What Is Combustion?

Combustion is a chemical reaction in which a substance reacts with oxygen and releases energy.

The substance being burned is called the fuel.

In a combustion reaction:

fuel + oxygen → products + energy

Energy is usually transferred to the surroundings as:

• heat
• light

Because combustion releases energy, it is an exothermic reaction.

Combustion reactions are important in transportation, electricity generation, heating, cooking, industry, and many other areas of everyday life.

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Fuels

A fuel is a substance that can release useful energy when it undergoes a chemical reaction.

Common fuels include:

• methane
• propane
• butane
• gasoline
• kerosene
• diesel
• coal
• biomass

Many common fuels contain hydrocarbons.

A hydrocarbon contains only:

carbon and hydrogen

Alkanes are an important family of hydrocarbon fuels.

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Why Alkanes Burn

Alkanes react with oxygen when sufficient activation energy is supplied.

A flame or spark can provide the initial energy needed to start the reaction.

During combustion:

• bonds in the fuel are broken
• bonds in oxygen molecules are broken
• new bonds form in the products

Overall, forming the new bonds releases more energy than is required to break the original bonds.

Therefore:

energy is released overall

and the reaction is:

exothermic

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Complete Combustion

Complete combustion occurs when a fuel burns with a sufficient supply of oxygen.

When a hydrocarbon undergoes complete combustion, the products are:

carbon dioxide + water

Therefore:

hydrocarbon + oxygen → carbon dioxide + water

For an alkane:

alkane + O₂ → CO₂ + H₂O

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Complete Combustion of Methane

Methane has the formula:

CH₄

The balanced equation for complete combustion is:

CH₄ + 2O₂ → CO₂ + 2H₂O

Check the atoms.

Left:

C = 1

H = 4

O = 4

Right:

C = 1

H = 4

O = 4

The equation is balanced.

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What Happens to the Atoms?

During combustion, atoms are rearranged.

They are not created or destroyed.

In methane combustion:

Carbon from methane becomes part of:

CO₂

Hydrogen from methane becomes part of:

H₂O

Oxygen from the air becomes part of both products.

This demonstrates the law of conservation of mass.

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Balancing Combustion Equations

A chemical equation must contain the same number of each type of atom on both sides.

A useful order for balancing alkane combustion equations is:

1. Balance carbon.

2. Balance hydrogen.

3. Balance oxygen last.

This method works well because oxygen appears in both products.

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Example: Combustion of Ethane

Start with:

C₂H₆ + O₂ → CO₂ + H₂O

Step 1: Balance carbon

There are 2 carbon atoms.

C₂H₆ + O₂ → 2CO₂ + H₂O

Step 2: Balance hydrogen

There are 6 hydrogen atoms.

We need:

3H₂O

So:

C₂H₆ + O₂ → 2CO₂ + 3H₂O

Step 3: Balance oxygen

Right side:

2CO₂ = 4 O atoms

3H₂O = 3 O atoms

Total:

7 O atoms

This requires:

7/2 O₂

So:

C₂H₆ + 7/2O₂ → 2CO₂ + 3H₂O

Multiply the entire equation by 2:

2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O

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Example: Combustion of Propane

Start:

C₃H₈ + O₂ → CO₂ + H₂O

Balance carbon:

C₃H₈ + O₂ → 3CO₂ + H₂O

Balance hydrogen:

C₃H₈ + O₂ → 3CO₂ + 4H₂O

Count oxygen on the right:

3CO₂ gives:

6 O atoms

4H₂O gives:

4 O atoms

Total:

10 O atoms

Therefore we need:

5O₂

Balanced equation:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

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Example: Combustion of Butane

Start:

C₄H₁₀ + O₂ → CO₂ + H₂O

Balance carbon:

C₄H₁₀ + O₂ → 4CO₂ + H₂O

Balance hydrogen:

C₄H₁₀ + O₂ → 4CO₂ + 5H₂O

Oxygen atoms required:

