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2 Answers

Nishant Vora IIT Patna
askIITians Faculty 2467 Points
6 years ago
A flame is just hot gas due to combustion so it doesn't really have a chemical formula. However, a lighter burns butane (I think) so the formula for the burning of that is:
Saurabh Kumar
askIITians Faculty 2411 Points
6 years ago
Fire cannot have a scientific formula, it is not a substance; it is a chemical reaction. Fire is very fast oxidization of a substance (usally). An example of a real and unfortunatly tragic event proving this is the deaths of astronauts in the 1950's when the velcro (not flammable in normal conditions) burned violently in the pure oxygen enviroment of the capsule they were in.

Combustion or burning is a complex sequence of chemical reactions between a fuel and an oxidant accompanied by the production of heat or both heat and light in the form of either a glow or flames.

Since not every oxidation process results in the production of heat (for example, corrosion), the term combustion can only be applied to exothermic processes that occur at a rate fast enough to produce heat.

In a complete combustion reaction, a compound reacts with an oxidizing element, and the products are compounds of each element in the fuel with the oxidizing element. For example:

CH_4 + 2O_2 \rightarrow \; CO_2 + 2H_2O + heat
CH_2S + 6F_2 \rightarrow \; CF_4 + 2HF + SF_6 + heat

In most cases, combustion uses oxygen (O2) obtained from the ambient air, which can be taken as 21 mole percent oxygen and 79 mole percent nitrogen (N2). Thus, when methane (CH4) is combusted using air as the oxygen source, the first example equation above becomes:

CH_4 + 2O_2 + 7.52N_2 \rightarrow \; CO_2 + 2H_2O + 7.52 N_2 + heat

As can be seen, when air is the source of the oxygen, nitrogen is by far the largest part of the products of combustion.

Chemical equation

Generally, the chemical equation for burning a hydrocarbon (such as octane) in oxygen is as follows:

C_xH_y + (x + \frac{y}{4})O_2 \rightarrow \; xCO_2 + (\frac{y}{2})H_2O

For example, the burning of propane is:

C_3H_8 + 5O_2 \rightarrow \; 3CO_2 + 4H_2O

The simple word equation for the combustion of a hydrocarbon in oxygen is:

Fuel + Oxygen \rightarrow \; Heat + Water + Carbon\ dioxide

If the combustion takes place using air as the oxygen source, the corresponding equations are:

C_xH_y + (x+ \frac{y}{4})O_2 + 3.76(x+ \frac{y}{4})N_2 \rightarrow \; xCO_2 + (\frac{y}{2})H_2O + 3.76(x + \frac{y}{4})N_2

For example, the burning of propane is:

C_3H_8 + 5O_2 + 18.8N_2 \rightarrow \; 3CO_2 + 4H_2O + 18.8N_2

The simple word equation for the combustion of a hydrocarbon in air is:

Fuel + Oxygen + Nitrogen \rightarrow \; Heat + Water + Carbon\ dioxide + Nitrogen

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