Question 1.
Covalent compounds have low melting and boiling point. Why? (2020)
Answer:
Covalent compounds have low melting and boiling points because the forces of attraction between molecules of covalent compounds are very weak. On applying a small amount of heat these molecular forces break.
Question 2.
What are covalent compounds? Why are they different from ionic compounds? List their three characteristic properties. (Delhi 2016)
Answer:
Covalent compounds are those compounds which are formed by sharing of valence electrons between the atoms e.g., hydrogen molecule is formed by mutual sharing of electrons between two hydrogen atoms.
They are different from ionic compounds as ionic compounds are formed by the complete transfer of electrons from one atom to another e.g., NaCl is formed when one valence electron of sodium gets completely transferred to outer shell of chlorine atom. The characteristic properties of covalent compounds are:
(i) They are generally insoluble or less soluble in water but soluble in organic solvents.
(ii) They have low melting and boiling points.
(iii) They do not conduct electricity as they do not contain ions.
Question 3.
What are covalent bonds? Show their formation with the help of electron dot structure of methane. Why are covalent compounds generally poor conductors of electricity? (Delhi 2013C)
Answer:
Covalent bonds are those bonds which are formed by sharing of the valence electrons between two atoms. Electron dot structure of methane is shown in the figure.
Covalent compounds are generally poor conductors ol electricity because they do not have tree electrons or ions.
Question 4.
Give reasons for the following:
(i) Element carbon forms compounds mainly by covalent bonding.
(ii) Diamond has high melting point.
(iii) Graphite is a good conductor of electricity. (3/5, Foreign 2011)
Answer:
(i) As carbon has four valence electrons and it can neither loose nor gain lour electrons thus, it attains noble gas configuration only by sharing of electrons. I bus, it forms covalent compounds.
(ii) In diamond, each carbon atom is bonded to four other carbon atoms forming a rigid three-dimensional structure. This makes diamond the hardest known substance. Thus, it has high melting point.
(iii) In graphite, each carbon atom is bonded to three other carbon atoms by covalent bonds in the same plane giving a hexagonal array. Thus, only three valence electrons are used for bond formation and hence, the fourth valence electron is free to move. As a result, graphite is a good conductor of electricity.
Question 5.
What is methane? Draw its electron dot structure. Name the type of bonds formed in this compound. Why are such compounds
(i) poor conductors of electricity and
(ii) have low melting and boiling points?
What happens when this compound burns in oxygen? (Delhi 2019)
Answer:
Methane is the first member of alkane series having formula CH4.
Refer to answer 3.
(ii) Refer to answer 1.
When methane is burnt in presence of oxygen then carbon dioxide will be produced.
CH4 + O2 → CO2 + H2O + heat + light
Question 6.
Elements forming ionic compounds attain noble gas electronic configuration by either gaining or losing electrons from their valence shells. Explain giving reason why carbon cannot attain such a configuration in this manner to form its compounds. Name the type of bonds formed in ionic compounds and in the compounds formed by carbon. Also explain with reason why carbon compounds are generally poor conductors of electricity. (Foreign 2015, AI 2014)
Answer:
Ionic compounds are formed either by gaining or losing electrons from the outermost shells, but carbon which has four electrons in its outermost shell cannot form ionic bonds because
1. If carbon forms ionic bonds by gaining four electrons to attain a noble gas configuration then it would be difficult for six protons in the nucleus to hold ten electrons.
2. If carbon forms ionic bonds by loss of four electrons then it would require a lot of energy to remove these electrons from outermost shell.
Due to these reasons carbon forms covalent bonds by sharing the valence electrons.
Type of bonds formed in ionic compounds are called electrovalent bonds and the type of bonds formed in carbon compounds are called covalent bonds.
Refer to answer 3.
Question 7.
State the reason why carbon can neither form C4+ cations nor C4- anions, but forms covalent compounds. Also state reasons to explain why covalent compounds :
(i) are bad conductors of electricity?
(ii) have low melting and boiling points? (Delhi 2014)
Answer:
Refer to answer 6.
(i) Refer to answer 3.
(ii) Refer to answer 1.
Question 8.
Name a cyclic unsaturated carbon compound. (2020)
Answer:
Question 9.
Assertion (A) : Following are the members of a homologous series :
CH3OH, CH3CH2OH, CH3CH2CH2OH
Reason (R) : A series of compounds with same functional group but differing by -CH2 unit is called homologous series.
