Carbon and its Compounds — Class 10 Science
Science · Chapter 4
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Chapter summary
Key ideas & terms
- Catenation
- Self-linking of carbon atoms into chains and rings.
- Covalent bond
- A bond formed by sharing electron pairs between atoms.
- Homologous series
- Compounds with the same functional group differing by –CH₂–.
- Micelle
- A cluster of soap molecules that traps oily dirt in water.
Extra questions & answers
Why does carbon form mostly covalent compounds?
With four valence electrons it can neither easily lose nor gain four electrons, so it shares them, forming covalent bonds.
What is the role of a micelle in cleaning?
The soap’s tails dissolve in oily dirt while the heads face water, lifting the dirt away as a suspended micelle.
Why does carbon form a very large number of compounds?
Because of catenation (its ability to bond with other carbon atoms forming long chains, branches and rings) and tetravalency (its capacity to form four covalent bonds).
What is a covalent bond, and why do carbon compounds have low melting points?
A bond formed by the sharing of electron pairs; carbon compounds are usually molecular with weak inter-molecular forces, so they melt and boil at low temperatures and conduct electricity poorly.
Distinguish between saturated and unsaturated hydrocarbons.
Saturated hydrocarbons (alkanes) have only single bonds between carbon atoms; unsaturated hydrocarbons (alkenes, alkynes) have at least one double or triple bond.
What is a functional group? Give two examples.
An atom or group of atoms that gives a compound its characteristic properties, e.g. the –OH (alcohol) group and the –COOH (carboxylic acid) group.
What is a homologous series?
A family of organic compounds with the same functional group and similar chemical properties, in which successive members differ by a –CH₂– unit.
Describe two important reactions of ethanol.
Ethanol burns in air (combustion) giving CO₂, water and heat, and on heating with excess concentrated sulphuric acid it undergoes dehydration to form ethene.
Why is ethanoic acid called glacial acetic acid, and how does it react with a base?
Pure ethanoic acid freezes in cold climates to an ice-like solid, hence "glacial"; with a base such as NaOH it gives sodium ethanoate (a salt) and water.
How do soaps clean, and why do they not work well in hard water?
Soap molecules form micelles that trap oily dirt with their tails while the heads stay in water, washing the dirt away; in hard water they react with calcium/magnesium ions to form scum, so detergents are used instead.
QUESTIONS
1. What would be the electron dot structure of carbon dioxide which has the formula
Carbon is the central atom and shares two pairs of electrons with each oxygen atom, forming two double covalent bonds. Each oxygen also has two lone pairs of electrons. The structure is
2. 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.)
The molecule is
QUESTIONS
1. How many structural isomers can you draw for pentane?
Pentane,
2. What are the two properties of carbon which lead to the huge number of carbon compounds we see around us?
The two properties are tetravalency and catenation. Tetravalency allows each carbon atom to form four covalent bonds, while catenation allows carbon atoms to form strong bonds with one another, producing long chains, branched chains and rings.
3. What will be the formula and electron dot structure of cyclopentane?
The molecular formula of cyclopentane is
4. Draw the structures for the following compounds.
(i) Ethanoic acid
(ii) Bromopentane*
(iii) Butanone
(iv) Hexanal.
*Are structural isomers possible for bromopentane?
(i) Ethanoic acid:
(ii) One structure of bromopentane is 1-bromopentane:
(iii) Butanone:
(iv) Hexanal:
Yes, structural isomers are possible for bromopentane because the bromine atom can occupy different positions and the five-carbon skeleton can also be straight or branched.
5. How would you name the following compounds?
(i)
(ii)
(iii)
(i) The compound
QUESTIONS
1. Why is the conversion of ethanol to ethanoic acid an oxidation reaction?
The conversion is an oxidation reaction because oxygen is added to ethanol and hydrogen is removed from it. In the presence of an oxidising agent, ethanol changes into ethanoic acid:
2. A mixture of oxygen and ethyne is burnt for welding. Can you tell why a mixture of ethyne and air is not used?
Ethyne burns completely in pure oxygen and produces a very hot, clean flame suitable for welding. Air contains only about one-fifth oxygen, so ethyne undergoes incomplete combustion in air and produces a yellow, sooty flame with less heat. Therefore, an ethyne–air mixture is unsuitable for welding.
QUESTIONS
1. How would you distinguish experimentally between an alcohol and a carboxylic acid?
Add sodium hydrogencarbonate to each substance. A carboxylic acid produces brisk effervescence of carbon dioxide, whereas an alcohol does not react. For ethanoic acid,
2. What are oxidising agents?
Oxidising agents are substances that cause oxidation by supplying oxygen to another substance or removing hydrogen from it. Alkaline potassium permanganate and acidified potassium dichromate are examples.
QUESTIONS
1. Would you be able to check if water is hard by using a detergent?
No. Detergents form lather in both hard and soft water because they do not produce insoluble precipitates with calcium and magnesium ions. Therefore, the amount of lather formed with a detergent cannot be used to test whether water is hard.
