Carbon Dioxide (CO2)
Carbon dioxide was first prepared by van Helmont in 1630 and named gassy Silvestre. Joseph Black (1754) called it fixed air. Lavoisier recognized the gas as an oxide of carbon in 1783 and called it acidic carbonic.
Carbon dioxide (CO2) is an important compound of carbon. CO2 constitutes approximately 0.3% by volume in air. CO2 gas is mainly produced due to the combustion of different carbonaceous fuels, respiration of animals and plants, decay of organic compounds, etc.
In a combined state, it occurs as carbonates, such as limestone, chalk, and marble. Marble exists in different forms such as calcium carbonate (CaCO3), magnesite (MgCO3), and dolomite (the double carbonate MgCO3, CaCO3).
Let us now discuss the laboratory preparation of CO2, its physical and chemical properties, and its application.
Read And Learn More WBBSE Solutions For Class 8 School Science Experiments Questions
Laboratory Method Of Preparation of CO2
A carbonate or bicarbonate of a metal in contact with a dilute mineral acid produces. carbon dioxide gas. In Laboratory, the gas is prepared by action of diluting hydrochloric acid on calcium carbonate (marble chips, CaCO3).
Apparatus and chemical required: Woulfe’s bottle fitted with a thistle funnel and delivery tube, gas jar with lid, marble chips (CaCO3), dilute hydrochloric acid (HCI).
Procedure: A few pieces of marble chips are taken in Woulfe’s bottle. Water is poured into the bottle so that the chips remain submerged in water.
Moderately concentrated HCI is then poured down the thistle funnel. Rapid effervescence occurs when the acid comes in contact with the marble chips.
The effervescence is due to the evolution of CO2 gas that evolves as small bubbles from the solution.
\(\mathrm{CaCO}_3+2 \mathrm{HCl} \rightarrow \mathrm{CaCl}_2+\mathrm{H}_2 \mathrm{O}+\mathrm{CO}_2\)
Collection: The gas passes through the delivery tube. It is allowed to escape for a few seconds to remove the air present initially in Woulfe’s bottle.
The delivery tube is now introduced in a gas jar where the gas is collected by upward displacement of air (since CO2 is heavier than air).
A burning piece of paper is held at the mouth of the gas jar. If the paper extinguishes, it indicates that the gas jar is full of CO2.
The delivery tube is withdrawn and the mouth of the jar is covered with a lid. Since the gas is fairly soluble in water, it is not collected by displacement of water.
Purification: The gas collected in this way contains vapors of HCI and water. If pure CO2 is required, before collection, the gas is passed through a solution of sodium bicarbonate (to remove HCl vapor) and then through
concentrated H2SO4 to remove water vapor.
Choice of mineral acid: Dilute sulphuric acid is not used as the mineral acid. Initially sulphuric acid reacts with CaCO3 to produce CO2 and CaSO4.
\(\mathrm{CaCO}_3+\mathrm{H}_2 \mathrm{SO}_4 \rightarrow \mathrm{CaSO}_4+\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O}\)
But after some time, the reaction stops. This is because dilute H2SO4 reacts with CaCO3 to produce calcium sulfate which has very low solubility in water and it forms an insoluble coating around the pieces of CaCO3, thus preventing further reaction between dilute H2SO4 and CaCO3. So the formation of CO2 is stopped.
Other Methods Of Preparation For CO2
When charcoal or any other fuel containing carbon is burnt in air or oxygen, carbon dioxide gas is produced. \(\mathrm{C}+\mathrm{O}_2 \rightarrow \mathrm{CO}_2\)
2. Metallic carbonates (except those of sodium, potassium, and barium) or metallic bicarbonates decompose on heating, liberating CO2 gas.
\(\begin{gathered}\mathrm{CaCO}_3 \rightarrow \mathrm{CaO}+\mathrm{CO}_2 \\
\mathrm{Ca}\left(\mathrm{HCO}_3\right)_2 \rightarrow \mathrm{CaCO}_3+\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \\
2 \mathrm{KHCO}_3 \rightarrow \mathrm{K}_2 \mathrm{CO}_3+\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O}
\end{gathered}\)
3. During the fermentation of sugar by the action of enzymes, present in yeast, CO2 is obtained.
\(\mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6 \rightarrow 2 \mathrm{C}_2 \mathrm{H}_5 \mathrm{OH} \text { (ethyl alcohol) }+2 \mathrm{CO}_2\)
Properties Of Carbon Dioxide
1. Physical Properties
1. Carbon dioxide (CO2) is a colorless gas having a faint pungent smell and an acidic taste.
2. It is fairly soluble in water and the solubility increases with pressure. Soda water is made by dissolving CO2 in water under high pressure.
