Energy from fuels: R1.3.1 Combustion fuels IB DP Chemistry Study Notes - New Syllabus 2025

Reactivity 1.3.1 — Combustion Reactions of Reactive Substances

Combustion of Reactive Substances

• Combustion is a rapid exothermic chemical reaction in which a substance reacts with oxygen to release heat and often light. It is a specific type of oxidation reaction where the oxidizing agent is oxygen.
• Combustion plays a crucial role in diverse contexts such as energy production (e.g., burning fuels), industrial synthesis, heating, engines, and biological systems like cellular respiration.
• The combustion process typically involves the breaking of chemical bonds in the reactant and the formation of new bonds in the products.
• The net result is the release of energy, primarily in the form of heat and sometimes visible light.
• Combustion always results in an increase in the temperature of the surroundings and is used widely for practical energy applications.

Types of Combustion

• Complete Combustion: Takes place in the presence of excess oxygen. The products are carbon dioxide (CO₂) and water (H₂O). It releases the maximum possible amount of energy from the fuel.
  Example: \( \text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O} \)
• Incomplete Combustion: Occurs when oxygen is limited. The products may include carbon monoxide (CO), carbon (soot), and water. This process releases less energy and produces toxic byproducts.
  Example: \( 2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O} \)

The distinction between complete and incomplete combustion is critical for understanding pollution, fuel efficiency, and safety hazards such as carbon monoxide poisoning.

Key Features of Combustion:

• Exothermic: Always releases heat energy.
• Oxidation process: Involves transfer of electrons; the substance combusting is oxidized.
• Activation energy required: Most combustion reactions require an initial energy input (e.g., a spark or heat source) to overcome the activation energy barrier. However, some highly reactive substances (such as sodium, potassium, or pyrophoric materials) can ignite spontaneously in air without an external trigger.

Combustion of different types of substances—metals, non-metals, hydrocarbons, and alcohols—shows variations in the type of oxides formed and the nature of the reaction, which are explored in the sections below.

Combustion of Metals

• Not all metals burn easily, but when they do, they typically react with oxygen to form ionic metal oxides. The ability to combust depends on the reactivity of the metal and physical form (powdered metals burn more readily due to higher surface area).
  • Highly reactive metals (e.g., Na, K, Mg): Combust vigorously with bright flames and form basic oxides.
  • Less reactive metals (e.g., Fe, Cu): Do not burn easily but oxidize slowly in air to form oxides without flame.

Example Reaction: \( 2\text{Mg}(s) + \text{O}_2(g) \rightarrow 2\text{MgO}(s) \)

Combustion of Non-Metals

• Non-metals, particularly those in Groups 14 to 16 (e.g., C, S, P), react with oxygen to form acidic oxides. These reactions are usually less energetic than metal combustion but have significant environmental implications.

Example: \( \text{S}(s) + \text{O}_2(g) \rightarrow \text{SO}_2(g) \)

• Other oxides formed: CO₂, SO₃, NO₂, P₄O₁₀

These oxides often dissolve in water to form acids such as sulfuric acid or nitric acid, contributing to acid rain. They are covalent and acidic in nature.

Combustion of Organic Compounds

Organic compounds, such as hydrocarbons and alcohols, are highly combustible due to their carbon-hydrogen structure. The combustion of these compounds is a major source of energy in industry and transportation.

Hydrocarbons:

• Composed entirely of carbon and hydrogen atoms.
• Undergo complete combustion to yield \( \text{CO}_2 \) and \( \text{H}_2\text{O} \).
• Incomplete combustion occurs when oxygen is insufficient, producing CO or soot (C).

Example: \( \text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O} \)

Alcohols:

• Contain carbon, hydrogen, and oxygen atoms (e.g., ethanol, methanol).
• Combust to form \( \text{CO}_2 \) and \( \text{H}_2\text{O} \), similar to hydrocarbons but require slightly less \( \text{O}_2 \).

Example: \( \text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O} \)

Combustion of organic compounds is fundamental to fuel chemistry and is closely tied to global concerns such as energy sustainability and climate change due to greenhouse gas emissions.

Deduction of Combustion Equations

In IB Chemistry, you are expected to write balanced chemical equations for the combustion of elements and compounds, especially hydrocarbons and alcohols. The process involves identifying the chemical formula of the substance, determining the products (typically CO₂ and H₂O), and balancing the equation accordingly.

Steps to Deduce a Combustion Equation:

1. Write the correct molecular formula of the fuel (e.g., hydrocarbon or alcohol).
2. Identify the products: complete combustion produces carbon dioxide and water.
3. Balance carbon atoms first, then hydrogen atoms, and finally oxygen atoms.
4. If necessary, use fractional coefficients and multiply the whole equation to eliminate fractions.

These equations are essential for calculating enthalpy changes of combustion and understanding energy transfer in chemical reactions. They also appear in thermochemical calculations and in environmental chemistry to analyze the release of greenhouse gases.