6) Combustion – similar to decomposition reaction, except that oxygen and heat are required for it to occur.
Oxygen combines with a carbon compound to form water and CO2.
Exothermic (heat producing)
Napthalene burning: C10H8 + 12 O2 —> 10 CO2 + 4 H2O
Cellular respiration (body temp burning of sugar)
C6H12O6 + 6O2 6CO2 + 6H2O + [energy stored in ATP]
Combustion of methane gas: CH4 + 2O2 CO2 + 2H2O
Hydrocarbons can combust (burn in the presence of oxygen)
There are many combustible fuels
Burning oil, paper or wood is almost never complete. There is always incomplete combustion.
Incomplete versus complete combustion
As such, one ends up with millions of different complex carbon molecules, collectively called “soot”.
Soot from a smokestack
Soot, or ash (product of fire, incineration or combustion)
Soot molecules formed from smaller molecules, like PAH
PAH = polycyclic aromatic hydrocarbons.
Organic compounds containing only C and H, and multiple aromatic rings.
Other forms of soot molecules
How do these reactions occur?
Single-replacement reactions, and combustion reactions, occur when one of the compounds oxidizes (loses electrons), and another compount is reduced (gains electrons.) These are called ReDox reactions and are studied in electrochemistry: ReDox reactions
What is fire made of?
This depends on the type of fuel involved in the combustion
Example: Burning candle.
The applied heat causes the fuel molecules in the candle wax to vaporize. In this state they can then readily react with oxygen in the air, which gives off enough heat in the subsequent exothermic reaction to vaporize yet more fuel, thus sustaining a consistent flame. The high temperature of the flame causes the vaporized fuel molecules to decompose, forming various incomplete combustion products and free radicals, and these products then react with each other and with the oxidizer involved in the reaction. Sufficient energy in the flame will excite the electrons in some of the transient reaction intermediates such as:
Methylidyne radical (CH)
Diatomic carbon (C2)
In most fires, most of the glowing gas is heated:
– Flame. (2016, November 22). In Wikipedia, The Free Encyclopedia
Is fire a plasma?
accorsing to this common explanation, “Yes”… but there is a reason to believe that this explanation is mistaken. Can flame be “Plasma”? – Arrow of Time
All gases have some tiny amount of ionization, so that in of itself doesn’t qualify any gas to be called a plasma.
See this further discussion https://www.quora.com/What-is-fire-What-are-the-physics-of-fire
Color of flames
2016 Massachusetts Science and Technology/Engineering Curriculum Framework
8.MS-PS1-2. Analyze and interpret data on the properties of substances before and after the substances interact to determine if a chemical reaction has occurred.
HS-PS1-2. Use the periodic table model to predict and design simple reactions that result in two main classes of binary compounds, ionic and molecular. Develop an explanation
based on given observational data and the electronegativity model about the relative
strengths of ionic or covalent bonds.
• Simple reactions include synthesis (combination), decomposition, single
displacement, double displacement, and combustion