4.3.1Reactivity 1 — What drives chemical reactions?
Combustion Reactions — IB Chemistry SL Revision Notes & Topical Questions
5 min read 12 parts Lesson 6 of 8 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 12Complete Combustion
Fuels are substances burned to provide useful energy.
Definition
- Definition Complete combustion is a rapid exothermic reaction with plenty of oxygen, in which carbon is oxidised to CO₂ and hydrogen to H₂O.
Hydrocarbons
- Hydrocarbons Alkanes burn as in CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l), releasing heat and giving only carbon dioxide and water.
Alcohols
- Alcohols Alcohols also burn completely, for example C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l), with their own oxygen contributing to the reaction.
Observation
- Observation Complete combustion gives a blue, non-smoky flame, whereas a yellow, sooty flame shows that some carbon has not fully oxidised.
Section 2 of 12Balancing Combustion Equations
1
Write the fuel and oxygen on the left, and carbon dioxide and water on the right.
2
Balance carbon first by putting the number of carbon atoms of the fuel in front of CO₂.
3
Balance hydrogen second by halving the number of hydrogen atoms to find the coefficient of H₂O.
4
Balance oxygen last by counting all oxygen atoms in the products and subtracting any already present in the fuel, then halve for O₂.
5
Double all coefficients if a fraction is left, and then add state symbols.
Section 3 of 12Energy Released
Energy Released
Per mole
- Per mole Larger molecules release more energy per mole because they have more bonds to be converted into C=O and O–H bonds.
Homologous series
- Homologous series Within a series such as the alkanes, the enthalpy of combustion becomes more negative with every added CH₂ group.
Per gram
- Per gram Hydrocarbons generally release more energy per gram than alcohols because alcohols already contain oxygen.
Selection of fuels
- Selection of fuels Fuels are chosen for energy density, cost, ease of storage and the pollutants produced.
Section 4 of 12Other Combustion
Other Combustion
Metals
- Metals Reactive metals such as magnesium burn in oxygen to form oxides, as in 2Mg(s) + O₂(g) → 2MgO(s), with bright flames.
Hydrogen
- Hydrogen Hydrogen burns to form only water, so no carbon dioxide is produced at the point of use.
Activation energy
- Activation energy A flame or spark is needed to start combustion, but the heat released then sustains the reaction.
Fire triangle
- Fire triangle Combustion needs fuel, oxygen and heat; removing any of these puts out a fire.
Section 5 of 12Worked Example
Question Write the balanced equation for the complete combustion of butan-1-ol, C₄H₉OH(l).
- Butan-1-ol is C₄H₁₀O, so 4 carbons gives 4CO₂ and 10 hydrogens gives 5H₂O.
- Oxygen atoms needed in the products = 8 + 5 = 13, and one of them comes from the alcohol itself.
- Oxygen atoms needed from O₂ = 12, so 6O₂ are required.
- Add state symbols.
Answer C₄H₉OH(l) + 6O₂(g) → 4CO₂(g) + 5H₂O(l)
Section 6 of 12Exam Checklist & Tips
4.3.1 – Key Points for the Exam
- Combustion A rapid exothermic reaction of a substance with oxygen.
- Complete combustion Combustion with excess oxygen giving carbon dioxide and water as the only products of an organic fuel.
- Fuel A substance burned to release useful energy.
- Enthalpy of combustion The enthalpy change when one mole of a substance burns completely in oxygen.
Common Mistakes
- Balancing oxygen before carbon and hydrogen, which makes errors likely.
- Forgetting that an alcohol supplies one oxygen atom of its own.
- Writing CO as a product of complete combustion.
- Leaving out state symbols or using H₂O(g) when the data refer to liquid water.
Section 7 of 12Complete Combustion
Fuels are substances burned to provide useful energy.
Definition
- Definition Complete combustion is a rapid exothermic reaction with plenty of oxygen, in which carbon is oxidised to CO₂ and hydrogen to H₂O.
Hydrocarbons
- Hydrocarbons Alkanes burn as in CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l), releasing heat and giving only carbon dioxide and water.
Alcohols
- Alcohols Alcohols also burn completely, for example C₂H₅OH(l) + 3O₂(g) → 2CO₂(g) + 3H₂O(l), with their own oxygen contributing to the reaction.
Observation
- Observation Complete combustion gives a blue, non-smoky flame, whereas a yellow, sooty flame shows that some carbon has not fully oxidised.
Section 8 of 12Balancing Combustion Equations
1
Write the fuel and oxygen on the left, and carbon dioxide and water on the right.
2
Balance carbon first by putting the number of carbon atoms of the fuel in front of CO₂.
3
Balance hydrogen second by halving the number of hydrogen atoms to find the coefficient of H₂O.
4
Balance oxygen last by counting all oxygen atoms in the products and subtracting any already present in the fuel, then halve for O₂.
5
Double all coefficients if a fraction is left, and then add state symbols.
Section 9 of 12Energy Released
Energy Released
Per mole
- Per mole Larger molecules release more energy per mole because they have more bonds to be converted into C=O and O–H bonds.
Homologous series
- Homologous series Within a series such as the alkanes, the enthalpy of combustion becomes more negative with every added CH₂ group.
Per gram
- Per gram Hydrocarbons generally release more energy per gram than alcohols because alcohols already contain oxygen.
Selection of fuels
- Selection of fuels Fuels are chosen for energy density, cost, ease of storage and the pollutants produced.
Section 10 of 12Other Combustion
Other Combustion
Metals
- Metals Reactive metals such as magnesium burn in oxygen to form oxides, as in 2Mg(s) + O₂(g) → 2MgO(s), with bright flames.
Hydrogen
- Hydrogen Hydrogen burns to form only water, so no carbon dioxide is produced at the point of use.
Activation energy
- Activation energy A flame or spark is needed to start combustion, but the heat released then sustains the reaction.
Fire triangle
- Fire triangle Combustion needs fuel, oxygen and heat; removing any of these puts out a fire.
Section 11 of 12Worked Example
Question Write the balanced equation for the complete combustion of butan-1-ol, C₄H₉OH(l).
- Butan-1-ol is C₄H₁₀O, so 4 carbons gives 4CO₂ and 10 hydrogens gives 5H₂O.
- Oxygen atoms needed in the products = 8 + 5 = 13, and one of them comes from the alcohol itself.
- Oxygen atoms needed from O₂ = 12, so 6O₂ are required.
- Add state symbols.
Answer C₄H₉OH(l) + 6O₂(g) → 4CO₂(g) + 5H₂O(l)
Section 12 of 12Exam Checklist & Tips
4.3.1 – Key Points for the Exam
- Combustion A rapid exothermic reaction of a substance with oxygen.
- Complete combustion Combustion with excess oxygen giving carbon dioxide and water as the only products of an organic fuel.
- Fuel A substance burned to release useful energy.
- Enthalpy of combustion The enthalpy change when one mole of a substance burns completely in oxygen.
Common Mistakes
- Balancing oxygen before carbon and hydrogen, which makes errors likely.
- Forgetting that an alcohol supplies one oxygen atom of its own.
- Writing CO as a product of complete combustion.
- Leaving out state symbols or using H₂O(g) when the data refer to liquid water.
Last updated:
\n