Chemistry Test 235

CONCEPT:

Coordination Number and Oxidation State in Complex Ions

• The coordination number refers to the number of ligands directly bonded to the central metal ion in a complex. In the given complex, the ligands are ammonia NH₃ and chloride Cl⁻.
• The oxidation state of the central metal ion cobalt in this case is calculated by considering the charges on the ligands and the overall charge of the complex.

EXPLANATION:

• For the complex ion CoNH₃₄Cl₂Cl:
  • Cobalt Co is the central metal ion.
  • Four NH₃ molecules neutral ligands are attached to Co, contributing no charge.
  • Two Cl⁻ ions are directly bonded to Co inside the square brackets, contributing a charge of -2.
  • The extra Cl⁻ outside the square brackets is an ionic part of the complex.
• The overall charge of the complex ion inside the square brackets is neutral, meaning the oxidation state of Co must be +3 to balance the -2 charge from the two Cl⁻ ions inside.
• The coordination number is determined by the number of ligands directly attached to the cobalt ion: 4 NH₃ molecules and 2 Cl⁻ ions, giving a coordination number of 6.

CONCLUSION:

• The coordination number is 6, and the oxidation state of cobalt is +3.
• The correct answer is: 6, +3.

CONCEPT:

Ligand Field Strength

• The field strength of a ligand refers to its ability to split the d-orbitals of a metal ion in a coordination complex.
• This splitting is explained by the spectrochemical series, which arranges ligands from weak to strong fields.
• Stronger field ligands cause a larger splitting of the d-orbitals.

EXPLANATION:

Spectro chemical series: 

• According to the spectrochemical series, CO is a strong field ligand, while H₂O, F^-, and S^2- are weaker.
• CO, being a π-acceptor ligand, has the highest field strength.

The correct answer is: CO (Option 1)

CONCEPT:

Resonance and Electromeric Effects of Substituents

• The -NH₂ group (amino group) attached to a benzene ring exhibits an electron-donating property through resonance.
• It donates electron density into the ring via its lone pair of electrons, particularly enhancing electron density at the ortho and para positions.
• This phenomenon is called the +R (positive resonance) effect, making the ring more reactive toward electrophiles.

EXPLANATION:

• The +R effect refers to electron donation into the ring, while the -E effect refers to electron withdrawal.
• The -NH₂ group increases reactivity in electrophilic aromatic substitution reactions through the +R effect.

The correct answer is: +R Effect (Option 4)

CONCEPT:

Dipole Moment of Molecules

• The dipole moment of a molecule depends on the difference in electronegativity between atoms and the molecular geometry.
• Polar molecules have a nonzero dipole moment due to the uneven distribution of charge.
• In symmetrical molecules, the dipoles may cancel out, leading to zero or minimal dipole moment.

CONCEPT:

sp³ Hybridization in Molecules

• Hybridization can be calculated using the formula:
  • Hybridization = (Number of atoms bonded to the central atom) + (Number of lone pairs on the central atom)
• If the total equals 4, the hybridization is sp³, corresponding to tetrahedral geometry.
• Each orbital involved forms one sigma bond or holds lone pair electrons.

CALCULATION OF HYBRIDIZATION:

CONCEPT:

Basicity of Conjugate Bases

• The basicity of a conjugate base depends on its ability to accept a proton (H⁺).
• Stronger bases are less stable and prefer to bind with protons more readily.
• Factors like electronegativity and resonance affect the basicity of conjugate bases.

EXPLANATION:

• OH^- is a strong base, followed by alkoxide ions (R-O^-).
• CH₃COO^- is resonance stabilized, making it less basic.
• Cl^- is very weakly basic due to high electronegativity and large size.

The correct order is: OH^- > R-O^- > CH₃COO^- > Cl^- (Option 3)

CONCEPT:

Calomel Electrode

• A calomel electrode is a reference electrode commonly used in electrochemical experiments.
• It consists of mercury (Hg) in contact with mercury(I) chloride (Hg₂Cl₂), also known as calomel, and a solution of chloride ions (Cl⁻).
• This forms a Metal – Insoluble Salt – Anion system, where the insoluble salt Hg₂Cl₂ is in equilibrium with Cl⁻ ions in solution.

EXPLANATION:

• In this type of electrode, the mercury metal is in contact with its own insoluble salt, Hg₂Cl₂, which dissociates into Cl⁻ ions in the solution.
• The reaction: Hg₂Cl₂ (s) + 2e⁻ ⇌ 2Hg (l) + 2Cl⁻ (aq)
• The electrode is classified as Metal – Insoluble Salt–anion because the metal (Hg) and its insoluble salt (Hg₂Cl₂) form an electrochemical system, with the anion Cl⁻ in equilibrium.
• The potential of the calomel electrode depends on the concentration of Cl⁻ ions in the solution.

The correct answer is: Metal – Insoluble Salt – Anion Electrodes (Option 1)

CONCEPT:

Acid Strength and Anion Stability

• The strength of an acid is also influenced by the stability of the conjugate base (anion) it forms after donating a proton (H⁺).
• The more stable the conjugate base, the stronger the acid.
• Stability of the conjugate base depends on factors such as resonance, electronegativity, and the ability to disperse negative charge.

EXPLANATION:

• HCl forms Cl⁻, a highly stable ion due to its large size and electronegativity.
• H₂SO₄ forms HSO₄⁻, which is stabilized by resonance, making H₂SO₄ a very strong acid.
• HNO₃ forms NO₃⁻, which is also resonance-stabilized, but slightly less stable than HSO₄⁻.
• CH₃COOH forms CH₃COO⁻, which has limited resonance and less stability, making it the weakest acid here.

The correct answer is: H₂SO₄

CONCEPT:

Boiling Points of Group 16 Hydrides

• The boiling point of a compound depends on the strength of the intermolecular forces.
• Hydrogen bonding significantly increases the boiling point of molecules like H₂O.
• In the absence of hydrogen bonding, the boiling point typically increases with molecular mass and polarizability.

EXPLANATION:

• H₂O forms strong hydrogen bonds, giving it the highest boiling point among these compounds.
• H₂S, H₂Se, and H₂Te exhibit weaker van der Waals forces since they do not form hydrogen bonds.
• Among H₂S, H₂Se, and H₂Te, the boiling point increases with molecular mass: H₂Te > H₂Se > H₂S.
• Therefore, H₂S has the lowest boiling point, as it has the smallest molecular mass and weakest intermolecular forces.

CONCLUSION:

The compound with the lowest boiling point is H₂S.

CONCEPT:

Oxidation States of Transition Elements

• Transition metals exhibit multiple oxidation states because they have unfilled d-orbitals and can lose different numbers of electrons.
• Elements like Scandium (Sc), Titanium (Ti), Iron (Fe), and Cobalt (Co) can show variable oxidation states, depending on their chemical environment.
• The +3 oxidation state is a common oxidation state for many transition metals, but not all transition metals can only exist in this state.

EXPLANATION:

• Scandium (Sc): Scandium can only exhibit a +3 oxidation state. After losing its 3 valence electrons (2 from the 4s and 1 from the 3d), it achieves a stable noble gas configuration (similar to Argon), and no other oxidation states are accessible.
• Titanium (Ti): Titanium can show multiple oxidation states, including +2, +3, and +4, making it more versatile in its chemistry.
• Iron (Fe): Iron commonly shows +2 and +3 oxidation states but can also exhibit other oxidation states in complex compounds.
• Cobalt (Co): Cobalt can display multiple oxidation states, with +2 and +3 being the most common.

CONCLUSION:

• Scandium (Sc) is the only element from the given list that can only exist in the +3 oxidation state.