12th Chemistry Pre Half Yearly 2025: Original Question with answer

 12th Chemistry Pre Half Yearly 2025: Original Question with official answer key | Download PDF

12th Chemistry Pre Half Yearly 2025: Original Question

Time ; 1.30Hrs                                                    Max.Marks : 50

Part - I
Answer all the questions. Choose the correct answer 20 x 1 = 20

1, Considering Ellingham diagram, which of the following metals can be used to reduce alumina?
a) Fe b) Cu c) Zn d) Mg

2, Most easily liquefiable gas is
a) Kr b) Ne c) He d) Ar

3, Which of the following is paramagnetic in nature?
a) ${\text{Zn(NH}}_3{\text{)}}_4^{2+}$ b) ${\text{Co(NH}}_3{\text{)}}_6^{3+}$ c) ${\text{Ni(H}}_2{\text{O)}}_6^{2+}$ d) ${\text{Ni(CN)}}_4^{2-}$

4, Cell equation : $A+2B^{-}\to A^{2+}+2B$ ; $A^{2+}+2c^{-}\to A$ $E^0=+0.34\text{V}$ and log $K=15.6\text{at 300K}$ for cell reactions find $E^0$ for $B^{+}+e^{-}\to B$
a) 1.26 b) 0.80 c) -0.54 d) -10.94

5, Williamson synthesis of preparing dimethyl ether is a / an /
a) $S_{N}1$ reactions b) electrophilic addition
c) $S_{N}2$ reaction d) electrophilic substitution

6, The general formula of Monosaccharide is
a) $C_n({\text{H}}_2\text{O}{)}_{2n}$ b) $C_n({\text{H}}_2\text{O}{)}_n$ c) $C_n({\text{H}}_2\text{O}{)}_{2n-1}$ d) $C_{n-1}({\text{H}}_2\text{O}{)}_n$

7, The formation of cyanohydrin from acetone is an example of
a) nucleophilic substitution b) electrophilic substitution
c) electrophilic addition d) Nucleophilic addition

8, Fog is colloidal solution of
a) liquid in gas b) gas in gas c) solid in gas d) gas in liquid

9, After 2 hours, a radioactive substance becomes $\frac{1}{16}$ of original amount. Then the half life (in min) is
a) 60 minutes b) 120 minutes c) 15 minutes d) 30 minutes

10, The vacant space in bcc lattice unit cell is
a) 32% b) 23% c) 48% d) 26%

11, Assertion: due to Frenkel defect, density of the crystalline solid decreases.
Reason: in Frenkel defect cation and anion leaves the crystal.
a) Both assertion and reason are true but reason is not the correct explanation of assertion
b) Both assertion and reason are true and reason is the correct explanation of assertion.
c) Both assertion and reason are false. d) Assertion is true but reason is false

12, All the elements of group 17 and 18 are
a) Metalloids b) Non-Metals c) Metals d) a) and b)

13, When aniline reacts with acetic anhydride the product formed is
a) o – aminoacetophenone b) m-aminoacetophenone
c) acetanilide d) p – aminoacetophenone

14, Which one of the following is a bio-degradable polymer?
a) HDPE b) PHBV c) Nylon 6 d) PVC

15, Permanganate ion changes to ______ in acidic medium
a) MnO${}_4^{2-}$ b) MnO${}_2$ c) Mn${}^{3+}$ d) Mn${}^{2+}$

16, Among the following which will not be hydrolysed
a) NaCl b) HCOONa c) HCOONH${}_4$ d) NH${}_4$NO${}_3$

17, The ratio of close packed atoms to tetrahedral hole in cubic packing is
a) 1:1 b) 1:4 c) 2:1 d) 1:2

18, Which of the following electrolytic solution has the least specific conductance
a) 0.002N b) 2N c) 0.02N d) 0.2N

19, Which of the following lanthanoid ions is diamagnetic?
a) Eu^{2+} b) Ce^{2+} c) Yb^{2+} d) Sen^{2+}

20, Which of the metal is extracted by Hall-Heroult process?
a) Ni b) Al c) Cu d) Zn

Part - II
Answer any Six questions 6 x 2 = 12

21, Give the uses of potassium dichromate

22, Give one test to differentiate [Co(NH_3)_5Cl]SO_4and [Co(NH_3)_5SO_4]Cl

23, What is Bragg's equation? Give its significance

24, What is packing efficiency ?

25, Explain the effect of catalyst on reaction rate with an example

26, What is flocculation value?

27, Write a note on denaturation of Proteins

28, Write a short note on peptide bond

29, What are Hormones? Give example

30, Mention the uses of Bakelites

Part - III
Answer any Six questions 6 x 3 = 18

31, What are the effects of lanthanide contraction?

32, Write the formula for the following coordination compounds. 

33, [Ti(H_2O)_6]^{3+} is coloured, while [Sc(H_2O)_6]^{3+} is colorless - Explain

34, Explain Schottky defect -

35, Show that in case of first order reaction, the time required for 99.9% completion is nearly ten times the time required for half completion of the reaction'

36, How colloids are used in tanning of leather and in Rubber industry?

