Carbon always forms Covalent Bonds
1. It could gain four electrons to form C4-ions having inert (i.e., neon) gas configuration with 8 electrons in the valence shell. But this anion would be highly unstable because it would be very difficult for a nucleus with only 6 protons to hold on 10 electrons, i.e., 4 extra electrons.
2. It could lose 4 electrons to form C4+ions having inert (i.e., helium) gas configuration with 2 electrons in the valence shell. But it would require a large amount of energy to remove four electrons because due to small size, the valence electrons of carbon are strongly held by the nucleus. Thus, formation of C4+cations is energetically not favourable.
A bond formed between two atoms by mutual sharing of electrons between them so as to complete their octets or duplets (in case of elements having only one shell) is called a covalent bond and the number of electrons contributed by each atom is known ascovalency.
Types of Covalent Bonds
The covalent bonds are of three types. If each atom contributes one electron, the covalent bond formed is called a single covalent bond.
Comparison of properties of Ionic and Covalent Compounds
|Ionic compounds||Covalent compounds|
|1. Mode of formation: They are formed by complete transfer of electrons from one atom to the other.For example,||1. Mode of formation: They are formed by mutual sharing of electrons between the two atoms.For example,|
|2. Physical state: Ionic compounds are generally solids. For example, NaCl (sodium chloride), MgO (magnesium oxide), MgCl2(magnesium chloride), etc.||2. Physical state: These compounds may be solid, liquid and gases. For example, Cl2(chlorine) is a gas, Br2(bromine) is a liquid while I2(iodine) is a solid.|
|3. Melting points and boiling points: Due to strong forces of attraction between positive and negative ions, the melting points and boiling points of ionic compounds are quite high.||3. Melting points and boiling points: Due to weak intermolecular forces of attraction, covalent compounds generally have low melting and boiling points.|
|4. Solubility: ‘Like dissolves like’ is the general rule of solubility. Thus, ionic compounds being polar are more soluble in polar solvents like water but are insoluble in non-polar or organic solvents such as alcohol, benzene, petrol, ether, chloroform, etc.||4. Solubility: Covalent compounds being non-polar are generally insoluble in polar solvents like water but are soluble in non-polar or organic solvents like alcohol, benzene, petrol, ether, chloroform, etc.|
|5. Electrical conductivity: Ionic compounds do not conduct electricity in the solid state but do so in the molten state or in their aqueous solutions. For example, solid sodium chloride does not conduct electricity because Na+and Cl– ions are strongly attracted by each other. However, in molten state, sodium chloride splits to form Na+ and Cl– ions. Similarly, in water, sodium chloride ionises to form Na+(aq) and Cl–(aq). Since ions carry current, therefore, sodium chloride conducts electricity both in the molten state as well as in the aqueous solution.||5. Electrical conductivity: Covalent compounds do not contain ions and hence are generally bad conductors of electricity.|
|6. Nature of reactions: Ionic compounds undergo ionic reactions which are very fast, almost instantaneous and always proceed to completion. For example,AgNO3 (aq) + NaCl (aq) → Silver nitrate Sodium chloride AgCl (s) ↓ NaNO3 (aq)Silver chloride (white ppt.) Sodium nitrate||6. Nature of reactions: Covalent compounds undergo molecular reactions which are slow and never proceed to completion. For example, CH4 (g) + Cl2 (g) Diffused sunlight→Methane Chlorine CH3Cl (g) + HCl (g)Methyl chloride Hydrogen chloride|
Allotropic forms of carbon
The phenomenon of existence of an element in two or more forms which have different physical properties but identical chemical properties is called allotropy and the different forms are called allotropic forms or simply allotropes.
Carbon occurs in three crystalline allotropic forms:
Comparison of properties of diamond and graphite
|1. Diamond has a three-dimensional network structure.||1. Graphite has a two-dimensional sheet like structure consisting of a number of benzene rings fused together.|
|2. It is the hardness natural substance known.||2. Graphite is soft and greasy and is used as solid lubricant for heavy machinery operating at high temperature.|
|3. It is a bad conductor of electricity but is a very good conductor of heat. Because of hardness and high thermal conductivity, diamond tipped tools do not overheat and hence are extensively used for cutting and drilling purposes.||3. It is a good conductor of both heat and electricity. Because of high electrical conductivity, graphite is used for making electrodes of battery and arcs.|
|4. It is a transparent substance with high refractive index. Therefore, it is used for making gemstones and jewellery.||4. It is an opaque greyish black substance.|
Versatile nature of carbon
1. Catenation: This unique property of self-linking of carbon atoms through covalent bonds to form long straight or branched chains and rings of different sizes is called catenation.
2. Tetracovalency of carbon: Carbon has a valency of four. Therefore, it is capable of bonding four other atoms of carbon or atoms of some other monovalent elements. Further, due to small size, the nucleus of carbon atom can hold its shared pairs of electrons strongly. As a result, the bonds that carbon forms with most of the other elements such as hydrogen, oxygen, nitrogen, sulphur, chlorine, etc. are very strong thereby making these compounds exceptionally stable. This further increases the number of carbon compounds.