From CO₂:

4 × 2 = 8

From H₂O:

5 × 1 = 5

Total:

13 oxygen atoms

So:

C₄H₁₀ + 13/2O₂ → 4CO₂ + 5H₂O

Multiply everything by 2:

2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O

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General Complete Combustion Pattern

For an alkane:

CₙH₂ₙ₊₂

complete combustion produces:

CO₂

and:

H₂O

The general pattern can be written as:

CₙH₂ₙ₊₂ + (3n + 1)/2 O₂ → nCO₂ + (n + 1)H₂O

For many introductory problems, however, it is usually easier and safer to balance each equation using:

carbon → hydrogen → oxygen

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Complete Combustion and Flames

Complete combustion is associated with efficient burning when sufficient oxygen is available.

For many gaseous hydrocarbon burners, a well-adjusted flame with good oxygen mixing is often:

blue

A blue Bunsen burner flame, for example, usually indicates relatively complete combustion.

However, flame appearance depends on the fuel and burner conditions, so colour alone should not be treated as a universal test for every combustion system.

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Incomplete Combustion

Incomplete combustion occurs when there is not enough oxygen for the fuel to burn completely.

Instead of all carbon atoms becoming carbon dioxide, some may form:

carbon monoxide (CO)

or:

carbon (C)

Water is still produced from the hydrogen in the hydrocarbon.

Therefore, possible products include:

carbon monoxide + water

and/or:

carbon + water

Some carbon dioxide may also form.

Real incomplete combustion can produce a mixture of products rather than only one carbon-containing product.

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Why Incomplete Combustion Happens

Incomplete combustion can occur when:

• oxygen supply is limited
• air and fuel do not mix properly
• a burner is poorly adjusted
• combustion occurs in a poorly ventilated space
• combustion conditions do not allow the fuel to burn completely

Less complete oxidation of the carbon occurs.

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Incomplete Combustion Producing Carbon Monoxide

Methane can undergo incomplete combustion to produce carbon monoxide.

One balanced equation is:

2CH₄ + 3O₂ → 2CO + 4H₂O

Check:

Left:

C = 2

H = 8

O = 6

Right:

C = 2

H = 8

O = 6

The equation is balanced.

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Incomplete Combustion Producing Carbon

With even more limited oxygen, solid carbon may form.

For methane:

CH₄ + O₂ → C + 2H₂O

This carbon can appear as:

soot

Soot is evidence that carbon-containing fuel has not been completely oxidized.

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Complete vs Incomplete Combustion

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Carbon Monoxide

Carbon monoxide, CO, is a colourless and odourless gas.

It is dangerous because it interferes with the blood's ability to transport oxygen.

Carbon monoxide can be produced by incomplete combustion in equipment such as:

• faulty heaters
• poorly maintained furnaces
• engines
• generators
• poorly ventilated combustion appliances

This is why proper ventilation and correctly maintained combustion equipment are important.

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Why Carbon Monoxide Is Dangerous

Red blood cells contain hemoglobin, which normally transports oxygen.

Carbon monoxide binds strongly to hemoglobin.

This reduces the amount of oxygen that the blood can transport effectively.

Because carbon monoxide has no colour or smell, people may not notice its presence without appropriate detection equipment.

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Soot

Soot consists largely of tiny carbon-rich particles formed during incomplete combustion.

It can:

• blacken surfaces
• reduce air quality
• enter the respiratory system
• contribute to atmospheric particulate pollution

Black carbon particles can also absorb sunlight and contribute to atmospheric warming.

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Energy from Combustion

Combustion reactions are exothermic.

Energy is released because the products are at a lower chemical energy than the reactants.

This released energy can be used for:

• heating
• cooking
• transportation
• generating electricity
• industrial processes

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Complete Combustion Releases More Useful Energy

Incomplete combustion does not fully oxidize the carbon in the fuel.

