Class 10 Science Chapter 4 Important Questions

Food, clothes, medicines, books, or many of the things that you listed are all based on this versatile element carbon. In addition, all living structures are carbon based. The amount of carbon present in the earth’s crust and in the atmosphere is quite meagre. The earth’s crust has only0.02% carbon in the form of minerals (like carbonates, hydrogen- carbonates, coal and petroleum) and the atmosphere has 0.03% of carbon dioxide. In spite of this small amount of carbon available in nature, the importance of carbon seems to be immense.

(i) Formation of chlorine molecules: Two chlorine atom shares one electron of each and acquire stable electronic configuration.

(ii) Formation of carbon tetrachloride: Carbon atom has four electrons in the outermost shell. In the formation of carbon tetrachloride, it shares four electrons with four chlorine atoms. In turn four chlorine atoms shares one electron each with one carbon atom. Thus in all one carbon atom and four chorine atom complete their octet by mutual sharing of electrons and carbon tetrachloride containing four covalent bond is formed.

(iii) Formation of ammonia molecules: Nitrogen(N) atom has 5 electrons (2, 5) in the outermost shell. It shares one electron each of three hydrogen atom to form ammonia molecules. One unshared paired of electron in ammonia molecule is not involved in bond formation and is called a lone pair of electrons.

The bond between two atoms formed by sharing of two pairs of electrons is called a double bond. Two horizontal lines between two atoms denote a double bond example O≡O
(O₂)

Examples:
(i) Formation of Oxygen atom has six electrons in its outermost shell, two short of forming a complete octet. Thus each oxygen atom in O₂ shares 2 electrons with the other oxygen atom attain stable configuration.
(ii) Formation of ethylene molecule (C₂H₄): In ethylene two carbon atom shares two pair of electron between them and the rest two electrons on each carbon atom share one electron each with two hydrogen atoms. Thus ethylene molecules has one double bond(C=C) and four single bonds (C-H)

The bond formed between two atoms through the sharing of three electron pairs is called a triple bond. Three horizontal lines between two atoms denote a triple bond example nitrogen molecule is given as N≡N.

Examples:
(i) Formation of N₂ molecules: Nitrogen atoms have five electrons in their outermost shells. Therefore, in N2 molecules two nitrogen atom shares 3 electrons with each other, Thus acquiring the stable structure and forming a N₂ molecule with triple bond.
(ii) Formation of acetylene molecule(C₂H₂): In acetylene molecule, each carbon atom satisfies its valency by sharing one electron with a hydrogen atom and sharing the other three electrons with the other carbon atom.

Covalent compound exhibit the following properties:
(i) These have low melting and boiling point.
(ii) These do not conduct electricity either in the solid state or in the molten state or in the solution form. They are non-electrolyte.
(iii) These are readily soluble in non-polar solvent like benzene, carbon tetrachloride etc., but do not dissolve in polar solvent like water.
(iv) The covalent bond is directional in nature.

Covalent compounds have strong bonds within the molecule and intermolecular force are very weak and so have low melting and boiling points.

Covalent compound are formed due to sharing of electron between atoms and no charged particle are formed, hence such compound is generally bad conductor of electricity.

Elements of stable configuration exist in atomic form. For example, inert gases He, Ne, Ar, etc. Elements which are short of inert configuration by 1,2 or 3 electron shell (contain 5, 6, 7 electrons) achieve stable configuration by sharing requisite number of electron with other atoms and exist in molecular form. For example, chlorine atom has 7 electrons in their outermost shell, thus two Cl atoms share one electron of each other and exist as Cl-Cl (Cl₂).

Carbon has two unique features:
(i) It is tetravalent.
(ii) It has the property of catenation.
The above two feature enables carbon to form a large numbers of compounds with varying size, shape that’s why carbon atom has long chain or even are arranged in branches or rings. Because of tetra-valency carbon atom in its compound are linked by single, double or triple bonds. Carbon atom are also bonded with oxygen, hydrogen, chlorine, nitrogen and sulphur giving rise to compounds with specific properties typical of the element other than carbon present in the molecule.

