Carbon and Its Inorganic Compounds

Lessons

Senior 3 · Chemistry

Carbon and Its Inorganic Compounds

5 lessons

Lesson 3 of 5

Carbon Dioxide: Preparation, Properties and Uses

25 min

A lake that holds a gas

Deep below the surface of Lake Kivu, under the pressure of hundreds of metres of water, lies an enormous amount of dissolved gas: an estimated 256 km³ of carbon dioxide, together with methane. At the surface you would never know it. In 1986 a much smaller lake in Cameroon, Lake Nyos, suddenly released the carbon dioxide from its deep water. The invisible gas poured down the valleys around the lake like water and killed 1,746 people and thousands of cattle. Why does a gas stay dissolved in deep water? And why would it flow downhill? The properties of carbon dioxide answer both questions.

Preparing carbon dioxide in the laboratory

In S2 you saw that any carbonate fizzes with an acid. The laboratory preparation uses marble chips (calcium carbonate) and dilute hydrochloric acid:

$\mathrm{CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)}$

Dilute sulfuric acid is not used, because insoluble calcium sulfate coats the chips and the reaction soon stops. The apparatus is shown in Figure 3.1.

The acid is added through a thistle funnel whose end dips below the surface of the acid. The liquid seals the funnel, so the gas cannot escape through it and must leave by the delivery tube.
Carbon dioxide is denser than air ( $M_{r} = 44$ , against about 29 for air), so it is collected by downward delivery: the delivery tube reaches to the bottom of an upright gas jar, and the gas fills the jar from the bottom, pushing the air out at the top.
It is only fairly soluble in water, so it can also be collected over water when a small loss does not matter.
If dry gas is needed, it is first bubbled through concentrated sulfuric acid or passed over anhydrous calcium chloride. Both drying agents can be used because carbon dioxide is an acidic gas and does not react with them.

Figure 3.1: Laboratory preparation of carbon dioxide.

Warning

Safety. Wear eye protection when handling acid. Carbon dioxide is not poisonous in small amounts, but it does not support breathing: in a closed room, a pit or a cellar it can collect at the bottom and suffocate anyone who enters. Never seal dry ice in a closed bottle or jar: as it turns to gas the pressure builds until the container bursts. Never hold dry ice in bare hands; at −78 °C it causes burns like frostbite.

Physical properties

Colourless, with no smell.
About 1.5 times as dense as air, so it can be poured from one container into another (Figure 3.2).
Fairly soluble in water, and much more soluble under pressure. Fizzy drinks are made by dissolving carbon dioxide under pressure; when you open the bottle the pressure falls and the gas bubbles out. The deep water of Lake Kivu holds its carbon dioxide in the same way.
At normal pressure it has no liquid state. Cooled to −78 °C it changes straight into a white solid, dry ice, which changes straight back into gas when it warms (it sublimes).

Chemical properties

An acidic oxide. Carbon dioxide dissolves in water to form a weak acid, carbonic acid. Damp blue litmus turns only slightly red, and universal indicator shows a pH of about 5 to 6:

$\mathrm{CO_2(g) + H_2O(l) \rightleftharpoons H_2CO_3(aq)}$

With alkalis it forms a carbonate, and with more carbon dioxide a hydrogencarbonate:

$\mathrm{CO_2(g) + 2NaOH(aq) \rightarrow Na_2CO_3(aq) + H_2O(l)}$

$\mathrm{Na_2CO_3(aq) + H_2O(l) + CO_2(g) \rightarrow 2NaHCO_3(aq)}$

The test for carbon dioxide is the same kind of reaction with calcium hydroxide solution (limewater), which turns milky because insoluble calcium carbonate forms:

$\mathrm{Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)}$

As you saw in S2, a large excess of carbon dioxide makes the milkiness disappear again, because soluble calcium hydrogencarbonate forms.

