ASSEB| CLASS 12| CHEMISTRY| SOLVED PAPER - 2023| H.S. 2ND YEAR
2023
CHEMISTRY
(Theory)
Full Marks: 70
Pass Marks: 21
Time: Three hours
The figures in the margin indicate
full marks for the questions.
1. Why ZnO is colourless at room temperature and turns yellow on heating? 1
Ans:- Zinc oxide (ZnO) is white or colourless at room
temperature because its crystal structure is regular and it has no significant
defects.
On heating, ZnO
turns yellow because some of the oxygen atoms leave the lattice, producing
oxygen vacancies (defects). These vacancies change the way ZnO absorbs and
reflects light, making it appear yellow. This change is due to a reversible
chemical change involving the loss and reabsorption of oxygen.
2. How much percentage of space is empty in a hexagonal
closed packed solid? 1
Ans:- The percentage of vacant space (vacancies) in a
hexagonal close packed (HCP) solid is 26%.
This means that
74% of the space is occupied by particles, and the remaining 26% is vacant
(vacancies).
3. Write the name of the catalyst used during synthesis
of Cl2 from HCI by Deacon's process. 1
Ans:- The catalyst used in Deacon process is cupric
chloride (CuCl2).
CuCl2
oxidises hydrochloric acid (HCl) with oxygen to produce chlorine gas (Cl₂).
4. Name the following chemical reaction: 1
Ans:- The chemical reaction shown in the image, where
sodium phenoxide reacts with carbon dioxide under pressure (400K / 4-7 atm) and
then undergoes acidification with dilute HCl to form salicylic acid, is known
as the Kolbe-Schmidt reaction or Kolbe reaction.
This reaction is
a type of carboxylation reaction used for the synthesis of salicylic acid, an
important precursor for the production of aspirin.
5. Write two significances of Hoffmann bromamide
degradation reaction. 1
Ans:- Two importance of Hofmann bromamide decomposition
reaction:-
(i) It is used
for the synthesis of primary amines from primary amides having one carbon atom
less, making it a valuable method in organic synthesis.
(ii) This
reaction helps in reducing the length of the carbon chain by one carbon atom,
allowing chemists to modify molecular structures efficiently.
6. Arrange the following in increasing order of their pKb
values in aqueous solution: 1
(C2
H5)3 N, NH3,(C2H5)2
NH and C2 H5 NH2
Ans:- In correct increasing order:
NH₃ <
(C₂H₅)₃N < C₂H₅NH₂ < (C₂H₅)₂NH.
7. Which of the following compound would undergo
Cannizzaro reaction? 1
(a)
Formaldehyde
(b)
Acetaldehyde
Ans:- (a) Formaldehyde
Reason:
Cannizzaro reaction occurs with those aldehydes which do not have α-hydrogen.
Formaldehyde fulfills this criterion, whereas acetaldehyde contains α-hydrogen
and does not undergo Cannizzaro reaction.
8. Among the isomeric alkanes of molecular formula C5
H12, identify the one that on photochemical chlorination yields a
single monochloride. 1
Ans:- Neopentane (2,2-dimethylpropane)
Reason:
In neopentane all hydrogen atoms are equivalent, so only one monochloride
product is formed on chlorination.
9. Element B crystallizes in body centered cubic (bcc)
unit cell. Calculate approximate number of unit cells in 9.2gm of element B. (Atomic
number of B= 23u) 2
Ans:-
Or
The edge length
of a face centered cubic cell of an ionic substance is 508 pm. If the radius of
the cation is 110 pm, then calculate the radius of the anion. 2
Ans:-
10. What is meant by negative deviation from Raoult’s law? What type of non-ideal solution is formed when ethanol is mixed with water? 1+1=2
Ans:- Negative deviation from Raoult's law occurs when the vapor pressure of a solution is less than the pressure predicted by Raoult's law. This happens because the intermolecular attraction between the molecules of the solute and the solvent is stronger than between the pure components.
When ethanol is added to water, the solution formed shows a negative deviation from Raoult's law, which means that it is an immiscible solution that has stronger intermolecular interactions than the pure substances.
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