Saturday, September 29, 2012

X CBSE | NCERT Solutions:Carbon and its compound Solved Important...

1.Q. Why carbon form covalent bond by sharing electron?

Ans: Carbon has four electrons in its outermost shell and needs to gain or lose four electrons to attain noble gas configuration.  (i) it could gain four electrons forming C4-anion. But it would be difficult for the nucleus with six protons to hold on to ten electrons, that is, four extra electrons. (ii) It could lose four electrons forming C4+cation. But it would require a large amount of energy to remove four electrons leaving behind a carbon cation with six protons in its nucleus holding on to just two electrons.
Hence, it is difficult for the carbon to lose or gain four electrons therefore  carbon formcovalent bond by sharing electron

2.Q. Why do covalent compounds are in gaseous or liquid state at normal temperature and pressure.

Ans: due to Weak Vanderwaal’s forces present between the molecules of covalent compounds.  

3.Q. Why do covalent compounds have less melting and boiling points?

Ans: As energy required breaking the crystal is less due to the presence of weak Vanderwaal’s force.

4.Q. Why do covalent compounds are bad conductors of electricity?

Ans: Due to the absence of free electrons or free ions.

5.Q. Why graphite has high melting point?

Ans: Within each layer of graphite, every carbon atom is joined to three others by strong covalent bonds. This forms a pattern of interlocking hexagonal rings. The carbon atoms are difficult to separate from one another. So graphite also has high melting point.

6.Q.why is  diamond known as the hardest metal?

Ans: each carbon is  linked to four other carbon atoms forming regular and tetrahedral arrangement and this network of carbon atoms extends in three dimensions and is very rigid. This strong bonding is the cause of its hardness and its high density.

7.Q.Define term  allotropy?

Ans: Allotropy is the property by virtue of which an element exist in more than one form and each form has different physical properties but identical chemical properties. These different forms are called allotropes. The two common allotropic forms of carbon are diamond and graphite.

8.Q. Name the products formed when methane burns in :
(a) Sufficient supply of air. (b) Insufficient supply of air. 
Write the chemical equations for above reactions. 

Ans:When methane burns in a sufficient supply of air then Carbon dioxide and water vapours are formed.
CH4 + 2O2 -------burning---->   CO2 + 2H2O.
When methane burns in an insufficient supply of air then carbon monoxide and water vapours are formed.
2CH4 + 3O2 ---------->  2CO + 4H2O

Q. 9 Give any one test to distinguish between methane and ethene gases in the lab. Name the reaction by which ethene molecule can be converted into polythene.

Ans. Ethene gas decolourises the bromine water but methane being  a saturated hydrocarbon doesn't decolourise the bromine water.
CH2 = CH2 + Br2 (aq)  ------àCH2Br-CH2Br (1,2-dibromo ethane).
 Polymerisation is the reaction by which ethene molecule get converted into polythene.

10.Q. What is meant by homologous series? State any four characteristics.

Ans. The member of the same class of organic compounds such as alkanes when arranged in ascending order of their molecular mass differ from  each other by (-CH2) group are called homologous and series is known as homologous series.
The characteristics of homologous series are :
a. The members of a homologous series are represented by the same general formula.
b. The members of homologous series differ by (-CH2) group.
c. The members of same homologous series show similar chemical properties.
d. The members of same homologous series show different physical properties.

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