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Showing posts with label ALAKANE:. Show all posts
Showing posts with label ALAKANE:. Show all posts

Wednesday, February 5, 2020

What is Octane number?

Octane number is a scale which is used to determine the quality of the fuel in an internal engine. It has been found that straight chain hydrocarbons have higher tendency to knock than branched chain hydrocarbons. Two pure hydrocarbons have been selected as standards.

(1) n-hexane: A straight chain hydrocarbon, knocks very badly when used as fuel. Due to its poor knocking properties it has been arbitrarily assigned an octane number of Zero.
(2) Isooctane: A branched chain hydrocarbon, has the highest antiknocking properties. This has been given an octane number of 100.Isooctane is a commercial name for the hydrocarbon and is not in accordance with common name system.
All other fuels are then graded on the basis of their octen number ranging from 0 to 100. In order to do so, various mixtures of these two hydrocarbon are tried in a test engine till the mixture is formed which produces the same knocking as the fu being tested. The percentage  of Isooctane in this mixture is then taken as the Octane Number.

Thus Octane Number may be defined as the percentage of Isooctane in the mixture of isooctane and n-heptane. Which has the same anti-knocking properties such as the fuel under examination. For example, a fuel is assigned as octane number of 90 if it has same anti-knocking properties as a mixture of 90% I and 10% n-heptane.

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Tuesday, February 4, 2020

What is Knocking?

"A sharp metallic sound produced in the internal combustion of engine is known as knocking."It results in a huge loss of energy.

Gasoline or petrol is used as a fuel in internal combustion of engine. The vapours of petrol mixed with air are compressed by piston and allowed to burn in the cylinder of engine after sparking. This produces large expansion resulting in the movement of piston and consequently the automobile.

But if the petrol is of such quality that its vapours mixed with air can ignite prior to high compression on passing of firing spark, the energy produced by burning of fuel is not utilized in moving the piston, rather. is lost in creating a metallic sound, called knocking. 

"It is infact maximum in petrol containing straight chain paraffins but is not produced in branched chain compounds."

The tendency to knock decreases in the order is straight chain alkanes > branched chain alkanes > olefins > cycloalkanes  aromatic hydrocarbons.

Related Questions:

How arrange the following compounds in order of their increasing boiling points ? n-pentane, 2-methyl pentane, n-hexane, 2,2-dimethyl pentane, ethane.

We know that boiling point on alkanes is directly proportional to their molecular weight and inversely proportional to branching. Therefore, the increasing order is

“Ethane < Pentane < 2-Methyl pentane < n-Hexane < 2, 2-Dimethyl pentane”

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How to arrange the isomeric hexanes in order of increasing stability at room temperature?

The stability of structural isomers generally increases with increased branching. Therefore the increasing order of stability is 

  “n-Hexane < 2-methylpentane < 2, 3-dimethylbutane < 2,2-dimethylbutane”

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