Combustion Reactions
Quick Notes
- Combustion = chemical reaction with oxygen, usually producing heat and light.
- Complete combustion of organic compounds produces CO2 and H2O.
- Reactive metals form metal oxides when combusted.
- Some non-metals (e.g., S, C) also burn in oxygen to form non-metal oxides (e.g., SO2, CO2).
- General formula for hydrocarbon combustion:
CxHy + (x + y/4) O2 → xCO2 + y/2 H2O - Alcohols combust similarly also forming carbon dioxide and water
- Balanced symbol equations should reflect the stoichiometry of the reaction.
Full Notes
What Is Combustion?
Combustion is a high-temperature exothermic reaction between a substance and oxygen. It often results in the production of a flame, light, and the release of energy.
Combustion of Reactive Metals
When reactive metals (e.g., Na, Mg, Ca) burn in oxygen, they form metal oxides.
Examples:
4Na(s) + O2(g) → 2Na2O(s)
2Mg(s) + O2(g) → 2MgO(s)
These reactions are vigorous and produce bright white light (e.g., Mg).
Combustion of Non-Metals
Some non-metals such as carbon and sulfur also combust in oxygen.
Examples:
C(s) + O2(g) → CO2(g)
S(s) + O2(g) → SO2(g)
These reactions form acidic oxides, which can contribute to acid rain when dissolved in water.
Combustion of Hydrocarbons
Hydrocarbons (compounds made up of only carbon and hydrogen) undergo complete combustion in a good supply of oxygen:
General equation: CxHy + O2 → xCO2 + (y/2)H2O
Examples:
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
C2H6(g) + (7/2)O2(g) → 2CO2(g) + 3H2O(g)
Note: If oxygen is limited, incomplete combustion can occur, forming CO or C (soot) – see Complete & Incomplete Combustion for more detail.
Combustion of Alcohols
Alcohols combust similarly to hydrocarbons but contain an –OH group.
Example:
C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g)
Like hydrocarbons, alcohols release a large amount of energy when burned, making them useful fuels.
Writing and Balancing Combustion Equations
Steps:
- Write the unbalanced equation with reactants and likely products (CO2 and H2O for organics).
- Balance carbon atoms (by adding nCO2 for every n carbon atoms in the reactants).
- Balance hydrogen atoms (by adding 0.5nH2O for every n hydrogen atoms in the reactants).
- Balance oxygen atoms (add up all the oxygen atoms in the products).
- Use fractions for O2 if needed, then multiply all coefficients to get whole numbers.
Summary
- Combustion is a reaction with oxygen, releasing heat and light.
- Metals form metal oxides and non-metals form acidic oxides.
- Hydrocarbons and alcohols combust to give CO2 and H2O.
- Equations must be balanced to reflect stoichiometry.
- Oxygen is the oxidising agent and the fuel is the reducing agent in combustion.
Why is high activation energy often considered to be a useful property of a fuel?
A high activation energy means the fuel will not react easily at low temperatures or without a spark, making it more stable and safer to store and handle. It prevents uncontrolled combustion, reducing the risk of accidental ignition.
Which species are the oxidising and reducing agents in a combustion reaction?
In a combustion reaction, the oxidising agent is oxygen (O2) because it gains electrons and is reduced. The fuel (e.g., a hydrocarbon) is the reducing agent, as it loses electrons and is oxidised.
For example:
In the combustion of methane, carbon in CH4 goes from an oxidation number of −4 to +4 in CO2 → oxidation. Oxygen in O2 goes from 0 to −2 in H2O and CO2 → reduction.
This change in oxidation numbers shows that the fuel is oxidised and oxygen is reduced, confirming their roles as reducing and oxidising agents, respectively.