Products such as:

CO

and:

C

can still undergo further oxidation.

Therefore, incomplete combustion generally releases less energy from a given amount of fuel than complete combustion.

This makes incomplete combustion:

less efficient

as a way of obtaining energy from the fuel.

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Carbon Dioxide and the Environment

Complete combustion of hydrocarbon fuels produces:

carbon dioxide, CO₂

Carbon dioxide is a greenhouse gas.

Greenhouse gases absorb and re-emit infrared radiation, helping keep Earth's surface and lower atmosphere warmer than they would otherwise be.

Increasing atmospheric carbon dioxide concentrations strengthen this greenhouse effect and contribute to climate change.

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The Carbon Cycle and Fossil Fuels

Carbon naturally moves between:

• atmosphere
• oceans
• living organisms
• soils
• rocks

This movement is part of the carbon cycle.

Fossil fuels contain carbon that has been stored underground for very long periods.

When fossil fuels are burned, carbon is transferred into the atmosphere mainly as:

CO₂

Large-scale fossil-fuel combustion therefore changes the amount of carbon dioxide in the atmosphere.

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Air Pollution from Combustion

Combustion can produce several pollutants depending on the fuel and conditions.

These can include:

• carbon monoxide
• particulate matter
• nitrogen oxides
• sulfur dioxide if sulfur-containing fuels are burned
• unburned hydrocarbons

Not all of these pollutants come directly from the alkane itself.

For example, nitrogen oxides can form when nitrogen and oxygen from the air react at the high temperatures found in engines and other combustion systems.

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Nitrogen Oxides

Air contains large amounts of:

nitrogen, N₂

and:

oxygen, O₂

At very high combustion temperatures, these gases can react and form nitrogen oxides, often written collectively as:

NOₓ

Nitrogen oxides contribute to problems including:

• poor air quality
• photochemical smog
• acid deposition

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Sulfur Dioxide

Some fossil fuels contain sulfur-containing impurities or compounds.

When sulfur is burned, sulfur dioxide can form:

S + O₂ → SO₂

Sulfur dioxide can contribute to:

• respiratory problems
• acid deposition
• environmental damage

Pure alkanes themselves contain only carbon and hydrogen, so sulfur dioxide is not a product of burning a pure alkane.

It results from sulfur-containing material in the fuel.

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Particulate Matter

Incomplete combustion can release very small solid or liquid particles into the air.

These are called:

particulate matter

Fine particles can penetrate deeply into the respiratory system.

Reducing incomplete combustion can therefore improve both fuel efficiency and air quality.

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Complete Combustion Is Not Pollution-Free

A common misconception is:

"Complete combustion is clean, so it has no environmental impact."

Complete combustion reduces products such as carbon monoxide and soot.

However, hydrocarbon complete combustion still produces:

carbon dioxide

Therefore, complete combustion can still contribute to greenhouse-gas emissions.

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Combustion in Vehicle Engines

Vehicle engines burn hydrocarbon-based fuels to release energy.

Ideally:

hydrocarbon + oxygen → carbon dioxide + water

Real engines may also produce:

• CO
• NOₓ
• unburned hydrocarbons
• particulate matter

Modern engine and exhaust-control technologies attempt to reduce some of these pollutants.

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Catalytic Converters

Many vehicles use a catalytic converter to reduce harmful exhaust emissions.

Catalytic converters can help convert:

carbon monoxide → carbon dioxide

nitrogen oxides → nitrogen

unburned hydrocarbons → carbon dioxide + water

They do not eliminate all environmental effects of fuel combustion, particularly the carbon dioxide produced from fossil carbon.

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Combustion in Power Generation

Fuels may also be burned to generate electricity.

Chemical energy in the fuel is converted through several stages:

chemical energy → thermal energy → mechanical energy → electrical energy

Some energy is transferred to the surroundings at each stage, so the entire process is not 100% efficient.