3. The gas is 1.5 times heavier than air. Its density is 1.958 grams per liter at 08C and 1 atmospheric pressure.
4. It can be liquefied by application of pressure at any temperature below 31°C.
5. The gas is not poisonous but causes suffocation in want of oxygen in an atmosphere of carbon dioxide.
Experiments show that CO2 is heavier than air.
Experiment -1
Let us take a balloon, a rubber band, sodium bicarbonate and vinegar, and a glass bottle. First, solid sodium bicarbonate and vinegar are taken together in the glass bottle and the balloon is fixed at the mouth of the bottle.
Due to the reaction between vinegar and sodium bicarbonate, CO2 gas is produced. This gas will inflate the balloon. Now the balloon is taken out and its mouth is tied tightly with a cord, and then it is released.
It is found that the balloon is not floating. Rather it just lies on the floor. This is because CO2 is heavier than air and that is why it did not float but settled on the floor.
Experiment -2
Let us take two gas jars A and B. “A” contains air and “B” contains CO2. Now a lighted jute stick is introduced in gas jar “A”.
The jute stick will remain lighted due to the presence of oxygen in the air. Now gas jar “B” is inverted over gas jar “A” and we find that the fire is extinguished.
CO2 being heavier than air will go downwards to gas jar “A” and displace air (which is lighter than CO2) from there. Within a very short time gas jar “A” is filled with CO2 gas.
Since CO2 does not support combustion, the fire is extinguished. Experiment to show that CO2 is soluble in water.
Experiment-3
A Wolfe’s bottle is used to produce CO2 gas and the gas formed is passed through a gas jar filled with water. After allowing CO2 gas to flow through the water for sufficient time, a blue litmus paper and a red litmus paper are dipped in that water.
The blue litmus paper turns red, indicating that the solution is acidic. Water becomes acidic because CO2 dissolves in water to produce carbolic acid (H2CO4). H2CO4 is a weak acid and it turns blue litmus paper red.
\(\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{H}_2 \mathrm{CO}_3\)
The solubility of CO2 in water is increased by applying pressure. By applying pressure, CO2 is dissolved in soft drinks. When the bottle is opened, the pressure is released, the solubility of CO2 in water decreases and bubbles of CO2 evolved from the bottle.
2. Chemical Properties
1. CO2 is neither combustible nor a supporter of combustion. When a lighted taper is introduced in a jar full of CO2, the taper is extinguished and the gas in the jar does not also catch fire.
2. Aqueous solution of CO2 turns blue litmus paper red. This proves that an aqueous solution of CO2 is acidic. When CO2 is dissolved in water, it forms carbonic acid (H2C03), which is a weak acid.
The formation of this acid makes the aqueous solution of CO2 acidic.
Although CO2 is not a supporter of combustion, when a few metals like magnesium (Mg) and sodium (Na) in a feebly burning state are introduced in a gas jar containing CO2, they burn more When a lighted magnesium filament is introduced in a gas jar filled with CO2, the metal filament burns very brightly.
This is because the heat produced during the burning of magnesium is sufficient to break up CO2 into carbon and oxygen. The metal burns more vigorously in presence of oxygen. Carbon is separated as black particles.
When a lighted magnesium filament is introduced in a gas jar filled with CO2, the metal filament burns very brightly.
This is because the heat produced during the burning of magnesium is sufficient to break up CO2 into carbon and oxygen. The metal burns more vigorously in presence of oxygen. Carbon is separated as black particles.
\(2 \mathrm{Mg}+\mathrm{CO}_2 \rightarrow 2 \mathrm{MgO}+\mathrm{C}\)
This is an example of a redox reaction. Here Mg is oxidized to MgO and CO2 is reduced to C. These reactions show that CO2 contains carbon. The burning of sodium or potassium continues in the CO2 atmosphere.
\(\begin{aligned}4 \mathrm{Na}+3 \mathrm{CO}_2 & \rightarrow 2 \mathrm{Na}_2 \mathrm{CO}_3+\mathrm{C} \\
4 \mathrm{~K}+3 \mathrm{CO}_2 & \rightarrow 2 \mathrm{~K}_2 \mathrm{CO}_3+\mathrm{C}
\end{aligned}\)
4. Reaction with base or alkali: CO2 is an acidic oxide. It reacts with a base to form carbonate compounds. For example, some lime water is taken in a test tube and CO2 gas is bubbled through it.
Lime water turns milky due to the formation of calcium carbonate (CaC03) which is insoluble and remains suspended in the solution.
\(\mathrm{Ca}(\mathrm{OH})_2+\mathrm{CO}_2 \rightarrow \mathrm{CaCO}_3+\mathrm{H}_2 \mathrm{O}\)
If more CO2 is passed through the milky water, calcium bicarbonate [Ca(HC03)2] is formed, which is soluble in water. So the solution becomes clear again.
\(\mathrm{Ca}(\mathrm{OH})_2+\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \rightarrow \mathrm{Ca}\left(\mathrm{HCO}_3\right)_2\)
If the clear solution is boiled, it again turns milky, since the bicarbonate dissociates to form CaC03 again.
