Combustion Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Combustion.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

A rapid exothermic chemical reaction in which a fuel reacts with oxygen gas, releasing large amounts of energy as heat and light.

Burning. When something burns, it's reacting with oxygen and releasing energy as heat and light.

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: Combustion starts by comparing the reactant-product pattern, charges, states, and conserved atoms.

Common stuck point: Students often know a formula related to combustion but skip the recognition step: Does the balanced equation match a recognizable pattern of reactants, products, ions, oxygen, or electron transfer? That leads to a correct-looking substitution attached to the wrong chemical model.

Sense of Study hint: Ask: Does the balanced equation match a recognizable pattern of reactants, products, ions, oxygen, or electron transfer?

Worked Examples

Example 1

easy
Write the balanced equation for the complete combustion of ethanol (C2H5OH\text{C}_2\text{H}_5\text{OH}).

Answer

C2H5OH+3O2→2CO2+3H2O\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}

First step

1
Complete combustion of a hydrocarbon/alcohol produces CO2\text{CO}_2 and H2O\text{H}_2\text{O}.

Full solution

  1. 2
    Unbalanced: C2H5OH+O2→CO2+H2O\text{C}_2\text{H}_5\text{OH} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}.
  2. 3
    Balance C: 2 CO2\text{CO}_2. Balance H: 6 H atoms → 3 H2O\text{H}_2\text{O}. Balance O: products have 2(2)+3(1)=72(2) + 3(1) = 7 O. Reactant has 1 O in ethanol, so need 6 more from O2\text{O}_2 → 3 O2\text{O}_2.
  3. 4
    C2H5OH+3O2→2CO2+3H2O\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}.
Combustion reactions are always exothermic. Complete combustion requires sufficient oxygen and produces only CO2\text{CO}_2 and H2O\text{H}_2\text{O}. Incomplete combustion produces CO or soot (C).

Example 2

medium
What is the difference between complete and incomplete combustion of methane (CH4\text{CH}_4)? Write equations for both.

Example 3

medium
Balance: C2H2+O2→CO2+H2O\text{C}_2\text{H}_2 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} (acetylene).

Example 4

hard
Balance: C6H12O6+O2→CO2+H2O\text{C}_6\text{H}_{12}\text{O}_6 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} (glucose).

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy
Balance the combustion of butane: C4H10+O2→CO2+H2O\text{C}_4\text{H}_{10} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}.

Example 2

hard
Ethanol (C2H5OH\text{C}_2\text{H}_5\text{OH}) is used as a fuel additive. Write the balanced equation for its complete combustion. If only limited oxygen is available, what products form instead (incomplete combustion)?

Example 3

easy
Classify: CH4+2O2→CO2+2H2OCH_4 + 2O_2 \rightarrow CO_2 + 2H_2O. What reaction type?

Example 4

easy
What two products form in complete combustion of a hydrocarbon?

Example 5

easy
Is O2O_2 always a reactant in combustion?

Example 6

easy
Balance: C+O2→CO2C + O_2 \rightarrow CO_2.

Example 7

easy
Complete combustion vs incomplete: what does incomplete combustion produce?

Example 8

easy
Complete: 2H2+O2→2H_2 + O_2 \rightarrow ? (combustion of hydrogen)

Example 9

easy
A fuel burns in oxygen, releasing heat and light. Name the reaction type.

Example 10

easy
Complete combustion of C2H6C_2H_6 gives which products?

Example 11

medium
Balance: CH4+O2→CO2+H2OCH_4 + O_2 \rightarrow CO_2 + H_2O.

Example 12

medium
Balance: C3H8+O2→CO2+H2OC_3H_8 + O_2 \rightarrow CO_2 + H_2O (propane).

Example 13

medium
Balance: C2H6+O2→CO2+H2OC_2H_6 + O_2 \rightarrow CO_2 + H_2O (ethane).

