CBSE - Carbon and its Compounds

The two features of carbon that give rise to a large number of compounds are as follows:
→ Catenation – It is the ability to form bonds with other atoms of carbon.
→ Tetravalency – With the valency of four, carbon is capable of bonding with four other atoms.

The formula for cyclopentane is C₅H₁₀. Its electron dot structure is given below.

(i)

(ii) There are many structural isomers possible for bromopentane. Among them, the structures of three isomers are given.

(iii)

(iv)

(i) Bromoethane
(ii) Methanal (formaldehyde)
(iii) Hexyne

CH₃CH₂OH + (Alkaline KMnO₄) → CH₃COOH
Since, in this reaction one oxygen is added to ethanol, hence it is an oxidation reaction.

2HC ≡ CH + 5O₂ → 4CO₂ + 2H₂O + Heat
When ethyne is burnt in air, it gives a sooty flame. This is due to incomplete combustion caused by limited supply of air. However, if ethyne is burnt with oxygen, it gives a clean flame with temperature 3000�C because of complete combustion. This oxy-acetylene flame is used for welding. It is not possible to attain such a high temperature without mixing oxygen. This is the reason why a mixture of ethyne and air is not used.

We can distinguish between an alcohol and a carboxylic acid on the basis of their reaction with carbonates and hydrogen carbonates. Acid reacts with carbonate and hydrogen carbonate to evolve CO₂ gas that turns lime water milky.

Alcohols, on the other hand, do not react with carbonates and hydrogen carbonates.

Oxidising agents are the substances that gain electrons in redox reaction and whose oxidation number is reduced.

Detergent gives lather with hard and soft water both, while a soap gives lather with soft water only. Thus, it is not possible to check if water is hard; by using a detergent.

A soap molecule has two parts namely hydrophobic and hydrophilic. With the help of these, it attaches to the grease or dirt particle and forms a cluster called micelle. These micelles remain suspended as a colloid. To remove these micelles, it is necessary to agitate clothes.

Ethane, with the molecular formula C₂H₆ has
(a) 6 covalent bonds.
(b) 7 covalent bonds.
(c) 8 covalent bonds.
(d) 9 covalent bonds.

Ans (b) 7 covalent bonds.

Butanone is a four-carbon compound with the functional group
(a) carboxylic acid.
(b) aldehyde.
(c) ketone.
(d) alcohol.

Ans (c) ketone.

While cooking, if the bottom of the vessel is getting blackened on the outside, it means that
(a) the food is not cooked completely.
(b) the fuel is not burning completely.
(c) the fuel is wet.
(d) the fuel is burning completely.

ANS (b) the fuel is not burning completely.

The structure of CH₃Cl is given below:


Carbon can neither lose four of its electrons nor gain four electrons as both the processes require extra amount of energy and would make the system unstable. Therefore, it completes its octet by sharing its four electrons with other carbon atoms or with atoms of other elements. The bonds that are formed by sharing electrons are known as covalent bonds. In covalent bonding, both the atoms share the valence electrons, i.e., the shared electrons belong to the valence shells of both the atoms.

Here, carbon requires 4 electrons to complete its octet, while each hydrogen atom requires one electron to complete its duplet. Also, chlorine requires an electron to complete the octet. Therefore, all of these share the electrons and as a result, carbon forms 3 bonds with hydrogen and one with chlorine. The bond between C and Cl atoms is covalent but due to higher value of electro-negativity of Cl, the C�Cl bond is polar in nature.

 (a) Ethanoic acid

(b) H₂S

(c) Propanone


(d) F₂ 

A homologous series is a series of carbon compounds that have different numbers of carbon atoms but contain the same functional group.

For example, methane, ethane, propane, butane, etc. are all part of the alkane homologous series. The general formula of this series is C_nH_2n+2.
Methane CH₄
Ethane CH₃CH₃
Propane CH₃CH₂CH₃
Butane CH₃CH₂CH₂CH₃
It can be noticed that there is a difference of -CH₂ unit between each successive compound.

