Alkynes: ethyne, its preparation and properties: WASSCE Chemistry

A welder cutting steel uses a flame hot enough to melt it: around 3,000 °C. The fuel is ethyne (acetylene), the simplest alkyne, burned in pure oxygen. Alkynes contain a carbon–carbon triple bond. This lesson covers their structure and the preparation and properties of ethyne.

Learning objectives

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

state the general formula of the alkynes and name the first members
describe the structure and shape of the C≡C triple bond
describe the laboratory preparation of ethyne from calcium carbide
explain how calcium carbide is manufactured
describe the physical properties and combustion of ethyne
state the uses of ethyne

Structure and names

Definition

Alkynes are unsaturated hydrocarbons that contain one carbon–carbon triple bond (C≡C). General formula: CₙH₂ₙ₋₂.

Name	Molecular formula	Condensed formula	Boiling point
Ethyne (acetylene)	C₂H₂	HC≡CH	−84 °C (sublimes)
Propyne	C₃H₄	CH₃C≡CH	−23 °C
But-1-yne	C₄H₆	HC≡CCH₂CH₃	8 °C
But-2-yne	C₄H₆	CH₃C≡CCH₃	27 °C

The triple bond is made of one σ bond and two π bonds. The two π bonds make alkynes highly unsaturated: they can add two molecules of hydrogen or bromine.

Shape. The two carbons of the triple bond and the atoms attached to them lie in a straight line: ethyne is a linear molecule with bond angles of 180° (sp hybridisation).

The C≡C bond (0.120 nm) is shorter and stronger than C=C (0.134 nm), which is shorter and stronger than C–C (0.154 nm).

Laboratory preparation of ethyne

Water drips from a dropping funnel onto calcium carbide in a flask; ethyne is collected over water

Figure 1. Preparing ethyne from calcium carbide and water. WASSCE Tutor diagram.

Drip water from a dropping funnel onto lumps of calcium carbide (CaC₂) in a flask. No heating is needed:

\text{CaC}_2\text{(s)} + 2\text{H}_2\text{O(l)} \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{C}_2\text{H}_2\text{(g)}

\begin{gathered}\text{CaC}_2\text{(s)} + 2\text{H}_2\text{O(l)} \\[6pt] \rightarrow \text{Ca(OH)}_2\text{(aq)} + \text{C}_2\text{H}_2\text{(g)}\end{gathered}

Water is added drop by drop because the reaction is vigorous and exothermic.
Ethyne is collected over water.
Gas made from commercial carbide smells of garlic because of impurities such as phosphine (PH₃) and hydrogen sulphide; these can be removed by passing the gas through acidified copper(II) sulphate solution.
Pure ethyne is odourless.

Manufacture of calcium carbide

Calcium carbide is made by heating calcium oxide (quicklime) with coke in an electric furnace at about 2,000 °C:

\text{CaO} + 3\text{C} \rightarrow \text{CaC}_2 + \text{CO}

\begin{gathered}\text{CaO} + 3\text{C} \\[6pt] \rightarrow \text{CaC}_2 + \text{CO}\end{gathered}

The calcium oxide comes from heating limestone. Ethyne can also be made industrially by heating methane strongly for a very short time.

Physical properties of ethyne

Colourless gas, odourless when pure
Slightly less dense than air
Slightly soluble in water, very soluble in acetone (propanone)
Explosive under pressure: it is stored in cylinders dissolved in propanone and soaked into a porous material

Combustion

In plenty of oxygen, ethyne burns with an extremely hot flame:

2\text{C}_2\text{H}_2 + 5\text{O}_2 \rightarrow 4\text{CO}_2 + 2\text{H}_2\text{O} \qquad \Delta H = -2600 \text{ kJ}

\begin{gathered}2\text{C}_2\text{H}_2 + 5\text{O}_2 \\[6pt] \rightarrow 4\text{CO}_2 + 2\text{H}_2\text{O} \\[6pt] \Delta H = -2600 \text{ kJ}\end{gathered}

In air, ethyne burns with a very luminous, very sooty flame, because it has a very high proportion of carbon (92.3% by mass).

Hydrocarbon	% carbon by mass	Flame in air
Ethane, C₂H₆	80.0%	Blue, non-luminous, clean
Ethene, C₂H₄	85.7%	Luminous, slightly smoky
Ethyne, C₂H₂	92.3%	Very luminous, very sooty

Uses of ethyne

Oxy-acetylene flame (about 3,000 °C) for welding and cutting metals
Carbide lamps: once used by miners and fishermen, burning ethyne from carbide and water
Making chloroethene (for PVC), ethanal and ethanoic acid
Ripening fruit (calcium carbide is misused for this; see below)

Worked example

Volume of ethyne from carbide. What volume of ethyne at s.t.p. is produced when 16 g of calcium carbide reacts with excess water? (CaC₂ = 64; molar volume = 22.4 dm³)

Moles of CaC₂ $= 16 \div 64 = 0.25$ mol
1 mol CaC₂ gives 1 mol C₂H₂, so moles of ethyne $= 0.25$ mol
Volume $= 0.25 \times 22.4 = 5.6$ dm³

In Sierra Leone

Some fruit sellers in West Africa use calcium carbide ("carbide") to ripen mangoes and bananas quickly. Moisture turns it into ethyne, which acts like ethene. But commercial carbide contains arsenic and phosphorus impurities, and its use on food is banned in many countries. Fruit ripened this way can look ripe outside but taste unripe inside. Natural ripening, or ripening with controlled ethene, is safer.

Common mistake

Ethyne is made from calcium carbide, CaC₂, not calcium carbonate, CaCO₃. Read the formula carefully.

Key points

Alkynes, CₙH₂ₙ₋₂, contain a C≡C triple bond (one σ and two π bonds) and are linear around it (180°).
Ethyne is prepared by dripping water onto calcium carbide: CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂; collected over water.
CaC₂ is made from CaO and coke in an electric furnace.
Ethyne burns in air with a very sooty flame (92.3% C) and in oxygen at about 3,000 °C (welding).
It is stored dissolved in propanone because it explodes under pressure.

WASSCE examination focus

WAEC tests this

Expect to describe the laboratory preparation of ethyne with an equation and a labelled diagram, to give the general formula and structure of the alkynes, to state uses of ethyne (especially the oxy-acetylene flame) and to calculate gas volumes from the mass of calcium carbide.

Quick check

Write the general formula of the alkynes.
Write the equation for the preparation of ethyne.
What is the shape of the ethyne molecule, and what are its bond angles?
Why does ethyne burn with a sootier flame than ethane?

Answers:

CₙH₂ₙ₋₂.
CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂.
Linear; 180°.
Ethyne has a much higher proportion of carbon (92.3% against 80%), so some carbon is not completely burned in air.

Alkynes: ethyne, its preparation and properties