10th Science Unit 11 Carbon and its Compounds Book Back Answers

1. a) alkane
2. b) alkene
3. c) alkyne
4. d) alcohol

1. Alkenes have the general formula $C_nH_{2n}$.

2. For $n=3$, $C_nH_{2n} = C_3H_6$, which matches the given formula (propene).

1. a) Aldehyde
2. b) Carboxylic acid
3. c) Ketone
4. d) Alcohol

1. The secondary suffix '-ol' stands for the $-OH$ functional group.

2. A compound whose name ends in '-ol' therefore belongs to the alcohol class.

1. a)
   • ol
2. b) – oic acid
3. c)
   • al
4. d)
   • one

1. In the IUPAC table of suffixes, alcohol is '-ol', aldehyde ($-CHO$) is '-al', ketone is '-one' and carboxylic acid ($-COOH$) is '-oic acid'.

2. Hence aldehyde takes the secondary suffix '-al', e.g. ethanal.

1. a) $\mathrm{C_3H_8}$ and $\mathrm{C_4H_{10}}$
2. b) $\mathrm{C_2H_2}$ and $\mathrm{C_2H_4}$
3. c) $\mathrm{CH_4}$ and $\mathrm{C_3H_6}$
4. d) $\mathrm{C_2H_5OH}$ and $\mathrm{C_4H_8OH}$

1. Successive members of a homologous series differ by one $-CH_2$ group (14 amu) and belong to the same class.

2. $C_3H_8$ (propane) and $C_4H_{10}$ (butane) are both alkanes differing exactly by $CH_2$.

1. a) Reduction of ethanol
2. b) Combustion of ethanol
3. c) Oxidation of ethanoic acid
4. d) Oxidation of ethanal

1. Ethanol is highly inflammable and burns in oxygen to give carbon dioxide and water.

2. Burning of a substance in oxygen with the formation of $CO_2$ and $H_2O$ is combustion.

1. a) 95.5 %
2. b) 75.5 %
3. c) 55.5 %
4. d) 45.5 %

1. Fractional distillation of the fermented 'wash' gives a mixture of 95.5% ethanol and 4.5% water.

2. This mixture is called rectified spirit; 100% ethanol is absolute alcohol.

1. a) Carboxylic acids
2. b) Ethers
3. c) Esters
4. d) Aldehydes

1. Among the daily-life uses of organic compounds, ethers are listed as anaesthetic agents and pain killers.

2. Hence ethers are used as anaesthetics.

1. a) mineral
2. b) vitamin
3. c) fatty acid
4. d) carbohydrate

1. TFM means Total Fatty Matter.

2. It shows the fatty acid content of the soap; higher TFM means a better bathing soap.

1. a) It is a sodium salt of long chain fatty acids
2. b) It is sodium salts of sulphonic acids
3. c) The ionic part in a detergent is $-\mathrm{SO_3^-Na^+}$
4. d) It is effective even in hard water.

1. Detergents are sodium salts of sulphonic acids with the ionic part $-SO_3^-Na^+$ and work even in hard water, so options b, c and d are correct.

2. Being a sodium salt of long chain fatty acids is the property of soap, not detergent, so statement (a) is wrong.

1. Such an atom or group decides the chemical properties of the compound, e.g. –OH, –CHO, –COOH.

1. Alkynes contain a carbon–carbon triple bond and follow the general formula $C_nH_{2n-2}$, e.g. acetylene $C_2H_2$.

1. The root word gives the number of carbon atoms in the parent chain (Meth-, Eth-, Prop-, ...), i.e. the carbon skeleton.