Example 14

medium
Balance the alcohol combustion: C2H5OH+O2→CO2+H2OC_2H_5OH + O_2 \rightarrow CO_2 + H_2O.

Example 15

medium
Why is incomplete combustion of CH4CH_4 dangerous?

Example 16

medium
Distinguish: is 2Mg+O2→2MgO2Mg + O_2 \rightarrow 2MgO combustion or synthesis?

Example 17

medium
Balance: C4H10+O2→CO2+H2OC_4H_{10} + O_2 \rightarrow CO_2 + H_2O (butane).

Example 18

medium
How many mol O2O_2 are needed to burn 2 mol CH4CH_4 completely?

Example 19

medium
Balance: C5H12+O2→CO2+H2OC_5H_{12} + O_2 \rightarrow CO_2 + H_2O (pentane).

Example 20

challenge
Burning 0.50 mol C3H8C_3H_8 completely. Mol CO2CO_2 produced and O2O_2 consumed?

Example 21

challenge
Complete combustion of 8.0 g CH4CH_4 (M=16) gives what mass of H2OH_2O (M=18)?

Example 22

challenge
A hydrocarbon CxHyC_xH_y produces 3 mol CO2CO_2 and 4 mol H2OH_2O per mol burned. Find the formula.

Example 23

easy
Complete combustion releases what two forms of energy?

Example 24

easy
Balance: S+O2→SO2S + O_2 \rightarrow SO_2.

Example 25

easy
Is 2Mg+O2→2MgO2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO} a combustion reaction?

Example 26

easy
Why is a blue flame on a gas stove preferred over a yellow flame?

Example 27

easy
Identify the fuel in C3H8+5O2→3CO2+4H2O\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}.

Example 28

medium
Balance: C6H14+O2→CO2+H2O\text{C}_6\text{H}_{14} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} (hexane).

Example 29

medium
Balance: C2H4+O2→CO2+H2O\text{C}_2\text{H}_4 + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} (ethene).

Example 30

medium
Balance: C3H7OH+O2→CO2+H2O\text{C}_3\text{H}_7\text{OH} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} (1-propanol).

Example 31

medium
Write the incomplete combustion of methane that produces CO instead of CO2_2, and balance it.

Example 32

medium
How many moles of O2_2 are needed to completely burn 3 mol of propane (C3_3H8_8)?

Example 33

medium
Burning 1 mol of CH4_4 produces how many mol of CO2_2 and how many of H2_2O?

Example 34

medium
True or false: combustion always involves a flame.

Example 35

medium
Balance: CH3OH+O2→CO2+H2O\text{CH}_3\text{OH} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} (methanol).

Example 36

medium
What gas detector should every home have to warn against incomplete combustion?

Example 37

hard
Burning 0.25 mol of octane (C8_8H18_{18}) completely: how many moles of CO2_2 form?

Example 38

hard
Complete combustion of 16 g of methane (M = 16 g/mol) requires what mass of O2_2 (M = 32 g/mol)?

Example 39

hard
Burning 0.10 mol of ethanol (C2_2H5_5OH) completely gives what mass of CO2_2 (M = 44 g/mol)?

Example 40

hard
A hydrocarbon produces 4 mol of CO2_2 and 5 mol of H2_2O per mol burned. Find its formula.

Example 41

hard
Burning 4.40 g of propane (M = 44 g/mol) completely. How much O2_2 (in mol) is needed?

Example 42

hard
Incomplete combustion of C3_3H8_8 may produce CO, C (soot), and H2_2O along with CO2_2. Why does this happen physically?

Example 43

hard
32 g of methane is burned completely. What mass of water (M = 18 g/mol) forms?

Example 44

challenge
1.00 mol of an unknown hydrocarbon Cx_xHy_y requires 6.5 mol O2_2 for complete combustion. If it forms 4 mol CO2_2, find xx and yy.
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Background Knowledge

These ideas may be useful before you work through the harder examples.

chemical reactionexothermic