Preparing Methane, and the Uses of Alkanes – Alkanes (Senior 3 Chemistry) | Ganzaa

Alkanes

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Senior 3 · Chemistry

Alkanes

4 lessons

Lesson 4 of 4

Preparing Methane, and the Uses of Alkanes

30 min

A gas field under a lake

Lake Kivu, on Rwanda's western border, holds a large amount of methane dissolved in its deep water. Since 31 December 2015 the KivuWatt plant has pumped up the gas-rich water, separated the methane and burned it to generate electricity for the national grid, with a gross capacity of 26 MW. The same gas can be made in a school laboratory in a few minutes, from two white solids and a Bunsen burner.

A methane extraction platform on Lake Kivu: a barge carrying a tall tower painted blue and yellow, with a Rwandan flag flying on the right. Photo: U.S. Department of the Treasury, public domain, via Wikimedia Commons.

Making methane in the laboratory

Methane is made by heating anhydrous sodium ethanoate (sodium acetate) with soda lime, a mixture of sodium hydroxide and calcium oxide. The sodium hydroxide removes the $\mathrm{-COONa}$ group as sodium carbonate and leaves an alkane with one carbon atom fewer:

\mathrm{CH_3COONa(s) + NaOH(s) \rightarrow CH_4(g) + Na_2CO_3(s)}

Mix about 5 g of anhydrous sodium ethanoate with about twice its mass of soda lime and put it in a hard-glass test tube.
Clamp the tube with its mouth tilted slightly down, so that any moisture cannot run back onto the hot glass and crack it.
Heat the mixture strongly, and collect the gas over water (Figure 4.1). Methane is almost insoluble in water.
Let the first jar of gas escape: it is mostly air from the apparatus.
Before you stop heating, lift the delivery tube out of the water.

Warning

Safety. (1) Soda lime is corrosive: wear eye protection and do not touch it. (2) Mixtures of methane and air explode when lit: keep flames away from the gas jars, and test only a small jar of gas, with its mouth pointing away from people. (3) Lift the delivery tube out of the water before you turn off the burner. As the tube cools, the gas inside contracts and cold water is sucked back into the hot tube, which can shatter it.

Definition

Soda lime: a solid mixture of sodium hydroxide and calcium oxide. It is used instead of sodium hydroxide alone because it does not absorb water from the air so easily and attacks the glass less. Example: heating sodium ethanoate with soda lime gives methane.

Decarboxylation: a reaction that removes a carboxyl group (as carbonate or carbon dioxide) from a compound. Example: sodium propanoate with soda lime gives ethane.

Testing the gas

The methane collected is colourless and has no smell. A lit splint at the mouth of a small jar makes it burn with a pale blue flame; shaking the jar with limewater afterwards turns the limewater milky. Bromine water shaken with the gas in the dark keeps its orange colour, which shows that the gas is saturated.

Worked example

Problem. Which alkane is formed when sodium propanoate, $\mathrm{CH_3CH_2COONa}$ , is heated with soda lime? Write the equation.

Solution. The $\mathrm{-COONa}$ group is removed and replaced by H, so the chain loses one carbon: three carbons in the salt give a two-carbon alkane, ethane.

\mathrm{CH_3CH_2COONa(s) + NaOH(s) \rightarrow C_2H_6(g) + Na_2CO_3(s)}

Worked example

Problem. What mass of methane, and what volume at room temperature and pressure, is made from 8.2 g of anhydrous sodium ethanoate? (C = 12, H = 1, O = 16, Na = 23; one mole of gas occupies 24 dm³ at r.t.p.)

Solution. $M_r(\mathrm{CH_3COONa}) = 24 + 3 + 32 + 23 = 82$ , so $n = \frac{8.2}{82} = 0.10$ mol. The equation gives 1 mol of methane per mole of salt: 0.10 mol of methane.

Mass $= 0.10 \times 16 = 1.6$ g. Volume $= 0.10 \times 24 = 2.4$ dm³.

