SHORT ANSWER -Class 10 – Science -Chemistry -Chapter 4 – Carbon and its Compounds

SHORT ANSWER -Class 10 – Science -Chemistry -Chapter 4 – Carbon and its Compounds

Q1. What are covalent compounds? Why are they different from ionic compounds? List their three characteristic properties. 

Ans:

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.

Q2. 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. 

Ans:

(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.

Q3. 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.

Ans:

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.

Q4. Draw the structural formula of all the possible isomers of the compound with the molecular formula C3 H6 O and also give electron dot structures.

Q5. Explain giving reasons, why carbon can neither form C4+ cation nor C4- anion but forms covalent compounds which are bad conductors of electricity and have low melting and boiling points.

Ans :Carbon cannot lose four electrons because high energy is needed to remove four electrons. It cannot gain 4 electrons because 6 protons cannot hold 10 electrons. It can share 4 electrons to form covalent bonds. Covalent compounds do not conduct electricity because these do not form ions. They have low melting and boiling points due to weak force of attraction between molecules.

Q6. Name two oxidising agents that are used to convert alcohols to acids. Distinguish between ethanol and ethanoic acid on the basis of

(a) litmus test (b) reaction with NaHCO3 .

Ans : These two oxidising agents will convert alcohols to acids

 (i) Alkaline KMnO4     

(ii) Acidified K2 Cr2O7 a. Litmus test: Acetic acid turns blue litmus red but ethanol does not. b. NaHCO3 test: Acetic acid will give brisk effervescence due to evolution of CO2 whereas ethanol will not react.

 Q7. What are covalent compounds? Why are they different from ionic compounds? List three characteristic properties.

Ans :   Those compounds in which bonds are formed by sharing of electrons are covalent compounds. While ionic compounds are formed by complete transfer of electrons.

 Q8.  a. Chemical properties of ethanol is different from methyl ethanoate. Justify the statement with proper reason.

b. Methyl ethanoate is used in making perfume. Justify.

c. Ethanol is converted into ethene when excess of cone. H2 SO4 is added. Justify with the help of reaction

 Ans :a. Chemical properties of a compound depends on its functional group. Ethanol and methyl ethanoate have different functional group thus behave differently. CH3 CH2 OH has functional group — OH. It has specific smell, reacts with Na metal to liberate H2 . CH2 COOCH3 has ester (RCOOR’) as functional group. It has pleasant fruity smell, undergoes saponification reaction with NaOH.

b. It is due to pleasant fruity smell.

c. Conc H₂ SO₄  dehydrates ethanol to ethene.

CH₃ CH₂ OH à CH₂ = CH₂ + H₂ O

Q9. C₃  H₆ , C₄ H₈ and C₅ H₁₀ belong to same 1 homologous series. a. Define homologous series.

b. Why the melting and boiling point of C₅ H₁₀ is higher than C₄ H₃ ?

c. Arrange these hydrocarbons in order of increasing boiling points.

Ans : a. The series of organic compounds having same functional group and similar chemical properties is called homologous series.

 b. C5 H10 has higher molecular mass, more surface area, more van der Waal’s forces of attraction, hence higher boiling point than C4 H8 .

c. C3 H6 < C4 H8 < C5 H10

Q10. What is difference between the molecule of soaps and detergents, chemically? Explain the cleansing action of soaps.

 Ans : Soaps are sodium or potassium salts of fatty acids. They contain —COONa group. Detergents are sodium or potassium salts of sulphonic acids. They contain —SO3 Na or —SO4 Na group. Soap has ionic end which is hydrophilic, interacts with water while carbon chain is hydrophobic interacts with oil, grease. The soap molecules orient themselves in a cluster in which hydrophobic tails are inside the cluster and ionic ends face outside. These cluster are called micelles. These attract oil which is washed away by water.

Q11. Give reasons for the following observations:

a. The element carbon forms a very large number of compounds.

b. Air holes of a gas burner have to be adjusted when the heated vessels get blackened by the flame.

c. Use of synthetic detergents causes pollution of water.

Ans : a. It is due to tetravalency of carbon and property of catenation shown by carbon to maximum extent.

Air holes must be kept open fully so that complete combustion of fuel takes place producing blue flame.

c. Some of detergents are not biodegradables, they create water pollution.