chemistry notes for class 10 chapter 2 hydrogen
Chemistry New Shams Notes
Q.1: What are hydrocarbons? Explain with examples.
“The compounds of carbon and hydrogenare called hydrocarbons.” Classification of hydrocarbons:
Hydrocarbons are classified into two main groups:
i. Saturated hydrocarbons (alkanes)
ii. Unsaturated hydrocarbons (alkenes and alkynes)
i. Saturated hydrocarbons:
“The hydrocarbons which contain all carbon-carbon single bonds are called saturated hydrocarbons or alkanes.”
They have the general formula of CnH2n+2, where n is the number of carbon atoms.
Methane (CH4), Ethane (C2H6), propane (C3H8) and butane (C41–110 etc. are the examples of saturated hydrocarbons.
ii. Unsaturated hydrocarbons:
“The hydrocarbons containing at least one carbon-carbon double or triple bond are called unsaturated hydrocarbons.”
They are further classified to:
“The hydrocarbons which contain at least one carbon-carbon double bond are called alkenes.”
They have the general formula of CnH2n, where n is the number of carbon atoms.
Ethene (C2H4), propene (C3H6), butene (C4H8) and pentene (C6H1o) are examples of alkenes.
Q.2: Write a short note on alkanes?
“The saturated hydrocarbons containing all carbon-carbon or carbon to hydrogen single bonds are called alkanes or paraffin.”
Reason for calling paraffin:
Paraffin is a Latin word meaning “little affinity”. Since alkanes contain C-C and C-H single bonds, therefore they have a little affinity towards chemical reactions that is why they are called paraffin.
Reason of calling saturated hydrocarbons:
Since in alkanes each carbon atom is bonded with four other atoms therefore no further atom can be added to alkanes. Hence, they are called saturated hydrocarbons.
The general formula of alkanes is CnH2n+2 where n is the number of carbon atoms e.g. if n=1 then the formula will be CH4, which is called methane. Similarly if n=2 then the formula will be C2H6 called ethane etc. Examples:
The name of the first ten alkanes along with their physical states, melting point, and boiling points are given in the table below:
Naming the longest chain:
Name the longest chain e.g propane for 3, butane for 4 carbon atoms etc.
It is called the base name or parent name of the compound.
For example, the parent name of the above example is hexane.
iv. Naming the branch:
Name each branch (substituent) attached to the longest chain and give its location by the number of the carbon atom to which it is attached. A hyphen is used between the number and name of the branch, and comma (,) is used between number and number.
For example, in the above example, 3-methyl is the position of the branch.
v. Writing the full name:
The parent name is written at the end, after mentioning the position of the substituent.
For example, the name of the above compound is 3–methyl hexane.
vi. Naming two or more identical branches:
When two or more identical branches are present then use prefixes di, tri, tetra, etc. A comma is used to separate the position numbers.
iii. Numbering the branches on same carbon:
When two branches are present on the same carbon uses the number of that carbon twice.
For example, in the above example two methyl groups are present on the same carbon therefore their position is mentioned as 2,2.