It does not burn and does not support burning. A burning splint goes out in it. Burning magnesium is the exception: it is so reactive that it tears the oxygen away from carbon dioxide and keeps burning, leaving white magnesium oxide speckled with black carbon:

$\mathrm{2Mg(s) + CO_2(g) \rightarrow 2MgO(s) + C(s)}$

Photosynthesis. Green plants use the energy of sunlight to turn carbon dioxide and water into glucose and oxygen:

$\mathrm{6CO_2(g) + 6H_2O(l) \rightarrow C_6H_{12}O_6(aq) + 6O_2(g)}$

Figure 3.2: Pouring carbon dioxide onto candles. It is denser than air and does not support burning.

Table 3.1: Carbon monoxide and carbon dioxide compared

Property	Carbon monoxide, $\mathrm{CO}$	Carbon dioxide, $\mathrm{CO_2}$
$M_{r}$ ; density compared with air (29)	28; about the same	44; about 1.5 times denser
Solubility in water	almost insoluble	fairly soluble; more under pressure
Type of oxide	neutral	acidic
Effect on limewater	none	turns it milky
Does it burn?	yes, with a blue flame	no, and it does not support burning
Reducing agent?	yes (reduces CuO, $\mathrm{Fe_2O_3}$ )	no
Effect on the body	very poisonous: blocks haemoglobin	not poisonous, but suffocates in high concentration
Collected by	over water	downward delivery (or over water)

Uses of carbon dioxide

Fire extinguishers. It is denser than air, so it settles over a fire like a blanket and shuts out oxygen; it does not burn; it leaves no mess; and it does not conduct electricity, so it is safe on electrical fires and burning liquids.
Fizzy drinks, dissolved under pressure.
Dry ice keeps vaccines, meat and ice cream cold during transport, and it leaves no water behind as it warms.
Food for plants: some greenhouses add carbon dioxide to speed up photosynthesis.
Baking: baking powder releases carbon dioxide, which makes bread, cakes and mandazi rise.

Worked example

Problem. A class needs five gas jars of carbon dioxide, each of $250\,\mathrm{cm^3}$ , at room temperature and pressure. What is the least mass of pure calcium carbonate needed? (Ca = 40, C = 12, O = 16; molar volume at room temperature and pressure $= 24\,\mathrm{dm^3\,mol^{-1}}$ )

Solution. Total volume $= 5 \times 250 = 1250\,\mathrm{cm^3} = 1.25\,\mathrm{dm^3}$ .

$n(\mathrm{CO_2}) = \frac{1.25\,\mathrm{dm^3}}{24\,\mathrm{dm^3\,mol^{-1}}} = 0.0521\,\mathrm{mol}$

From the equation, 1 mol $\mathrm{CaCO_3}$ gives 1 mol $\mathrm{CO_2}$ , so 0.0521 mol $\mathrm{CaCO_3}$ is needed. $M(\mathrm{CaCO_3}) = 40 + 12 + 3 \times 16 = 100\,\mathrm{g\,mol^{-1}}$ , so the mass is $0.0521 \times 100 = 5.21\,\mathrm{g}$ . In practice more is used, because the first gas pushed out is air, and some carbon dioxide escapes or dissolves.

Worked example

Problem. A burning splint is lowered into a gas jar of carbon dioxide and goes out. A piece of burning magnesium ribbon lowered into another jar keeps burning, and afterwards the jar contains a white powder with black specks. Explain both observations.

Solution. The splint goes out because carbon dioxide contains no free oxygen, and carbon, already fully oxidised in $\mathrm{CO_2}$ , cannot be oxidised further. Magnesium is far more reactive than carbon: its reaction with oxygen releases so much energy that burning magnesium can take the oxygen from carbon dioxide. The white powder is magnesium oxide and the black specks are carbon:

$\mathrm{2Mg(s) + CO_2(g) \rightarrow 2MgO(s) + C(s)}$

So carbon dioxide extinguishers must never be used on burning magnesium.

Tip

Heavy, acidic, and it clouds lime. Three facts answer most exam questions on carbon dioxide: it is denser than air (downward delivery, fire blanket), it is an acidic oxide (reacts with alkalis), and it turns limewater milky (the test).