Saturated hydrocarbons:
A hydrocarbon in which each carbon atom is attached to four other atoms is known as saturated hydrocarbon. The bonds so formed are single covalent bonds. These hydrocarbons are also called alkanes.
Unsaturated hydrocarbons:
Hydrogens containing either a carbon double bond (C=C) or a carbon-carbon triple bond in their molecules are called unsaturated hydrocarbon.

Carbon atom has a small size and this enables its nucleus to hold on the shared pair of electron quite strongly. This is why carbon forms strong bond with most other elements and the compounds formed are exceptionally stable as compared to covalent bond formed by other elements.

Saturated hydrocarbon having to carbon atom is ethane. Its chemical formula is C₂H₆.

(a) Compound containing carbon and hydrogen are called hydrocarbon example methane (CH₄).

(b) Structural difference- Saturated hydrocarbon have all its carbon connected to each other by only single bond whereas in unsaturated hydrocarbons there are one or more double or triple bonds between two carbon atoms.
Saturated hydrocarbon- methane (CH₄), ethane (C₂H₆).
Unsaturated hydrocarbon- ethane (C₂H₄), ethyne (C₂H₂).

Hydrocarbon can be classified either on the basis of nature of bonds or nature of chains between carbon atoms.

(a) On the basis of bonds, hydrocarbons are classified as
(i) Saturated hydrocarbon. Example CH₄ (methane).
(ii) Unsaturated hydrocarbon. Example C₂H₄ (ethene).

(b) On the basis of chains, hydrocarbons are classified as
(i) Aliphatic hydrocarbon: They have straight or branched chain of carbon atom. Example: butane and isobutene.
(ii) Cyclic hydrocarbon: These contain rings of carbon atoms. Example: Cyclohexane and benzene.

Silicon forms compound which have chains like carbon. Compound formed by silicon are few and are not so reactive.

We first write the carbon atoms joined together in a chain and then fill the remaining valences with hydrocarbon atoms. In this way we get following structures for saturate hydrocarbon having 3,4,5 and 6 carbon atoms respectively. Then name these according to the number of carbon atoms example propane, butane, pentane and hexane for saturate hydrocarbon containing 3,4,5 and 6 carbon atoms respectively.
Propane or C₃H₈
Butane or C₄H₁₀
Pentane or C₅H₁₂
Hexane or C₆H₁₄

The molecular formula for an alkane 4 carbon atom is C₄H₁₀. An alkene contains 2 hydrogen atom less than the corresponding alkane and an alkyne contain 4 hydrogen atom less than corresponding alkanes. Therefore, molecular formula of
(i) alkene is C₄H₈
(ii) alkyne is C₄H₆

The compound having a triple bond is called an alkyne. It has 4 hydrogen atom less than the corresponding alkane. Therefore, C₃H₄ is an alkyne. (Its corresponding alkane is C₃H₈).

Functional group.

A group which determines the chemical nature of an organic compound is called a functional group. For example, Cl in CH₃Cl, OH in CH₃OH are functional group.

The general formula for alkanes CₙH₂ₙ₊₂. Since an alkene contain two hydrogen atom less than the corresponding alkane, therefore the general formula for alkenes is CₙH₂ₙ. Further since an alkyne contain 2 hydrogen atoms less than the corresponding alkene hence the general formula for alkynes is CₙH₂ₙ₋₂.

(i) CH₄, C₂H₆, C₄H₁₀ represent homologous series called alkanes.

(ii) C₂H₄, C₃H₆ represent homologous series called alkenes.

(iii) C₂H₂, C₃H₄ represent homologous series called alkyne.

Two successive homologous should differ:
(i) by 14 in molecular mass and
(ii) one carbon atom and two hydrogen atom in terms of atoms in their molecules.

The phenomenon in which two or more compounds having same molecular formula may exist with different structural formulae is called isomerism.

