Tag: chemistry

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.