Chapter 4 · Carbon and its Compounds

NCERT Exemplar Class 10 Science Chapter 4 Long Answer Questions

All 11 questions of NCERT Exemplar Class 10 Science Chapter 4 Long Answer Questions (Carbon and its Compounds), in the book's order, each with its answer and a step-by-step solution.

1. Q47
   A salt X is formed and a gas is evolved when ethanoic acid ​reacts with sodium ​hydrogencarbonate. Name the salt X and the gas evolved. Describe an activity and draw the ​diagram of the apparatus ​to prove that the ​evolved gas is ​the one which you have ​named. Also, write chemical equation of ​the reaction involved.
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   X is sodium ethanoate and the gas is carbon dioxide, shown by lime water turning milky.

   The salt X is sodium ethanoate (sodium acetate), ${\text{CH}}_3\text{COONa}$, and the gas evolved is carbon dioxide, ${\text{CO}}_2$. Activity: take about 1 g of sodium hydrogencarbonate in a test tube A clamped to a stand and close it with a cork carrying a thistle funnel and a delivery tube. Pour about 2 mL of ethanoic acid down the thistle funnel, whose lower end dips below the liquid so that no gas escapes back through it; a brisk effervescence begins. Lead the delivery tube into a second test tube B containing freshly prepared lime water. The lime water turns milky, which proves that the gas is carbon dioxide; if the gas is passed in excess the milkiness disappears again as soluble calcium hydrogencarbonate is formed. The reactions are ${\text{CH}}_3\text{COOH}+{\text{NaHCO}}_3⟶{\text{CH}}_3\text{COONa}+{\text{H}}_2\text{O}+{\text{CO}}_2$ in tube A, and ${\text{Ca(OH)}}_2+{\text{CO}}_2\to {\text{CaCO}}_3\downarrow +{\text{H}}_2\text{O}$ in tube B.

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2. Q48
   1. (a)What are hydrocarbons? Give examples.
   2. (b)Give the structural ​differences between ​saturated and ​unsaturated hydrocarbons with two ​examples each.
   3. (c)What ​is a functional group? ​Give examples of ​four different functional groups.
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   Hydrocarbons contain only carbon and hydrogen; saturated ones have single bonds, unsaturated ones a double or triple bond; functional groups give characteristic properties.

   (a) Hydrocarbons are compounds made up of carbon and hydrogen only, with no other element present. Examples: methane ${\text{CH}}_4$, ethane ${\text{C}}_2{\text{H}}_6$, ethene ${\text{C}}_2{\text{H}}_4$, ethyne ${\text{C}}_2{\text{H}}_2$ and benzene ${\text{C}}_6{\text{H}}_6$. (b) In a saturated hydrocarbon every carbon-carbon bond is a single bond, so each carbon carries as many hydrogen atoms as it can; general formula ${\text{C}}_n{\text{H}}_{2n+2}$; examples methane ${\text{CH}}_4$ and ethane ${\text{CH}}_3-{\text{CH}}_3$. In an unsaturated hydrocarbon at least one pair of carbon atoms is joined by a double or a triple bond, so those carbons carry fewer hydrogen atoms; examples ethene ${\text{CH}}_2={\text{CH}}_2$, an alkene of formula ${\text{C}}_n{\text{H}}_{2n}$, and ethyne $\text{HC}\equiv \text{CH}$, an alkyne of formula ${\text{C}}_n{\text{H}}_{2n-2}$. Saturated hydrocarbons give substitution reactions and burn with a clean blue flame; unsaturated ones give addition reactions, decolourise bromine water and burn with a yellow sooty flame. (c) A functional group is an atom or group of atoms present in a molecule that gives the compound its characteristic chemical properties, whatever the length of its carbon chain. Four examples: the alcohol group $-\text{OH}$, as in ethanol; the aldehyde group $-\text{CHO}$, as in ethanal; the ketone group $>\text{C}=\text{O}$, as in propanone; and the carboxylic acid group $-\text{COOH}$, as in ethanoic acid.

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3. Q49
   Name the ​reaction which is commonly used in the ​conversion ​of ​vegetable oils to fats. ​Explain ​the reaction involved ​in detail.
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   Hydrogenation: hydrogen adds across the double bonds of vegetable oil with a nickel catalyst at about 473 K, giving a solid saturated fat.