1. The number of carbon atom in a hydrocarbon (or any other organic compounds) is indicated by the following steam:
One carbon atom is indicated by writing “Meth”
Two carbon atoms is indicated by writing “Eth”
Three carbon atoms is indicated by writing “Prop”
Four carbon atoms is indicated writing “But”
Five carbon atoms is indicating by writing “Pent”

2. Straight chain hydrocarbons
(i) A straight chain saturated hydrocarbon containing single bonds is indicating by writing the word “ane” after stem. For example, a four carbon atom saturated hydrocarbon(C₄H₁₀) is called butane and a-5c atom straight saturated hydrocarbon is called pentane (C₅H₁₂).
(ii) An unsaturated hydrocarbon containing a double bond is indicating by writing the word “ene” after the stem
(iii) An unsaturated hydrocarbon containing a triple bond is indicating by “yne” after the steam.

3. Branched chain saturated hydrocarbon: The following rule should be followed for naming branched chain saturated carbon.
(i)First find out the longest chain of carbon atoms in the structure of the compound. The compound is then named on the basis of number of carbon atoms present in the longest chain. (This is called parent hydrocarbon).
(ii) The methyl groups present as side chains are considered substituents and named separately, e.g. methyl (CH₃-) or ethyl (C₂H₅-) groups.
(iii) The carbon atoms of the parent hydrocarbon are numbered in such a way that the alkyl groups (substituents) get the lowest possible number. That is, the terminal carbon atom which is nearest to the carbon atom containing an alkyl group (or a substituent) is given number 1.
(iv) The position of the alkyl group is indicated by writing the number of carbon atom to which it is attached.
(v) The IUPAC name of the compound is obtained by writing the (position and name of alkyl group just before the name of the parent hydrocarbon.

4. Hydrocarbons containing functional groups: In case a functional group is present, it is indicated in the name of the compound with a prefix (for halides) and a suffix for other functional groups. If the name is to be given by a prefix, then halo (chloro, bromo, iodo) is written before the name of the corresponding hydrocarbon, e.g. chloropropane is CH₃CH₂CH₂Cl. If the name of the functional group is to be given by a suffix, the name of the carbon chain is modified by deleting the final “e” and substituting the appropriate suffix. For example, a two carbon chain with an aldehyde group would be named as:
(i) Ethane – e = ethan + “al” = ethanol
(ii) A three carbon chain with a ketone group
Popane – e = propan + “one” = propanone
(iii)A three carbon chain with an alcoholic group
Propane – e = propan + “ol” = propanol
(iv) If the carbon chain is unsaturated then final “ane” is the name of carbon is substituted by “ene” or “yne”; a three carbon chain with a double or triple bond.
Propane – ane = prop + “ene” = propane (double bond)
Propane –ane = prop + “yne” = propyne (triple bond)

(i) The given compound satisfied the formula of alkenes CₙH₂ₙ and has one double bond and is named as an alkene. Therefore, ending ene- is to be used.
(ii)C₃H₆ contains 3 carbon atoms in the main chain and the Greek word for 3 is prop. Therefore, the compound is propane.
(iii) The position of double bond is to be indicated and the carbon atom joined with a double bond should appear at the lowest possible number. When the carbon atoms are numbered from left hand side, the double bond appears between carbon 2 and 3. When the carbon atoms are numbered from the right hand side, the double bond appears between carbon atom 1 and carbon atom 2. Since we have to name it in such a way that the carbon joined by a double bond carries the lowest possible numbers. Therefore, the correct IUPAC name of this compound is 1-propane or simply propane.

The concept of functional group is important in organic chemistry for the following reasons:
(i) Functional groups are taken as the basis for naming of the organic compounds.
(ii) Organic compounds are classified on the basis of their functional groups.
(iii) Functional groups are the sites of chemical reactions. Compounds containing the same functional group show similar chemical behavior.

