chemistry notes for class 10 chapter 2 hydrogen
Chemistry New Shams Notes
Q.1: What are hydrocarbons? Explain with examples.
Ans: Hydrocarbons:
“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.”
General formula:
They have the general formula of CnH2n+2, where n is the number of carbon atoms.
Examples:
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:
a. Alkenes:
“The hydrocarbons which contain at least one carbon-carbon double bond are called alkenes.”
General formula:
They have the general formula of CnH2n, where n is the number of carbon atoms.
Examples:
Ethene (C2H4), propene (C3H6), butene (C4H8) and pentene (C6H1o) are examples of alkenes.
Q.2: Write a short note on alkanes?
Ans: 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.
General formula:
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.
Example:
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.
Example: •
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.
Example:
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.
Example:
iii. Numbering the branches on same carbon:
When two branches are present on the same carbon uses the number of that carbon twice.
Example:
For example, in the above example two methyl groups are present on the same carbon therefore their position is mentioned as 2,2.