(a) Both (A) and (R) are true and (R) is the correct explanation of the assertion (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation of the assertion (A).
(c) (A) is true, but (R) is false.
(d) (A) is false, but (R) is true. (2020)
Answer:
(a): The given compounds are members of homologous series of alcohol.
Question 10.
Write the molecular formula of first two members of homologous series having functional group -Cl. (Delhi 2017)
Answer:
The molecular formula of first two members of homologous series having -Cl functional group are CH3Cl and CH3CH2Cl.
Question 11.
Write the molecular formula of first two members of homologous series having functional group -OH. (Delhi 2017)
Answer:
The molecular formula of first two members of homologous series having -OH functional group are CH3OH and CH3CH2OH.
Question 12.
Write the molecular formula of the 2nd and 3rd member of the homologous series whose first member is ethene. (AI 2017)
Answer:
Homologous series of alkenes have general formula, CnH2n whose first member is ethene.
2nd member of homologous series of alkenes is C3H6 i.e., propene.
3rd member of homologous series of alkenes is C4H8 i.e., butene.
Question 13.
Write the molecular formula of the 2nd and 3rd member of the homologous series whose first member is methane. (AI 2017)
Answer:
Methane, CH4 is an alkane. Alkanes have general formula, CnH2n+2.
2nd member of homologous series of alkanes is C2H6 i.e., ethane.
3rd member of homologous series of alkanes is C3H8 i.e., propane.
Question 14.
Write the next homologue of each of the following:
(i) C2H4
(ii) C4H6 (Delhi 2016)
Answer:
(i) C2H4 belongs to alkene series having general formula, CnH2n.
Thus, next homologue will be C3H2×3 = C3H6
(ii) C4H6 belongs to alkyne series having general formula, CnH2n-2.
Thus, next homologue will be C5H2×5-2 = C5H8
Question 15.
Name the following compounds :
(a) CH3 – CH2 – OH
Answer:
(a) CH3 – CH2 – OH : Ethanol
Question 16.
Select saturated hydrocarbons from the following : C3H6; C5H10; C4H10; C6H14; C2H4
Answer:
Saturated hydrocarbons have general formula, CnH2n+2.
Among the given compounds only C4H10 and C6H14 satisfy the above formula. Thus, these are saturated hydrocarbons.
Question 17.
Write the name and structure of an alcohol with three carbon atoms in its molecule. (AI 2016)
Answer:
An alcohol with three carbon atoms in its molecule is propanol. The structure of propanol is
Question 18.
Write the name and structure of an alcohol with four carbon atoms in its molecule. (AI 2016)
Answer:
An alcohol with four carbon atoms is butanol and its structure is :
Question 19.
Write the name and structure of an aldehyde with four carbon atoms in its molecule. (AI 2016)
Answer:
An aldehyde with four carbon atoms is butanal and its structure is
Question 20.
Which element exhibits the property of catenation to maximum extent and why? (Foreign 2016)
Answer:
Carbon has the unique ability to form bonds with other atoms of carbon, giving rise to large molecules. This property is called catenation. Carbon shows catenation due to its small size and Stronger carbon-carbon bond strength.
Question 21.
Write the name and molecular formula of the fourth member of alkane series. (Foreign 2016)
Answer:
The general formula of the alkane series is CnH2n+2. For fourth member of alkane series, n = 4
∴ C4H2×4+2 = C4H10 i.e., butane.
Question 22.
What is homologous series of carbon compounds? (Foreign 2016)
Answer:
A homologous series is the family of organic compounds having the same functional group, similar chemical properties but the successive (adjacent) members of the series differ by a -CH2 unit or 14 mass units.
Question 23.
Write the name and formula of the 2nd member of homologous series having general formula CnH2n. (Delhi 2015)
Answer:
Refer to answer 12.
Question 24.
Write the name and formula of the 2nd member of homologous series having general formula CnH2n+2. (Delhi 2015)
Answer:
Refer to answer 13.
Question 25.
Write the name and formula of the 2nd member of homologous series having general formula CnH2n-2. (Delhi 2015)
Answer:
General formula, CnH2n-2 belongs to alkyne series. The second member of this series is propyne i.e., (C3H4) or CH3 – C ≡ CH. Question 26.
Write the number of covalent bonds in the molecule of ethane. (AI2015, Delhi 2014)
Answer:
The structural formula of ethane (C2H6) is
There are total 7 covalent bonds. Six C – H covalent bonds and one C – C covalent bond.