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Testing the Products of Complete Combustion

The products of complete hydrocarbon combustion are:

carbon dioxide

and:

water

Carbon dioxide can be tested using limewater.

Carbon dioxide causes limewater to turn:

milky/cloudy

Water can be detected using suitable chemical tests, such as cobalt chloride paper or anhydrous copper(II) sulfate in laboratory contexts.

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Balancing Strategy for Complete Combustion

Consider:

C₅H₁₂ + O₂ → CO₂ + H₂O

Step 1: Carbon

There are 5 carbon atoms.

Write:

5CO₂

So:

C₅H₁₂ + O₂ → 5CO₂ + H₂O

Step 2: Hydrogen

There are 12 hydrogen atoms.

Write:

6H₂O

So:

C₅H₁₂ + O₂ → 5CO₂ + 6H₂O

Step 3: Oxygen

Products contain:

5CO₂ → 10 O atoms

6H₂O → 6 O atoms

Total:

16 O atoms

Therefore:

8O₂

Final equation:

C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O

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Another Example: Hexane

Start:

C₆H₁₄ + O₂ → CO₂ + H₂O

Balance carbon:

C₆H₁₄ + O₂ → 6CO₂ + H₂O

Balance hydrogen:

C₆H₁₄ + O₂ → 6CO₂ + 7H₂O

Oxygen required:

6CO₂ → 12 O

7H₂O → 7 O

Total:

19 O atoms

So:

C₆H₁₄ + 19/2O₂ → 6CO₂ + 7H₂O

Multiply by 2:

2C₆H₁₄ + 19O₂ → 12CO₂ + 14H₂O

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Worked Example 1: Identify the Reaction

Methane reacts with oxygen and releases heat.

What type of reaction is this?

Combustion

If sufficient oxygen is available and the products are CO₂ and H₂O, it is:

complete combustion

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Worked Example 2: Identify the Products

What are the products of complete combustion of propane?

Propane is a hydrocarbon.

Complete hydrocarbon combustion produces:

carbon dioxide + water

Therefore:

C₃H₈ + O₂ → CO₂ + H₂O

Balanced:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

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Worked Example 3: Balance Methane Combustion

Unbalanced:

CH₄ + O₂ → CO₂ + H₂O

Balanced:

CH₄ + 2O₂ → CO₂ + 2H₂O

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Worked Example 4: Balance Ethane Combustion

Unbalanced:

C₂H₆ + O₂ → CO₂ + H₂O

Balanced:

2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O

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Worked Example 5: Complete or Incomplete?

A hydrocarbon flame produces visible soot.

This suggests:

incomplete combustion

because some carbon has not been completely oxidized to carbon dioxide.

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Worked Example 6: Carbon Monoxide

Why might a poorly ventilated fuel-burning appliance be dangerous?

Limited oxygen can cause:

incomplete combustion

which can produce:

carbon monoxide

Carbon monoxide interferes with oxygen transport in the blood.

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Worked Example 7: Environmental Impact

A natural-gas heater burns methane completely.

Does this mean it has no environmental impact?

No.

Complete combustion produces:

CO₂

Carbon dioxide is a greenhouse gas.

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Worked Example 8: Conservation of Mass

Consider:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

Count the atoms.

Reactants:

C = 3

H = 8

O = 10

Products:

C = 3

H = 8

O = 10

The equation obeys the:

law of conservation of mass

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Worked Example 9: Incomplete Combustion

Balance:

CH₄ + O₂ → CO + H₂O

Start by balancing carbon and hydrogen:

CH₄ + O₂ → CO + 2H₂O

There are 3 oxygen atoms on the right.

Use:

3/2O₂

Then multiply everything by 2:

2CH₄ + 3O₂ → 2CO + 4H₂O

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Worked Example 10: Pentane Combustion

Write the balanced complete-combustion equation for pentane.