   The reaction is hydrogenation, an addition reaction; industrially it is called the hardening of oils. Vegetable oils are esters of glycerol with long-chain carboxylic acids whose chains contain carbon-carbon double bonds, so they are unsaturated and are liquid at room temperature. When hydrogen gas is bubbled through the oil in the presence of a catalyst of finely divided nickel at about 473 K, hydrogen adds across each double bond, one atom going to each of the two carbon atoms, and the double bond becomes a single bond: $\text{R}-\text{CH}=\text{CH}-{\text{R}}^{\prime }+{\text{H}}_2\underset{473\text{K}}{\overset{\text{Ni}}{\to }}\text{R}-{\text{CH}}_2-{\text{CH}}_2-{\text{R}}^{\prime }$. Nothing leaves the molecule, which is why this is an addition and not a substitution. Once the chains are saturated the substance is solid at room temperature; the product is vanaspati ghee. The nickel is a catalyst and is recovered unchanged.

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4. Q50
   1. (a)Write ​the formula and draw electron dot ​structure ​of carbon tetrachloride.
   2. (b)​What is saponification? ​Write ​the ​reaction involved in this process.
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   ${\text{CCl}}_4$ has carbon sharing a pair with each of four chlorines; saponification is hydrolysis of an ester by an alkali, giving soap from fats.

   (a) The formula of carbon tetrachloride is ${\text{CCl}}_4$. Carbon has 4 valence electrons and each chlorine has 7, so the molecule holds $4+4\times 7=32$ valence electrons, that is 16 pairs. In the electron dot structure the carbon atom is at the centre and shares one electron pair with each of the four chlorine atoms, which uses 4 pairs; the remaining 12 pairs are drawn as 3 lone pairs on each chlorine atom. The carbon then has 4 shared pairs, that is 8 electrons, and each chlorine has 1 shared pair and 3 lone pairs, also 8 electrons, so every atom has an octet. (b) Saponification is the hydrolysis of an ester by an alkali: an ester heated with a solution of sodium hydroxide gives the sodium salt of the carboxylic acid together with the alcohol. ${\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+\text{NaOH}\stackrel{\text{heat}}{\to }{\text{CH}}_3\text{COONa}+{\text{C}}_2{\text{H}}_5\text{OH}$. When the ester is a fat or a vegetable oil, an ester of glycerol with long-chain carboxylic acids, the salt formed is soap and the alcohol is glycerol, which is how the process gets its name: fat or oil $+3\text{NaOH}\to$ soap $+$ glycerol.

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5. Q51
   Esters ​are ​sweet-smelling substances and are ​used in making perfumes. Suggest ​some activity and the ​reaction involved for ​the preparation of an ester with well ​labeled diagram.
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   Warm ethanol and glacial ethanoic acid with conc. sulphuric acid in a water bath; a fruity smell shows ethyl ethanoate formed.

   Activity: take about 1 mL of ethanol and 1 mL of glacial ethanoic acid in a clean dry test tube and add a few drops of concentrated sulphuric acid. Clamp the tube in a beaker of water so that the water surrounds the mixture, and warm this water bath over a burner for about five minutes; the tube is never heated directly in the flame, because ethanol and the ester are volatile and highly inflammable. Then pour the contents into a beaker holding 20 to 50 mL of water and smell it: a sweet fruity smell shows that the ester ethyl ethanoate has been formed. The reaction is ${\text{CH}}_3\text{COOH}+{\text{CH}}_3{\text{CH}}_2\text{OH}\stackrel{{\text{conc. H}}_2{\text{SO}}_4}{\to }{\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+{\text{H}}_2\text{O}$, with the concentrated sulphuric acid acting as catalyst and dehydrating agent.

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6. Q52
   A compound ​C (molecular formula, ${\text{C}}_2{\text{H}}_4{\text{O}}_2$) reacts with Na - metal to form ​a compound ​R and evolves a gas which burns with a pop ​sound. Compound C on treatment with an alcohol A in presence of an acid forms a sweet smelling compound S (molecular formula, ${\text{C}}_3{\text{H}}_6{\text{O}}_2$). On addition of NaOH to C, it ​also gives R and water. S on treatment with NaOH solution gives ​back R and A. ​Identify C, R, A, S and ​write down the reactions involved.
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   C is ethanoic acid, R sodium ethanoate, A methanol and S methyl ethanoate, with four reactions written from the clues.