Ethanol and Ethanoic acid be differentiated on the basis of their following properties:

→ Ethanol is a liquid at room temperature with a pleasant smell. Ethanoic acid has a melting point of 17�C. Since it is below the room temperature so, it freezes during winter. Moreover, ethanoic acid has a smell like vinegar.
→ Ethanol does not react with metal carbonates while, ethanoic acid reacts with metal carbonates to form salt, water and carbon dioxide. For example,
2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O

→ Ethanol does not react with NaOH while ethanoic acid reacts with NaOH to form sodium ethanoate and water. For example,
CH₃COOH + NaOH → CH₃COONa + H₂O→ Ethanol is oxidized to give ethanoic acid in presence of acidified KMnO₄ while, no reaction takes place with ethanoic acid in presence of acidified KMnO₄.

Soap molecule has two ends. One end is hydrophilic and another end is hydrophobic. When soap is dissolved in water and clothes are put in the soapy solution, soap molecules converge in a typical fashion to make a structure; called micelle. The hydrophobic ends of different molecules surround a particle of grease and make the micelle; which is a spherical structure. In this, the hydrophilic end is outside the sphere and hydrophobic end is towards the centre of the sphere. This is why micelle formation takes place when soap is added to water.

Since ethanol is not as polar as soap, so micelles will not be formed in other solvents such as ethanol.

Carbon and its compounds give large amount of heat on combustion due to high percentage of carbon and hydrogen. Carbon compounds used as fuel have optimum ignition temperature with high calorific values and are easy to handle. Their combustion can e controlled. Therefore, carbon and its compounds are used as fuels.

Hard water often contains salts of calcium and magnesium. Soap molecules react with the salts of calcium and magnesium and form a precipitate. This precipitate begins floating as an off-white layer over water. This layer is called scum. Soaps lose their cleansing property in hard water because of formation of scum.

Since soap is basic in nature, it will turn red litmus blue. However, the colour of blue litmus will remain blue.

Hydrogenation is the chemical reaction between hydrogen and other compounds in the presence of catalyst. Hydrogenation is used mainly to reduce saturated hydrocarbons. Hydrogenation is an addition reaction.For Example: When ethene is heated with the catalyst nickel it is reduced to ethane.

Industrial application:

→ >Hydrogenation is used in many industrial applications. For example; in Petrochemical Industry, hydrogenation is used to convert alkenes into alkanes (paraffins) and cycloalkanes.
→ It is also used to prepare vegetable ghee from vegetable oils.

Unsaturated hydrocarbons undergo addition reactions. Being unsaturated hydrocarbons, C₃H₆ and C₂H₂ undergo addition reactions.

Butter contains saturated fats. Therefore, it cannot be hydrogenated. On the other hand, oil has unsaturated fats. That is why it can be hydrogenated to saturated fats (solids).

The dirt present on clothes is organic in nature and insoluble in water. Therefore, it cannot be removed by only washing with water. When soap is dissolved in water, its hydrophobic ends attach themselves to the dirt and remove it from the cloth. Then, the molecules of soap arrange themselves in micelle formation and trap the dirt at the centre of the cluster. These micelles remain suspended in the water. Hence, the dust particles are easily rinsed away by water.

Cleansing action of soaps 

1. Carbon exists in the atmosphere in the form of
(a) carbon monoxide only
(b) carbon monoxide in traces and carbon dioxide
(c) carbon dioxide only
(d) coal
Ans. (c) carbon dioxide only

2. Which of the following statements are usually correct for carbon compounds? These

(i) are good conductors of electricity
(ii) are poor conductors of electricity
(iii) have strong forces of attraction between their molecules
(iv) do not have strong forces of attraction between their molecules
(a) (i) and (iii)
(b) (ii) and (iii)
(c) (i) and (iv)
(d) (ii) and (iv)
Ans. (d) (ii) and (iv)
Explanation: Organic compound are made of covalent bonds. These bonds have weaker forces of attraction. Covalent compounds are poor conductors of electricity.

3. A molecule of ammonia (NH3) has
(a) only single bonds
(b) only double bonds
(c) only triple bonds
(d) two double bonds and one single bond
Ans. (a) Only single bonds
Explanation: The valency of nitrogen is 2 and that of hydrogen is one. Three atoms of hydrogen combine with 1 atom of nitrogen to make ammonia. Hence, there are three single bonds in ammonia.