37, What is inversion of phase ? Give an example

38, Give any three difference between DNA and RNA

39, Differentiate thermoplastic and thermosetting

40, How is Terylene prepared? Mention its uses

12th Chemistry Pre Half Yearly Original Question click here to download

12th Chemistry Pre Half Yearly 2025: Official Answer key

1. d) Mg

2. a) Kr

3. c) ${\text{Ni(H}}_2{\text{O)}}_6^{2+}$

4. b) 0.80

5. c) $S_{N}2$ reaction

6. b) $C_n({\text{H}}_2\text{O}{)}_n$

7. d) Nucleophilic addition

8. a) liquid in gas

9. d) 30 minutes

10. a) 32%

11. c) Both assertion and reason are false.

12. b) Non-Metals

13. c) acetanilide

14. b) PHBV

15. d) Mn${}^{2+}$

16. a) NaCl

17. c) 2:1

18. a) 0.002N

19. c) Yb${}^{2+}$

20. b) Al

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Part - II & III: Short Answers (21-40)

21. Uses of potassium dichromate:

• As an oxidizing agent in laboratories and industries.

• In the tanning of leather.

• In the preparation of other chromium compounds.

22. Test to differentiate the complexes:
Add barium chloride (BaCl₂) solution. The complex that gives a white precipitate of BaSO₄ is [Co(NH₃)₅SO₄]Cl, as SO₄²⁻ ions are outside the coordination sphere.

23. Bragg's equation:

• Equation: $n\lambda =2d\sin \theta$

• Significance: It is used to determine the interplanar spacing (d) in a crystal by using X-ray diffraction.

24. Packing efficiency:
The percentage of total space in a crystal lattice that is occupied by the constituent particles.

25. Effect of catalyst:
A catalyst provides an alternative reaction pathway with a lower activation energy, increasing the reaction rate without being consumed. Example: MnO₂ catalyzes the decomposition of KClO₃ to O₂.

26. Flocculation value:
The minimum concentration of an electrolyte (in millimoles/L) required to cause the coagulation or flocculation of a sol.

27. Denaturation of Proteins:
The process where a protein loses its native 3D structure (and thus its biological function) due to factors like heat or pH change. The primary structure remains intact.

28. Peptide bond:
A covalent chemical bond (C–N) formed between the carboxyl group of one amino acid and the amino group of another, with the loss of a water molecule.

29. Hormones:
They are chemical messengers secreted by endocrine glands directly into the blood, which regulate physiological processes. Example: Insulin.

30. Uses of Bakelite:
Used for making electrical switches, handles for cookware, and radio and telephone cabinets because it is a poor conductor of heat and electricity.

31. Effects of lanthanide contraction:

• Similar atomic radii of 4d and 5d transition series elements.

• Difficulty in separating lanthanides.

• Basicity decreases from La³⁺ to Lu³⁺.

32. Formulas:
a) K₄[Fe(CN)₆]
b) [Fe(CO)₅]
c) [Co(NH₃)₅(NO₂)]²⁺

33. Colour explanation:
[Ti(H₂O)₆]³⁺ is coloured because Ti³⁺ has one d-electron, which can undergo d-d transition by absorbing visible light. [Sc(H₂O)₆]³⁺ is colourless because Sc³⁺ has no d-electrons, so no d-d transitions are possible.

34. Schottky defect:
A type of point defect in ionic crystals where a pair of cations and anions of equal charge is missing from the lattice, thus maintaining electrical neutrality. It decreases the density of the crystal.

35. First order reaction time:
For a first-order reaction, t₁/₂ = 0.693/k and t₉₉.₉% = 6.909/k. The ratio t₉₉.₉% / t₁/₂ ≈ 10, proving the statement.

36. Uses of colloids:

• Tanning: Animal hides are colloidal in nature. Tanning uses positively charged colloidal tannin to coagulate and harden the protein in the hide.

• Rubber Industry: Latex (a colloidal solution of rubber) is coagulated by electrophoresis to deposit rubber on molds.

37. Inversion of sucrose:
The hydrolysis of sucrose (dextrorotatory) produces a mixture of glucose and fructose (laevorotatory). This change in the sign of rotation from '+' to '-' is called inversion. Example: Sucrose hydrolysis with dilute acid.

38. Differences between DNA and RNA:

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Strands	Double-stranded	Usually single-stranded

39. Thermoplastic vs. Thermosetting:

Feature	Thermoplastic	Thermosetting
Behaviour on heating	Softens on heating	Hardens on heating
Structure	Linear or branched chains	Cross-linked or 3D network
Reusability	Can be reshaped and reused	Cannot be remoulded

40. Preparation of Terylene:
It is prepared by the condensation polymerization of ethylene glycol and terephthalic acid.
Uses: Making synthetic fibers (e.g., Dacron), magnetic recording tapes, and bottles.

12th Chemistry Pre Half Yearly Original Question click here to download.