2. Prefix shows substituents and suffix shows bonding and functional group.

1. Unsaturated compounds contain double or triple bonds, so they react with bromine water and decolourise it.

2. Saturated compounds do not decolourise bromine water.

1. Ethanol heated with conc. $H_2SO_4$ at 443 K loses a water molecule.

2. $CH_3CH_2OH \rightarrow CH_2=CH_2 + H_2O$

1. Rectified spirit (95.5% ethanol) refluxed over quicklime and distilled gives 100% pure alcohol.

1. Ethanoic acid (acetic acid) is a carboxylic acid, so it is acidic in nature.

2. Acids turn blue litmus red.

1. Oils or fats are boiled with an alkali (NaOH or KOH) to give soap.

2. This alkaline hydrolysis is called saponification.

1. Straight (linear) hydrocarbon chains are easily broken down by bacteria.

2. Branched chains are not degraded, so those detergents are non-biodegradable.

1. Functional group –OH — the class of compounds containing the –OH group is Alcohol (R–OH, e.g. ethanol).

2. Heterocyclic — a ring containing carbon along with another atom like oxygen; Furan has oxygen in its ring.

3. Unsaturated — contains a C=C double bond; Ethene ($CH_2=CH_2$) is unsaturated.

4. Soap — sodium or potassium salt of a long chain fatty acid; Potassium stearate is such a salt.

5. Carbocyclic — ring made of carbon atoms only; Benzene is a carbocyclic (aromatic) compound.

1. a) A and R are correct, R explains the A.
2. b) A is correct, R is wrong.
3. c) A is wrong, R is correct.
4. d) A and R are correct, R doesn't explains A.

1. Assertion is correct: detergents clean effectively even in hard water, while soaps lose their cleansing power in hard water.

2. Reason is correct: the calcium and magnesium salts of detergents are soluble in water, so detergents do not form precipitates (scum) with $Ca^{2+}$ and $Mg^{2+}$ ions.

3. Soaps, being salts of fatty acids, form insoluble calcium and magnesium salts (scum) in hard water, which reduces their cleansing action.

4. So the solubility of their Ca and Mg salts is the very reason detergents work better in hard water — R correctly explains A.

1. a) A and R are correct, R explains the A.
2. b) A is correct, R is wrong.
3. c) A is wrong, R is correct.
4. d) A and R are correct, R doesn't explains A.

1. Assertion is correct: alkanes contain only carbon–carbon single bonds, so they are saturated hydrocarbons (general formula $C_nH_{2n+2}$).

2. Reason is correct: all hydrocarbons (alkanes, alkenes and alkynes) are made up of covalent bonds.

3. But the presence of covalent bonds is common to all hydrocarbons; it does not tell why alkanes alone are saturated.

4. Saturation is decided by the presence of only single bonds, not by the covalent nature of the bonds. Hence R does not explain A.

1. A ketone has the functional group $-\mathrm{CO}-$ (a carbonyl group attached to two carbon atoms), so the smallest possible ketone must have at least three carbon atoms.

2. Hence the simplest ketone is propanone (acetone), molecular formula $\mathrm{C_3H_6O}$, condensed formula $\mathrm{CH_3COCH_3}$.

3. Its structural formula is $\mathrm{CH_3-\overset{\overset{\textstyle O}{\|}}{C}-CH_3}$, i.e. two methyl groups joined to a $>\!\mathrm{C}=\mathrm{O}$ group.

4. Its IUPAC name is propanone (root word 'prop' + 'an' + secondary suffix 'one').

1. (i) Propane – acyclic (open chain), saturated compound; all carbons are joined by single bonds. Structure: $\mathrm{CH_3-CH_2-CH_3}$.

2. (ii) Benzene – cyclic, carbocyclic (homocyclic) and aromatic compound; a six-carbon ring with alternate double bonds, $\mathrm{C_6H_6}$.

3. (iii) Cyclobutane – cyclic, carbocyclic and alicyclic compound; a saturated ring of four $\mathrm{CH_2}$ groups, $\mathrm{C_4H_8}$.

4. (iv) Furan – cyclic, heterocyclic compound; a five-membered ring containing four carbon atoms and one oxygen atom, $\mathrm{C_4H_4O}$.

1. Ethanoic acid is manufactured on a large scale by the oxidation of ethanol.

2. The oxidising agent used is alkaline potassium permanganate ($\mathrm{KMnO_4/OH^-}$) or acidified potassium dichromate ($\mathrm{K_2Cr_2O_7/H^+}$).

3. Equation: $\mathrm{CH_3CH_2OH} \xrightarrow[\text{2[O]}]{\mathrm{KMnO_4/OH^-}} \mathrm{CH_3COOH} + \mathrm{H_2O}$.

4. With acidified $\mathrm{K_2Cr_2O_7}$ the orange colour changes to green, so this reaction is also used to identify alcohols.

1. Some detergents have highly branched hydrocarbon chains. These are non-biodegradable, i.e. they cannot be broken down by the micro-organisms (bacteria) present in water.

