Answer the questions.[Che-3M]

Question 13 Marks

How can a layer of aluminium oxide on an aluminium object be made thicker? What is this process called?

Answer

Aluminium develops a thin oxide layer when exposed to air. This aluminium oxide coat makes it resistant to further corrosion. The resistance can be improved further by making the oxide layer thicker. This process is called anodising. During anodising, a clean aluminium article is made the anode and is electrolysed with dilute sulphuric acid. The oxygen gas evolved at the anode reacts with aluminium to make a thicker protective oxide layer. This oxide layer can be dyed easily to give aluminium articles an attractive finish.

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Question 23 Marks

i. Where do compounds of carbon find applications?
ii. Draw the electron dot structures of CO₂ and state the type of bonding

Answer

i. The carbon compounds are being increasingly used as a source of energy, as medicines, colours, textiles, plastics, food preservatives etc. Wood contains carbon in the form of cellulose. Plastic contains carbon in long chains called polymers. Steel contains carbon sandwiched between Iron molecules. Graphite is pure carbon. Diamonds are pure carbon (in a different crystalline structure than graphite).
ii. :O:: C : : O: [CO₂]

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Question 33 Marks

i. Define covalent bond. Explain with the help of examples.
ii. Discuss the important characteristics of covalent compounds.

Answer

i. Covalent bond: The bond formed by equal contribution and mutual sharing of electrons between two atoms so that both the atoms acquire the stable nearest noble gas configuration i.e. get their octet complete is called covalent bond.
The mutually shared electrons become the common property of both the bonded atoms.
The number of electrons contributed by an atom of the element for mutual sharing during the formation of a covalent bond is called its covalency.
Each pair of shared electrons is represented by putting a single line (-) between two atoms.
In the example given below :

Element	At no.	Electronic configuration
H	1	1
C	6	2,4
O	8	2,6
N	7	2,5

Examples,
a. Formation of a hydrogen molecule $\left(\mathrm{H}_{2}\right)$ : At no. of hydrogen $=1$. It has one electron is the first orbit. When two hydrogen atoms approach each other they share their single electron present in their first orbits. Each hydrogen atom can now be thought of as having noble gas configuration of helium. It may represented as:

Formation of $\mathrm{H}_{2}$ molecule
b. Formation of chlorine molecule. Two chlorine atoms combine with each other to form a molecule of chlorine. In this case, both the atoms have seven electrons in their outermost shell and they contribute one electron each to form a covalent bond. Thus, both the chlorine atoms acquire noble gas configuration of argon. This may be depicted as:

Formation of $\mathrm{Cl}_{2}$ molecule
c. Formation of HCl molecule. A covalent bond is formed not only between similar atoms but it may be formed between dissimilar atoms also. For example, hydrogen and chlorine form a covalent bond between their atoms. Hydrogen atom has only one electron and chlorine atom has seven electrons in its valence shell. Therefore, by mutual sharing of electron pair between a hydrogen and a chlorine atom both the atoms acquire nearest noble gas configuration. Hydrogen atom acquires electronic configuration of helium whereas chlorine atom gets electronic configuration of argon.
ii. The important characteristics of covalent compounds are as follows :
a. Covalent compounds consist of molecules. Covalent compounds do not have any ions. Therefore, they consist of molecules. For example, $\mathrm{H}_{2}, \mathrm{Cl}_{2}, \mathrm{H}_{2} \mathrm{O}, \mathrm{NH}_{3}$ etc.
b. Covalent compounds are liquids or gases in nature. We have studied that the ionic compounds are crystalline solids. But only a few covalent compounds are solids (e.g. sugar, glucose, iodine). These are mostly liquids (water, ethyl alcohol) or gases (oxygen, hydrogen, ammonia) at room temperature. Actually, the attractive forces in covalent molecules are weak and these molecules are not as close lo one another as the ionic solids.
c. Covalent compounds have low melting and boiling points. As covalent molecules do not have ions, the attractive forces among them are weak. Therefore, the covalent molecules can be easily separated from each other. In other words, they have low melting and boiling points.
d. Covalent compounds do not conduct electricity. Covalent compounds normally do not conduct electricity. Some of them are poor conductors of electricity. The current is carried by the ions. As covalent compounds do not have ions, these are poor conductors of electricity.
e. Covalent compounds are insoluble in water. Covalent compounds generally do not dissolve in water. They are soluble in alcohol, ether, benzene etc. which are called organic solvents. However some of them such as ammonia and ethyl alcohol are water soluble.

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