4. Buckminsterfullerene is an allotropic form of
(a) phosphorus
(b) sulphur
(c) carbon
(d) tin
Ans. (c) Carbon
5. Which of the following are correct structural isomers of butane?


(a) (i) and (iii)
(b) (ii) and (iv)
(c) (i) and (ii)
(d) (iii) and (iv)
Ans. (c) (i) and (ii)
Explanation: Formula of butane is C4H10. Options (iii) and (iv) have 8 hydrogen atoms each, hence, they are incorrect.


6. In the given reaction, alkaline KMnO₄ acts as

(a) reducing agent
(b) oxidising agent
(c) catalyst
(d) dehydrating agent
Ans. (b) Oxidising agent
Explanation: Two hydrogen atoms are replaced by an atom of oxygen in this reaction. This shows oxidization of ethanol. Addition oxygen is being provided by potassium permanganate.

7. Oils on treating with hydrogen in the presence of palladium or nickel catalyst form
fats. This is an example of
(a) Addition reaction
(b) Substitution reaction
(c) Displacement reaction
(d) Oxidation reaction
Ans. (a) Addition reaction
Explanation: In this reaction, hydrogen is added to oil.

8. In which of the following compounds, — OH is the functional group?
(a) Butanone
(b) Butanol
(c) Butanoic acid
(d) Butanal
Ans. (b) Butanol
Explanation: Compounds with -OH as functional group have ‘ol’ as suffix.

9. The soap molecule has a
(a) hydrophilic head and a hydrophobic tail
(b) hydrophobic head and a hydrophilic tail
(c) hydrophobic head and a hydrophobic tail
(d) hydrophilic head and a hydrophilic tail
Ans. (a) Hydrophilic head and hydrophobic tail
Explanation: Oil and grease is trapped inside a micelle because of hydrophobic tail. Hydrophilic head makes the outer surface of micelle. Thus, a micelle is easily washed by water.

10. Which of the following is the correct representation of electron dot structure of nitrogen?

Ans. (d)
Explanation: In this structure, each nitrogen atom gets 8 electrons. This is not the case in other options.

11. Structural formula of ethyne is

Ans. (a)
Explanation: The formula of ethyne is C₂H₂.

12. Identify the unsaturated compounds from the following
(i) Propane
(ii) Propene
(iii) Propyne
(iv) Chloropropane

(a) (i) and (ii)
(b) (ii) and (iv)
(c) (iii) and (iv)
(d) (ii) and (iii)
Ans. (d) (ii) and (iii)
Explanation: Propene has one double bond, while propyne has one triple bond. Hence,
they are unsaturated compounds.
13. Chlorine reacts with saturated hydrocarbons at room temperature in the
(a) absence of sunlight
(b) presence of sunlight
(c) presence of water
(d) presence of hydrochloric acid
Ans. (b) Presence of sunlight
Explanation: Chlorine reacts with saturated hydrocarbons in the presence of sunlight and displaces hydrogen atoms one by one.

14. In the soap micelles
(a) the ionic end of soap is on the surface of the cluster while the carbon chain is in the interior of the cluster.
(b) ionic end of soap is in the interior of the cluster and the carbon chain is out of the cluster.
(c) both ionic end and carbon chain are in the interior of the cluster.
(d) both ionic end and carbon chain are on the exterior of the cluster.
Ans. (a) the ionic end of soap is on the surface of the cluster while the carbon chain is in the interior of the cluster.

15. Pentane has the molecular formula C₅H₁₂. It has
(a) 5 covalent bonds
(b) 12 covalent bonds
(c) 16 covalent bonds
(d) 17 covalent bonds
Ans. (c) 16 covalent bonds
Explanation: Following is the structural formula of pentane. It shows 16 covalent bonds:


16. Structural formula of benzene is

Ans. (c)
Explanation: Formula of benzene is C6H6. Option (a) and (d) show the same formula. In option c, all the arms of carbon atoms are occupied.