(i) The general formula of alkyl halides is CₙH₂ₙ₊₁.X, where X is the halogen atom (F, Cl, Br, I).
(ii) Longest carbon chain is chosen and the alkyl halide is named by adding the prefix: Fluro (F), Chloro (Cl), Bromo (Br) and Indo (I) before the alkane of the main chain.
(iii) The position of halogen atom is indicated by a number assigned to the carbon atom to which it is attached.
(iv) The prefixes di-or tri- are used if 2 or 3 halogen atoms of the same element are attached to the amin carbon chain.

(i) The functional group in CH₃CHBr is bromine.
(ii) Since it has two C-atoms and 2-Br atoms, it should be named as dibromo ethane.
(iii) Both the bromine atoms are attached to the same C-atom and so the carbon chain should be numbered from the C-atom attaching bromine atoms.
Common name: Ethylidene dibromide.
IUPAC name: 1, 1 dibromo ethane.
IUPAC name of BrCH₂CH₂Br: 1, 2 dibromo ethane.

(i) The functional group in CH₃CHO is the aldehyde group (-CHO.
(ii) CH₃CHO contains 2 atoms and is therefore, named as derivative of ethane.
(iii) Since CH₃CHO is an aldehyde, the suffix –e of ethane is replaced by the suffix-al.
(iv) The IUPAC name of CH₃CHO is Ethanal.
(v) The common name of CH₃CHO is Acetaldehyde.

General formula of carboxylic acid is R-COOH/ They are named by replacing suffix “e” of corresponding alkane as given from the total number of carbon atoms by “oic” and then adding at the end.
Examples:
Formula Common name Name (IUPAC)
HCOOH Formic acid Methanoic acid
CH₃COOH Acetic acid Ethanoic acid
C₂H₅COOH Propionic acid Propanoic acid

HCHO, CH₃COOH re aldehydes. HCOOH, CH₃COOH are carboxylic acids. CH₃OH and C₂H₅OH are alcohols. Out of these only carboxylic acids would turn blue litmus solution red. So HCOOH and Ch₃COOH would turn blue litmus solution red.

Next homologue of propanol-butanol (CH₃CH₂CH₂CH₂OH).

Next homologue of butanal-pentanal (CH₃CH₂CH₂CH₂CHO).

Four.

Fourth member-C₄H₉OH

Fifth member-C₅H₁₁OH.

(i) Wood, charcoal when ignited burn with a flame in the beginning due to vapourisation of volatile substances and then these just glows red and gives out heat without a flame.

(ii) Candle, LPG gas stove burns with a flame. This is because in these cases gaseous substance burn.

(iii) If there is insufficient oxygen/air soot is produced.

(i) Saturated hydrocarbons burn with a blue flame.

(ii) Unsaturated hydrocarbons will give a yellow flame lots of black smoke.

In all the above cases provision are made for the adequate supply of supporter substance, air (oxygen) that’s why
(i)In wood fired chullhas gaps are left between the logs so air can enter the burning site and can provide oxygen for the combustion of logs.
(ii) Grate is made in a coal or charcoal sigree or antigrithee so that supply of air(oxygen) will be adequate for the combustion of coal or charcoal.
(iii) A performed sheet is kept around the wicks so that air can reach the wicks easily through perforations or holes and supply of oxygen remains adequate for the combustion of kerosene.

Combustion is the burning of compounds in air to give CO₂ and water with the liberation of heat and light.
Oxidation is the reaction in which carbon compounds take up oxygen in the presence of oxidising agents to give another carbon compound. Thus all combustion reactions are also oxidation reaction but oxidation are not combustion reactions. In fact combustion means complete combustion.
For example
Combustion:
(i) Carbon compound on combustion(complete oxidation) CO₂ and H₂O
CH₄ + 2O₂ → CO₂ +2H₂O
Oxidation:
(ii) Alcohols are oxidised to acid in the presence of alkaline potassium permanganate or acidified potassium dichromate.
CH₃CH₂OH → CH₃COOH
(iii) With cold dilute alkaline potassium permanganate, ethene(ethylene) is oxidised to ethylene glycol.
CH₂ = CH₂ → CH₂OH

