A polymer forms when many small molecules join into a long chain, and a fuel is chosen by how much energy it releases and what its combustion produces. Both ideas come from the compound’s structure.
This lesson is part of carbon compounds. It builds on comparing saturated and unsaturated compounds.
How does a polymer form?
Ethene has a carbon-carbon double bond. When the double bond opens, each molecule can link to its neighbours, forming a long chain.
The small starting molecule is the monomer and the long product is the polymer. If 1 000 ethene molecules (mass 28 each) join without losing any atoms, the polymer mass is 1 000 × 28 = 28 000 in the same units. No atoms are lost, so the masses add.
Worked example: choosing a fuel from data
A class is given energy released per gram: Fuel A releases 50 kJ per gram and Fuel B releases 30 kJ per gram. They need 600 kJ.
Fuel A. Mass needed = 600 ÷ 50 = 12 g.
Fuel B. Mass needed = 600 ÷ 30 = 20 g.
Check: 12 × 50 = 600 and 20 × 30 = 600. Fuel A gives the same energy from less mass.
Answer sentence. Fuel A releases more energy per gram, so less is needed for the same 600 kJ. The data does not give cost or pollution, so it alone cannot say which fuel is the better choice.
That last sentence states the assumption: the comparison uses only energy per gram.
The mistake that costs marks
Students write that burning a fuel “uses up carbon dioxide”. Combustion of a carbon fuel produces carbon dioxide and water when oxygen is plentiful.
| Wrong | Right | |
|---|---|---|
| Products, plenty of oxygen | carbon dioxide used up | carbon dioxide and water formed |
| Limited oxygen | same products | possible carbon monoxide and soot |
| Polymer made from | different monomers each time | the same monomer repeated |
Check yourself
Fuel C releases 40 kJ per gram and Fuel D releases 25 kJ per gram. Which mass of each releases 500 kJ?
Answer
Fuel C: 500 ÷ 40 = 12.5 g. Fuel D: 500 ÷ 25 = 20 g.
Check: 12.5 × 40 = 500 and 20 × 25 = 500. Fuel C needs less mass.
What to study next
Continue with interpreting carbon-cycle information, where combustion appears as one flow among several. Then try the carbon compounds practice set. The graph evidence comparison lab helps with comparing data fairly.
For a teacher to go through these questions with you, see online one-to-one Science tuition.