AHSEC| 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 colurless at room temperature turns yellow on heating? 1
Ans:- Zinc oxide (ZnO) appears colorless at room
temperature because of its wide bandgap, which prevents it from absorbing
visible light. Upon heating, ZnO undergoes changes in its defect structure,
leading to the formation of zinc vacancies and interstitial zinc atoms. This
results in the absorption of light in the visible spectrum, making the material
appear yellow. Upon cooling, ZnO returns to its original colorless state as
these defects are eliminated.
2. How much percentage of space is empty in a hexagonal
closed packed solid? 1
Ans:- In a hexagonal close-packed (HCP) structure,
about 26% of the space is empty (or vacant), meaning that about 74% of the
volume is filled with atoms. This high packing efficiency is characteristic of
close-packed structures, which maximize the number of atoms in a given volume.
3. Write the name of the catalyst used during synthesis
of Cl2 from HCl by Deacon’s process. 1
Ans:- The catalyst used in Deacon's process for the
synthesis of chlorine gas (Cl2) from hydrochloric acid (HCl) is
usually copper (II) chloride (CuCl2). This process involves the
oxidation of HCl using oxygen, which is facilitated by this catalyst.
4. Name the following chemical reaction: 1
Ans:- The chemical reaction in which sodium phenoxide
reacts with carbon dioxide at 400 K and 4-7 atm pressure to form sodium
salicylate is called Kolbe reaction.
5. Write two significances of Hoffmann bromamide
degradation reaction. 1
Ans:- Significances of Hofmann Bromamide Degradation
Reaction:-
(i)
Conversion of primary amide to primary amine: The Hofmann bromamide
degradation reaction is important for its ability to convert primary amides
into primary amines with one fewer carbon atom. This transformation is useful
in organic synthesis, allowing chemists to effectively modify molecular
structures while retaining the nitrogen functionality.
(ii)
Synthetic applications: This reaction is used in the synthesis of a variety
of compounds, including pharmaceuticals and agrochemicals. For example, it can
be used to prepare anthranilic acid and other important amines, making it a
valuable tool in medicinal chemistry and materials science.
6. Arrange the following in increasing order of their pKb
values aqueous solution: 1
(C2H5)3N,
NH3, (C2H5)2 NH and C2H5NH2
Ans:-
7. Which of the following compound would undergo
Cannizzaro reaction? 1
(a) Formaldehyde
(b)
Acetaldehyde
Ans:- The Cannizzaro reaction is a chemical process
in which aldehydes, which do not have alpha-hydrogens, undergo
disproportionation in the presence of a strong base, giving an alcohol and a
carboxylic acid.
Compounds
under consideration:-
(i)
Formaldehyde (HCHO): This compound does not have any alpha-hydrogen, which
makes it suitable for the Cannizzaro reaction.
(ii)
Acetaldehyde (CH₃CHO): This compound has an alpha-hydrogen (hydrogen
attached to the carbon next to the carbonyl group), which means it cannot
undergo the Cannizzaro reaction.
Conclusion:-
Only
formaldehyde will undergo the Cannizzaro reaction, while acetaldehyde will not
due to the presence of alpha-hydrogen.
8. Among the isomeric alkanes of molecular formula C5H12
identify the one that on photochemical chlorination yields a single
monochloride. 1
Ans:- Isomeric Alkenes of C5H12 and Monochlorination:-
Of the isomeric
alkanes with molecular formula C5H12, 2,2-dimethylpropane
is the one that gives a single monochloride on photochemical chlorination. This
is due to its symmetric structure, which means that any chlorine substitution
will result in the same product, specifically 1-chloro-2,2-dimethylpropane.
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:-
Negative
deviation from Raoult's Law occurs when the vapor pressure of a solution is
lower than expected based on the vapor pressures of its pure components. This
usually occurs when there are strong intermolecular forces between the
different components in the solution that are stronger than those between the
pure components.
Types of
Non-Ideal Solutions with Ethanol and Water:-
When ethanol
is mixed with water, a negative deviation from Raoult's Law is observed. This
is because the hydrogen bonding between water and ethanol molecules is stronger
than the hydrogen bonding present in pure water or pure ethanol. As a result,
fewer molecules escape into the vapor phase, causing the vapor pressure to be
lower than predicted.
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