Q1. Covalent compounds have low melting and boiling points. Why? (Term II, 2021-22, 2020)
Ans. Covalent compounds have low melting and boiling points because the forces of attraction between molecules of covalent compounds are very weak. On applying a small amount of heat these molecular forces break.
Q2. Write the name of an allotrope of carbon. (2021C)
Q3. How are covalent bonds formed? (2020)
Ans. Covalent bonds are formed by sharing of electron pair between two atoms. Covalently bonded molecules are observed to have stronger bonds within the molecule but intermolecular forces are weak.
Q4. Covalent compounds are generally poor conductors of electricity. Why? (2020)
Ans. Covalent compounds do not contain ions and hence, are generally poor conductors of electricity.
Q5. Name a cyclic unsaturated carbon compound. (2020)
Ans. A cyclic unsaturated carbon compound is a compound that contains a carbon ring with one or more double or triple bonds between the carbon atoms. An example of a cyclic unsaturated carbon compound is benzene, which has a ring of six carbon atoms with alternating double bonds.
Q6. Write the molecular formula of first two members of homologous series having functional group -CI. (Delhi 2017)
Ans. The molecular formula of first two members of homologous series having - Cl functional group are CH3CI and CH3CH2CI.
Q7. Write the molecular formula of first two members of homologous series having functional group -OH. (Delhi 2017)
Ans. The molecular formula of first two members of homologous series having - OH functional group are CH3OH and CH3CH2OH.
Q8. Write the molecular formula of the 2nd and 3rd member of the homologous series whose first member is ethene. (AI 2017)
Ans. Homologous series of alkenes have general formula, CnH2n whose first member is ethene.
2nd member of homologous series of alkenes is C3H6 i.e., propene.
3rd member of homologous series of alkenes is C4H8 i.e., butene.
Q9. Write the molecular formula of the 2nd and 3rd member of the homologous series whose first member is methane. (AI 2017)
Ans. Methane, CH4 is an alkane. Alkan es have general formula, CnH2n+2.
2nd member of homologous series of alkanes is C2H6 i.e., ethane.
3rd member of homologous series of alkanes is C3H8 i.e., propane.
Q10. Write the next homologue of each of the following: (Delhi 2016)
Ans. (i) C2H4 belongs to alkene series having general formula, CnH2n.
Thus, next homologue will be C3H2x3 = C3H6
(ii) C4H6 belongs to alkyne series having general formula, CnH2n-2.
Thus, next homologue will be C5H2x5-2 = C5H8
Q11. Name the following compounds: (Delhi 2016)
Ans. (a) CH3 - CH2 - OH: Ethanol (Alcohol)
Q12. Select saturated hydrocarbons from the following: (Delhi 2016)
C3H6; C5H10; C4H10; C6H14; C2H4
Ans. Saturated hydrocarbons (alkanes) have general formula, CnH2n+2. Among the given compounds, only C4H10 and C6H14 satisfy the above formula. Thus, these are saturated hydrocarbons. Unsaturated hydrocarbons have the general formula CnH2n (alkene) and CnH2n-2 (alkyne).
Q13. Write the name and structure of an alcohol with three carbon atoms in its molecule. (Al 2016)
Ans. An alcohol with three carbon atoms in its molecule is propanol. The structure of propanol is
Q14. Write the name and structure of an alcohol with four carbon atoms in its molecule. (AI 2016)
Ans. An alcohol with four carbon atoms is butanol and its structure is:
Q15. Write the name and structure of an aldehyde with four carbon atoms in its molecule. (AI 2016)
Ans. An aldehyde with four carbon atoms is butanal and its structure is:
Q16. Which element exhibits the property of catenation to maximum extent and why? (Foreign 2016)
Ans. Carbon has the unique ability to form bonds with other atoms of carbon, giving rise to large molecules. This property is called catenation. Carbon shows catenation due to its small size and stronger carbon-carbon bond strength.
Q17. Write the name and molecular formula of the fourth member of alkane series. (Foreign 2016)
Ans. The general formula of the alkane series is CnH2n+2. For fourth member of alkane series, n = 4
∴ C4H2 x 4 + 2 = C4 H10 i.e., butane.
