Combustion Reactions: Metals, Non-Metals, and Organic Compounds
- Combustion reactions are a cornerstone of chemical processes that power modern life.
- From igniting a match to fueling vehicles, these reactions release energy through the interaction of substances with oxygen.
- But how do combustion reactions vary for metals, non-metals, and organic compounds?
What Is Combustion?
Combustion
Combustion is a chemical process where a substance, known as the fuel, reacts with oxygen (O₂), releasing energy in the form of heat and light.
This reaction requires three essential components:
- Fuel: The material undergoing combustion.
- Oxygen: The reactant that facilitates the reaction.
- Activation Energy: An initial input of energy, such as a spark or flame, to initiate the reaction.
The products of combustion depend on the type of fuel and the oxygen supply:
Complete combustion
Complete combustion occurs with excess oxygen, producing carbon dioxide (CO₂) and water (H₂O).
Incomplete combustion
Incomplete combustion happens with limited oxygen, resulting in byproducts like carbon monoxide (CO) and soot (C).
Combustion of Reactive Metals
- Reactive metals like lithium and magnesium combust readily in oxygen to form metal oxides.
- These reactions are examples of redox reactions, where the metal is oxidized (loses electrons) and oxygen is reduced (gains electrons).
$$\text{metal} + \text{oxygen} \rightarrow \text{metal oxide}$$
- Lithium Combustion:$$\text{4Li (s)} + \text{O}_2\text{(g)} \rightarrow 2\text{Li}_2\text{O (s)}$$
- Lithium burns with a red flame, forming lithium oxide.
- Magnesium Combustion:$$\text{2Mg (s)} + \text{O}_2\text{(g)} \rightarrow 2\text{MgO (s)}$$
- Magnesium burns with a bright white light, producing magnesium oxide.
Metal oxides formed during combustion are ionic compounds, consisting of metal cations and oxide anions.
Combustion of Non-Metals
- Non-metals also undergo combustion, forming non-metal oxides.
- These reactions are significant in both natural processes and industrial applications.
$$\text{non-metal} + \text{oxygen} \rightarrow \text{non-metal oxide}$$
Sulfur Combustion
- Sulfur combusts in oxygen to form sulfur dioxide (SO₂):$$\text{S (s)} + \text{O}_2\text{(g)} \rightarrow \text{SO}_2\text{(g)}$$
- In the atmosphere, sulfur dioxide can oxidize further to form sulfur trioxide (SO₃), which reacts with water to produce sulfuric acid (H₂SO₄).
- This sequence contributes to acid rain.
Non-metal oxides such as SO₂ and CO₂ are covalent compounds, not ionic. Avoid assuming they form ionic bonds.
Combustion of Organic Compounds
Organic compounds, particularly hydrocarbons, release substantial energy during combustion, making them vital for energy production.
Complete Combustion of Hydrocarbons
Hydrocarbons, composed of carbon and hydrogen, combust fully in oxygen to produce carbon dioxide and water.
$$\text{hydrocarbon} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$$
Methane (CH₄), a primary component of natural gas, combusts as follows:
$$\text{CH}_4\text{(g)} + 2\text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} + 2\text{H}_2\text{O (g)}$$
Write the balanced equation for the complete combustion of propane (C₃H₈).
Solution
$$\text{C}_3\text{H}_8\text{(g)} + 5\text{O}_2\text{(g)} \rightarrow 3\text{CO}_2\text{(g)} + 4\text{H}_2\text{O (g)}$$
Combustion of Alcohols
Alcohols, such as ethanol (C₂H₅OH), combust to produce carbon dioxide and water.
Ethanol Combustion
$$\text{C}_2\text{H}_5\text{OH (l)} + 3\text{O}_2\text{(g)} \rightarrow 2\text{CO}_2\text{(g)} + 3\text{H}_2\text{O (l)}$$
Ethanol is considered carbon-neutral because the CO₂ released during combustion is offset by the CO₂ absorbed by plants during photosynthesis.
The Role of Activation Energy
- Combustion reactions require an initial input of energy, known as activation energy, to begin.
- This energy is necessary to break bonds in the reactants, allowing the reaction to proceed.
High activation energy prevents spontaneous combustion, ensuring fuels can be stored and transported safely without igniting unintentionally.
Oxidizing and Reducing Agents in Combustion
Combustion is fundamentally a redox reaction:
- Oxidizing agent: Oxygen (O₂), which gains electrons and is reduced.
- Reducing agent: The fuel, which loses electrons and is oxidized.
Redox reactions will covered in Reactivity 3.2.
In methane combustion:
- Methane (CH₄) acts as the reducing agent, with carbon oxidized from -4 in CH₄ to +4 in CO₂.
- Oxygen (O₂) serves as the oxidizing agent, reduced from 0 in O₂ to -2 in H₂O and CO₂.
