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ScienceGrade 10 5 min read

🛢️ Organic Chemistry & Fuels: The Chemistry of Carbon and Combustion

Explore how carbon's unique bonding gives rise to hydrocarbons and fuels, and learn to represent, name, and analyse the combustion reactions that power our world.

Learning Goals

By the end of this lesson, you will be able to:

✓
Describe why carbon forms the basis of organic molecules and draw the structures of simple alkanes.
✓
Write balanced equations for the complete and incomplete combustion of hydrocarbon fuels.
✓
Explain the relationship between a hydrocarbon's structure and its properties as a fuel.
✓
Interpret molecular, structural, and displayed formulae of organic compounds.

Before You Begin

Before this lesson, you should know:

→
Covalent bonding and how atoms share electron pairs to achieve full outer shells.
→
The meaning of a chemical formula and how to balance simple chemical equations.
→
The concept of a reactant, a product, and conservation of mass in reactions.

You should already be confident with the following ideas:

Notation & Symbols

Key notation and symbols used in this topic:

C_{n} H_{2 n + 2}

General formula for the alkanes, where

n

is the number of carbon atoms.

\to

Reaction arrow — separates reactants from products in an equation.

(g), (l)

State symbols indicating a gas or a liquid respectively.

Δ H

Enthalpy change — the heat energy released or absorbed by a reaction.

–

A single covalent bond shown in a displayed (structural) formula.

Familiarise yourself with the symbols used throughout organic chemistry.

Carbon, Hydrocarbons, and Fuels

Organic chemistry is the study of carbon-based compounds; a hydrocarbon is a molecule composed only of carbon and hydrogen atoms, and many hydrocarbons are used as fuels because their combustion releases large amounts of energy.

Carbon's bonding: Each carbon atom has four outer electrons, allowing it to form four strong covalent bonds. This lets carbon build long chains and branched structures — the basis of millions of organic molecules.
Alkanes: A homologous series of saturated hydrocarbons with the general formula $C_{n} H_{2 n + 2}$ . Examples include methane $C H_{4}$ , ethane $C_{2} H_{6}$ , and propane $C_{3} H_{8}$ .
Saturated vs unsaturated: Saturated molecules contain only single C–C bonds; unsaturated molecules (alkenes) contain at least one C=C double bond and are more reactive.
Complete combustion: When a hydrocarbon burns in plenty of oxygen it produces carbon dioxide and water, releasing energy: this is why hydrocarbons are valuable fuels.
Incomplete combustion: In a limited oxygen supply, combustion also produces carbon monoxide $C O$ and carbon (soot), releasing less energy and forming toxic products.

Example

The complete combustion of methane is written as

C H_{4} (g) + 2 O_{2} (g) \to C O_{2} (g) + 2 H_{2} O (l)

Here one molecule of methane reacts with two molecules of oxygen, confirming that atoms are conserved: 1 carbon, 4 hydrogen, and 4 oxygen on each side.

100%

Displayed formulae of methane, ethane, and propane, showing carbon's four covalent bonds.

Worked Example

Basic

Write the molecular formula and name the first three members of the alkane homologous series.

Worked Example

Intermediate

Balance the equation for the complete combustion of propane

C_{3} H_{8}

Worked Example

Advanced

Compare the products of complete and incomplete combustion of butane

C_{4} H_{10}

, and write a balanced equation for incomplete combustion producing carbon monoxide.

Why It Works

Understanding the reasoning behind the rule:

The general formula

C_{n} H_{2 n + 2}

for alkanes arises directly from carbon's bonding capacity and the structure of a saturated chain.

Your Turn

Try this one yourself — use the hint if you get stuck:

Write the balanced equation for the complete combustion of ethane

C_{2} H_{6}

Watch Out For...

✗

Confusing complete and incomplete combustion products — writing

C O_{2}

when limited oxygen produces

C O

or carbon (soot).

Check the oxygen supply. Complete combustion (plenty of

O_{2}

) gives

C O_{2}

and

H_{2} O

; incomplete combustion (limited

O_{2}

) gives

C O

and/or

C

plus

H_{2} O

✗

Not balancing combustion equations correctly, especially the oxygen coefficient.

Balance carbon first, then hydrogen, then oxygen last. For alkanes use

C_{n} H_{2 n + 2} + \frac{3 n + 1}{2} O_{2} \to n C O_{2} + (n + 1) H_{2} O

✗

Assuming all hydrocarbons have the general formula

C_{n} H_{2 n + 2}

Only alkanes fit

C_{n} H_{2 n + 2}

. Alkenes are

C_{n} H_{2 n}

because of the

C = C

double bond, which reduces the number of hydrogens.

In the Real World

⛽

Petroleum refineries use fractional distillation to separate crude oil into fractions (petrol, kerosene, diesel) by boiling point, which depends on molecular chain length and intermolecular forces.

🏭

Carbon monoxide from incomplete combustion in engines and boilers is a toxic, colourless gas — a key driver of emissions regulations and catalytic converter engineering.

🌍

The

C O_{2}

released per joule of energy differs between fuels; this drives the shift toward hydrogen and lower-carbon fuels in climate policy and energy engineering.

Extended Practice

Test yourself with a mix of multiple choice and short-answer questions:

Q1.

Which is the molecular formula of butane, an alkane with 4 carbon atoms?

MCQ

Q2.

Which products indicate incomplete combustion?

MCQ

Q3.

Write and balance the complete combustion equation for methane

C H_{4}

C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O

Carbon and hydrogen balance first, then oxygen requires 2

O_{2}

molecules.

Q4.

Explain why alkenes are described as unsaturated while alkanes are saturated.

Alkenes contain at least one

C = C

double bond, so they can add more atoms across that bond (unsaturated). Alkanes have only single

C - C

bonds and hold the maximum number of hydrogens (saturated).

Q5.

How many hydrogen atoms are in the alkene with 5 carbon atoms (pentene)?

MCQ

Extension: Comparing Fuel Energy

Advanced

A harder challenge that connects to the next skill:

Propane

C_{3} H_{8}

undergoes complete combustion. Write the balanced equation, then explain why longer-chain hydrocarbons tend to release more energy per mole when burned.

Key Takeaways

1.Alkanes are saturated hydrocarbons with general formula $C_{n} H_{2 n + 2}$ ; alkenes are unsaturated with $C_{n} H_{2 n}$ due to a $C = C$ double bond.
2.Complete combustion in plentiful oxygen produces $C O_{2}$ and $H_{2} O$ ; incomplete combustion in limited oxygen gives $C O$ and/or carbon.
3.Balance combustion equations in the order: carbon, hydrogen, then oxygen.
4.Energy release in combustion comes from forming stronger product bonds than those broken, and generally increases with hydrocarbon chain length.

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🔋 Electrochemistry & Cells