form water; a reducing agent (CuO → Cu); forms hydrides with reactive metals.
Uses: ammonia, margarine, HCl, rocket fuel, fuel cells.

WASSCE examination focus

WAEC tests this

Expect to describe and draw the laboratory preparation of hydrogen, to explain the method of collection and drying, to give the test, and to write equations showing hydrogen as a reducing agent. Questions often ask why nitric acid or sodium cannot be used.

Quick check

Write an equation for the preparation of hydrogen from zinc and dilute sulphuric acid.
Why is hydrogen collected over water?
Describe what is seen when hydrogen is passed over heated copper(II) oxide.
Give two uses of hydrogen.

Answers:

Zn + H₂SO₄ → ZnSO₄ + H₂.
It is only slightly soluble in water.
The black solid turns reddish-brown (copper) and droplets of water form.
Any two: making ammonia, hydrogenating oils (margarine), making HCl, rocket fuel, fuel cells, welding.

Hydrogen: preparation, properties and uses