Chemistry Test 245

Concept:

In colloidal chemistry, the charge on sol particles is usually developed due to the adsorption of ions from the surrounding solution onto the surface of the colloidal particles. The type of ion adsorbed depends on the nature of the colloidal particles and the ions present in the solution during the formation of the sol.

As₂S₃ (arsenious sulphide) is a lyophobic colloid. It is formed due to the hydrolysis of As2O3 (arsenious oxide) in boiled distilled water. Further H2S gas is passed through the solution. The reaction is as follows:

As₂O₃ + 3H₂O → 2As(OH)3

2As(OH)₃ + 3H₂S → As₂S₃ + 6H₂O

The particles in arsenious sulphide colloidal solution are surrounded by HS – ions which are formed due to the dissociation of H2S. The sulphide ion layer is surrounded by H+ ions.

Explanation:

Arsenic sulphide sol is prepared by passing hydrogen sulfide gas (H₂S) through a solution of arsenic trioxide (As₂O₃). The chemical reaction involved in this process is:

As₂O₃ + 3 H₂S → As₂S₃ + 3 H₂O

During this process, arsenic sulphide (As₂S₃) forms as colloidal particles. The surface of these newly formed colloidal particles adsorbs ions from the solution, which imparts a charge to them. Hydrogen sulfide (H₂S) dissociates in water to form H⁺ and S^2- ions.

The arsenic sulphide sol particles are more likely to adsorb the S^2- ions due to their affinity for negatively charged species. This adsorption of S^2- ions leads to the development of a negative charge on the surface of the colloidal particles.

Conclusion:

Based on the reaction and the ions present in the solution, the charge on the arsenic sulphide sol particles is primarily developed due to the adsorption of S^2- ions. Therefore, the correct answer is S^2-

Concept:

The given reaction involves the reduction of oxygen gas to water under acidic conditions. This type of reaction typically occurs in galvanic cells involving oxygen gas at the cathode, hydrogen ions (H⁺) in the solution, and a platinum electrode as the inert conductor. Platinum is commonly used as the electrode for redox reactions involving gases, as it does not interfere with the chemical process.

• Galvanic Cell Components: In galvanic cells, oxidation takes place at the anode and reduction occurs at the cathode. The oxygen reduction reaction (ORR) occurs at the cathode, while the anode typically involves a different redox process, depending on the setup of the cell.

• Platinum Electrode: In this reaction, platinum is used as the inert electrode, and H⁺ ions in the acidic medium are reduced to form water, making the oxygen gas reduce at the cathode.

Concept:

Dmitri Mendeleev developed the Periodic Law, which states that the properties of the elements are a periodic function of their atomic weights.

Explanation:

Mendeleev created an early version of the periodic table by arranging elements in order of increasing atomic weight. He observed that elements with similar chemical properties occurred at regular intervals (periodically). This arrangement allowed him to predict the existence and properties of undiscovered elements.

Conclusion:

According to Mendeleev, the properties of the elements are a periodic function of their: Atomic weight

CONCEPT:

Precipitation Reactions

• A precipitate forms when two aqueous solutions react to form an insoluble solid.
• In this case, we are looking for the formation of Silver Bromide (AgBr), which is an insoluble compound.
• Silver nitrate (AgNO₃) and potassium bromide (KBr) are both soluble in water, and when mixed, they undergo a double displacement reaction, forming the insoluble precipitate AgBr.
• Other options involve solids or non-reactive species that will not form a precipitate of AgBr.

EXPLANATION:

• AgNO₃(aq) + KBr(aq) — This combination will give a precipitate of AgBr because Ag⁺ from AgNO₃ reacts with Br⁻ from KBr to form insoluble AgBr.
• AgCl(s) + NaBr(aq) — AgCl is a solid and does not react with NaBr to form AgBr.
• Ag₂SO₄(s) + KBr(aq) — Ag₂SO₄ is a solid and will not dissolve or react with KBr to form AgBr.
• Ag₂PO₄(s) + KBr(aq) — Ag₃PO₄ is a solid and will not react with KBr to form AgBr.

CONCLUSION:

• AgNO₃(aq) + KBr(aq) is the correct combination that will give a precipitate of AgBr.

Concept:

Optical isomerism occurs in complexes when the complex lacks a plane of symmetry, meaning its mirror images cannot be superimposed. This is often found in octahedral complexes with bidentate ligands, such as ethylenediamine (en), that create chirality within the structure.

• Bidentate Ligands: Ligands like ethylenediamine (en) form two bonds to the metal ion, often resulting in chiral complexes in octahedral geometry.

