Sunday, October 23, 2011

Class 10 - Science - CH4 - Carbon and Its Compounds (Q & A)

Class 10 - Carbon and Its Compounds
cellulose
cellulose
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Q1. What is next homologue of C3H7OH is called?
Answer: The next homologue of C3H7OH is called butanol C4H9OH.

Q2. What is the molecular formula of the alcohol which can be derived from propane?

Answer: Propane: CH3-CH2-CH3 or C3H8
Alcohol obtained from propane is C3H7OH.

Q3. What is meant by the term functional group?

Answer: An atom or a group of atoms, which makes a carbon compound reactive and decides its
properties. is called a functional group.

Q4. Give the names of the functional groups: (CBSE 2007)
          i) -CHO  (ii  -C=O

Answer: (i) Aldehydic group (ii) Ketonic group.

Q5. Give the names of the following functional groups: (CBSE 2007)
         i) —OH (ii) —COOH

Answer: (i) Alcoholic (ii) Carhoxylic.



Q6. Which functional groups always occur at the terminal position of a carbon chain?

Answer: Aldehydic group R—CHO (R is tue alkyl group), and
       Carboxyl group   R—COOH (R is the alkyl group).

Q7. Why a candle flame burns yellow, while a highly-oxygenated gas-fuel flame burns blue?

Answer: The most important factor determining color of the flame is oxygen supply and the extent of fuel-oxygen pre-mixing, which determines the rate of combustion and thus the temperature and reaction paths, thereby producing different color hues. In case of candle, it is an incomplete combustion and the flame temperature is not high. This gives a yellow flame. While a highly-oxygenated gas (e.g. ethyne) flame burns blue because of complete combustion raising a very high temperature.

Q8. Why is the reaction between methane and chlorine considered a substitution reaction? [CBSE 2008]

Answer: Methane reacts with chlorine in the presence of sunlight to form chloromethane and hydrogen chloride.
        CH4+ Cl2    →   CH3Cl  + HCl
 With the excess of chlorine, aH the four hydrogen atoms of methane are replaced by chlorine atoms to form carbon tetrachloride (CCl4). This reaction is considered as substitution reactions because hydrogen of methane is substituted by chlorine.

Q9: Why does Carbon form compounds mainly by covalent bonding?

Answer.  Being tetravalent carbon atom, it is neither capable of losing all of its four valance electrons nor it can easily accept four electrons to complete its octet. Both of these are requirements of ionic bond formation and are energetically less favourable. Carbon completes its octet by sharing of electrons and hence covalent bonding is preferred.


Q10. Give a chemical test to distinguish between saturated and unsaturated hydrocarbons.

Answer. When saturated compounds are burnt in air, they give a clear (blue) flame but the burning of unsaturated compounds (akenes and alkynes) gives a sooty (yellowish) flame. Actually, saturated compounds contain comparatively less percentage of carbon which get completely oxidized by the oxygen present in air. On the other hand, the percentage of carbon in unsaturated compounds is more and it requires more oxygen to get completely oxidized that is not fulfilled by air. So due to incomplete oxidation they burn with a sooty flame.

Bromine-water test:  Br2water is a brown coloured liquid.
(a) Unsaturated hydrocarbons give addition reaction with Br2. So the colour of Br2-water gets decolourized.
            R-C=C-R + Br2 →R-BrC-CBr-R
(b) Saturated hydrocarbons do not react with Br2-water, so the colour of Br2.-water does not get decolourised.

Q11: What do you mean by Octane Rating?

Answer: Gasolines are rated on a scale known as octane rating, which is based on the way they burn in an engine. The higher the octane rating, the greater the percentage of complex-structured hydrocarbons that are present in the mixture, the more uniformly the gasoline burns, and the less knocking there is in the automobile engine. Thus, a gasoline rated 92 octane will burn more smoothly than one rated 87 octane.

Q12: Two compounds A and B react with each other in the presence of a dehydrating agent to produce an ester. Both react with Na to evolve hydrogen gas. On reaction with Na2CO3 only A evolves CO2. Identify the functional groups present in A and B giving reason for your answer.

Answer: Compound A contains –COOH group while compound B contains –OH group.

Since carboxylic acids and alcohols react with each other to form an ester, out of A and B, one is an alcohol and the other is a carboxylic acid. This is further strengthened by reaction of both with Na to evolve hydrogen gas.
Only carboxylic acids react with Na2CO3 to evolve CO2, A contains –COOH group while B contains –OH group.

Following video posted on You tube by tutor vista nicely summarizes  this chapter:



Q13: An organic compound 'X' is widely used as a preservative in pickles and has a molecular formula C2H2O2. This compound reacts with ethanol to form a sweet smelling compound ‘Y'.
   a. Identify the compound ‘X’
   b. Write the chemical equation for its reaction with ethanol to form compound ‘Y’.
   c. How can we get compound ‘X’ back from ‘Y’?
   d. Name the process and write corresponding chemical equation.
   e. Which gas is produced when compound ‘X’ reacts with washing soda? Write the chemical equation.