Q18. What is homologous series of carbon compounds? (Foreign 2016)
Ans. A homologous series is the family of organic compounds having the same functional group, similar chemical properties but the successive (adjacent) members of the series differ by a —CH2 unit or 14 mass units.
Q19. Write the name and formula of the 2nd member of homologous series having general formula CnH2n. (Delhi 2015)
Ans. Homologous series of alkenes have general formula, CnH2n whose first member is ethene.
2nd member of homologous series of alkenes is C3H6 i.e., propene.
Q20. Write the name and formula of the 2nd member of homologous series having general formula CnH2n+2. (Delhi 2015)
Ans. Alkanes have general formula, CnH2n + 2.
2nd member of homologous series of alkanes is C2H6 i.e., ethane.
Q21. Write the name and formula of the 2nd member of homologous series having general formula CnH2n-2. (Delhi 2015)
Ans. General formula, CnH2n-2 belongs to alkyne series. The second member of this series is propyne i.e., (C3H4) or CH3—C≡CH.
Q22. Write the number of covalent bonds in the molecule of ethane. (Al 2015, Delhi 2014)
Ans. The structural formula of ethane (C2H6) is
There are total 7 covalent bonds. Six C — H covalent bonds and one C — C covalent bond.
Q23. Write the number of covalent bonds in the molecule of butane, C4H10. (AI 2015)
Ans. Butane (C4H10) has the following structural formula as:
Total number of covalent bonds is 13 in which there are 10C — H and 3C - C covalent bonds.
Q1. "Carbon prefers to share its valence electrons with other atoms of carbon or with atoms of other elements rather than gaining or losing the valence electrons in order to attain noble gas configuration.” Give reasons to justify this statement. (Term II, 2021-22)
Ans. Carbon has atomic number 6 with four electrons in the valence shell. If it gains four electrons in order to complete its octet, it will lead to the formation of C4- anion. The addition of four electrons in valence shell j will result in strong electronic repulsions between eight I electrons now present in valence shell. Hence, it will be : difficult for the nucleus to hold on to ten electrons. On the other hand, if it loses four electrons, it forms C4+ cation. It would require a large amount of energy to remove four electrons. Hence, carbon prefers to share its valence electrons with other atoms of carbon to attain noble gas configuration. The bond formed is known as covalent bond.
Q2. Draw the electron dot structure of the molecules of (a) Oxygen, and (b) Nitrogen. The atomic numbers of oxygen and nitrogen are 8 and 7 respectively. (Term II, 2021-22 C)
Ans. (a) Formation of oxygen molecule:
(b) Formation of nitrogen molecule:
Q3. Carbon forms compounds mainly by covalent bonding. Why? (Term II, 2021-22 C)
Ans. Carbon can form only covalent compounds because carbon can neither gain nor lose four electrons to acquire stable octet. The only way by which it can acquire the nearest noble gas configuration is by sharing its valence electrons with other C-atoms or atoms of other elements. Hence, carbon forms compounds mainly by covalent bonding.
Q4. State the reason why
(i) carbon compounds have low melting and boiling points.
(ii) carbon compounds do not conduct electricity.
(iii) carbon can form only covalent compounds. (Term II, 2021-22)
Ans. (i) Due to weak intermolecular forces of attraction, carbon compounds generally have low melting and boiling points.
(ii) Carbon compounds do not contain ions and hence, are generally poor conductors of electricity.
(iii) Carbon can form only covalent compounds because carbon can neither gain nor lose four electrons to acquire stable octet. The only way by which it can acquire the nearest noble gas configuration is by sharing its valence electrons with other C-atoms or atoms of other elements.
Q5. (a) Draw the electron dot structure for ethyne. (Term II, 2021-22)
(b) List two differences between the properties exhibited by covalent compounds and ionic compounds.
Q6. (a) Write the molecular formula of the following carbon compounds:
(b) Carbon compounds have low melting and boiling points. Why? (Term II, 2021-22)
Ans. (a) (i) Methane (CH4)
(ii) Propane (CH3CH2CH3) or C3H8
(b) Due to weak intermolecular forces of attraction, covalent compounds generally have low melting and boiling points.
Q7. Consider the carbon compounds having following molecular formula:
(a) State the number of double covalent bonds present in C3H6.
(b) Write the formula of first member of the homologous series to which the carbon compound C4H6 belongs.
(c) Which one of the above compounds forms ring structure of carbon atoms?
(d) Identify, which of the above compounds, is a member of alkane series. (Term II, 2021-22)
Ans. (a) C3H6(or CnH2n, n = 3) i.e, alkene series thus, has one double covalent bond.
(b) C4H6 (or CnH2n-2, n = 4) i.e., alkyne series.
The first member of alkyne series is ethyne (C2H2) HC ≡ CH.
(c) C6H12 can form ring structure of C-atoms.
(d) Alkane series: Cn H2n+2
Only C3H8 is a member of alkane series CH3 — CH2 — CH3
Q8. The molecular formulae of two alkynes, A and B are CxH2 and C3Hy respectively.
(a) Find the values of x and y.
(b) Write the names of A and B. (Term II, 2021-22)
Ans. (a) General formula of alkyne CnH2n - 2
For CxH2, 2n - 2 = 2 ⇒ n = 2 ∴ x = 2.
For C3Hy, n = 3, y = 2n - 2 = 2 x 3 - 2 = 4
Hence, x = 2, y = 4
(b) A is ethyne (C2H2) and B is propyne (C3H4).
Q9. What is a homologous series? Find the difference in molecular mass between the two consecutive members of a homologous series. State how in a homologous series of carbon compounds the following properties vary with increase in molecular mass: (Term II, 2021-22)
(i) Melting and boiling points
(ii) Chemical properties
Ans. A homologous series is the family of organic compounds having the same functional group, similar chemical properties but the successive (adjacent members of the series differ by a —CH2 unit or 14 mas units.
(i) As the molecular mass increases in a homologous series, melting and boiling points also increases.
(ii) Chemical properties remains same for the members of homologous series because they all have same functional group.
Q10. Draw two different possible structures of a saturated hydrocarbon having four carbon atoms in its molecule. What are these two structures of the hydrocarbon having same molecular formula called? Write the molecular formula and the common name of this compound. Also write the molecular formula of its alkyne. (Term II, 2021-22)
These are called structural isomers as they have the same j molecular formula i.e., C4H10 but different structures. As i the molecular formula is C4H10, common name of this | compound is butane. i The alkyne of four carbon atoms is butyne. Its structure i is as follows:
The molecular formula of butyne is C4H6.
Q11. (i) Write the molecular formula of benzene and draw its structure.
(ii) Write the number of single and double covalent bonds present in a molecule of benzene.
(iii) Which compounds are called alkynes? (Term II, 2021-22)
Ans. (i) Molecular formula of benzene is C6H6.
(ii) Number of single bonds = 9
Number of double bonds = 3
(iii) Alkynes are the unsaturated hydrocarbons that contain at least one triple bond between two carbon atoms (—C ≡ C—)
Q12. Consider the following organic compounds:
(a) Name the functional group present in these compounds.
(b) Write the general formula for the compounds of this functional group.
(c) State the relationship between these compounds and draw the structure of any other compound having similar functional group (Term II, 2021-22)
Ans. (a) Aldehyde (—CHO) group.
(c) Compound (i) is propanal, and compound (ii) is ethanal. They belong to the same homologous series where each successive compound differs from each other by a—CH2 unit.
Other member of same homologous series:
Q13. Carbon, a member of group 14, forms a large number of carbon compounds estimated to be about three million. Why is this property not exhibited by other elements of this group? Explain. (2020)
Ans. Carbon has the unique ability to form bonds with other atoms of carbon, giving rise to large molecules. This property is called catenation. Carbon shows catenation due to its small size and stronger carbon-carbon bond strength. As we move down the group, the element-element bond energies decrease rapidly. For this reason, other elements of this group show little or no catenation property.
Q14. 3 mL of ethanol is taken in a test tube and warmed gently in a water bath. A 5% solution of alkaline potassium permanganate is added first drop by drop to this solution, then in excess.
(i) How is 5% solution of KMnO4 prepared?
(ii) State the role of alkaline potassium permanganate in this reaction. What happens on adding it in excess?
(iii) Write chemical equation of this reaction. (2020)
Ans. (i) 5% solution of KMnO4 is prepared by adding 5 g of KMnO4 in 95 g of water.
(ii) Here alkaline KMnO4 acts as an oxidising agent. It oxidises ethanol to ethanoic acid by donating nascent oxygen. If excess of KMnO4 is added the purple colour will persist indicating no more alcohol is left and reaction stops.
Q15. What is a homologous series of carbon compounds? Give an example and list its three characteristics. (2019)
Ans. A homologous series is defined as a group of compounds having the same functional group, similar chemical properties in which the successive members differ by a -CH2 group or 14 mass unit.
For example, in alkane homologous series, the general formula is CnH2n+2 i.e.; first three members are CH4. C2H6 and C3H8 where two successive members differ by -CH2 group.
Characteristics of homologous series:
Q16. Write the molecular formula of ethene and draw its electron dot structure. (2019)
Ans. Molecular formula of ethene is C2H4. Its electron dot structure is:
Q17. A compound 'X' on heating with excess conc. sulphuric acid at 443 K gives an unsaturated compound 'Y'. 'X' also reacts with sodium metal to evolve a colourless gas 'Z'. Identify 'X', 'Y and 'Z. Write the equation of the chemical reaction of formation of 'Y' and also write the role of sulphuric acid in the reaction. (2018)
Ans. As X reacts with conc. H2SO4 to give an alkene so it should be an alcohol as cone. H2SO4 acts as a dehydrating agent. The reaction of X with Na also confirms that it is an alcohol because alcohols react with Na metal to evolve colourless hydrogen gas.
Here, cone H2SO4 acts as a dehydrating agent i.e., helps in the removal of water.
Q18. (a) Why are most carbon compounds poor conductors of electricity?
(b) Write the name and structure of a saturated compound in which the carbon atoms are arranged in a ring. Give the number of single bonds present in this compound. (2018)
Ans. (a) Due to catenation, carbon forms covalent bonds with the constituent elements in the carbon compounds, hence it does not have mobile electrons and carbon compounds do not dissociate themselves into ions and hence, they are poor conductor of electricity.
Number of single bonds: 15
Q19. Two carbon compounds X and Y have the molecular formula C4H8 and C5H12 respectively. Which one of these is most likely to show addition reaction? Justify your answer. Also give the chemical equation to explain the process of addition reaction in this case. (Delhi 2017)
Ans. All unsaturated hydrocarbons (containing double or triple bonds) have tendency to get converted to saturated hydrocarbons (single bonds) by adding small molecules such as hydrogen (H2), halogens (X2), etc. Such reactions are called addition reactions. Compound with the molecular formula C4H8 belongs to alkene series (CnH2n). Hence, it will undergo addition reaction.
Q20. What happens when (write chemical equation in each case) (a) ethanol is burnt in air?
(b) ethanol is heated with excess cone. H2SO4 at 443 K?
(c) a piece of sodium is dropped into ethanol? (Al 2017)
(c) When a small piece of sodium is dropped into ethanol then hydrogen gas is liberated which burns with a pop sound.
2C2H5OH + 2Na → 2C2H5C-Na+ + H2↑
Q21. Distinguish between esterification and saponification reaction with the help of the chemical equations for each. State one use of each (i) esters, and (ii) saponification process. (Al 2017)
Use of esters: They are used for making perfumes or used as artificial flavouring substances.
Use of saponification process: This process is used in making soaps.
Q22. Explain esterification reaction with the help of a chemical equation. Describe an activity to show esterification. (Al 2017)
Ans. When alcohol is added to carboxylic acid in the presence of acid catalyst then a fruity smelling ester is formed. This process is called esterification.
Aim: To demonstrate esterification process using ethanol and acetic acid.
Materials required: Beaker, water, test tube, ethanol, acetic acid. cone. H2SO4, tripod stand, burner, w ire gauze, etc.
Observations: Pleasant fruity smelling compound (called ester) is formed.
Conclusion: Carboxylic acid reacts with alcohol in presence of cone. H2SO4 which acts as a dehydrating agent to form esters.
Q23. When ethanol reacts with ethanoic acid in the presence of cone. H2SO4, a substance with fruity smell is produced. Answer the following:
(i) State the class of compounds to which the fruity smelling compounds belong. Write the chemical equation for the reaction and write the chemical name of the product formed.
(ii) State the role of cone. H2SO4 in this reaction. (Delhi 2016)
Ans. (i) When ethanol reacts with ethanoic acid in presence of cone. H2SO4, ethyl ethanoate is formed which belongs to the class of ester compounds, having fruity smell.
(ii) The above reaction is called esterification which occurs in presence of cone. H2SO4 which acts as a dehydrating agent and helps in the removal of water. Cone. H2SO4 also acts as a catalyst to speed up the reaction.
Q24. Write chemical equation of the reaction of ethanoic acid with the following:
(b) Sodium hydroxide;
Write the name of one main product of each reaction. (Al 2016)
Ans. Ethanoic acid reacts with sodium as well as sodium hydroxide to form sodium ethanoate.
Q25. Write three different chemical reactions showing the conversion of ethanoic acid to sodium ethanoate. Write balanced chemical equation in each case. Write the name of the reactants and the products other than ethanoic acid and sodium ethanoate in each case. (Al 2016)
Ans. Ethanoic acid reacts with Na2CO3 to form sodium ethanoate and CO2 gas is liberated.
With sodium hydrogen carbonate it forms sodium ethanoate.
With NaOH It forms sodium ethanoate.
Q26. List two tests for experimentally distinguishing between an alcohol and a carboxylic acid and describe how these tests are performed. (Al 2015)
Ans. Tests for distinguishing between an alcohol and a carboxylic acid are:
(i) Litmus test: When we place a drop of carboxylic acid on blue litmus paper it turns red while alcohol will not change the colour of blue litmus paper.
(ii) Sodium hydrogen carbonate test/sodium carbonate
test: If a pinch of NaHCO3 or Na2CO3 is added to two test tubes containing alcohol and carboxylic acid respectively, then test tube containing carboxylic acid will show the evolution of colourless gas with brisk effervescence while test tube containing alcohol does not show any reaction.
Q27. What are esters? How are they prepared? List two uses of esters. (Delhi 2014)
Ans. Esters are compounds with functional group COOR i.e.
R and generally volatile liquids having pleasant fruity smell.
Esters are prepared when a carboxylic acid reacts with an alcohol in the presence of small amount of concentrated H2SO4. For example, when ethanoic acid reacts with ethanol it forms an ester (i.e. ethyl ethanoate).
Uses of ester:
1. It is used in making perfumes.
2. It is used in making artificial flavours and essences used in ice-creams, sweets and cold drinks.
Q1. Explain why carbon forms compounds mainly by covalent bonds. Explain in brief two main reasons for carbon forming a large number of compounds. Why does carbon form strong bonds with most other elements? (2023)
Ans. As carbon has four valence electrons and it can neither loose nor gain four electrons thus, it attains noble gas configuration only by sharing of electrons. Thus, it forms covalent compounds. The existence of large number of compounds is due to some unique properties of carbon which are:
(i) Carbon atoms possess an unique property to link together to form very long chains. This property is referred to as catenation.
A large number of carbon atoms can join together to form straight chains, branched chains and rings as shown below:
(ii) Due to small size and presence of four valence electrons, a carbon atom can form multiple bonds with some other carbon atoms as well as with other atoms like oxygen, nitrogen etc., This increases the variety of compounds formed by it and hence the number of compounds is tremendously increased.
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, etc. are very strong there by making these compounds exceptionally stable.
Q2. (i) Draw the structure of the following compounds:
(a) Butanoic acid
(ii) How are structure (i) and structure (ii) given below related to one another?
Give reason to justify your answer.
Draw one more possible structure for above case.
(iii) Differentiate between saturated and unsaturated carbon compounds on the basis of their general formula. (2023)
Ans. (i) (a) Butanoic acid : CH3CH2CH2COOH
(b) Chloropentane: CH3CH2CH2CH2CH2CI
(ii) Structures (i) and (ii) are chain isomers.
Chain isomers have the same molecular formula but differ in the order in which the carbon atoms are bonded to each other.
One more structure for the given case is shown below:
(iii) The general formula of saturated carbon compounds: CnH2n + 2 (Alkane)
The general formula of unsaturated carbon compounds: CnH2n(Alkene), CnH2n-2 (Alkyne)
Q3. (i) Draw two structural isomers of butane.
(ii) Draw the structures of propanol and propanone.
(iii) Name the third homologue of:
(iv) Name the following:
(b) CH3 - CH2CH = CH2
(v) Show the covalent bond formation in nitrogen molecule. (2023)
Ans. (i) Structural isomers of butane are the following:
(iii) (a) Three homologue of alcohol are the following:
CH3OH, CH3CH2OH, CH3CH2CH2OH
Third homologue of alcohol is CH3CH2CH2OH
(b) Three homologue of aldehyde are the following:
HCHO, CH3CHO, CH3CH,CHO
Third homologue of aldehyde is CH3CH2CHO.
(iv) (a) Benzene (b) But-1-ene
(v) Z = 7,
Q4. (i) Write the name and general formula of a chain of hydrocarbons in which an addition reaction with hydrogen can take place. Stating the essential condition required for an addition reaction to occur, write the chemical equation giving the name of the reactant and product of such a reaction. How is an addition reaction different from a substitution reaction?
(ii) Write the structure of benzene. (2023)
Ans. (i) In addition reactions, an unsaturated hydrocarbon combines with other molecules to give a single product. Addition reactions are characteristic reactions of unsaturated hydrocarbons as other molecules are added at the multiple bond to give a saturated product. Addition reactions are given by all unsaturated hydrocarbons (alkenes, CnH2n and alkynes, C,H2n-2)
The essential conditions tor addition reaction are:
- Presence of unsaturated hydrocarbon.
- Presence of catalyst such as Ni/Pt/Pd.
Addition reaction of ethene with hydrogen: Ethene combines with hydrogen when heated in presence of nickel catalyst to give ethane.
The addition of hydrogen molecule to an unsaturated hydrocarbon to obtain a saturated hydrocarbon is called hydrogenation. The process requires the presence of nickel or palladium metals as catalyst.
Substitution reactions 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. Whereas addition reactions involve addition of a reagent to unsaturated hydrocarbons to form a single product.
(ii) The structure of benzene (C6H6) is
Q5. Write the chemical equation for the following:
(i) Combustion of methane
(ii) Oxidation of ethan
(iii) Hydrogenation of ethene
(iv) Esterification reaction
(v) Saponification reaction (2023)
Ans. The process in which compounds of carbon react with oxygen to give carbon dioxide, water, heat and light, is known as combustion. Alkanes burn in air and release large amount of heat, therefore can be used as excellent fuels.
(ii) Oxidation is a process in which oxygen is added to a substance.
(iii) Hydrogenation means addition of hydrogen to an unsaturated compound.
(iv) When alcohol is added to carboxylic acid in the presence of acid catalyst then, a fruity smelling ester is formed. This process is called esterification.
(v) Esters react in the presence of an acid or a base to give the alcohol and carboxylic acid. This reaction is known as saponification because it is used in the preparation of soap.
Q6. (i) What happens when a small piece of sodium is dropped in ethanol? Write the equation for this reaction.
(ii) Why is glacial acetic called so?
(iii) What happens when ethanol is heated at 443 K in the presence of cone. H2SO4? Write the role of cone. H2SO4 in this case.
(iv) Write an equation showing saponification. (2023)
Ans. (i) Ethanol reacts with sodium to produce sodium ethoxide and hydrogen gas.
2C2H5OH + 2Na → 2C2H5ONa + H2
(ii) Pure acetic acid is known as glacial acetic acid. The melting point of pure ethanoic acid is 290 K and hence it often freezes during winter in cold climates. This gives rise to its name glacial acetic acid.
(iii) CH3CH2OH + H2SO4(conc.) CH2 = CH2 + H2O
Ethanol on heating with excess of concentrated sulphuric acid at 443 K, releases water molecule to form ethene. In this reaction, concentrated sulphuric acid acts as dehydrating agent.
(iv) Esters in the presence of alkali (NaOH) give alcohol and sodium salt of carboxylic acid. This process is known as saponification because it is used in the preparation of soaps.
Q7. (a) Define isomerism. Draw all possible isomers of butane.
(b) "A compound ‘X’ on combustion gives a yellow flame with lots of smoke.” What inference would you draw from this statement ?
(c) State the role of alkaline KMnO4 in the reaction involving conversion of an alcohol to corresponding carboxylic acid. (2020)
Ans. (a) Isomers are those molecules which have the same molecular formula but different structural formulae i.e., show different properties and the phenomenon is called isomerism. The structures of possible isomers of butane (C4H10) are:
(b) The compound 'X’ is an unsaturated compound (alkene or alkyne) which burn in air with a yellow sooty flame (producing black smoke).
(c) Alkaline KMnO4 acts as an oxidising agent which oxidise alcohol (—OH) to corresponding carboxylic acid (—COOH).
Q8. (a) What is a homologous series? Explain with an example.
(b) Define the following terms giving one example of each.
(ii) Addition reaction (2020)
Ans. (a) A homologous series is the family of organic compounds having the same functional group, similar chemical properties but the successive (adjacent) members of the series differ by a —CH2 unit or 14 mass units.
For example, alkane series has general formula CnH2n + 2.
First member of homologous series of alkane is methane, i.e., CH4.
Second member of homologous series of alkane is ethane, i.e., C2H6.
Third member of homologous series of alkane is propane i.e., C3H8.
(b) (i) Carboxylic acids react with alcohols in the presence of a little concentrated sulphuric acid to form pleasant smelling esters. This reaction is called esterification reaction.
(ii) Those reactions in which atoms or group of atoms are simply added to a double or triple bond without the elimination of any atom or molecule, are known as addition reactions.
Q9. (a) Carry out following conversions:
(i) Ethanol to ethene
(ii) Ethanol to ethanoic acid
(b) Differentiate between addition reaction and substitution reaction. Give one example of each. (2020)
Ans. (a) (i) When ethanol is heated with cone. H2SO4 at 443 K, ethene is obtained due to dehydration of ethanol.
(ii) When 5 % alkaline KMn04 solution is added drop by drop to warm ethanol then it gets oxidised to ethanoic acid.
(b) Addition reactions: Those reactions in which atoms or group of atoms are simply added to a double or triple bond without the elimination of any atom or molecule, are known as addition reactions.
Substitution reactions: The reactions which involve the displacement or substitution of an atom or a group of atoms in an organic compound by another atom or group of atoms, are known as substitution reactions. Saturated hydrocarbons are fairly unreactive and inert in the presence of most of the reagents. However, in presence of sunlight, hydrocarbons undergo rapid substitution reactions. e.g.,
Q10. (a) A compound 'X' undergoes addition reaction with H2 to form a compound Y having molecular mass 30 g mol-1. ‘X’ decolorises bromine water and burns with a smoky flame. Identify ‘X’ and Y and write chemical equations of the reactions involved.
(b) Write the structural formulae of (i) Butanone, and (ii) Pentanoic acid.
(c) Would you be able to check if water is hard by using a detergent ? Give reason to justify your answer. (2020 C)
Ans. (a) As the molecular mass of Y is 30 g mol-1, it is ethane (C2H6 = 12 x 2 + 6 x 1 = 30).
‘X’ is ethene (CH2 = CH2) which decolourises Br2- water and burns with a smoky flam.
(c) No. we are not able to check if water is hard by using a detergent as detergent works well in hard water as well. This is because calcium and magnesium salts of detergents are soluble in water and hence, detergents can be used for washing even in hard water.
Q11. (a) Carry out the following conversions giving complete conditions for the reaction to take place in each case:
(i) Ethanoic acid from Ethanol
(ii) Ethane from Ethene
(iii) Ester from Ethanoic acid and ethanol Also state the names given to all the above conversions.
(b) Detergents are preferred over soaps. Why? (Give one reason) (2019 C)
Ans. (a) (i) Oxidation of ethanol to ethanoic acid:
Here , alkaline KMnO4 or acidified K2Cr2O7 dichromate are oxidising agents.
(ii) Addition reaction:
(b) Detergents work well with hard water and are biodegradable.
Q12. (a) Compare soaps and detergents on the basis of their composition and cleansing action in hard water.
(b) What happens when ethanol is treated with sodium metal? State about the behaviour of ethanol in this reaction.
(c) Draw the structure of cyclohexane.
(d) Name the following compound. (2018)
(b) when ethanol is treated with sodium metal observation received are-
Hydrogen gas is evolved and it behaves like an acid.
(c) structure of cyclohexane is below-
(d) Name of the compound is Ethanal and it is an Acetaldehyde
Q13. Soaps and detergents are both types of salts. State the difference between the two. Write the mechanism of the cleansing action of soaps. Why do soaps not form lather (foam) with hard water? Mention any two problems that arise due to the use of detergents instead of soaps. (Delhi 2017, Al 2015)
Ans. The molecules of soap are sodium or potassium salts of long-chain carboxylic acids. Detergents are generally ammonium or sulphonate salts of long chain carboxylic acids.
Difference between Soap and Detergent
(i) Soaps are sodium salts of long chain carboxylic acids.
(ii) The ionic group in soap is -COO-Na+.
(iii) Soaps are not useful when water is hard.
(i) Detergents are the odium salt of long chain benzene sulphonic acids.
(ii) The ionic groups in detergents is SO3-Na+ or SO4-Na+.
(iii)Detergent can be used for washing purpose even when water is hard
Cleansing action of soap can be described as follows:
The soap molecule is generally represented as RCOONa. In solution, it ionises to form RCOO- and Na+. Each soap molecule has a polar head group (carboxylate ion, COO group) and a long nonpolar hydrocarbon tail (R group from long chain fatty acid). The polar head attracts the polar water molecule and is called hydrophilic end and the nonpolar tail attracts the water-insoluble oily or greasy dirt particles.
Q14. Why are certain compounds called hydrocarbons? Write the general formula for homologous series of alkanes, alkenes and alkynes and also draw the structure of the first member of each series. Write the name of the reaction that converts alkenes into alkanes and also write a chemical equation to show the necessary conditions for the reaction to occur. (AI 2017)
Ans. Compounds consisting of carbon and hydrogen are known as hydrocarbons.
(a) Saturated hydrocarbons: Alkanes (CnH2n+2) are the compounds of carbon that have a single bond.
(b) Unsaturated hydrocarbons: The compounds of carbon having double bonds are alkene (CnH2n) and having triple bonds are alkyne (CnH2n-2). The reaction which converts unsaturated hydrocarbons to saturated hydrocarbons i.e. alkenes to alkane is known as hydrogenation reaction. It is used to obtain ghee from oil.
Q15. What are micelles? Why does it form when soap is added to water? Will a micelle be formed in other solvents such as ethanol also? State briefly how the formation of micelles help to clean the clothes having oily spots. (Foreign 2016)
Ans. A micelle is a submicroscopic aggregate of molecules with non-polar groups on the inside and hydrophilic groups on the outside. If ethanol is used in place of water, micelle will not be formed as the solution will not have any polar part.
Q16. With the help of an example, explain the process of hydrogenation. Mention the essential conditions for the reaction and state the change in physical property with the formation of the product. (Delhi 2015)
Ans. The process of adding hydrogen to an unsaturated compound is called hydrogenation. For example, the hydrogenation of ethene leads to the formation of ethane.
Conditions for Hydrogenation
(i) Presence of an unsaturated compound (i.e. an unsaturated hydrocarbon)
(ii) Presence of a catalyst such as finely divided palladium or nickel
In case of the hydrogenation of vegetable oils, liquid unsaturated fatty acids are converted into solid saturated fatty acids.
|1. What is carbon?|
|2. What are compounds?|
|3. Why is carbon important in organic chemistry?|
|4. How many valence electrons does carbon have?|
|5. Give an example of a carbon compound.|