• Optical Activity: A complex that can exist in two non-superimposable mirror images is considered optically active and shows optical isomerism.

Explanation: 

• The complex [Cr(en)₃]³⁺ exhibits optical isomerism because it has three bidentate ligands (ethylenediamine), which result in a chiral arrangement. The complex can form two non-superimposable mirror images, making it optically active.

• In contrast, complexes like [Co(NH₃)₄Cl₂]Cl, [Ni(CN)₄]^2-, and [Cu(NH₃)₄]²⁺ do not exhibit optical isomerism. They either lack chirality or possess symmetry that prevents the formation of non-superimposable mirror images.

Conclusion:

The correct answer is: (3) [Cr(en)₃]³⁺, which exhibits optical isomerism due to the presence of bidentate ligands and its chiral geometry.

Concept:

1. Nucleophilic Substitution Reaction:
   - In aromatic nucleophilic substitution reactions, electron-withdrawing groups (EWGs) enhance the reactivity of the aromatic ring towards nucleophiles by stabilizing the negative charge in the intermediate.
2. Electron-Withdrawing Groups (EWGs):
   - Nitro groups (−NO₂) are strong EWGs that increase reactivity towards nucleophilic substitution. - The position of these nitro groups (ortho, meta, para) relative to the leaving group (chlorine) affects the reactivity.

Explanation-

Multiple EWGs in positions ortho and para to the leaving group significantly increase reactivity towards nucleophilic substitution. - Thus, 2,4,6-trinitrochlorobenzene (with three nitro groups) is highly reactive. - 2,4-dinitrochlorobenzene (with two nitro groups) is also very reactive but less so than the trinitro compound. - 4-nitrochlorobenzene (with one nitro group in the para position) is less reactive than those with multiple nitro groups. - 3-nitrochlorobenzene (single nitro group in the meta position) is the least reactive because the meta position has less electron-withdrawing influence on the chlorine atom.

Least Reactive Compound:

The compound that is least reactive towards nucleophilic substitution reaction, when treated with aqueous NaOH, is  3-nitrochlorobenzene

CONCEPT:

sp² Hybridization

• sp² hybridization occurs when one s orbital and two p orbitals from the central atom mix to form three hybrid orbitals, each having 33.33% s-character and 66.67% p-character.
• It typically leads to a trigonal planar geometry with bond angles of approximately 120°.
• sp² hybridization is commonly associated with the presence of a double bond or a delocalized π-system.

EXPLANATION:

• Option 1: The molecule has a trigonal planar geometry — This is a correct criterion for sp² hybridization because sp² hybridized atoms generally arrange themselves in a trigonal planar structure.
• Option 2: The central atom must have at least one double bond — This is also a typical feature of sp² hybridization, though it’s not mandatory. Many sp² hybridized atoms do have a double bond.
• Option 3: The bond angles are approximately 109.5° — This is NOT a criterion for sp² hybridization. Bond angles of 109.5° are typical of sp³ hybridization, which leads to a tetrahedral geometry, not sp².
• Option 4: Each bonded atom must have a lone pair — This is also incorrect. sp² hybridization does not require the presence of lone pairs on the bonded atoms, though lone pairs can sometimes be present.

CONCLUSION:

• Option 3 (The bond angles are approximately 109.5°) is NOT a criterion for sp² hybridization.

CONCEPT:

Adiabatic Reversible Expansion in Thermodynamics

• An adiabatic process is one in which no heat is exchanged between the system and its surroundings (Q = 0). In other words, the entire change in internal energy is due to work done on or by the system.
• In a reversible adiabatic process for an ideal gas, the relationship between pressure ( p) and volume ( V ) follows a specific expression due to the first law of thermodynamics and the ideal gas behavior.

CONCEPT:

Ionic Compounds and Their Properties

• Ionic compounds consist of positively and negatively charged ions held together by strong electrostatic forces of attraction.
• These electrostatic forces, also known as ionic bonds, are generally very strong.
• Due to the strength of these ionic bonds, a considerable amount of energy is required to overcome these attractions, leading to high melting and boiling points for ionic compounds.

EXPLANATION:

• Statement I: Ionic compounds generally have high melting and boiling points.
• Reason: The strong electrostatic forces of attraction between oppositely charged ions in ionic compounds require a significant amount of energy to overcome.
• Statement II: This is the fundamental reason behind the high melting and boiling points of ionic compounds.

The correct answer is Both Statement I and Statement II are true.