2. Such detergents remain in rivers and lakes for a long time, form thick foam on the water surface and pollute the water, harming aquatic life.

3. Remedy: manufacture and use biodegradable detergents, which have straight (linear) hydrocarbon chains that bacteria can easily degrade.

4. Other measures: use detergents only in the required quantity, and treat the waste water from homes and industries before it is let into water bodies.

1. Composition: Soap is the sodium (or potassium) salt of long chain fatty acids; a detergent is the sodium salt of sulphonic acid.

2. Ionic part: In soap it is $-\mathrm{COO^-Na^+}$; in detergent it is $-\mathrm{SO_3^-Na^+}$.

3. Raw material: Soap is prepared from animal fats or vegetable oils; detergent is prepared from hydrocarbons obtained from crude oil (petroleum).

4. Action in hard water: Soap loses its effectiveness and forms a scum in hard water; a detergent works well even in hard water and forms no scum.

5. Foaming and degradation: Soap has poor foaming capacity and is biodegradable; detergent has rich foaming capacity but most detergents are non-biodegradable.

(i)

1. Ethanoic acid is a carboxylic acid and sodium hydroxide is a base; they react to form a salt (sodium ethanoate) and water.

2. Equation: $CH_3COOH + NaOH \rightarrow CH_3COONa + H_2O$

(ii)

1. Ethanoic acid reacts with sodium bicarbonate (a weaker base) and liberates carbon dioxide with brisk effervescence.

2. Equation: $CH_3COOH + NaHCO_3 \rightarrow CH_3COONa + CO_2\uparrow + H_2O$

(iii)

1. Ethanol is oxidised to ethanoic acid by acidified potassium dichromate, which supplies nascent oxygen [O].

2. Equation: $CH_3CH_2OH \xrightarrow[2[O]]{K_2Cr_2O_7/H^+} CH_3COOH + H_2O$

3. During this reaction the orange colour of $K_2Cr_2O_7$ changes to green, so this reaction is used to identify alcohols.

(iv)

1. Ethanol is a highly inflammable liquid; it burns in oxygen to form carbon dioxide and water.

2. Equation: $C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O$

(i)

1. $\mathrm{C_4H_{10}O}$ is a saturated alcohol with 4 carbon atoms in a chain and one –OH group.

2. The –OH group is on carbon number 2, so it is attached to the second carbon of the butane chain.

3. Structural formula: $\mathrm{CH_3-CH(OH)-CH_2-CH_3}$

4. Expanded form: $\mathrm{CH_3 - \underset{|}{\overset{OH}{C}}H - CH_2 - CH_3}$

(ii)

1. Root word: the longest chain has 4 carbon atoms, so it is 'But'.

2. Primary suffix: all bonds between carbon atoms are single bonds, so 'ane' is used.

3. Secondary suffix: the functional group is –OH (alcohol), so 'ol' is used with locant number 2.

4. Joining them (terminal 'e' of 'ane' is removed): But + an + 2-ol.

(iii)

1. All the carbon atoms in the chain are joined only by single bonds (C–C).

2. A compound in which all carbon atoms are connected by single bonds is called saturated.

3. It will not decolourise bromine water.

(i)

1. The molecular formula $\mathrm{C_2H_4O_2}$ belongs to ethanoic acid (acetic acid), $\mathrm{CH_3COOH}$.

2. In lower concentration it is called vinegar, which is used as a food additive, flavouring agent and preservative.

3. It reacts with alcohols to give sweet (fruity) smelling esters, which matches the given clue.

(ii)

1. Ethanoic acid reacts with ethanol in the presence of conc. $\mathrm{H_2SO_4}$.

2. This reaction is called esterification and it produces an ester with a fruity odour and water.

3. Equation: $\mathrm{CH_3COOH + C_2H_5OH \xrightarrow{conc.\,H_2SO_4} CH_3COOC_2H_5 + H_2O}$

4. So compound 'B' is ethyl ethanoate ($\mathrm{CH_3COOC_2H_5}$).

(iii)

1. The reaction of an alcohol with a carboxylic acid giving a sweet smelling compound (ester) is called esterification.

2. Conc. $\mathrm{H_2SO_4}$ acts as the dehydrating agent/catalyst in this process.