Ncert Solutions Chemistry Class 12th

14.18

Open structure of D-glucose could not explain the following reactions:

(i) Despite having the aldehyde group, glucose does not give 2,4 DNP test and Schiff's

(ii) Glucose does not react with sodium hydrogen sulphite to form addition

(iii) The absence of free –CHO group is shown when the penta-acetate of glucose does not react with hydroxyl

(iv) When glucose is heated with methanol in the presence of dry HCl gas, it forms two isomeric monomethyl derivatives known as? -D-glucoside and? -D-glucoside. Since only one molecule of methanol is used for the formation of methyl glucosided, these must be hemiacetals.

14.17

When D-glucose is heated and treated with HI for a long period of time, then n-hexane is formed, which shows that all the six-carbon atoms are linked in a straight

2. When D-glucose is treated with Br₂ water i.e., bromine water which is a mild oxidising agent, then we get D-gluconic acid as one of the product. This reaction assures the presence of carbonyl group which is available as an aldehydic group.

3. On being treated with HNO₃, D-glucose get oxidised which gives saccharic acid as final Saccharic acid is a di-carboxylic acid. This reaction confirms the presence of alcoholic group (- OH) in the glucose.

14.16

Starch consists of two components – amylase and amylopectin. Amylose is a long linear chain of α–D-(+)-glucose units joined by C₁-C₄glycosidic linkage (α -link).

Amylopectin is a branched-chain polymer of –D-glucose units, in which the chain is formed by C1-C4 glycosidic linkage and the branching occurs by C₁-C₆ glycosidic linkage.

On the other hand, cellulose is the main structural material of tree and other plants. Wood is 50% cellulose, while cotton wool is almost pure cellulose. It is linear chain natural polymers of β-D-glucose units joined by 1, 4-glycosidic linkage (natural linear polymers).

14.15

Hydrolysis is the process of using water to break down a molecule into two parts. It is usually a type of decomposition reaction where one reactant is water, where water is used to break chemical bonds in the other reactant. It can be considered as reverse of a condensation reaction.

The general formula of a hydrolysis reaction is:

XY + H₂O → XH + YOH

(i) On hydrolysis with dilute acids, sucrose yields an equimolecular mixture of α –D glucose and β–D- fructose.

(ii) The hydrolysis of lactose gives β–D-galactose and β–D-glucose as final products.

Equivalent weight is Ag, E_Ag = 180/1  = 180

Equivalent weight is Cu, E_Cu = 63.5 / 2 = 31.75

Equivalent weight is Zn, E_Zn= 65/2 = 32.5

Using Faraday's second law of electrolysis, to find the mass of Cu and Zn, we use Equation 1,

∴ W_Zn = 0.436 g

To find the time of current flow, using Faraday's first law of electrolysis we get,

M = Z *I *t ⇒ Equation 2

? Z = Equivalent Weight / 96487, Equation 2 becomes,

M = 108 / 96487 X 1.5 X t

t = 1.45 X 96487 / 108X 1.5

t = 864 seconds.

The time of current flow, t = 864 seconds, the mass of Cu is 0.426 g and mass of Zn is 0.436 g

Quantity of electricity passed = 5 A * (20 * 60 sec)
= 6000 C ⇒ Equation 1
The electrode reaction is written as,
Ni²⁺ + 2e → Ni
Thus, the quantity of electricity required = 2F
= 2*96487 C
= 192974 C
? 192974 C of electricity deposits 1 mole of Ni, which is 58.7 g ⇒ Equation 2
Thus, equating equations 1 and 2, we get

192974 C of electricity deposits = 58.7 g
6000 C of electricity will deposit = 58.7 X 6000 / 192974
= 1.825g of Ni
The mass of Ni deposited at the cathode is 1.825g of Ni

(i) The electrode reaction for 1 mole of H₂O is given as,

H₂O → H₂ + 1/2O₂

i.e., O^2- →1/2 O₂ + 2e ^-

∴ The quantity of electricity required = 2F

= 2*96487 C

= 192974 C

The quantity of electricity required in coulomb for the oxidation of 1 mol of H₂O to O₂ is 192974 C

(ii) The electrode reaction for 1 mole of FeO is

FeO + 1/2 O₂ → 1/2 Fe₂O₃

i.e., Fe²⁺ → Fe³⁺ + e ^-

∴ The quantity of electricity required = 1F

= 1*96487 C

= 96487 C

The quantity of electricity required in coulomb for the oxidation of 1 mol of FeO to Fe₂O₃ is 96487 C

14.14

Glycogen is a polysaccharide-type of carbohydrate. In animals, carbohydrates are stored as glycogen.

But starch is a carbohydrate which consists of two components –amylase (15 -20 %) and amylopectin (80 – 85%). However, glycogen is also like amylopectin but branching will take place after every 5 to 6 glucose unit. Also, glycogen is highly branched.