Chemistry of combustion
Introduction:-
The relation between man and fire is very old. fire has been of great assistance to the mankind. it is one of the five basic elements of human body. If fire is taken away the life is no more. Fire is an essential element for survival provided it is is under control, neither less not more.
At the same time fire becomes destructive when it goes out of control. It is is this aspect of fire witch is is discussed in the book
2.0 fire triangle
Fire is an external sign of chemical reaction, usually the combination of carbon and or hydrogen with oxygen, resulting in the release of energy in the form of heat, light and sound.
- Hence there are three basic elements required for fire. These are
2. A source of ignition (heat)
3. The presence of oxygen (Air)
These three elements can be e represented by by a triangle popularly known as "fire triangle".
3.0 fire tetrahedron or fire square
Besides three basic elements i.e.; fuel, heat and oxygen (Air) there is a fourth element which is essential for continuation of fire, once ignited. This is is known as the "chain reaction".
Q:- what is chain reaction
Ans:- once a fuel is heated in the presence of air or oxygen the atoms of fuel are liberated. Mostly these atoms are of carbon and hydrogen. Most of these atoms are highly unstable as well as reactive. They are react with the oxygen atoms molecules of air. Because of these reaction i.e.; the oxidation reaction, being exothermic in nature, heat is is produced. This heat raises the temperature of fuel by radiation and liberates more reactive and unstable atoms of carbon and hydrogen. These again react with oxygen of air. Thus this process of liberation of carbon, hydrogen atoms and their reaction with atmospheric oxygen continuous as long as fuel is available and oxygen or air is present. This is called a chain reaction. Fire cannot continuous if chain reaction is broken. The sequence of chain reaction that it takes place is is illustrated as follows, in case of combustion of hydrogen atom.
- The chemical reaction of hydrogen occurs because its atoms are very reactive. Being unstable they quickly combined with other atoms. Adding the single hydrogen atom to the mixture of hydrogen and oxygen lead to a series of chemical reaction.
- The single hydrogen atom (H) reacts with a molecule of oxygen (O2), producing a molecule of hydroxide radical (OH) and an atom of Oxygen (O).
Both OH & O are highly reactive.
H + O2>OH+O
- The OH radical reacts rapidly with a hydrogen molecule producing water H2O and another hydrogen atom (H). The oxygen atom liberated in step 2 reacts with hydrogen molecule (H2) producing a new OH and H radicals.
O+H2>OH+H
- The new OH molecule reacts with H2 molecule producing another molecule of water (H2O) and yet another hydrogen atom (H)
- The hydrogen atoms liberated by this reaction quickly react with O2 molecules to produce several H atoms and the process repeats in cycle order.
This is known as chain reaction.
Hence fire now can be represented as a tetrahedron the four faces of which indicate
1. Fuel
2. Heat
3. Oxygen (Air)
4. Chain reaction.
It can also be represented as four sides of a square to indicate 1. fuel 2.heat 3.oxygen (Air) 4.chain reaction.
Fuels:-
A matter which can readily burn is called fuel. It can be in any of the following physical States.
1.solid fuels
2. Liquid fuels
3. Gaseous fuels
1. Solid fuels:- ordinary carbonaceous solid fuels such as wood, paper textiles etc.
Solid fuels are further categorized as follows:
2. Liquid fuels:- kerosene oil, petrol, diesel oil are the example of liquid fuels.
3. Gaseous fuels:- hydrogen, acetylene, LPG are the example of gaseous fuels.
5.0 AIR:-
Atmospheric air is composed of two main gases, oxygen and nitrogen. Other gases such as CO2, helium, etc are negligible in proportion. If we ignore presence of minor gases we can say that air contains 21% Oxygen and 79% nitrogen. Oxygen is a a non flammable gas but it is a good supporter of the combustion process.
Minimum percentage of oxygen in air required to support combustion is 16 % the rate of combustion increases with the increase in the percentage of oxygen in air.
6.0:- HEAT:-
A. Definition:- Heat is a form of energy
B. Units of measurement:-
- Calory: the amount of heat required to raise the temperature of 1 gram of water by 1 degree centigrade is equal to 1 calorie.
- BTU (British thermal unit): the amount of heat required to raise the temperature of one pound of water by 1 degree. Fahrenheit is equal to 1 BTU.
C. Effects of heat:
- It raises temperature
- It expands all matter
- It changes color
- It provides thermo couple effects
- It changes the physical state of matter.
- Specific heat: imagine a stool of Steel and a stool of wood are lying In sun for a few hours. The Steel stool gets more hot as a compared to the wooden stool, why?
Definition:
Specific heat is the amount of heat required to raise the temperature of 1 gram of any substance by 1 degree centigrade.
D. Latent heat:
The meaning of the word latent is hidden. When a substance is heated its physical States changes at a certain temperature, from solid to liquid and liquid to gas (vapor). During change of state the temperature remains constant. For example water begins to change to vapor or gas at 100 degrees Celsius. This is the boiling point of water. This temperature remains constant at 100 degree Celsius till the entire quantity of water is converted to vapor. The heat given to water during this period of time is hidden as there is no rise in temperature. Hence the quantity of heat given to each gram of water to convert it from liquid state to vapor state is called the the latent heat of vaporization of water. This is approximately 540 calories per gram of water.
Similarly when water is cooled to 0° Celsius begins to convert into solid (ice) state. The temperature remains 0 degree Celsius till the entire amount of water is converted into ice. The quantity of hear removed from water after reaching 0 degree Celsius for each gram is the latent heat freezing of water. This is approximately 80 calories per gram.
F. Transmission of heat:
Most fire start Propagate and extinguish due to the transmission of heat. there are 3 modes by which heat travels. they are:
- Conduction: in conduction heat travels through the molecules of the substance without the movement of the molecules themselves. Solids are generally hated in this way. There are good or bad conductors of heat. The ability to conduct heat varies from material to material. There are materials such as iron Rod, copper wire which conducts heat with much ease. These are known as good conductors of heat.
At the same time there are materials such as wood, plastic etc. Which conduct heat with great difficulty or too slowly. These are known as bad or poor conductors of heat.
- Convection: only liquid and gases use this way for transfer of heat. Liquids and gases are termed as"Fluids" also. When the fluid is heated, the heated molecules expand and hands become light and move upward. To take their place the cold molecules come in. These to move up to make place for the cold molecules. This process continuous in circulation till a uniform temperature is reached throughout the fluid. This way of heat transfer by the actual movement of the molecules is called convection. In fire situation in a building convection currents can convey hot gases, produced by combustion, upwards throw stairways open lift shafts etc, thereby spreading fire to the upper floors. Intern fresh air is drawn in through doors or windows which help fire to burn vigorously.
- Radiation: in this way heat travels without any help of the medium. Radiated heat does not heat the intervening medium appreciably. Heat travels in in straight lines and is known radiant heat. This process of heat transfer is called "radiation". Transmission of heat from sun is by radiation.
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