17. Ethanol reacts with sodium and forms two products. These are
(a) sodium ethanoate and hydrogen
(b) sodium ethanoate and oxygen
(c) sodium ethoxide and hydrogen
(d) sodium ethoxide and oxygen
Ans. (c) Sodium ethoxide and hydrogen
Explanation: This reaction can be shown by following equation:

2Na + 2CH₃CH₂OH → 2CH₃CH₂ONa + H₂

18. The correct structural formula of butanoic acid is


Ans. (d)
Explanation: Formula of butanoic acid is C₃H₇COOH.

19. Vinegar is a solution of
(a) 50% - 60% acetic acid in alcohol
(b) 5% - 8% acetic acid in alcohol
(c) 5% - 8% acetic acid in water
(d) 50% - 60% acetic acid in water
Ans. (c) 5% - 8% acetic acid in water

20. Mineral acids are stronger acids than carboxylic acids because
(i) mineral acids are completely ionized
(ii) carboxylic acids are completely ionized
(iii) mineral acids are partially ionized
(iv) carboxylic acids are partially ionized
(a) (i) and (iv)
(b) (ii) and (iii)
(c) (i) and (ii)
(d) (iii) and (iv)
Ans. (a) (i) and (iv)
Explanation: Mineral acids are completely ionized but carboxylic acids are partially ionized. Hence, mineral acids are stronger acids than carboxylic acids.

21. Carbon forms four covalent bonds by sharing its four valence electrons with four univalent atoms, e.g. hydrogen. After the formation of four bonds, carbon attains the electronic configuration of
(a) helium
(b) neon
(c) argon
(d) krypton
Ans. (b) Neon
Explanation: Electronic configuration of carbon is 2, 4 and after sharing four electrons from four univalent atoms, its electronic configuration becomes 2, 8 which is same as that of Neon.


22. The correct electron dot structure of a water molecule is
(a) (b)  (c)  (d)


Ans. (c) Explanation: This dot structure shows a complete octet after oxygen shared two electrons with two univalent atoms of hydrogen.

23. Which of the following is not a straight chain hydrocarbon?


Ans. (d)
Explanation: In this option, three carbon atoms are sharing electrons with univalent atoms. In other options, only two carbon atoms are sharing electrons with univalent atoms.


24. Which among the following are unsaturated hydrocarbons?

(a) (i) and (iii)
(b) (ii) and (iii)
(c) (ii) and (iv)
(d) (iii) and (iv)
Ans. (c) (ii) and (iv)
Explanation: These compounds show double and triple bonds. Hence they are unsaturated hydrocarbons

25. Which of the following does not belong to the same homologous series?
(a) CH₄
(b) C₂H₆
(c) C₃H₈
(d) C₄H₈
Ans. (d) C₄H₈

Explanation: The general formula for this compound is C_nH_2n, while that of other options is C_nH_2n+2.


26. The name of the compound CH₃— CH₂— CHO is
(a) Propanal
(b) Propanone
(c) Ethanol
(d) Ethanal
Ans. (b) Propanone
Explanation: This compound has −CHO as active radical. Hence, one suffix is used.


27. The heteroatoms present in CH₃— CH₂— O — CH₂— CH₂Cl are
(i) oxygen
(ii) carbon
(iii) hydrogen
(iv) chlorine
(a) (i) and (ii)
(b) (ii) and (iii)
(c) (iii) and (iv)
(d) (i) and (iv)
Ans. (d) (i) and (iv)
Explanation: Oxygen and chlorine are not among essential components of hydrocarbons.

28. Which of the following represents saponification reaction?


Ans. (d)
Explanation: When ester is treated with an alkali, the reaction gives ethanol and sodium ethanoate. This reaction is called saponification reaction.


29. The first member of alkyne homologous series is
(a) ethyne
(b) ethene
(c) propyne
(d) methane
Ans. (a) Ethyne

H :C ::: C :H  

Ans. alkyne

Carboxylic acid is ethanoic acid and its structural formula is as follows:

Alcohol is ethanol and its structural formula is as follows:

Compound X is as ester (ethyl ethanoate) and its structural formula is as follows:

Detergents are generally ammonium or sulphonate salts of long chain carboxylic acids. The charged ends of these compounds do not form insoluble precipitate with calcium or magnesium ions. Calcium and magnesium ions are present in hard water. Due to this, detergents are effective even in hard water. Hence, detergents are better cleansing agents than soaps.