In an addition reaction an atom or group of atoms are added to an unsaturated compound while in substitution reaction an atom or a group present in saturated compound is replaced by another atom or group without the compound undergoing any change in its burn with yellow or sooty flame.
Examples:
(i) (a) Addition of chlorine by ethylene
CH₂ = CH₂ + Cl₂ → CH₂Cl-CH₂Cl
(b) Unsaturated hydrocarbon add up hydrocarbon in the presence of catalyst such as palladium or nickel to give saturated hydrocarbon.
CH₂ = CH₂ → CH₃ – CH₃
(ii) Substitution of chlorine for hydrogen in ethane.
C₂H₆ + Cl₂ → C₂H₅Cl + HCl

Physical properties of alcohols are:
(i) State: Most common alcohols are liquid.
(ii) Electrical conductivity: Like hydrocarbon they do not conduct electricity.
(iii) Solubility: Lower member of the series example methanol, ethanol, propanol are quite soluble in water. As the number of carbon increases the solubility readily decreases.
(iv) Action on sodium: They react with sodium produce hydrogen.
(v) Action of heat: They readily burns like hydrocarbons.
(vi) Boiling point: Boiling point of alcohol are higher than the hydrocarbon of the same molecular weight.

(i) It is a very important solvent for many organic compounds.
(ii) It is used to sterilise wounds and syringes.
(iii) It is used as solvent for laquers, varnishes, perfumes, medicine etc.
(iv) It is used in the manufacture of paints, dyes, medicines and a large number of organic compounds.
(v) It is used for making antifreeze mixture which are used in the radiators of motor vehicle in cold countries because water with ethyl alcohol has a much lower freezing point than water itself.
(vi) It is used for alcoholic drink like whisky, wine, beer other liquors.
(vii) It is also used as a fuel.

Fermentation: It is a slow process of breaking down of organic compounds by microorganism to simpler molecules. It is an exothermic process and is carried out at 20-30 degree Celsius.
Examples:
(i) Production of ethyl alcohol from molasses (cane sugar)
C₁₂H₂₂O₁₁ + H₂O → 4C₂H₅OH + 4CO₂.
(ii) Production of acetic acid from fermented liquors (15% ethyl alcohol)
CH₃CH₂OH → CH₃COOH

After the crystallization of cane sugar from syrup of sugarcanes, the solution still contains 60-70% solute comprising mainly of cane sugar fructose and glucose. This solution is known as molasses and is used from the production of alcohol.

Harmful effects of alcohol on human being:
(i) It is an intoxicant. Under the influence a person loss mental balance to distinguish between right and wrong.
(ii) Excessive of alcoholism affects the lungs of a person and may lead to his death.
(iii) It make the person to addicted to its uses.
(iv) Its consumption causes continuous economic loss to the family.
(v) If a person take alcohol adulterated with methanol or pyridine he is liable to lose his eye sight.

When a piece of sodium metal is poured over ethyl alcohol, vigorous reaction occurs and hydrogen gas is bubbled out.
2C₂H₅OH + 2Na → 2C₂H₅ONa + H₂

The reaction in which an alcohol combine with a carboxylic acid in the conc. sulphuric acids is called Esterification.

Water freeze at 0 degree Celsius. Thus in cold countries where the temperature falls below 0 degree Celsius, water cannot be used in radiators. Therefore, a mixture of water and alcohol is used because (i) water and alcohol are miscible in all proportions and (ii) a mixture of water and alcohol has much lower freezing point than that of water.

(i) To 1 cc of the given compound add 0.5 g sodium acetate and a few drops of concentrated sulphuric acid, pleasant fruit smell of an esters shows the presence of –OH, the hydroxyl group or alcoholic group in the compound.
(ii) To 2-3 cc of the compound add a few pieces of anhydrated CaCl₂. Keep it for a few minutes and then filter in a dry test tube. To the filtrate add a small piece of sodium metal. Evolution of hydrogen gas with brisk effervescence indicates the presence of an alcoholic group in the compound.