Answer:
a. Compound X is ethanoic acid which gives and ester (Y) when reacts with ethanol.
b. CH3COOH + CH3CH2OH → CH3COOC2H5
c. Esters gives back alcohol and carboxylic acid in the presence of acid or base.
d. CH3COOC2H5 ---NaOH → C2H5OH + CH3COOH
e.CO2  gas is released.
      CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2

Q14(NCERT): What would be the electron dot structure of carbon dioxide which has the formula CO2?

Answer: Electron dot structure of CO2 is  O=C=O
Electron dot structure of CO2

Q15(NCERT): What would be the electron dot structure of a molecule of sulphur which is made up of eight atoms of sulphur? (Hint – The eight atoms of sulphur are joined together in the form of a ring.)

Answer:  S8 electron dot structure are in two forms:


Q16(NCERT): How many structural isomers can you draw for pentane?

Answer: There are three structural isomers of pentane:
(i) Pentane
     CH3‒ CH2‒ CH2‒ CH2‒ CH3

(ii) 2-methyl Butane (iso-pentane)

     CH3‒CH2‒CH‒CH3
               |
               CH3
(iii)  2-2 dimethyl propane (neo-pentane)

          CH3
          | 
     CH3‒ CH ‒ CH3
          |
          CH3

Q17(NCERT): What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?

Answer: Two properties of carbon that attributes to its large family of carbon compounds are:
  1. Catenation: Ability to form chains with other atoms of carbon by covalent bonding.
  2. Tetravalency: Valency = 4 i.e. it forms four covalent bonds with four other atoms.

Q18: What is Covalent bonding?

Answer: The chemical bonding occurs due to mutual sharing of  electron pairs of two or more atoms of different elements is called covalent bonding. By mutual sharing of electron pairs atom attain noble gas configuration. e.g. In Hydrogen molecule (H2), the two H-atoms combine by covalent bonding (H-H).

Q19: What are Hydrocarbons? Give examples.

Answer: Compounds of carbon and hydrogen are called hydrocarbons. Methane, Ethane, Butane, ethyne, propene, benzene, petroleum products all are examples of hydrocarbons.


Q20(CBSE 2011): What are saturated hydrocarbons?

Answer: The hydrocarbons in which valency of carbon is satisfied by a single covalent bond are called saturated hydrocarbons. Alkanes like methane (CH4), ethane(C2H6), propane(C3H8) etc. are examples of saturated hydrocarbons. Saturated hydrocarbons will generally give a clean flame.

Q21(CBSE 2011): "Saturated hydrocarbons burn with a blue flame while unsaturated hydrocarbons burn with a sooty flame". Why?

Answer: Saturated hydrocarbons have only C-C and C-H single bonds and thus contain the maximum
possible number of hydrogen atoms per carbon atom. With sufficient oxygen, saturated hydrocarbons burn completely and give blue flame.
CH4 + 2O2 → CO2 + 2H2O
Unsaturated hydrocarbons contain a carbon-carbon double bond (C=C) or triple bond (C≡C). Hence they contain less number of hydrogen than carbon. Unsaturated hydrocarbons undergoe incomplete combustion and give yellow flame along with black sooty(carbon).
C2H4 + O2 → CO2 + 2H2O + C(s)

Q22: What are isomers?

Answer: Compounds with identical molecular formula but different structures are called structural isomers. Organic compounds show a great level of isomerism. Isomers may be structural (due to difference in arrangement of C atoms forming chain) or stereo (due to arrangement of bonds in chain). With increase in number of carbon atoms in a molecular formula leads to increase in number of isomers.
              C‒C‒C‒C           OR       C-C-C
                                           |
                                           C 


Q23(NCERT)Why do ionic compounds have high melting points?

Answer: Ions have strong electrostatic forces of attraction among them forming ionic compounds. It requires lot of energy to break these ionic bonds or forces. That's why ionic bonds have high melting points.


Q24 What are the characteristics of Homologue series?

Answer: Important characteristics of Homologue series are:
  1. Have same general formula.
  2. Belong to same functional group.
  3. Have general methods of separation.
  4. Have similar chemical properties.
  5. Show similar gradation of physical properties. e.g. boiling points of alcohol increase with increase in their molecular weights.
  6. Similarly solubility decreases with increase in  molecular weights.

Q25 What is the general formula of Alkanes?

Answer:  CnH2n+2


Q26 What is a hetroatom? What is the hetroatom in alcohol functional group?

Answer:  In a hydrocarbon chain, one or more hydrogen atoms can be replaced by other atoms according to their valencies. The element which replaces hydrogen in the chain is called a heteroatom.
e.g. in  alcohol (-OH) functional group, oxygen is the hetroatom.




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