Class 10 Science

# Carbon and Its Compounds NCERT Exemplar Problems

Question 8: A compound C (molecular formula, C2H4O2) reacts with Na – metal to form a compound R and evolves a gas which burns with a pop sound. Compound C on treatment with an alcohol A in presence of an acid forms a sweet smelling compound S (molecular formula, C3H6O2). On addition of NaOH to C, it also gives R and water. S on treatment with NaOH solution gives back R and A. Identify C, R, A, S and write down the reactions involved.

Answer: C is ethanoic acid. R is sodium acetate and hydrogen gas is evolved. A is methanol and S is ester (methyl acetate). The reactions involved in this process are as follows:

2CH3COOH + 2Na → 2CH3COONa + H2

CH3COOH + CH3OH + Conc. H2SO4 → CH3COOCH3 + H2O

CH3COOH + NaOH → CH3COONa + H2O

CH3COOH + NaOH → CH3COONa+ CH3OH

Question 9: Look at the given figure and answer the following questions

Ref: NCERT Exemlpar Problems

(a) What change would you observe in the calcium hydroxide solution taken in tube B?

Answer: Calcium hydroxide solution will turn milky.

(b) Write the reaction involved in test tubes A and B respectively.

Answer: 2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2(Test tube A)

Ca(OH)2 + CO2 → CaCO3 + H2O (Test tube B)

The white colour of the liquid disappears with excess CO2:

CaCO3 + H2O + CO2 → Ca(HCO2)2

(c) If ethanol is given instead of ethanoic acid, would you expect the same change?

Answer: We know that ethanol does not react with sodium carbonate. Due to this; similar change cannot be expected.

Question 10: How can a solution of lime water be prepared in the laboratory?

Answer: For preparing lime water in the laboratory; calcium oxide (quick lime) is dissolved in water and the mixture is allowed to settle. The supernatant liquid is then decanted to obtain lime water.

Question 11: How would you bring about the following conversions? Name the process and write the reaction involved.

(a) Ethanol to ethene.

Answer: Ethanol is treated with concentrated sulphuric acid to obtain ethene. This can be shown by following equation:

CH3CH2OH + conc. H2SO4 → CH2=CH2 + H2O

(b) Propanol to propanoic acid.

Answer: Propanol is treated with potassium permanganate to obtain propanoic acid. This can be shown by following equation:

CH3CH2CH2OH + alk KMnO4 → CH3CH2OOH

Question 12: Draw the possible isomers of the compound with molecular formula C3H6O and also give their electron dot structures.

Ref: NCERT Exemlpar Problems

Question 13: Explain the given reactions with the examples

(a) Hydrogenation reaction

Answer: When hydrogen is added to unsaturated hydrocarbon in the presence of a catalyst; like nickel or palladium; this reaction is called hydrogenation reaction. This reaction is used for converting unsaturated hydrocarbon into saturated hydrocarbon. The following equation shows a hydrogenation reaction.

(b) Oxidation reaction

Answer: When two hydrogen atoms of an organic compound are substituted by an oxygen atom, the reaction is called oxidation reaction. The following equation shows an oxidation reaction.

CH3CH2CH2OH + alk KMnO4 → CH3CH2OOH

(c) Substitution reaction

Answer: When hydrogen of an organic compound is substituted by another element, this reaction is called substitution reaction. The following equation is an example of substitution reaction.

CH4 + Cl2 + sunlight → CH3Cl + HCl

(d) Saponification reaction

Answer: When an ester is treated with a base, the ensuing reaction produces carboxylic acid and ethanol. This reaction is called saponification reaction. This can be shown by following equation.

CH2COOC2H5 + NaOH → CH3COONa + C2H5OH

(e) Combustion reaction

Answer: When oxidation is accompanied by production of heat and light, the reaction is called combustion. The following equation is an example of combustion.

CH4 + 2O2 → CO2 + 2H2O + Heat and light

Question 14: An organic compound A on heating with concentrated H2SO4 forms a compound B which on addition of one mole of hydrogen in presence of Ni forms a compound C. One mole of compound C on combustion forms two moles of CO2 and 3 moles of H2O. Identify the compounds A, B and C and write the chemical equations of the reactions involved.

Answer: Compound A is ethanol. Compound B is ethene and compound C is ethane. The following equations show the reactions involved in this process.

C2H5OH + conc. H2SO4 → C2H4 + H2O

C2H44 + H2 + Ni → C2H6

2C2H6 + 7O2 → 4CO2 + 6H2O + Heat and light