SOLUTIONS
2023 BOARD EXAM
CBSE CLASS 12-PCM CHEMISTRY Board Paper 2023 — Set 5
2023
CHEMISTRY
CLASS 12-PCM
CBSE EXAMINATION PAPER-2023
CHEMISTRY
(Solved)
General Instructions :
Read the following instructions carefully and follow them :
- This question paper contains 15 questions. All questions are compulsory.
- This question paper is divided into 3 sections.
- Section A – questions number 1 to 1 are case based questions
- Section B – questions number 2 to 13 are multiple choice questions
- Section C – questions number 14 to 15 are very short answer
- There is no overall choice given in the question paper. However, an internal choice has been provided in few questions.
- Use of calculator is NOT allowed.
Section A
Section B
Which of the following molecules has a chiral centre correctly labelled with an asterisk (*)?
The correct answer is HOCH₂C*H(OH)CH₂OH. A chiral center is a carbon atom that is bonded to four different groups. In this molecule, the carbon atom marked with an asterisk is attached to four distinct substituents: a hydroxymethyl group (HOCH₂), a hydroxyl group (OH), a hydrogen (H), and a methylene group (CH₂OH), making it chiral. The other options do not present a carbon atom bonded to four different groups.
Which of the following alcohols will not undergo oxidation?
2-Methylbutan-2-ol is the correct answer because it is a tertiary alcohol. Tertiary alcohols are resistant to oxidation under normal conditions, unlike primary and secondary alcohols, which can be oxidized to aldehydes or ketones respectively. The context mentions that alkanes having tertiary hydrogen can be oxidized, but it must be noted that tertiary alcohols generally do not undergo oxidation readily.
A voltaic cell is made by connecting two half cells represented by half equations below: Sn²⁺(aq) + 2e⁻ → Sn(s) E° = – 0.14 V Fe³⁺(aq) + e⁻ → Fe²⁺(aq) E° = + 0.77 V Which statement is correct about this voltaic cell?
The correct answer is 'Sn is oxidised and the voltage of the cell is 0.91 V.' In the voltaic cell, Sn is oxidised from Sn(s) to Sn²⁺(aq), while Fe³⁺ is reduced to Fe²⁺. To find the cell voltage, we subtract the standard reduction potential of the oxidised species (Sn, -0.14 V) from that of the reduced species (Fe³⁺, +0.77 V): 0.77 V - (-0.14 V) = 0.91 V.
Four half reactions I to IV are shown below:
I. 2Cl⁻ → Cl₂ + 2e⁻
II. 4OH⁻ → O₂ + 2H₂O + 2e⁻
III. Na⁺ + e⁻ → Na
IV. 2H⁺ + 2e⁻ → H₂ Which two of these reactions are most likely to occur when concentrated brine is electrolysed?
The correct answer is I and IV. During the electrolysis of concentrated brine, chlorine is generated at the anode (I: 2Cl⁻ → Cl₂ + 2e⁻) and hydrogen is produced at the cathode (IV: 2H⁺ + 2e⁻ → H₂). Reaction II (4OH⁻ → O₂ + 2H₂O + 2e⁻) is not preferred due to higher overpotential for oxygen generation compared to chlorine, and reaction III is not relevant in this context since sodium does not get reduced in the electrolysis of brine.
Which property of transition metals enables them to behave as catalysts?
The correct option is 'Variable oxidation states'. Transition metals are known for their ability to adopt multiple oxidation states, which allows them to participate in various chemical reactions and form intermediate complexes, making them effective catalysts.
In the two tetrahedral structures of dichromate ion,
All Cr – O bonds are non-equivalent. In the dichromate ion (Cr2O7^2-), the two tetrahedral structures result in Cr–O bonds that are not all the same in length due to the shared oxygen and the differing angles that arise from its structure, specifically the Cr–O–Cr bond angle of 126°. This creates a situation where some of the bonds behave differently due to the geometry and resonance in the molecule.
1 mole of liquid A and 2 moles of liquid B make a solution having a total vapour pressure 40 torr. The vapour pressure of pure A and pure B are 45 torr and 30 torr respectively. The above solution
The correct option is 'shows positive deviation.' The total vapour pressure (40 torr) is lower than what would be expected based on Raoult’s law for an ideal solution, which indicates that the interactions between the molecules of A and B are weaker than those in the pure components. Thus, the solution exhibits positive deviation from Raoult’s law.
Which of the following would not be a good choice for reducing nitrobenzene to aniline?
The correct option is LiAlH₄. While H₂/Ni, Sn and HCl, and Fe and HCl are effective methods for reducing nitrobenzene to aniline, LiAlH₄ is a strong reducing agent that would more likely reduce nitro compounds to amines but is generally not preferred for this specific reduction, as it can lead to over-reduction or undesired side reactions.
Hydrolysis of sucrose is called
The correct answer is 'inversion' because hydrolysis of sucrose involves the breakdown of sucrose into its component monosaccharides, glucose and fructose, resulting in a mixture that is sweeter and has different optical rotation compared to the original disaccharide. This process is specifically referred to as 'inversion'.
Which one of the following has lowest pKₐ value?
The compound with the lowest pKₐ value is O₂N–CH₂–COOH (Nitroacetic acid). The presence of the electron-withdrawing nitro group (-NO₂) increases the acidity of the carboxylic acid, resulting in a lower pKₐ compared to the other options which have less electron-withdrawing substituents.
Which of the following cell was used in Apollo space programme?
The correct answer is H₂–O₂ Fuel cell, as it directly produces electricity from the reaction of hydrogen and oxygen, which was a key technology utilized during the Apollo space program for powering spacecraft.
The magnetic moment of [NiCl₄]²⁻ is [Atomic number : Ni = 28]
Section C
However, the melting point of the para isomer is higher because p-dichlorobenzene molecules are more symmetrical and pack more efficiently in the solid state, resulting in a more stable crystalline structure and higher melting point.
Give the reaction of glucose with hydrogen cyanide. Presence of which group is confirmed by this reaction ?
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