(i) Reactified spirit: It is ethyl alcohol containing 4.4% water.
(ii) Methylated spirit: It is reactified spirit to which 5-10% of poisonous substance such as methanol, pyridine or mineral naptha has been added to prevent its misuse for drinking.
(iii) Absolute alcohol: It is 100% ethyl alcohol totally free form of water or any other impurities.

Uses of Acetic acid or ethanoic acid are as follows:
(i) It is used as vinegar for preparing pickles.
(ii) It is used for the preparation of aspirin used for relieving headache.
(iii) It is used for preparing cellulose acetate which is an important artificial fibre.
(iv) It is used to coagulate rubber from latex.

Esters are the sweet smelling substances. These are used in making perfumes and flavouring agents. Esters are obtained when an alcoholic is warmed with a carboxylic acid in the presence of conc. sulphuric acid.
CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O
Esters react with acids or bases to give back the alcohol and the carboxylic acid example:
CH₂COOC₂H₅ + NaOH/HCl → CH₃COOH + C₂H₅OH

The reaction of an esters to react with an acid or base to give back the alcohol and carboxylic acid is called Saponification. This is so called because this reaction is used in the preparation of soap.

When soap is added to a sample of hard water, calcium and/or magnesium ions present in hard water react with soap forming insoluble calcium/magnesium soap which is sticky and greasy mass and thus no lather is formed.
Ca²⁺ + 2C₁₇H₃₅COONa → (C₁₇H₃₅COO)₂Ca + 2Na⁺

Soap or detergent consist of two part with distinct properties. One part or end consist of a large hydrocarbon tail which is hydrophobic. When a soap is added to the water it does not form a true solution but is dispersed in a way that the ionic end is in water and the hydrophobic part produce out of water. Inside water this given rise to the formation of clusters or molecules in which the hydrophobic tails are in the interior of the cluster and the ionic ends are on the surface of the cluster. This formation with a hundred or soap molecules is called a micelle.
Soap in the form of micelles is able to clean oil and dirt since it will be collected in the center of the micelle. When water is agitated the micelle with the oily dirt tend to lift off from the dirty surface and splits into fragments. This give opportunity to other tail to stick to oil if left. The negatively charged head present in water prevent the small globules so formed forming aggregates /precipitate because of ions repulsion. Thus dirt suspended in micelles is easily rinsed away.

The structure of synthetic detergent is similar to that of soaps. They have a long hydrocarbon chain which is water repelling and a short ionic part which is water attracting. The water attracting group in a synthetic detergent usually a sodium sulphonate group (SO₃⁻Na⁺) or sodium sulphate group(SO₄⁻Na⁺) The cleaning action of soap and detergent are similar. Thus chemically these they are not different. Difference lies only in the hydrocarbon part. Synthetic detergent made from long chain hydrocarbon obtained from petroleum whereas hydrocarbon part of soap or vegetable oils.
In soaps water attracting part is –COONa whereas in detergent it is –SO₃Na.
Detergent is being preferred to soap because these form lather even with hard water and have better cleaning action.

Detergents are the sodium salt of a long chain benzene sulphonic acids or a longer chain alkyl hydrogen sulphate. Most of the detergent have C₈ to C₁₂ hydrocarbon chain (a non-popular group) and a polar group SO₄⁻Na⁺. For example, sodium n-dodecyl benzene sulphonate and sodium n-dodecyl sulphate (or sodium lauryl sulphate).
Advantages of detergents: Ordinary washing soap (natural detergent) does not form lather in hard water or water containing Ca⁺⁺ or Mg⁺⁺ ions washing of clothes became difficult.
Synthetic detergents form lather even with hard water and wash clothes well.

Soap
1. When used with hard water a lot of it is wasted in removing Ca⁺⁺/Mg⁺⁺ salts as curdy precipitate.
2. It is completely oxidised by bacteria present in sewage and so it does not create any pollution problem in river.

Detergents
1. It forms good lather with hard water and thus can be safely used even with hard water.
2. Its excess creates pollution problem in river since it is not fully biodegradable.