Class 10 Science

# Carbon and Its Compounds

## NCERT Exemplar Problems

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

Look at the given figure and answer the following questions Ref: NCERT Exemlpar Problems

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

Answer: Calcium hydroxide solution will turn milky.

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

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.

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.

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

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

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

Draw the possible isomers of the compound with molecular formula C3H6O and also give their electron dot structures. Ref: NCERT Exemlpar Problems

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

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