(a) CH₃ CO CH₂ CH₂ CH₂ CH_3x

Ans. ketone

(b) CH₃ CH₂ CH₂ COOH

Ans. carboxylic acid

(c) CH₃ CH₂ CH₂ CH₂ CHO 

Ans. aldehyde

(d) CH₃ CH₂ OH

Ans. alcohol

Ethanol is heated at 443 k in presence of excess of concentrated sulphuric acid to obtain ethene. This reaction can be given by following equation:

Methanol is oxidized to methanol in liver. Methanal coagulates the cytoplasm in cells; which kills the cells. Methanal also acts on optic nerves and causes blindness. Thus, intake of even small quantity of methanal can be lethal.

When ethanol reacts with sodium, then hydrogen gas is evolved. This reaction can be given by following equation:

2 Na + CH₃CH₂OH → 2CH₃CH₂ONa +H₂

In this reaction, sulphuric acid. What is the role of sulphuric acid in this hydrogen from saturated hydrocarbon to form unsaturated hydrocarbon.

(a) chlorine (Group 17 of Periodic Table)

Ans. carbon tetrachloride (CCl₄)

(b) oxygen (Group 16 of Periodic Table)

Ans. Carbon dioxide (CO₂)

(a) The atomic number of chlorine is 17. Write its electronic configuration
Ans. Electronic configuration of chlorine: 2,8,7
(b) Draw the electron dot structure of chlorine molecule.
Ans. Electron dot structure of chlorine molecule:

Both carbon and silicon show the ability of catenation. But compounds made by silicon are reactive and less stable. On the other hand, bonds formed by carbon are very strong and hence organic compounds are more stable than silicon compounds. It can be said that carbon shows better catenation ability than silicon.

Saturated hydrocarbons generally burn with a clean flame. Unsaturated hydrocarbons burn with yellow flame and produce lot of soot. So, when ethane is burnt, it burns with a clean flame and no soot is produced. When ethene is burnt, it burns with yellow flame and produces lot of soot.

Match the reactions given in Column (A) with the names given in column (B).

Ans. (a)→(iv) (b)→(i) (c)→(ii) (d)→(iii)

you have named. Also, write chemical equation of the reaction involved. Ans. Salt X is sodium ethanoate and the evolved gas is carbon dioxide. Take a test tube and take ethanoic acid in it. Add sodium bicarbonate to the acid. Close the mouth of the test tube with a cork and attach a delivery tube to it. Take lime water in another test tube and attach this with another and of delivery tube.

You will observe that the lime water turns milky. This shows that the evolved gas is carbon dioxide. Following reaction takes place between ethanoic acid and sodium hydrogen carbonate.

CH₃ COOH + NaHCO₃ → CH₃ COONa + CO₂ + H₂ O

Ans (a). Compounds of hydrogen and carbon are called hydrocarbons, e.g. methane, ethane, ethene, etc.

Ans (b). All the bonds in saturated hydrocarbons (except in active radicals) are single bonds
but double or triple bond is present in unsaturated hydrocarbons.
 Examples of saturated hydrocarbons: Ethane and methane
 Examples of unsaturated hydrocarbons: Ethyne and ethene 

Ans(c). Functional group – An atom/group of atoms joined in a specific manner which is
responsible for the characteristic chemical properties of the organic compounds. 
Examples are hydroxyl group (–OH), aldehyde group (–CHO), carboxylic group (–COOH) etc. 
 

Addition reaction is used in the conversion of vegetable oils to fats. This is called hydrogenation of oil. Vegetable oils generally have long chains of unsaturated carbons, while animal fats have long chains of saturated carbons. Addition reaction is carried out in the presence of Ni as a catalyst. This reaction can be given by following equation:

It is important to note that unsaturated fat is healthy for eating. But saturated fat is unhealthy.

Ans. Carbon tetrachloride: CCl4

Ans(b). When ester treated with an alkali, the reaction gives ethanol and sodium ethanoate. This reaction is called saponification reaction because this reaction forms the basis of preparation of soap. This can be given by following equation. CH₃COOC₂H₅ +  NaOH → CH₃COONa + C₂H₅OH

Take 1 ml ethanol and 1 ml glacial acetic acid in a test tube. Pour a few drops of conc. sulphuric acid in the test tube.