   C is ethanoic acid, ${\text{CH}}_3\text{COOH}$; R is sodium ethanoate, ${\text{CH}}_3\text{COONa}$; A is methanol, ${\text{CH}}_3\text{OH}$; and S is methyl ethanoate, ${\text{CH}}_3{\text{COOCH}}_3$. The reactions are: $2{\text{CH}}_3\text{COOH}+2\text{Na}\to 2{\text{CH}}_3\text{COONa}+{\text{H}}_2$; ${\text{CH}}_3\text{COOH}+{\text{CH}}_3\text{OH}\stackrel{{\text{conc. H}}_2{\text{SO}}_4}{\to }{\text{CH}}_3{\text{COOCH}}_3+{\text{H}}_2\text{O}$; ${\text{CH}}_3\text{COOH}+\text{NaOH}\to {\text{CH}}_3\text{COONa}+{\text{H}}_2\text{O}$; and ${\text{CH}}_3{\text{COOCH}}_3+\text{NaOH}\to {\text{CH}}_3\text{COONa}+{\text{CH}}_3\text{OH}$.

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7. Q53
   Look at Figure 4.1 and answer the following questions

   1. (a)What change would you ​observe in the calcium ​hydroxide solution taken ​in tube B?
   2. (b)Write the reaction involved in test tubes ​A and B respectively.
   3. (c)If ethanol ​is given instead of ethanoic acid, ​would you expect the same change?
   4. (d)How can a solution of lime water be prepared ​in the laboratory?

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   Lime water in tube B turns milky; ethanol would not show this change; lime water is made from quick lime and water.

   (a) The clear calcium hydroxide solution in tube B turns milky, because insoluble calcium carbonate is precipitated; on passing the gas for a long time the milkiness disappears again as soluble calcium hydrogencarbonate is formed. (b) In tube A: $2{\text{CH}}_3\text{COOH}+{\text{Na}}_2{\text{CO}}_3⟶2{\text{CH}}_3\text{COONa}+{\text{H}}_2\text{O}+{\text{CO}}_2$; in tube B: ${\text{Ca(OH)}}_2+{\text{CO}}_2⟶{\text{CaCO}}_3\downarrow +{\text{H}}_2\text{O}$. (c) No. Ethanol is neutral and is a far weaker acid than carbonic acid, so it does not react with sodium carbonate; no carbon dioxide is evolved and the lime water in tube B remains clear. This is the very test by which a carboxylic acid is distinguished from an alcohol. (d) Take a small quantity of quick lime, $\text{CaO}$, in a beaker and add water; it reacts with hissing and much heat to give slaked lime, $\text{CaO}+{\text{H}}_2\text{O}\to {\text{Ca(OH)}}_2$. Stir, let the undissolved solid settle, then filter or decant off the clear liquid above; that clear saturated solution is lime water. Slaked lime shaken with water and allowed to settle serves equally well.

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8. Q54
   How would you bring about the following conversions? Name ​the process ​and write the reaction involved. ​

   1. (a)ethanol to ethene.
   2. (b)propanol to propanoic acid. Write ​the reactions.
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   Ethanol to ethene by dehydration with hot concentrated sulphuric acid; propanol to propanoic acid by oxidation with alkaline potassium permanganate.

   (a) Ethanol to ethene: the process is dehydration. Ethanol is heated at 443 K with excess concentrated sulphuric acid, which removes a molecule of water from it. ${\text{CH}}_3{\text{CH}}_2\text{OH}\underset{443\text{K}}{\overset{{\text{conc. H}}_2{\text{SO}}_4}{\to }}{\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\text{O}$. (b) Propanol to propanoic acid: the process is oxidation. Propanol is warmed with alkaline potassium permanganate, or with acidified potassium dichromate, which supplies the oxygen that converts the $-{\text{CH}}_2\text{OH}$ group into a $-\text{COOH}$ group. ${\text{CH}}_3{\text{CH}}_2{\text{CH}}_2\text{OH}\underset{\text{heat}}{\overset{{\text{alk. KMnO}}_4}{\to }}{\text{CH}}_3{\text{CH}}_2\text{COOH}$.

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9. Q55
   Draw the ​possible ​isomers ​of ​the ​compound with molecular formula ${\text{C}}_3{\text{H}}_6\text{O}$ and also ​give their electron ​dot structures.
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   Two isomers: propanal ${\text{CH}}_3{\text{CH}}_2\text{CHO}$ and propanone ${\text{CH}}_3{\text{COCH}}_3$, each with 24 valence electrons (12 pairs) in the dot structure.

