The exercise

1. Write the molecular formulas of propane and butane.
2. Draw the structures of propane and butane.
3. If burning these compounds gives only carbon dioxide (CO₂) and water (H₂O), write a balanced equation for each reaction.
4. Is using LP gas as a fuel better for the environment than using firewood? Give your view.

(i) Propane C₃H₈; butane C₄H₁₀. (ii) The structures:

(iii) The balanced equations:

LP gas burns cleanly and completely in a stove, giving mainly carbon dioxide and water, with no smoke, soot or ash; it also gives more heat for its mass. Firewood burns less completely and gives smoke and soot that harm the lungs, and cutting trees for firewood destroys forests. Both give carbon dioxide, and LP gas is a fossil fuel that will not be replaced, so a fair view is that LP gas is cleaner to use, while wood from replanted trees is renewable.

1. If petrol burns completely in an engine, what products can form?
2. Name two harmful substances released into the environment when petrol burns incompletely.
3. How would you know that the gas in a household LP gas cooker is burning incompletely?

1. Carbon dioxide and water: 2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O.
2. Carbon monoxide (CO), a poisonous gas, and soot (unburnt carbon).
3. The flame turns yellow or orange instead of blue, it gives off smoke, and soot blackens the bottom of the pans.

1. What is the chemical name of polythene?
2. Draw the structure of the monomer polythene is made from, and name it.
3. Give two advantages and two disadvantages of polythene.

1. Polyethene.
2. Ethene, C₂H₄, drawn below.
3. Advantages: light, waterproof, lasts long, cheap. Disadvantages: does not biodegrade, so it pollutes; gives off poisonous gases when burned.

1. Give three reasons why PVC pipes are better than iron pipes for water.
2. What is the name of the monomer used to make PVC?
3. Draw the structure of that monomer.

1. PVC does not rust or corrode in water (iron does, as in chapter 12); it is much lighter and easier to cut, join and carry; it is cheaper; and it does not conduct electricity.
2. Chloroethene (vinyl chloride).
3. Drawn below.

1. Name three natural polymers you know.

1. Any three of: rubber, protein, starch, cellulose, DNA, RNA.