Class 12 Chemistry MCQ

Chemistry-12 MCQ Q: Which of the following metal(s) show(s) hexagonal close packed structure (hcp) and which show face centred cubic (fcc) structure? a) Ag, Zn Mg, Cub) Mg, Zn Ag, Cuc) Cu, Fe Al, Snd) Na, Li Zn, CuCorrect Answer: Mg, Zn Ag, Cu Class 12 Chemistry MCQs for CBSE, Bihar, UP Board Explanation: Hexagonal close-packed (hcp) and face-centered cubic (fcc) Hexagonal close-packed (hcp) and face-centered cubic (fcc) are two types of crystal structures found in metals. Understanding which metals adopt these structures helps in determining their physical properties such as strength, ductility, and melting points. In the given options: – Magnesium (Mg) and Zinc (Zn) both crystallize in the hcp structure. The hcp arrangement allows these metals to have a high packing efficiency with atoms arranged in a hexagonal pattern, which contributes to their specific mechanical properties. – Silver (Ag) and Copper (Cu) crystallize in the fcc structure. The fcc structure is characterized by atoms positioned at each of the corners and the centers of all the cube faces of the unit cell. This arrangement provides high ductility and malleability, which is why metals like silver and copper are easily shaped and formed. Therefore, Option B, which lists Mg, Zn, Ag, and Cu, correctly identifies that Magnesium and Zinc have a hexagonal close-packed structure, while Silver and Copper have a face-centered cubic structure.

Chemistry-12 MCQ Q: Each of the following solids show, the Frenkel defect except a) ZnSb) AgBrc) AgId) KClCorrect Answer: KCl Class 12 Chemistry MCQs for CBSE, Bihar, UP Board Explanation: Frenkel Defect A Frenkel defect is a type of point defect in a crystal structure where an ion, typically the smaller cation, leaves its regular position in the lattice and moves to an interstitial site. This creates a vacancy at the original site and an interstitial defect where the ion relocates. Frenkel defects are more common in ionic crystals where there is a significant size difference between the cations and anions, allowing the smaller ions to move more easily within the lattice. Let’s examine each of the solids listed: ZnS (Zinc Sulfide): ZnS has a structure where zinc ions (Zn²⁺) are relatively small compared to the sulfide ions (S²⁻). This size difference allows Zn²⁺ ions to migrate to interstitial sites, leading to the formation of Frenkel defects. AgBr (Silver Bromide) and AgI (Silver Iodide): In both AgBr and AgI, silver ions (Ag⁺) are smaller and can move into interstitial positions within the crystal lattice. This mobility of Ag⁺ ions results in Frenkel defects, where vacancies and interstitials are created. KCl (Potassium Chloride): Unlike the other solids mentioned, KCl has ions that are more similar in size (K⁺ and Cl⁻) and a crystal structure that does not favor the movement of potassium ions to interstitial sites. Instead of forming Frenkel defects, KCl primarily exhibits Schottky defects, where both potassium and chloride ions leave their lattice positions, creating vacancies without the formation of interstitials. Therefore, among the options provided, KCl does not show Frenkel defects because its ionic structure and ion sizes do not support the formation of such defects.

Chemistry-12 MCQ Q: Hexagonal close packed arrangement of ions is described as  a) ABC ABAb) ABC ABCc) ABABAd) ABBABCorrect Answer: ABABA Class 12 Chemistry MCQs for CBSE, Bihar, UP Board Explanation:Hexagonal close packed (HCP) arrangement is one of the most efficient ways to pack spheres (such as ions) in a crystal structure. In HCP, each layer of ions is arranged in a hexagonal pattern, and each subsequent layer fits into the depressions of the layer below it to maximize packing efficiency. The stacking sequence describes how these layers are placed on top of each other. In HCP, the stacking follows a repeating pattern of two distinct layers. This pattern can be represented as ABABA. Here’s what this means: – A Layer: This is the first layer where ions are placed in a hexagonal arrangement.– B Layer: The second layer is placed so that the ions fit into the pockets of the A layer.– A Layer: The third layer repeats the first layer’s arrangement, aligning directly above the first A layer. This ABABA sequence continues, maintaining the hexagonal close packing structure. Looking at the options provided: – Option A: ABC ABA suggests a two-part repeating sequence, which doesn’t correctly represent the HCP structure.– Option B: ABC ABC represents the face-centered cubic (FCC) arrangement, not HCP.– Option C: ABABA correctly represents the HCP stacking sequence.– Option D: ABBAB does not follow a consistent pattern suitable for HCP. Therefore, the correct answer is ABABA.