Where alkanes come from and what they are used for

Most alkanes come from crude oil and natural gas, formed over millions of years from the remains of tiny sea organisms buried under rock. Crude oil is a mixture of hundreds of hydrocarbons, mainly alkanes. A refinery separates it into fractions by fractional distillation: hydrocarbons with short chains have low boiling points and rise to the top of the column; long ones condense near the bottom. Methane is also made today from waste: bacteria in a biogas digester break down cow dung and food waste without air and give biogas, which is mostly methane.

Table 4.1: Uses of alkanes

Alkane or mixture	Main uses
methane (natural gas, biogas)	fuel for cooking, heating and generating electricity; making hydrogen for ammonia fertiliser
propane and butane (LPG)	cooking gas in cylinders, lighters, camping stoves; 2-methylpropane as a refrigerant
petrol, kerosene, diesel	fuels for vehicles, aircraft, lamps, stoves and generators
lubricating oil, paraffin wax	engine oil and grease; candles and polishes
bitumen	road surfaces and waterproof roofing
long-chain alkanes	cracked to make petrol and alkenes, the raw material for plastics
chloroalkanes such as dichloromethane	solvents in laboratories and industry

Warning

Common mistakes. (1) The alkane made by decarboxylation has one carbon fewer than the sodium salt: sodium ethanoate (2 C) gives methane (1 C), not ethane. (2) Methane is collected over water because it is insoluble, not because it is "heavier than water". (3) Fractions of crude oil are mixtures with a range of boiling points, not pure compounds.

Tip

Burning alkanes and the climate. Every carbon atom in a burnt alkane ends up as a molecule of carbon dioxide, a greenhouse gas. Methane that escapes unburnt is a stronger greenhouse gas than carbon dioxide, so leaks from pipes, digesters and gas fields matter too.

FuseSchool: Fractional Distillation. Watch how a fractionating column separates crude oil into fractions, and why the shortest molecules come out at the top.

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Practice

Name the two reagents used to make methane in the laboratory and write the equation with state symbols.
Give the reason for each step: (a) the test tube is tilted with its mouth slightly down; (b) the first jar of gas is not kept; (c) the gas is collected over water.
Explain why the delivery tube is taken out of the water before the burner is turned off.
Which alkane is made by heating sodium butanoate, $\mathrm{CH_3CH_2CH_2COONa}$ , with soda lime? Write the equation.
Calculate the mass and the volume at r.t.p. of methane made from 16.4 g of anhydrous sodium ethanoate. (C = 12, H = 1, O = 16, Na = 23; 24 dm³ per mole)
Name the crude oil fraction used for: (a) road surfaces; (b) cooking gas cylinders; (c) generators and buses; (d) aircraft.
Explain why biogas is a renewable fuel but natural gas is not.
Methane from Lake Kivu is used to generate electricity in Rwanda. Give one benefit of this and one environmental concern.

Show answersHide answers

Anhydrous sodium ethanoate and soda lime: $\mathrm{CH_3COONa(s) + NaOH(s) \rightarrow CH_4(g) + Na_2CO_3(s)}$ .
(a) Moisture cannot run back onto the hot glass and crack it. (b) It is mostly air from the apparatus. (c) Methane is almost insoluble in water.
When heating stops, the gas in the tube cools and contracts, and cold water would be sucked back into the hot tube and crack it.
Propane: $\mathrm{CH_3CH_2CH_2COONa(s) + NaOH(s) \rightarrow C_3H_8(g) + Na_2CO_3(s)}$ .
$n = \frac{16.4}{82} = 0.20$ mol; mass $= 0.20 \times 16 = 3.2$ g; volume $= 0.20 \times 24 = 4.8$ dm³.
(a) bitumen; (b) refinery gas (LPG); (c) diesel; (d) kerosene.
Biogas is made from dung and plant waste, which are produced again every day. Natural gas took millions of years to form and is used much faster than it forms.
Benefit: electricity from a local fuel, so less fuel is imported. Concern: burning methane releases carbon dioxide, and extraction must not upset the layers of the lake, so it has to be monitored.

1A gas field under a lake

2Making methane in the laboratory

3Testing the gas

4Where alkanes come from and what they are used for

A gas field under a lake

Making methane in the laboratory

Testing the gas

Where alkanes come from and what they are used for