Question 27. Write the number of covalent bonds in the molecule of butane, C4H10. (AI 2015) Answer: Butane (C4H10) has the following structural formula as:
Total number of covalent bonds is 13 in which there are 10 C – H and 3 C – C covalent bonds.
Question 28.
Write the name of each of the following functional groups: (Foreign 2015, Delhi 2013)
(a) -OH
Answer:
(a) -OH : Alcohol
Question 29.
Write the name and molecular formula of the first member of the homologous series of alkynes. (Foreign 2015)
Answer:
General formula for alkyne is CnH2n-2
First member of homologous series of alkyne has the formula, C2H2×2-2 = C2H2 i.e., ethyne. Question 30.
Define the term functional group. Identify the functional group present in
Answer:
An atom or a group of atoms present in a molecule which largely determines its chemical properties, is called functional group.
Question 31.
Name the functional group present in each of the following organic compounds:
(i) C2H5Cl
(ii) C2H5OH (Delhi 2012)
Answer:
(i) C2H5Cl contains -Cl (chloro) group which belongs to halo functional group.
(ii) C5H5OH contains -OH group which belongs to alcoholic functional group.
Question 32.
Write the name and formula of the second member of the carbon compounds having functional group -OH. (AI 2012)
Answer:
Those having -OH as functional group belong to alcohol family. Second member of this family is ethanol, C2H5OH.
Question 33.
Write the name and formula of the first member of the series of carbon compounds having functional group (Foreign 2012)
Answer:
Carbon compound containing
group is called carboxylic acid. The first member of this family is methanoic acid (HCOOH).
Question 34.
Butanone is a four-carbon per molecule compound. Name the functional group present in it. (Foreign 2011)
Answer:
Question 35.
State two properties of carbon which lead to a very large number of carbon compounds. (2/5, AI 2011)
Answer:
Carbon forms a large number of carbon compounds like long chains which may be straight or branched chains or ring of different sizes due to its tetravalency ahd unique property of catenation. Carbon due to its small size forms exceptionally stable compounds by forming strong bonds.
Question 36.
Carbon, a member of group 14, forms a large number of carbon compounds estimated to be about three million. Why is this property not exhibited by other elements of this group? Explain. (2020)
Answer:
Refer to answer 20.
As we move down the group, the element-element bond energies decrease rapidly. For this reason other elements of this group show little or no catenation property.
Question 37.
(a) Why are most carbon compounds poor conductors of electricity?
(b) Write the name and structure of a saturated compound in which the carbon atoms are arranged in a ring. Give the number of single bonds present in this compound. (2018)
Answer:
(a) Due lo catenation, carbon forms covalent bonds with the constituent elements in the carbon compounds, hence it does not have mobile electrons and carbon compounds do not dissociate themselves into ions and hence, they are poor conductor of electricity.
Name: Cyclopentane
Number of single bonds : 15
Question 38.
An aldehyde as well as a ketone can be represented by the same molecular formula, say C3H6O. Write their structures and name them. State the relation between the two in the language of science. (AI 2016)
Answer:
The aldehyde and ketone represented by the
molecular formula, C3H6O.
In the language of science, they are called as isomers because both have same molecular formula but different structural formulae (having different functional groups.)
Question 39.
What is meant by isomers? Draw the structures of two isomers of butane, C4H10. Explain why we cannot have isomers of first three members of alkane series. (Delhi 2015, Foreign 2014)
Answer:
Isomers are those molecules which have the same molecular formula but different structural formula i.e., show different properties.
The structures of possible isomers of butane (C4H10) are:
The first three members of alkane series are :
(i) CH4 (methane)
(ii) C2H6 (ethane)
(iii) C3H8 (propane)
In the above members of alkane series, it is not possible to have different arrangements of carbon atoms. Thus, we cannot have isomers of first three members of alkane series.
Question 40.
Write the molecular formula of the following compounds and draw their electron-dot structures:
(i) Ethane
(ii) Ethene
(iii) Ethyne (Foreign 2015)
Answer:
(i) Molecular formula of ethane is C2H6.
Its electron dot structure is :
(ii) Molecular formula of ethene is C2H4. Its electron dot structure is :
(iii) Molecular formula of ethyne is C2H2. Its electron dot structure is :
Question 41.
What is meant by functional group in carbon compounds? Write in tabular form the structural formula and the functional group present in the following compounds :
(i) Ethanol
(ii) Ethanoic acid (Foreign 2015)
Answer:
Refer to answer 30.
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