Pentane:

C₅H₁₂

Start:

C₅H₁₂ + O₂ → CO₂ + H₂O

Balance C:

C₅H₁₂ + O₂ → 5CO₂ + H₂O

Balance H:

C₅H₁₂ + O₂ → 5CO₂ + 6H₂O

Balance O:

Right side contains:

10 + 6 = 16 oxygen atoms

Therefore:

8O₂

Final:

C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O

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Predicting Complete Combustion Products

For any alkane:

Step 1: Recognize that the fuel contains carbon and hydrogen.

Step 2: Check that sufficient oxygen is available.

Step 3: Carbon becomes:

CO₂

Step 4: Hydrogen becomes:

H₂O

Therefore:

alkane + oxygen → carbon dioxide + water

Then balance the equation.

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A Reliable Combustion Equation Strategy

Step 1: Write the correct formula for the fuel.

Step 2: Add O₂ as a reactant.

Step 3: For complete combustion, write CO₂ and H₂O as products.

Step 4: Balance carbon atoms first.

Step 5: Balance hydrogen atoms second.

Step 6: Balance oxygen atoms last.

Step 7: If a fractional O₂ coefficient appears, multiply the entire equation to obtain whole-number coefficients when required.

Step 8: Count every atom on both sides to check the equation.

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Complete vs Incomplete Combustion: Cause and Effect

Sufficient oxygen

↓

Complete combustion

↓

CO₂ + H₂O

↓

More complete release of chemical energy

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Limited oxygen

↓

Incomplete combustion

↓

CO and/or C + H₂O, often with some CO₂

↓

Less complete energy release + additional harmful pollutants

This cause-and-effect relationship is important for understanding both chemistry and environmental impacts.

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Environmental Impacts Summary

Combustion can affect the environment in several ways.

Carbon dioxide

• greenhouse gas
• contributes to climate change when atmospheric concentrations increase

Carbon monoxide

• toxic to humans and other animals
• associated with incomplete combustion

Soot and particulate matter

• reduce air quality
• can harm respiratory and cardiovascular health
• black carbon can contribute to warming

Nitrogen oxides

• contribute to smog
• contribute to acid deposition
• affect air quality

Sulfur dioxide

• may form from sulfur-containing fuels
• contributes to acid deposition
• affects air quality

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Reducing the Environmental Impact of Combustion

Several approaches can reduce combustion-related environmental impacts:

• improving energy efficiency
• reducing unnecessary fuel use
• ensuring sufficient oxygen for efficient combustion
• maintaining engines and combustion equipment
• using catalytic converters and pollution-control systems
• reducing sulfur in fuels
• using lower-carbon energy sources where appropriate
• replacing some combustion-based processes with renewable electricity
• using public transportation or other efficient transport systems
• developing cleaner industrial technologies

Different approaches target different pollutants, so there is no single solution to every environmental impact of combustion.

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Common Mistakes

Mistake 1: Saying combustion means burning without mentioning oxygen

Combustion involves reaction with:

oxygen

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Mistake 2: Saying complete combustion produces carbon monoxide

Complete hydrocarbon combustion produces:

CO₂ + H₂O

Carbon monoxide is associated with:

incomplete combustion

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Mistake 3: Forgetting water

Hydrocarbons contain hydrogen.

During combustion, hydrogen forms:

H₂O

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Mistake 4: Changing subscripts when balancing equations

Never change:

CH₄

into:

CH₂

to balance an equation.

Changing a subscript changes the substance.

Change coefficients, not chemical formulas.

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Mistake 5: Balancing oxygen first

For hydrocarbon combustion, it is usually easier to balance:

carbon → hydrogen → oxygen

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Mistake 6: Thinking a yellow flame always proves exactly which products are present

A luminous or sooty flame can indicate incomplete combustion, but actual combustion products depend on the fuel and conditions.

