Chapter 4: Carbon and its Compounds
Covalent bonding and properties of carbon
Carbon has 4 electrons in its outermost shell, so it forms covalent bonds by sharing electrons. Carbon shows catenation (forming long chains and rings) and has a valency of 4. Covalent compounds have low melting and boiling points and are poor conductors of electricity. Allotropes of carbon are diamond, graphite and fullerene.
Hydrocarbons and homologous series
Saturated (alkanes, CnH2n+2): methane CH4, ethane C2H6, propane C3H8, butane C4H10. Unsaturated: alkenes (CnH2n, double bond, such as ethene C2H4) and alkynes (CnH2n-2, triple bond, such as ethyne C2H2). In a homologous series, compounds have the same functional group and successive members differ by CH2.
Functional groups and chemical reactions
Alcohol -OH (ethanol C2H5OH), aldehyde -CHO, ketone C=O, carboxylic acid -COOH (ethanoic acid CH3COOH).
Addition reaction: unsaturated hydrocarbon + H2 (Ni catalyst) โ saturated hydrocarbon.
Esterification, soaps and detergents
Esterification: acid + alcohol โ ester + water (in the presence of concentrated H2SO4). Saponification: ester + NaOH โ alcohol + soap. Soap molecules form micelles that remove dirt. In hard water soap forms scum, but detergents do not.
Important questions
Q1. Why does carbon form covalent bonds?
Carbon has 4 electrons in its outermost shell. It neither gives nor takes electrons easily, so it shares electrons.
Q2. What is catenation?
The property of carbon atoms joining with each other to form long chains, branches and rings.
Q3. Name two allotropes of carbon.
Diamond and graphite (fullerene is another).
Q4. What is a covalent bond?
A bond formed by the sharing of a pair of electrons between two atoms.
Q5. Why do covalent compounds have low melting and boiling points?
Because the forces of attraction between their molecules are weak.
Q6. Why are most carbon compounds poor conductors of electricity?
They do not form ions because electrons are shared, so there are no charged particles to carry current.
Q7. What are saturated hydrocarbons? Give an example.
Hydrocarbons that have only single bonds between carbon atoms, such as methane. They are called alkanes.
Q8. Write the general formulae of alkanes, alkenes and alkynes.
Alkanes: CnH2n+2. Alkenes: CnH2n. Alkynes: CnH2n-2.
Q9. What is special about the structure of ethene (C2H4)?
There is a double bond between its two carbon atoms, so it is an unsaturated hydrocarbon.
Q10. What is a homologous series?
A series of compounds with the same functional group, in which successive members differ by a CH2 unit.
Q11. Name the functional groups in ethanol and ethanoic acid.
Ethanol has -OH (alcohol) and ethanoic acid has -COOH (carboxylic acid).
Q12. Write the formulae of ethanol and ethanoic acid.
Ethanol: C2H5OH. Ethanoic acid: CH3COOH.
Q13. What is combustion of carbon compounds?
Burning in oxygen to give carbon dioxide, water, heat and light. CH4 + 2O2 โ CO2 + 2H2O.
Q14. What is an addition reaction? State one use.
Addition of hydrogen to an unsaturated hydrocarbon in the presence of a nickel catalyst. This is how vegetable oils are turned into vanaspati ghee.
Q15. What is a substitution reaction?
A reaction in which hydrogen atoms of a hydrocarbon are replaced by other atoms. For example, CH4 + Cl2 โ CH3Cl + HCl (in sunlight).
Q16. What is esterification?
The formation of an ester from an acid and an alcohol in the presence of concentrated H2SO4: CH3COOH + C2H5OH โ CH3COOC2H5 + H2O. Esters have a sweet smell.
Q17. What is saponification?
The reaction of an ester with sodium hydroxide that gives an alcohol and the sodium salt of the acid (soap).
Q18. How does ethanoic acid react with sodium hydrogencarbonate?
CO2 is released with brisk effervescence: CH3COOH + NaHCO3 โ CH3COONa + H2O + CO2.
Q19. What are micelles and how does soap clean?
Soap molecules form spherical clusters (micelles) whose water-hating tails trap oil and dirt inside while the water-loving heads stay outside, so the dirt is washed away with water.
Q20. Soap does not work well in hard water but detergents do. Why?
Soap forms an insoluble scum with calcium and magnesium salts. The corresponding salts of detergents are soluble in water.
Quiz
1. What is the general formula of alkanes?
The general formula of alkanes is CnH2n+2.
2. Which is the functional group of an alcohol?
Alcohols contain the -OH group.
3. Which is the hardest allotrope of carbon?
Diamond is the hardest allotrope of carbon.
4. What is formed on burning methane?
CH4 + 2O2 โ CO2 + 2H2O.
5. Which gas does ethanoic acid release when it reacts with NaHCO3?
CO2 is released with effervescence.