   ${\text{C}}_3{\text{H}}_6\text{O}$ is two hydrogen atoms short of the saturated ${\text{C}}_3{\text{H}}_8\text{O}$, so it contains one double bond, and with an oxygen present that is a carbonyl group. On a three-carbon chain the carbonyl group can sit at an end or in the middle, which gives the two isomers. (i) Propanal, ${\text{CH}}_3-{\text{CH}}_2-\text{CHO}$, an aldehyde, in which the carbonyl carbon carries one hydrogen. (ii) Propanone, ${\text{CH}}_3-\text{CO}-{\text{CH}}_3$, a ketone, in which the carbonyl carbon carries two methyl groups. Each molecule has $3\times 4+6\times 1+6=24$ valence electrons, that is 12 pairs. In both electron dot structures these appear as 6 shared pairs between carbon and hydrogen, 2 shared pairs between the carbon atoms, 2 shared pairs in the carbon-oxygen double bond and 2 lone pairs on the oxygen, so every carbon has an octet, the oxygen has an octet and every hydrogen a duplet.

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10. Q56
    Explain the ​given reactions ​with the ​examples ​

    1. (a)​Hydrogenation reaction
    2. (b)Oxidation reaction
    3. (c)Substitution reaction
    4. (d)Saponification ​reaction
    5. (e)Combustion reaction
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    Hydrogenation adds hydrogen; oxidation adds oxygen; substitution replaces an atom; saponification is alkaline hydrolysis of an ester; combustion is burning in oxygen.

    (a) Hydrogenation is the addition of hydrogen to an unsaturated compound in the presence of a catalyst such as nickel; the multiple bond becomes a single bond and nothing leaves the molecule. Example: ${\text{CH}}_2={\text{CH}}_2+{\text{H}}_2\stackrel{\text{Ni}}{\to }{\text{CH}}_3-{\text{CH}}_3$; it is used to harden vegetable oils into vanaspati. (b) Oxidation is the gain of oxygen or the loss of hydrogen, brought about by an oxidising agent; alcohols are oxidised to carboxylic acids. Example: ${\text{CH}}_3{\text{CH}}_2\text{OH}\underset{\text{heat}}{\overset{{\text{alk. KMnO}}_4}{\to }}{\text{CH}}_3\text{COOH}$. (c) In a substitution reaction one atom or group of a saturated compound is replaced by another, the displaced atom leaving with the reagent. Example: ${\text{CH}}_4+{\text{Cl}}_2\stackrel{\text{sunlight}}{\to }{\text{CH}}_3\text{Cl}+\text{HCl}$. (d) Saponification is the hydrolysis of an ester by an alkali, giving the salt of the carboxylic acid and the alcohol. Example: ${\text{CH}}_3{\text{COOC}}_2{\text{H}}_5+\text{NaOH}\to {\text{CH}}_3\text{COONa}+{\text{C}}_2{\text{H}}_5\text{OH}$; with a fat or oil it gives soap and glycerol, which is how the process is named. (e) Combustion is the burning of a substance in oxygen with the evolution of heat and light, carbon compounds giving carbon dioxide and water. Example: ${\text{CH}}_4+2{\text{O}}_2\to {\text{CO}}_2+2{\text{H}}_2\text{O}+$ heat and light.

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11. Q57
    An ​organic compound A on heating with concentrated ${\text{H}}_2{\text{SO}}_4$ forms a compound B which on addition of one mole ​of hydrogen in ​presence ​of ​Ni forms a compound C. One mole of compound C on combustion forms two moles of ${\text{CO}}_2$ ​and 3 moles of ${\text{H}}_2\text{O}$. Identify the compounds A, ​B and C and write the chemical equations of the reactions involved.
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    A is ethanol, B is ethene and C is ethane, found from the combustion data and the reactions.

    A is ethanol, ${\text{C}}_2{\text{H}}_5\text{OH}$; B is ethene, ${\text{C}}_2{\text{H}}_4$; and C is ethane, ${\text{C}}_2{\text{H}}_6$. One mole of C gives 2 mol of carbon dioxide and 3 mol of water, so one molecule of C has 2 carbon atoms and $2\times 3=6$ hydrogen atoms, that is ethane; B is C less one molecule of hydrogen, that is ethene; and the compound that gives ethene on being heated with concentrated sulphuric acid is ethanol. The reactions are ${\text{C}}_2{\text{H}}_5\text{OH}\underset{443\text{K}}{\overset{{\text{conc. H}}_2{\text{SO}}_4}{\to }}{\text{C}}_2{\text{H}}_4+{\text{H}}_2\text{O}$, then ${\text{C}}_2{\text{H}}_4+{\text{H}}_2\stackrel{\text{Ni}}{\to }{\text{C}}_2{\text{H}}_6$, and finally $2{\text{C}}_2{\text{H}}_6+7{\text{O}}_2⟶4{\text{CO}}_2+6{\text{H}}_2\text{O}$.

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