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Mistake 7: Saying complete combustion has no environmental effects

Complete hydrocarbon combustion still produces:

carbon dioxide

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Mistake 8: Saying pure alkane combustion directly produces sulfur dioxide

Pure alkanes contain only carbon and hydrogen.

Sulfur dioxide comes from burning sulfur-containing material.

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Mistake 9: Thinking carbon monoxide can be detected by smell

Carbon monoxide is:

colourless and odourless

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Mistake 10: Forgetting conservation of mass

Every balanced chemical equation must contain the same number of each type of atom on both sides.

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Did You Know?

Combustion has powered human technology for thousands of years, from early fires to modern engines and power stations.

However, the large-scale combustion of fossil fuels has also created major environmental challenges.

Understanding combustion chemistry helps scientists and engineers design systems that:

• use fuel more efficiently
• produce fewer harmful pollutants
• reduce greenhouse-gas emissions
• recover more useful energy
• transition toward alternative energy technologies

Combustion chemistry therefore connects molecular reactions with some of the largest energy and environmental issues faced by society.

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Key Terms

• Combustion: Reaction of a substance with oxygen that releases energy.
• Fuel: Substance used to release useful energy.
• Hydrocarbon: Compound containing only carbon and hydrogen.
• Complete combustion: Combustion with sufficient oxygen.
• Incomplete combustion: Combustion occurring when oxygen is insufficient for complete oxidation.
• Exothermic: Describes a reaction that transfers energy to the surroundings.
• Carbon dioxide (CO₂): Product of complete hydrocarbon combustion and a greenhouse gas.
• Carbon monoxide (CO): Toxic gas that may form during incomplete combustion.
• Soot: Carbon-rich particulate material that can form during incomplete combustion.
• Particulate matter: Tiny solid or liquid particles suspended in air.
• Greenhouse gas: Atmospheric gas that absorbs and re-emits infrared radiation.
• Nitrogen oxides (NOₓ): Pollutants that can form during high-temperature combustion.
• Sulfur dioxide (SO₂): Pollutant produced when sulfur-containing material burns.
• Catalytic converter: Device that reduces several harmful vehicle exhaust pollutants.
• Fossil fuel: Carbon-rich fuel formed from ancient biological material over geological time.
• Conservation of mass: Principle that atoms are rearranged but not created or destroyed in chemical reactions.

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Key Equations

General complete hydrocarbon combustion:

hydrocarbon + oxygen → carbon dioxide + water

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Methane:

CH₄ + 2O₂ → CO₂ + 2H₂O

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Ethane:

2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O

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Propane:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O

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Butane:

2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O

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Pentane:

C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O

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Example incomplete methane combustion producing CO:

2CH₄ + 3O₂ → 2CO + 4H₂O

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Key Takeaways

• Combustion is a reaction between a fuel and oxygen.
• Combustion reactions are generally exothermic and release energy.
• Alkanes are important hydrocarbon fuels.
• Complete combustion occurs when sufficient oxygen is available.
• Complete combustion of hydrocarbons produces carbon dioxide and water.
• Incomplete combustion occurs when oxygen is limited.
• Incomplete combustion may produce carbon monoxide, carbon/soot, water, and some carbon dioxide.
• Carbon monoxide is toxic because it interferes with oxygen transport in the blood.
• Soot and other particulate matter can reduce air quality and affect health.
• Combustion equations must obey the law of conservation of mass.
• A reliable balancing order is carbon → hydrogen → oxygen.
• Coefficients may be changed when balancing equations; chemical subscripts should not be changed.
• Carbon dioxide from fossil-fuel combustion contributes to increased atmospheric greenhouse-gas concentrations.
• High-temperature combustion can produce nitrogen oxides.
• Sulfur-containing fuels can produce sulfur dioxide.
• Complete combustion reduces CO and soot formation but does not eliminate the environmental effects of burning fossil fuels.
• Understanding combustion connects chemical equations with energy production, air quality, climate, transportation, and everyday fuel use.