Combustion of Hydrocarbons - Class XI

When most fuels burn, the products include carbon dioxide and water.
Fuels are hydrocarbons. Combustion of hydrocarbons releases carbon dioxide and water.
$\displaystyle  CH _4 + 2O _2 \rightarrow CO _2 + 2H _2O$

Petrol engine gives off gaseous oxides of nitrogen.
During combustion of fuel at high temperature, nitrogen and oxygen of air combine to form $NO$ and $\displaystyle NO _2$ and are released with the exhaust gas.

Burning of coal and diesel release, sulphur dioxide gas, which is extremely corrosive and suffocating in nature.

Use of petrol and diseal as fuel in automobiles are now replaced by CNG.
CNG is compressed natural gas (methane stored at high pressure).
Combustion of CNG results in fewer unwanted gases than petrol and diseal.
Also CNG is safer than petrol and diseal as CNG is lighter than air and disperses quickly when released.

A chemical process in which a substance reacts with oxygen to give off heat is combustion.
For example, when C is burnt in excess air, it gives carbon dioxide and heat.
$\displaystyle C + O _2 \rightarrow CO _2 + $ heat

Nitrogen oxides is emitted in car exhaust and is a cause of acid rain.
Fuels such as coal for engine and petroleum for vehicle having little quantity of sulphur and nitrogen. On combustion, such fuels form oxides of sulphur and nitrogen which is environmental hazard.

Complete combustion of alkene require sufficient oxygen.
Alkene on combustion in presence of sufficient oxygen will give carbon dioxide and water.
$\displaystyle CH _2=CH _2 + 3O _2 \rightarrow 2CO _2 + 2H _2O$

The reaction $\displaystyle 2C _3H _8 + 7O _2 \rightarrow 2CO _2 + 8H _2O + 2CO + 2C$  is an example of incomplete combustion reaction. Propane, on incomplete combustion in  presence of excess oxygen gives carbon dioxide, water, carbon monoxide and carbon.

Alkanes are used as fuel, because there is evolution of large amount of heat during combustion. Here, the purpose is to obtain as much heat as possible from given mass of fuel.

LPG burns more cleanly than higher molecular weight hydrocarbon because it releases less pollutants.  Being a gas, LPG does not cause ground or water pollution hazards, but it  causes air pollution.
Note: LPG is liquefied petroleum gas which is compound of butane and propane used in cooking fuel ( in heating appliances, cooking equipment) , motor fuel (in vehicles) and refrigeration.

Benzene when heated in air burns with sooty flame and produces $\displaystyle CO _2 $ and $\displaystyle H _2O$.
$\displaystyle C _6H _6 + \dfrac {15}{2} O _2 \rightarrow 6CO _2 + 3H _2O$
Note: Incomplete combustion of unsaturated hydrocarbons give sooty flame due to presence of black unburnt carbon particles.

LPG releases 81% of the $CO _2$ per Kwh produced by oil.
LPG, at atmospheric pressure, has a higher calorific value 94 MJ/m$^3$ which equivalent to 26.1 kWh/m$^3$.

The unit of calorific value is kJ/kg or J/kg.
Calorific value is the heat produced when unit mass of fuel undergoes complete combustion.

Carbon monoxide is produced by incomplete combustion of fuels.
The complete combustion of fuels will give carbon dioxide.
$\displaystyle C + \dfrac {1}{2}O _2 \rightarrow CO$
$\displaystyle C +  O _2 \rightarrow CO _2$

LPG is liquefied petroleum gas which is a compound of butane and propane. It is used as cooking fuel (in heating appliances, cooking equipment), motor fuel (in vehicles) and refrigeration.

The reaction $\displaystyle C _3H _8 + 5O _2\rightarrow  3CO _2 + H _2O$ is an example of combustion reaction. Propane on complete combustion in  presence of excess oxygen gives carbon dioxide and water.

LPG has a lower energy density than either petrol or fuel-oil, so the equivalent fuel-consumption is higher.
Lower energy density means lesser amount of energy is released during combustion of unit mass of fuel. Hence, to obtain certain quantity of fuel more fuel should be combusted.

LPG can be used as a power source for combined heat and power technologies (CHP).
Combined  heat and power (CHP) is the simultaneous generation of electricity and useful heating from the combustion of a fuel. It is a thermodynamically efficient use of fuel.

Statement 1 : When a hydrocaron is combusted in air, $CO _2$ is the major product whereas the $CO$ is produced if there is incomplete combustion of some hydrocarbon.
Statement 2: Air contains much more nitrogen than air.
$N _2= 78$%
$O _2 = 21$%