Making lime water and testing for carbon dioxide gas (8 min). Watch how clear limewater is made from solid calcium hydroxide, then how it turns milky when carbon dioxide from baking soda and vinegar is bubbled through it.

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Practice

Write the equation, with state symbols, for the laboratory preparation of carbon dioxide from marble chips.
In Figure 3.1, why must the end of the thistle funnel be below the surface of the acid?
Give two reasons why carbon dioxide is collected by downward delivery rather than by upward delivery.
Name a drying agent for carbon dioxide, and explain why calcium oxide would not be suitable.
Write equations for the reaction of carbon dioxide with (a) water, (b) sodium hydroxide solution, (c) limewater.
What volume of carbon dioxide, at room temperature and pressure, is given off when 2.5 g of calcium carbonate reacts completely with excess dilute hydrochloric acid? ( $\mathrm{CaCO_3} = 100$ ; molar volume $24\,\mathrm{dm^3\,mol^{-1}}$ )
Explain why a carbon dioxide extinguisher is better than a water extinguisher for a fire in a computer room.
Use the properties of carbon dioxide to explain why the gas released from Lake Nyos in 1986 killed people in the valleys below the lake, several kilometres away.

Show answersHide answers

$\mathrm{CaCO_3(s) + 2HCl(aq) \rightarrow CaCl_2(aq) + H_2O(l) + CO_2(g)}$ .
The acid seals the end of the funnel, so the gas cannot escape up the funnel and is forced along the delivery tube.
It is denser than air ( $M_{r}$ 44 against 29), so it sinks and fills an upright jar from the bottom; in an upside-down jar it would fall out. (It is also fairly soluble, so collection over water loses some gas.)
Concentrated sulfuric acid or anhydrous calcium chloride. Calcium oxide is a base and would react with the acidic carbon dioxide, absorbing it.
(a) $\mathrm{CO_2(g) + H_2O(l) \rightleftharpoons H_2CO_3(aq)}$ ; (b) $\mathrm{CO_2(g) + 2NaOH(aq) \rightarrow Na_2CO_3(aq) + H_2O(l)}$ ; (c) $\mathrm{Ca(OH)_2(aq) + CO_2(g) \rightarrow CaCO_3(s) + H_2O(l)}$ .
$n = \frac{2.5}{100} = 0.025\,\mathrm{mol}$ ; volume $= 0.025 \times 24 = 0.60\,\mathrm{dm^3}$ (600 cm³).
Water conducts electricity, so it could give the firefighter an electric shock, and it damages equipment. Carbon dioxide does not conduct, leaves no residue, and smothers the fire by keeping out oxygen.
Carbon dioxide is denser than air, so the huge cloud flowed downhill along the valleys and stayed near the ground instead of mixing upwards. It does not support breathing, so people and animals in its path suffocated for lack of oxygen, and it is colourless and odourless, so they had no warning.

Sources and credits

Lake Kivu (the estimate of dissolved carbon dioxide) and Lake Nyos disaster (date and number of deaths), Wikipedia contributors, Wikipedia, licensed under CC BY-SA 4.0. Adapted by Ganzaa.
Photos: Lake Kivu, Western Rwanda.jpg, Synkubito, CC0; Dry Ice.jpg, APN MJM, CC BY-SA 3.0; Water and Carbon Dioxide fire extinguishers, Eric Fischer, CC BY 2.0; all via Wikimedia Commons.
Video: Making lime water and testing for carbon dioxide gas, Xa15Sci0, embedded from YouTube.
The diagrams, table, worked examples and Practice items were written and drawn by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1A lake that holds a gas

2Preparing carbon dioxide in the laboratory

3Physical properties

4Chemical properties

5Uses of carbon dioxide

A lake that holds a gas

Preparing carbon dioxide in the laboratory

Physical properties

Chemical properties

Uses of carbon dioxide