Chemistry-12 MCQ Q: Which of the following statement is not correct? a) Paramagnetic substances lose their magnetism in the absence of magnetic field.b) Diamagnetic substances are weakly magnetised in magnetic field in opposite direction.c) Ferromagnetic substances becomes paramagnetic on heating.d) In antiferromagnetism domains are oppositely oriented and cancel out each other’s magnetic moment.Correct Answer: c) Ferromagnetic substances becomes paramagnetic on heating. Class 12 Chemistry MCQs for CBSE, Bihar, UP Board Explanation:Option A: Paramagnetic substances lose their magnetism in the absence of a magnetic field. This statement is correct. Paramagnetic materials are attracted to magnetic fields when they are present, but they do not retain any magnetism once the external field is removed. Option B: Diamagnetic substances are weakly magnetised in a magnetic field in the opposite direction. This statement is correct. Diamagnetic materials create an induced magnetic field in a direction opposite to an externally applied magnetic field, resulting in a weak repulsion. Option C: Ferromagnetic substances become paramagnetic on heating. This statement is not correct. Ferromagnetic materials retain their magnetic properties up to a certain temperature known as the Curie temperature. When heated above the Curie temperature, they lose their ferromagnetic properties and become paramagnetic. However, the statement as presented implies that all heating causes ferromagnetic substances to become paramagnetic, which is not accurate. They only undergo this change when heated beyond their specific Curie temperature. Option D: In antiferromagnetism, domains are oppositely oriented and cancel out each other’s magnetic moment. This statement is correct. In antiferromagnetic materials, adjacent atomic magnetic moments align in opposite directions, effectively canceling each other out and resulting in no net macroscopic magnetization. Therefore, Option C is the incorrect statement among the given choices.

Chemistry-12 MCQ Q: An element occuring in the bcc structure has 12.08 × 10^23 unit cells. The total number of atoms of the element in these cells will be a) 24.16 × 10^23b) 36.18 × 10^23c) 6.04 × 10^23d) 12.08 × 10^23 Correct Answer: a) 24.16 × 10^23 Class 12 Chemistry MCQs for CBSE, Bihar, UP Board Explanation:In a body-centered cubic (bcc) crystal structure, each unit cell contains a specific number of atoms. Here’s how to determine the total number of atoms given the number of unit cells: 1. Understand the BCC Structure: – In a bcc structure, there are atoms at each of the eight corners of the cube and one atom in the center of the cube.– Each corner atom is shared among eight adjacent unit cells. Therefore, each corner contributes 1/8 of an atom to a single unit cell.– There are 8 corner atoms, so their total contribution is 8 × (1/8) = 1 atom.– The center atom is entirely within the unit cell, contributing 1 full atom.– Total atoms per unit cell in BCC = 1 (from corners) + 1 (center) = 2 atoms. 2. Calculate the Total Number of Atoms: – Given the number of unit cells is 12.08 × 10²³.– Since each unit cell has 2 atoms, multiply the number of unit cells by 2.– Total atoms = 12.08 × 10²³ unit cells × 2 atoms/unit cell = 24.16 × 10²³ atoms. Therefore, the total number of atoms in 12.08 × 10²³ unit cells of a bcc structure is 24.16 × 10²³.

Chemistry-12 MCQ Q: Which of the following is the correct increasing order of packing efficiency for hcp, bcc and simple cubic lattice? a) hcp < bcc < simple cubicb) bcc < hcp < simple cubicc) simple cubic < bcc < hcpd) simple cubic < hcp < bcc Correct Answer: simple cubic < bcc < hcp Class 12 Chemistry MCQs for CBSE, Bihar, UP Board Explanation:Packing efficiency refers to the percentage of space in a crystal structure that is occupied by its atoms. It is a measure of how tightly the atoms are packed together in the lattice. There are three common types of lattice structures to consider: 1. Simple Cubic (SC): In this structure, each unit cell has one atom at each of the eight corners of a cube. However, each corner atom is shared among eight adjacent cubes, so effectively there is only one atom per unit cell. The packing efficiency of the simple cubic lattice is about 52%, which means just over half of the space is occupied by atoms, and the rest is empty space. This makes it the least efficient among the three. 2. Body-Centered Cubic (BCC): The BCC structure also has atoms at each of the eight corners of the cube, but it includes an additional atom at the center of the cube. This results in a total of two atoms per unit cell (each corner atom contributes 1/8th of an atom, and there are eight corners). The packing efficiency of BCC is higher, around 68%, making it more efficient in space utilization compared to simple cubic. 3. Hexagonal Close-Packed (HCP): The HCP structure is arranged so that the atoms are packed closely together in a hexagonal pattern. Each unit cell in HCP contains a larger number of atoms compared to SC and BCC, and its packing efficiency is approximately 74%. This is the highest packing efficiency among the three structures, meaning it uses space most effectively. Putting it all together, the packing efficiencies increase in the order of simple cubic being the least efficient, followed by body-centered cubic, and hexagonal close-packed being the most efficient. Therefore, the correct increasing order of packing efficiency is: simple cubic < bcc < hcp

Chemistry-12 MCQ Q: A crystalline solid a) changes abruptly from solid to liquid when heatedb) has no definite melting pointc) undergoes deformation of its geometry easilyd) has an irregular 3-dimensional arrangements Correct Answer: changes abruptly from solid to liquid when heated Class 12 Chemistry MCQs for CBSE, Bihar, UP Board Explanation: Crystalline Solid Properties A crystalline solid is characterized by its highly ordered and repeating arrangement of atoms or molecules in a fixed geometric pattern. One of the key properties of crystalline solids is that they have a definite melting point. This means that when a crystalline solid is heated, it remains solid up to a specific temperature. At this precise temperature, the solid abruptly transitions into a liquid state. This sharp change occurs because the regular structure of the crystalline solid breaks down suddenly once the thermal energy overcomes the forces holding the particles in place. Option B is incorrect because crystalline solids do have a definite melting point. Option C is wrong because crystalline solids do not easily deform; they are typically rigid and maintain their shape until they reach their melting point. Option D is also incorrect because crystalline solids have a regular and orderly three-dimensional arrangement, not an irregular one. Therefore, the correct answer is that a crystalline solid changes abruptly from solid to liquid when heated.