Gently warm the test tube over a water bath for about 5 minutes. After that, take out the test tube and transfer its contents in a beaker. Take the smell of the content. The sweet smell shows that ester has been made. This reaction can be given by following equation:

C is ethanoic acid. It reacts with sodium to form sodium ethanoate. Hence, compound R is sodium ethanoate or sodium acetate. We know that hydrogen gas burns with a pop sound. This reaction can be given by following equation:

2CH₃COOH  + 2Na → 2CH₃COONa + H₂ 

When ethanoic acid reacts with methanol in the presence of an acid, we get (methyl ethanoate) ester which is a sweet smelling substance. Hence, compound S is methyl ethanoate and A is methanol. This reaction can be given as follows:

CH₃COOH  + CH₃OH  --Acid → CH₃COOH-CH₃ 

When sodium hydroxide is added to ethanoic acid, it gives sodium ethanoate and water.

CH₃COOH  + NaOH  → CH₃COONa  + H₂O 

When methyl ethanoate is treated with NaOH solution, it gives back methanol and sodium ethanoate.

CH₃COO-CH₃ + NaOH  → CH₃OH + CH₃COONa

Compound A is ethanol. When it is heated with concentrated sulphuric acid, we get ethene. So, compound B is ethene. This can be shown by following equation

CH₃CH2OH  ------Conc.H₂SO₄→ CH₂  = CH₂ + H₂O

When ethene is heated in the presence of nickel, we get ethane. This can be shown by following equation.

CH₂  = CH₂  ------Ni→ C₂H₆ (Compound C)

When 1 mole of ethane is burnt, we get 2 moles of carbon dioxide and 3 moles of water; as shown by following equation.

C₂H₆ + 7O₂ → 4CO₂ + 6H₂O

Addition of hydrogen to unsaturated hydrocarbon to obtain a saturated hydrocarbon is called hydrogenation reaction. Hydrogenation is used for converting unsaturated vegetable oil into fat. This can be shown by following equation:

When oxygen is added to alcohol to make carboxylic acid, this is called oxidation reaction. Following is an example of oxidation of alcohol

Saturated hydrocarbons are fairly unreactive. But in the presence of sunlight, chlorine substitutes hydrogen one by one.

CH₄ + Cl₂ → CH₃Cl + HCl (in the presence of sunlight) 

This substitution can go on till, carbon tetrachloride is obtained.

When ester is treated with an alkali, the reaction gives ethanol and sodium ethanoate. This reaction is called saponification reaction because this reaction forms the basis of preparation of soap. This can be given by following equation. 

CH₃COOC₂H₅  + NaOH → CH₃COONa + C₂H₅OH

When a substance burns in the presence of oxygen, the reaction is called combustion. Following is an example of combustion reaction.

CH₄ + O₂ →CO+ H₂O + heat and light

(a) What change would you observe in the calcium hydroxide solution taken in tube B?
Ans. Calcium hydroxide solution in the test tube B becomes milky.
(b) Write the reaction involved in test tubes A and B respectively.
Ans. Reaction in test tube A:

CH₃COOH + NaHCO₃ → CH₃COONa + CO₂ + H₂O

Reaction in test tube B:
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

(c) If ethanol is given instead of ethanoic acid, would you expect the same change?
Ans. Ethanol does not react with sodium hydrogen carbonate. Hence, same change cannot be observed.

(d) How can a solution of lime water be prepared in the laboratory?
Ans. For this, take distilled water in a beaker and mix calcium carbonate powder in it. After stirring thoroughly, wait till the mixture settles. Decant the clear liquid from the beaker. This liquid is lime water.

(a) ethanol to ethene. Ans. Ethanol is heated at 443 k in the presence of excess of conc. sulphuric acid. This reaction is called dehydrogenation. This reaction can be given by following equation:

(b) propanol to propanoic acid. Ans. Propanol is treated with alkaline potassium permanganate or acidified potassium dichromate to obtain propanoic acid.

Following are the isomers of this compound.

Following is the electron dot structure: