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ICSE Class 10 Chemistry Question 3 of 10

Organic Chemistry - Unit Test Paper — Question 3

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Question 3

Give balanced equations for the following conversions:

  1. 1,2 dibromoethane→AAcetylene→BSilver acetylide\text{1,2 dibromoethane} \xrightarrow{\text{A}} \text{Acetylene} \xrightarrow{\text{B}} \text{Silver acetylide}
  2. Ethanol→CEthene←DEthyl iodide\text{Ethanol} \xrightarrow{\text{C}} \text{Ethene} \xleftarrow{\text{D}} \text{Ethyl iodide}
  3. Bromoethane→EEthane←FSodium propanoate\text{Bromoethane} \xrightarrow{\text{E}} \text{Ethane} \xleftarrow{\text{F}} \text{Sodium propanoate}
  4. Sodium ethanoate→GMarsh gas→G1Methanol→G2Methanal→G3Methanoic acid\text{Sodium ethanoate} \xrightarrow{\text{G}} \text{Marsh gas} \xrightarrow{\text{G}_1} \text{Methanol} \xrightarrow{\text{G}_2} \text{Methanal} \xrightarrow{\text{G}_3} \text{Methanoic acid}
  5. Sodium acetate + H2←HAcetic acid→H1Ethyl ethanoate\text{Sodium acetate + H}_2 \xleftarrow{\text{H}} \text{Acetic acid} \xrightarrow{\text{H}_1} \text{Ethyl ethanoate}
Answer

1. 1,2 dibromoethane→AAcetylene\text{1,2 dibromoethane} \xrightarrow{\text{A}} \text{Acetylene}

Write balanced chemical equation for the preparation of Ethyne from 1, 2 – dibromoethane and alcoholic potassium hydroxide. Organic Chemistry, Simplified Chemistry Dalal Solutions ICSE Class 10

Acetylene→BSilver acetylide\text{Acetylene} \xrightarrow{\text{B}} \text{Silver acetylide}

HC ≡ CH ethyne+2AgNO3+2NH4OH⟶Ag-C ≡ C-AgSilver Acetylide+2NH4NO3+2H2O\underset{\text{ ethyne}}{\text{HC ≡ CH}} + 2\text{AgNO}_3 + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{Silver Acetylide}}{\text{Ag-C ≡ C-Ag}} + 2\text{NH}_4\text{NO}_3 + 2\text{H}_2\text{O}

2. Ethanol→CEthene\text{Ethanol} \xrightarrow{\text{C}} \text{Ethene}

C2H5OH ethyl alcohol→170°CConc. H2SO4[excess]C2H4ethene+H2O \underset{\text{ ethyl alcohol}}{\text{C}_2\text{H}_5\text{OH}} \xrightarrow[170\degree\text{C}]{\text{Conc. H}_2\text{SO}_4\text{[excess]}} \underset{ \text{ethene}}{\text{C}_2\text{H}_4} + \text{H}_2\text{O}\

Ethyl iodide→DEthene\text{Ethyl iodide} \xrightarrow{\text{D}} \text{Ethene}

C2H5I ethyl iodide+KOH alcoholic→boilC2H4 ethene+KI+H2O\underset{\text{ ethyl iodide}}{\text{C}_2\text{H}_5\text{I}} + \underset{ \text{ alcoholic}}{\text{KOH}} {\xrightarrow{\text{boil}}} \underset{ \text{ ethene}}{\text{C}_2\text{H}_4} + \text{KI} +\text{H}_2\text{O}

3. Bromoethane→EEthane\text{Bromoethane} \xrightarrow{\text{E}} \text{Ethane}

C2H5Brbromoethane+2[H]nascent hydrogen→alcoholZn/Cu coupleC2H6ethane+HBr\underset{\text{bromoethane}}{\text{C}_2\text{H}_5\text{Br}} + \underset{\text{nascent hydrogen}}{2\text{[H]}} \xrightarrow[\text{alcohol}]{\text{Zn/Cu couple}} \underset{\text{ethane}}{\text{C}_2\text{H}_6} +\text{HBr}

Ethane←FSodium propanoate\text{Ethane} \xleftarrow{\text{F}} \text{Sodium propanoate}

C2H5COONasodium propanoate+NaOHsodalime→300°CCaOC2H6↑ethane+Na2CO3\underset{\text{sodium propanoate}}{\text{C}_2\text{H}_5\text{COONa}} + \underset{\text{sodalime}}{\text{NaOH}} \xrightarrow[300\degree\text{C}]{\text{CaO}} \underset{\text{ethane}}{\text{C}_2\text{H}_6 \uparrow} + \text{Na}_2\text{CO}_3

4. Sodium ethanoate→GMarsh gas\text{Sodium ethanoate} \xrightarrow{\text{G}} \text{Marsh gas}

CH3COONa sodium acetate+NaOHsodalime→300°CCaOCH4↑methane+Na2CO3\underset{\text{ sodium acetate}}{\text{CH}_3\text{COONa}} + \underset{\text{sodalime}}{\text{NaOH}} \xrightarrow[300\degree\text{C}]{\text{CaO}} \underset{\text{methane}}{\text{C}\text{H}_4 \uparrow} + \text{Na}_2\text{CO}_3

Methane to Methanol to Methanal to Methanoic acid

CH4 Methane→K2Cr2O7[O]CH3OHmethanol→K2Cr2O7[O]HCHOmethanal→K2Cr2O7[O]HCOOHmethanoic acid\underset{\text{ Methane} }{\text{CH}_4} \xrightarrow[\text{K}_2\text{Cr}_2\text{O}_7]{\text{[O]}} \underset{\text{methanol}}{\text{CH}_3\text{OH}} \xrightarrow[\text{K}_2\text{Cr}_2\text{O}_7]{\text{[O]}} \underset{\text {methanal}}{\text{HCHO}} \xrightarrow[\text{K}_2\text{Cr}_2\text{O}_7]{\text{[O]}} \underset{\text{methanoic acid}}{\text{HCOOH}}

5. Sodium acetate + H2←HAcetic acid\text{Sodium acetate + H}_2 \xleftarrow{\text{H}} \text{Acetic acid}

2CH3COOHAcetic acid+2Na⟶2CH3COONasodium acetate+H22\underset{\text{Acetic acid}}{\text{CH}_3\text{COOH}} + 2\text{Na} \longrightarrow \underset{\text{sodium acetate}}{2\text{CH}_3\text{COONa}} + \text{H}_2

Acetic acid→H1Ethyl ethanoate\text{Acetic acid}\xrightarrow{\text{H}_1} \text{Ethyl ethanoate}

C2H5OHethyl alcohol+CH3COOHAcetic acid→Conc. H2SO4CH3−COO−C2H5ethyl ethanoate+H2O\underset{\text{ethyl alcohol}}{\text{C}_2\text{H}_5\text{OH}} + \underset{\text{Acetic acid}}{\text{CH}_3\text{COOH}} \xrightarrow{\text{Conc. H}_2\text{SO}_4} \underset{\text{ethyl ethanoate}}{\text{CH}_3-\text{COO}-\text{C}_2\text{H}_5} + \text{H}_2\text{O}

Chapter Overview: Organic Chemistry

Organic Chemistry is the study of carbon compounds. Carbon's unique ability to form four covalent bonds and catenate (form long chains) makes organic chemistry vast and diverse. The ICSE syllabus covers hydrocarbons (alkanes, alkenes, alkynes), their nomenclature (IUPAC), structural formulae, isomerism, and characteristic reactions. Alkanes (CnH2n+2) are saturated hydrocarbons that undergo substitution reactions. Alkenes (CnH2n) and alkynes (CnH2n−2) are unsaturated and undergo addition reactions. Students learn homologous series, functional groups, and the distinction between saturated and unsaturated compounds. The chapter introduces alcohols (with −OH group) and carboxylic acids (with −COOH group) as basic functional group chemistry. Students must write structural formulae, name compounds using IUPAC rules, and understand reactions like combustion, substitution, and addition. Practical tests like decolourising bromine water or acidified KMnO4 to distinguish between saturated and unsaturated compounds are important.

Key Concepts & Homologous Series

Term / Series Details
CatenationAbility of carbon to form bonds with other carbon atoms, creating chains and rings
Homologous SeriesFamily of compounds with same general formula and functional group, differing by CH2
AlkanesCnH2n+2; single bonds only; saturated (e.g., CH4, C2H6)
AlkenesCnH2n; one C=C double bond; unsaturated (e.g., C2H4, C3H6)
AlkynesCnH2n−2; one C≡C triple bond; unsaturated (e.g., C2H2, C3H4)
IsomerismCompounds with same molecular formula but different structural arrangements
Functional GroupAtom or group responsible for characteristic chemical properties (−OH, −COOH, C=C)
IUPAC NamingPrefix (substituent) + Root (chain length) + Suffix (functional group)

Must-Know Concepts

  • Carbon prefixes: Meth- (1C), Eth- (2C), Prop- (3C), But- (4C), Pent- (5C)
  • Combustion: CH4 + 2O2 → CO2 + 2H2O (complete); 2CH4 + 3O2 → 2CO + 4H2O (incomplete)
  • Substitution: CH4 + Cl2 → CH3Cl + HCl (in presence of UV light)
  • Addition: C2H4 + Br2 → C2H4Br2 (ethene decolourises bromine water)
  • Unsaturated compounds decolourise bromine water and acidified KMnO4; saturated compounds do not
  • Ethanol: C2H5OH; Ethanoic acid: CH3COOH (vinegar); Ester: CH3COOC2H5 (fruity smell)
  • Isomers of butane (C4H10): n-butane and isobutane (2-methylpropane)

Saturated vs Unsaturated Hydrocarbons

Feature Saturated (Alkanes) Unsaturated (Alkenes/Alkynes)
BondsOnly single bonds (C−C)Double (C=C) or triple (C≡C) bonds
Typical ReactionSubstitutionAddition
Bromine WaterNo decolourisationDecolourised
CombustionClean blue flameSmoky/luminous flame (higher C%)

Important Diagrams to Practice

  • Structural formulae of first five members of alkanes, alkenes, and alkynes
  • Isomers of butane and pentane with structural formulae
  • Laboratory preparation of ethylene from ethanol (dehydration)

Common Mistakes

  • Writing wrong general formulae (alkenes are CnH2n, NOT CnH2n+2)
  • Confusing substitution (alkanes) with addition (alkenes) reactions
  • Not showing all bonds in structural formulae
  • IUPAC naming errors: not selecting the longest carbon chain or wrong numbering
  • Forgetting conditions (UV light for substitution, Ni catalyst for hydrogenation)

Scoring Tips

  • Draw clear structural formulae showing every C-H and C-C bond
  • For IUPAC naming: identify longest chain, number from the end nearest to substituent/functional group
  • Always mention conditions (catalyst, temperature, UV) in reaction equations
  • Practice writing isomers for C4H10, C5H12, and C4H8

Frequently Asked Questions

Why is carbon's chemistry so vast?

Carbon has four valence electrons and can form four strong covalent bonds. Its small size allows strong C-C bonds, enabling catenation (long chains, branches, rings). This versatility leads to millions of organic compounds.

How do you test whether a hydrocarbon is saturated or unsaturated?

Add bromine water to the hydrocarbon. If the orange-brown colour of bromine water is decolourised, the compound is unsaturated (alkene or alkyne). If the colour persists, it is saturated (alkane).

What is the difference between structural isomers?

Structural isomers have the same molecular formula but different structural arrangements of atoms. For example, n-butane has a straight chain while isobutane (2-methylpropane) has a branched chain. They have different physical properties despite the same formula.