\(\mathrm{Ca}\left(\mathrm{HCO}_3\right)_2 \rightarrow \mathrm{CaCO}_3+\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O}\)
But when CO2 gas is bubbled through NaOH solution, soluble sodium carbonate is produced. Passage of excess CO2 gas produces sodium bicarbonate whose solubility is low compared to that of Na2C03 It remains suspended in the solution.
\(\begin{aligned}& 2 \mathrm{NaOH}+\mathrm{CO}_2 \rightarrow \mathrm{Na}_2 \mathrm{CO}_3+\mathrm{H}_2 \mathrm{O} \\
& \mathrm{Na}_2 \mathrm{CO}_3+\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \rightarrow 2 \mathrm{NaHCO}_3
\end{aligned}\)
Metallic oxides are basic. So, acidic CO2 reacts with them to form a carbonate compound.
For example,
\mathrm{Na}_2 \mathrm{O}+\mathrm{CO}_2 & \rightarrow \mathrm{Na}_2 \mathrm{CO}_3 \\
\mathrm{CaO}+\mathrm{CO}_2 & \rightarrow \mathrm{CaCO}_3
\end{aligned}\)
5. Oxidizing property of CO2: In powder or iron dust, the metals are oxidized to metal oxides and CO2 is reduced to carbon or carbon monoxide When CO2 gas is passed over red hot carbon, C is oxidized to CO, and CO2 is reduced to CO.
6. CO2 gas is absorbed by caustic potash (KOH) Solution. \(2 \mathrm{KOH}+\mathrm{CO}_2 \rightarrow \mathrm{K}_2 \mathrm{CO}_3+\mathrm{H}_2 \mathrm{O}\)
Tests Of Carbon Dioxide
1. Carbon dioxide is a colorless and odorless gas that extinguishes a lighted taper.
Carbon dioxide turns clear lime water milky and the milkiness disappears by the passage of excess the gas
Uses of Carbon dioxide
1. It is used to extinguish the fire, since, it is not a supporter of combustion.
Fire extinguisher: It is a strong metallic vessel containing a solution of sodium carbonate. There is sulphuric acid in a glass bottle kept separated from the solution.
The glass bottle can be broken with the help of a rod to which is attached a knob outside. When required, the bottle is broken by giving a blow to the knob.
The acid in contact with the sodium carbonate solution reacts vigorously liberating CO2 gas that passes through the nozzle. The emitting gas is directed to the fire-affected place to douse the fire.
\(\mathrm{Na}_2 \mathrm{CO}_3+\mathrm{H}_2 \mathrm{SO}_4=\mathrm{Na}_2 \mathrm{SO}_4+\mathrm{CO}_2+\mathrm{H}_2 \mathrm{O} \text {. }\)
2. Dry ice: Below the critical temperature (31°C) of CO2, it can be liquefied with pressure. On evaporation of liquid CO2 at 0°C temperature and 40 atmospheric pressure, a part of it solidifies, producing a solid form of carbon dioxide called dry ice.
During the evaporation of liquid CO2 latent heat of evaporation is taken from it and so it solidifies. The solid CO2 directly passes into the gaseous state on heating, without passing through the liquid phase. For this
2. Carbon dioxide turns clear lime water milky and the milkiness disappears by the passage of excess gas. the reason, the term dry is used.
Its temperature is – 79°C. It is used for preserving ice cream, packing fish, etc.
3. CO2 is used to prepare aerated water, such as soda water and lemonade. The solubility of CO2 gas in water increases with the increase in pressure. In soda water, lemonade, etc.,
the gas is dissolved in excess under high pressure. So when a bottle of soda water or lemonade is opened, the pressure on the contents of the bottle decreases to normal atmospheric pressure.
Now, due to the decrease in pressure, the solubility of CO2 decreases, and the excess CO2 gas comes out through the liquid, creating effervescence.
4. In the manufacture of sodium carbonate (Solvay process) and salicylic acid, CO2 is required.
5. Another important use of carbon dioxide is that the gas is absorbed by plants and trees for photosynthesis. During photosynthesis, oxygen gas is given out by the plants that help the purification of air. Thus human beings and animals are benefited.
6. Preparation of Urea: Urea is the most commonly used nitrogen fertilizer. Because nitrogen content is high and is readily converted to ammonia in the soil, it is very useful as fertilizer.
It is also starting material for the manufacture of plastics and drugs. It is produced by the reaction of carbon dioxide with ammonia.
\(\mathrm{CO}_2+2 \mathrm{NH}_3 \frac{200^{\circ} \mathrm{C}}{150 \text { atm pressure }} \mathrm{CO}\left(\mathrm{NH}_2\right)_2+\mathrm{H}_2 \mathrm{O}\) Urea