Year 9 Science · Unit 2 · Lesson 14

Combustion of Hydrocarbons

Foundation Worksheet

Name

Date

Class

Learning Goals

Name the products of complete and incomplete combustion.
Write and balance combustion equations, counting atoms on each side.
Use flame and heating results to tell complete from incomplete combustion.

Warm Up

True or false? Fix the false ones

Circle T or F. If a statement is false, correct it on the line.

Complete combustion of methane produces carbon dioxide and water.

Correct it:

Incomplete combustion produces the same products as complete combustion, just less of them.

Correct it:

Carbon monoxide (CO) is toxic because it is a greenhouse gas.

Correct it:

A blue flame is a sign of complete combustion.

Correct it:

Combustion is an endothermic reaction.

Correct it:

Fill the gap

Choose the correct word from the word bank. Each word is used once. Two words will not be used.

ionic sublimation oxygen complete carbon dioxide exothermic monoxide incomplete water

Combustion is the reaction of a hydrocarbon with that releases energy as heat and light. When plenty of oxygen is available, combustion occurs, and the only products are and . When the oxygen supply is limited, combustion occurs instead. Then carbon (CO) and soot can form, and the flame is yellow or orange. Because combustion releases energy to the surroundings, it is an reaction.

Your Turn

1. Write the word equation for the complete combustion of methane.

Recall 1 mark

2. When methane burns with a good air supply, CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(g). Count the atoms on each side to show this equation is balanced.

Apply 2 marks

Atom	Left side	Right side
C
H
O

3. Pentane, C₅H₁₂, is a petrol molecule. Use the three steps to finish its complete-combustion equation: C₅H₁₂ + O₂ → CO₂ + H₂O

Apply 3 marks

Carbon: C₅H₁₂ has 5 C atoms, so it makes CO₂.
Hydrogen: C₅H₁₂ has 12 H atoms, and each H₂O holds 2, so it makes H₂O.
Oxygen: count the O atoms on the right side = . Each O₂ holds 2, so O₂ are needed.

Write the balanced equation:

Go Deeper

Investigation: does the air hole matter?

A class heated 200 mL of water from 20 °C to 60 °C with a Bunsen burner running on natural gas (methane). They used the same burner, gas tap setting and beaker each time, and changed only the air hole.

Table 1. Heating 200 mL of water from 20 °C to 60 °C

Air hole	Flame, and the bottom of the beaker	Time (s)
Open	blue; beaker clean	128
Half open	blue with a yellow tip; faint black mark	156
Closed	yellow; thick black soot	214

Source: sample class results.

4. Which air hole setting gave complete combustion? Give two pieces of evidence from Table 1.

Apply 2 marks

5. With the air hole closed, the water took 86 s longer to heat. Use the idea of complete and incomplete combustion to explain why.

Analyse 2 marks

6. The black soot is carbon. With very little air, methane gives only carbon and water. Fill the gap, then check by counting atoms: CH₄(g) + O₂(g) → C(s) + 2H₂O(g)

Analyse 2 marks

Show What You Know

7. A family’s gas heater now burns with a yellow flame, and the wall above it has turned black. Explain what is happening in the heater, which products are going into the room, why this is dangerous, and what the family should do.

Analyse 4 marks