2. People use a variety of methods to wash clothes. Usually after adding the soap, they ‘beat’ the clothes on a stone, or beat it with a paddle, scrub with a brush or the mixture is agitated in a washing machine. Why is agitation necessary to get clean clothes?
Agitation helps loosen oily dirt from the fibres and brings it into contact with soap molecules. The hydrophobic ends of the soap molecules attach to the dirt while the ionic ends remain in water. Micelles then surround the loosened dirt, keeping it suspended in water so that it can be rinsed away.
EXERCISES
1. Ethane, with the molecular formula
(a) 6 covalent bonds.
(b) 7 covalent bonds.
(c) 8 covalent bonds.
(d) 9 covalent bonds.
(b) 7 covalent bonds. Ethane contains six
2. Butanone is a four-carbon compound with the functional group
(a) carboxylic acid.
(b) aldehyde.
(c) ketone.
(d) alcohol.
(c) ketone. Butanone contains the carbonyl group
3. 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.
(b) the fuel is not burning completely. An insufficient supply of oxygen causes incomplete combustion and the formation of soot, which blackens the vessel.
4. Explain the nature of the covalent bond using the bond formation in
Carbon has four valence electrons, each hydrogen atom has one, and chlorine has seven. In
5. Draw the electron dot structures for
(a) ethanoic acid.
(b)
(c) propanone.
(d)
(a) Ethanoic acid has the structure
(b)
(c) Propanone has the structure
(d)
6. What is an homologous series? Explain with an example.
A homologous series is a family of organic compounds having the same functional group and general formula, in which successive members differ by a
7. How can ethanol and ethanoic acid be differentiated on the basis of their physical and chemical properties?
Physically, ethanol has a characteristic alcoholic smell, whereas ethanoic acid has a sharp vinegar-like smell. Pure ethanoic acid may freeze in cold weather near
8. Why does micelle formation take place when soap is added to water? Will a micelle be formed in other solvents such as ethanol also?
A soap molecule has a hydrophilic ionic end that interacts with water and a hydrophobic hydrocarbon tail that avoids water. In water, many soap molecules arrange themselves with their tails directed inward and their ionic ends facing outward, forming a micelle. Ordinary micelles are not formed in ethanol because the hydrocarbon tails have appreciable interaction with ethanol and are not forced to gather away from the solvent in the same manner.
9. Why are carbon and its compounds used as fuels for most applications?
Carbon and many of its compounds release a large amount of heat when they burn. They generally have high calorific values, are readily available, and can often be stored and transported conveniently. When sufficient oxygen is supplied, many carbon fuels burn steadily to form mainly
10. Explain the formation of scum when hard water is treated with soap.
Hard water contains calcium and magnesium ions. These ions react with the soluble sodium or potassium salts present in soap to form insoluble calcium or magnesium salts of long-chain carboxylic acids. The insoluble, curdy precipitate produced is called scum. This reaction also reduces the amount of soap available for forming lather.
11. What change will you observe if you test soap with litmus paper (red and blue)?
Soap solution is alkaline. It turns red litmus paper blue, while blue litmus paper remains blue.
12. What is hydrogenation? What is its industrial application?
Hydrogenation is the addition of hydrogen to an unsaturated compound in the presence of a catalyst such as nickel or palladium. For example,
13. Which of the following hydrocarbons undergo addition reactions:
The unsaturated hydrocarbons
14. Give a test that can be used to differentiate between saturated and unsaturated hydrocarbons.
Add bromine water to the hydrocarbon and shake it. An unsaturated hydrocarbon decolourises bromine water because bromine adds across its double or triple bond. A saturated hydrocarbon does not decolourise bromine water under ordinary conditions. Alkaline potassium permanganate may also be used; its colour is discharged by an unsaturated hydrocarbon.
15. Explain the mechanism of the cleaning action of soaps.
A soap molecule contains a hydrophobic hydrocarbon tail and a hydrophilic ionic end. The tail attaches itself to oily dirt, while the ionic end remains in water. On agitation, many soap molecules surround each dirt particle, with their tails pointing inward and ionic ends pointing outward. This forms a micelle containing the dirt at its centre. The charged outer surface keeps the micelles suspended and prevents them from joining together, so the dirt can be washed away with water.
Group Activity
I Use molecular model kits to make models of the compounds you have learnt in this Chapter.
Models can be constructed for methane, ethane, ethene, ethyne, straight-chain and branched hydrocarbons, cyclic compounds, ethanol and ethanoic acid. The models should show carbon as tetravalent and should correctly represent single, double and triple covalent bonds.
II Take about
You can cut out the soap in fancy shapes. You can also add perfume to the soap before it sets.
Heating the vegetable oil with sodium hydroxide causes saponification. The fats or oils react with the alkali to produce soap and glycerol. Adding common salt reduces the solubility of the soap and separates it from the mixture. On cooling, the soap solidifies and can be cut into shapes; perfume may be added before it sets.