Combustion: what it is, types, reaction and enthalpy

Combustion is an exothermic chemical reaction between two regents, fuel and oxidizer, in which energy is released in the form of heat.

• Fuel: It is the oxidizable substance, responsible for fueling combustion. Examples: gasoline, wood, cooking gas, alcohol and diesel.
• oxidizing: It is the substance that enhances combustion. In most cases, the oxidizer is oxygen O gas₂.

Combustion only takes place in the presence of fuel and oxidizer. Thus, it is only terminated or interrupted due to the lack of one of these reagents.

Combustion is very common in everyday life, it is present in the burning of cooking gas, automobile fuels, candles, wood, paper, among others.

Burning wood is an example of combustion

Types

Organic compounds have two types of combustion, complete and incomplete.

Complete combustion

Complete combustion is that which has enough oxygen to consume the fuel. It presents as products the CO₂ (Carbon dioxide) and the H₂O (Water).

Complete combustion has greater heat release.

Examples:

a) Complete combustion of ethanol (C₂H₆O):

Ç₂H₆O+O₂ → CO₂ + H₂O
When balancing the reaction:
Ç₂H₆O + 3 O₂ → 2 CO₂ + 3 H₂O

In this reaction, the amount of oxygen was enough to consume all the methanol and originate as products the CO_{2 and} H₂O.

b) Complete combustion of methane (CH₄):

CH₄ + O₂ → CO₂ + H₂O
CH₄ + O₂ → CO₂ + 2 H₂O

Incomplete combustion

In incomplete combustion there is not enough oxygen to completely consume the fuel.

It has two types of products: the CO (Carbon monoxide) or soot (C), substances that are toxic to the environment and harmful to health.

Incomplete combustion has less heat release.

Examples:

a) Incomplete combustion of ethanol:

Ç₂H₆O + 2 O₂ → 2 CO + 3 H₂O = Production of CO and H₂O.

Ç₂H₆O+O₂ → 2 C + 3 H₂O = Production of soot and H₂O.

Note that between the two reactions there is a decrease in the amount of oxygen, this represents a smaller amount of heat released.

b) Incomplete combustion of methane:

CH₄ + 3/2 O₂ → CO + 2 H₂O
CH₄ + O₂ → C + 2 H₂O

Read too:

• Oxidation
• Chemical reactions
• Endothermic and exothermic reactions
• thermochemistry

Combustion enthalpy

The enthalpy (H) of combustion or heat of combustion is the energy released from burning 1 mole of fuel under standard state conditions (Temperature: 25 °C; Pressure: 1 atm).

Because combustion is an exothermic reaction, the enthalpy change (∆H) will always have a negative value.

The enthalpy of combustion can be calculated using the following formula:

ΔH = H_products - H_reagents

Spontaneous combustion

Spontaneous combustion is the one that takes place without the existence of an external flammable source.

This happens with some materials capable of accumulating a lot of heat inside them, increasing the speed of chemical reactions. This condition increases the temperature of the material until combustion takes place.

There is still evidence of spontaneous human combustion (CHE), in which the body combusts without any influence from external sources.

The first record of this case would have happened to a woman, while she was sleeping, in 1663. Other similar cases were also related to spontaneous human combustion.

However, science is still trying to understand how the process takes place in the human body. At the moment, there are only a few theories to explain the phenomenon.

See too: Fire