Carbon and some of its compounds: Carbon (IV) oxide

6.0 Carbon and some of its compounds

6.3 Carbon (IV) oxide

6.3.1 Preparation of carbon (IV) oxide

Questions 6.3.1(a)

What are the general products of reaction between acids and carbonates?

Answers to Questions 6.3.1a

We use our knowledge of reactions between acids and carbonates to prepare carbon (IV) oxide. This, we do by selecting and reacting a suitable carbonate (e.g. CaCO₃) and dilute acid (e.g. HCl(aq)).

Set-up

Water (sometimes sodium hydrogencarbonate solution) is used to absorb hydrogen chloride spray from hydrochloric acid.

Questions 6.3.1(b)

1. State the function of concentrated sulphuric acid in this set-up. Name an alternative substance for concentrated sulphuric acid.

2. Write equation for the reaction between hydrochloric acid and calcium carbonate.

3. Name the method of delivery shown in the diagram.

4. Explain why this method of delivery is suitable for collecting carbon (IV) oxide.

5. What is the effect of the gas on wet blue litmus paper? What does this show?

6. What is the effect of inverting the gas jar over the flame of a burning candle?

7. Normally carbon (IV) oxide does not support burning. But ignited magnesium continues to burn in carbon (IV) oxide with a very hot flame. Suggest a reason for this. What are the other observations? Identify the other products observed.

Answers to Questions 6.3.1b

Although all dilute acids produce carbon (IV) oxide when reacted with carbonates, not all of them are suitable. Dilute sulphuric acid, for example, forms an insoluble (only slightly soluble) calcium sulphate which stops the reaction immediately by coating the unreacted carbonate. Note that calcium chloride (CaCl₂) is soluble. For the same reason, carbonates of lead and barium should not be reacted with sulphuric acid.

6.3.2 Reactions of carbon (IV) oxide

We have already observed some important reactions of carbon (IV) oxide in Section 6.3.1.

Questions 6.3.2

Write equations to represent the following reactions of carbon (IV) oxide.

1. Carbon (IV) oxide slightly dissolves in water to form carbonic acid (H₂CO₃) which is a weak acid and turns blue litmus pink.

   Equation:

2. Carbon (IV) oxide reacts with calcium hydroxide solution to form a white precipitate, which is calcium carbonate.

   Equation:

3. Excess of carbon (IV) oxide causes the carbonate to dissolve by forming calcium hydrogencarbonate, Ca(HCO₃)₂. Water also takes part in this reaction.

   Equation:

4. In the same manner, carbon (IV) oxide reacts with sodium hydroxide solution to form sodium carbonate (Na₂CO₃). But, unlike calcium carbonate, this is soluble; so it is in solution.

   Equation:

5. With excess of carbon (IV), sodium carbonate (Na₂CO₃) reacts to form sodium hydrogencarbonate solution (NaHCO₃). Water also takes part in this reaction.

   Equation:

6. Carbon (IV) oxide reacts with burning magnesium to form a white solid, which is magnesium oxide, and carbon as black specks.

   Equation:

Answers to Questions 6.3.2

Note that the formation of white precipitate with calcium hydroxide, used to test for carbon (IV) oxide, occurs only during the first few minutes of bubbling the gas through the solution. On further bubbling of the gas (excess of CO₂), the precipitate dissolves into a colourless solution (calcium hydrogencarbonate), a two-stage reaction.

Because of the two-stage reactions with carbon (IV) oxide gas, sodium hydroxide solution is a good absorber of the gas. Potassium hydroxide solution (KOH(aq)) reacts with carbon (IV) oxide in the same manner as NaOH.

2KaOH(aq)  +   CO₂(g)   ⟶   K₂CO₃(aq)  +   H₂O(l)

K₂CO₃(aq)  +   CO₂(g) + H₂O(l)   ⟶   2KHCO₃(aq)

6.3.3 Properties of carbon (IV) oxide

1. Colourless and odourless gas
2. Denser than air
3. Extinguishes a burning splint or candle (does not support combustion)
4. Reacts with/dissolves in sodium hydroxide solution to form sodium carbonate and sodium hydrogencarbonate (baking powder)
5. Sublimes (with a cooling effect)
6. Forms a white precipitate with calcium hydroxide solution, which dissolves in excess of the gas. This is the usual test for the gas.

6.3.4 Uses of carbon (IV) oxide

Questions 6.3.3

From the properties stated, suggest 3-4 uses of carbon (IV) oxide.

Answers to Questions 6.3.3

1. Carbon (IV) is used as a fire extinguisher. Why? Because it is denser than air and does not support combustion, carbon (IV) oxide acts as a blanket over the fire, cutting off oxygen to stop the fire.




Figure 6.3.4(a) Fire extinguisher




When extinguishing a fire, the jet from the gas canister should be directed at the base of the fire. If directed above the flame, it will heat up, expand, become less dense are rise without putting out the fire.



2. Substances which readily evaporate or sublime do so by absorbing heat; so they have a cooling effect. Therefore solid carbon (IV) oxide, which sublimes, is used as a refrigerant.


3. It is used to make carbonated fizzy drinks. The gas is dissolved in water under pressure to make soda water, which helps in digestion and preservation. During use, some of the gas escapes as bubbles. Watch the video of a fizzy drink below.

(courtesy Youtube-Fizzy drink, carbonated drink by Joseph Rabari)