3. Tendency to form multiple bonds: Due to small size, carbon also forms multiple (double and triple) bonds with other carbon atoms, oxygen and nitrogen. This multiplicity of carbon-carbon, carbon-oxygen and carbon-nitrogen bonds further increases the number of carbon compounds.
4. Isomerism: If a given molecular formula represents two or more structures having different properties, the phenomenon is called isomerism and the different structures are called isomers.
Vital force theory
Organic compounds are produced only under the influence of some mysterious force existing in the living organisms. This mysterious force was called the vital force.
Alkanes, Alkenes and Alkynes
Compounds of carbon and hydrogen are called hydrocarbons. These are of the following two types:
1. Saturated hydrocarbons
2. Unsaturated hydrocarbons
Saturated hydrocarbons containing open chain structures are called alkanes. Their general formula is CnH2n+2where n= 1, 2, 3…., etc. For examples, methane, ethane, propane, etc. On the other hand, saturated hydrocarbons containing cyclic structures are called cycloalkanes. Their general formula is CnH2nwhere n=3, 4, 5…., etc. For example, cyclopropane, cyclobutane, cyclopentane, cyclohexane, etc.
Unsaturated hydrocarbons containing open chain structures are called alkenes and alkynes. Those which contain one double bond are called alkenes (e.g., ethane, propene, etc.) while those containing one or more triple bond are called alkynes (e.g., ethane, propene, etc.) while those containing one or more triple bond are called alkynes (e.g., ethyne, propyne, etc.). The general formula of alkenes is CnH2nand those of alkynes is CnH2n-2where n= 2, 3, 4….., etc.
A functional group may be defined as an atom or a group of atoms present in a molecule which largely determines its chemical properties.
A homologous series may be defined as a family of organic compounds having the same functional group, similar chemical properties and the successive (adjacent) members of which differ by a CH2unit or 14 mass units.
Nomenclature of carbon compounds
The prefixes and suffixes of the common functional groups are given below:
|6. Carboxylic acid||—-||oic acid|
|7. Double bond (alkenes)||—-||ene|
|8. Triple bond (alkynes)||—-||yne|
Since carbon dioxide is formed by addition of oxygen to carbon, therefore, combustion may be regarded as an oxidation reaction.
Addition of oxygen to any substance is called oxidation and the substances which are capable of adding oxygen to other substances are called oxidising agents.
Reactions which involve addition of two reactants to form a single product are called addition reactions.
Catalysts are substances which can change usually increase the speed of a chemical reaction without being used up in that reaction.
Reactions which involve the direct replacement (displacement or substitution) of an atom or a group of atoms in an organic molecule by another atom or group of atoms without any change in the rest of the molecule are called substitution reactions.
CH4 + Cl2 sunlight or 520-670K → CH3Cl + HCl
Methane Chlorine Chloromethane Hydrogen chloride
Physical properties of Ethanol
1. Physical state, melting point and boiling point: Ethanol is a colourless liquid at room temperature. Its freezing point is 156K while its boiling point is 351K.
2. Smell and taste: It has a distinct smell and a burning taste.
3. Solubility: Ethanol in soluble in water in all proportions.
4. Physiological action: Ethanol is commonly called alcohol. It is an active ingredient of all alcoholic drinks such a beer, rum, whisky, brandy, etc. Consumption of small quantities of dilute ethanol causes drunkenness.
Uses of Ethanol
1. Ethanol in form of rectified spirit (95% alcohol + 5% water) is used as an antiseptic for wounds. It is also used for sterilizing skin before giving an injection.
2. Ethanol is used in alcoholic beverages, i.e., beer, rum, whisky, brandy, etc.
3. Ethanol is widely used in industry as a solvent for paints, lacquers, tincture iodine, cough syrups, perfumes, etc.
4. It is also used in the preparation of dyes, cosmetics and transparent soaps.
5. It is used in the manufacture of number of other chemicals such as chloroform, iodoform, ether, acetic acid, acetaldehyde, etc.
6. It is used as an important laboratory reagent for carrying out organic reactions and for crystallization of organic compounds.
7. Ethanol is used as a fuel in internal combustion engines in form of power alcohol.
8. In cold countries, ethanol is used as an antifreeze in the radiators of automobiles.
9. It is used in spirit levels and low temperature thermometers.
Harmful effects of drinking alcohol
Consumption of small quantities of alcohol causes intoxication. When large quantities of alcohol are consumed, it tends to slow down metabolic processes and tends to depress the central nervous system. This results in lack of coordination, mental confusion, drowsiness, lowering of the normal inhibitors, and finally stupor (lethargy). The individual may feel relaxed but does not realise that his sense of judgement, sense of timing and muscular coordination has been seriously impaired. The consumption of alcohol in large quantities may even cause death by damaging liver and kidney.
This reaction between a carboxylic acid and an alcohol to form an ester is called the esterification reaction.
Soaps and detergents
Soaps and detergents are cleansing agents. Soaps are sodium or potassium salts of long chain carboxylic acids. These long chain carboxylic acids are also called fatty acids because they are obtained by hydrolysis of oils and fats.
Detergent are usually used to make shampoos and other products for cleaning clothes.
Q1. What is the properties of Ionic and Covalent Compounds.
Q2. What is Catenation?
Q3. Difference between detergents and Soaps?