Question

Identify the wrong statements. (a) On dissolving a detergent in water, its surface tension is increased. (b) Phenol and benzoic acid can be distinguished using \(\mathrm{NaOH}\) aqueous solution. (c) Langmuir adsorption theory explains chemical adsorption. (d) Valence bond theory of complexes explain the colour of complexes.

Short answer

Statements (a), (c), and (d) are incorrect.

Step-by-step solution

Analyzing Option (a)

Surface tension is the property of a liquid that allows it to resist an external force due to cohesive interactions among the molecules of the liquid. Typically, surfactants such as detergents decrease the surface tension of water because they disrupt these cohesive interactions by accumulating at the surface. Therefore, the statement that detergent increases the surface tension is incorrect.

Analyzing Option (b)

Phenol can react with \(\mathrm{NaOH}\) to form sodium phenoxide, whereas benzoic acid forms sodium benzoate. However, both phenol and benzoic acid can undergo reactions that are somewhat comparable, making it less straightforward to distinguish them solely based on \(\mathrm{NaOH}\) aqueous solution. This statement could be considered incorrect if it implies that \(\mathrm{NaOH}\) is unique in distinguishing these two.

Analyzing Option (c)

Langmuir adsorption theory primarily describes physical adsorption, which involves weak van der Waals forces, rather than chemical adsorption, which involves stronger covalent or ionic bonding. Therefore, the statement that Langmuir adsorption theory explains chemical adsorption is incorrect.

Analyzing Option (d)

Valence bond theory explains the bonding in complexes, but it does not directly explain the color. The color in complexes is typically explained by Crystal Field Theory, which details how the energy of d-orbitals is affected by ligand arrangements. Thus, this statement is incorrect.

Key concepts

Surface Tension

Surface tension is a fascinating and essential property of liquids that acts like an invisible skin on a liquid's surface. It arises due to cohesive forces, which are the molecular attractions between the molecules within the liquid.
These forces make the surface molecules experience a net inward force, creating a barrier that can resist external forces. Imagine walking on a trampoline and feeling the tension under your feet—this is somewhat similar to what happens with surface tension.

• In pure water, cohesive forces are strong because water molecules are attracted to each other.
  
• When a detergent or any surfactant is dissolved in water, it interferes with these cohesive forces.
  
• Surfactants reduce surface tension by lining up along the surface and breaking the strong interactions between water molecules.

This is why detergents make water spread easier, allowing for more effective cleaning. Contrary to the claim that detergents increase surface tension, they actually decrease it.

Langmuir Adsorption Theory

The Langmuir adsorption theory is an early twentieth-century theory explaining how molecules from a gas or liquid adsorb onto a solid surface. It's named after Irving Langmuir, who developed this concept while working on gas absorption.
Langmuir theory assumes that the surface has a fixed number of identical sites where adsorption can occur.

• Adsorption in this theory is typically characterized as 'physical adsorption,' which involves weak van der Waals forces.
  
• This differs from 'chemical adsorption,' where stronger chemical bonds are formed.
  
• Langmuir adsorption model provides a simple equation describing how adsorbate binds to the surface and how coverage evolves with pressure or concentration.

In practice, Langmuir adsorption is widely used to understand and design catalysts and filters, although it's not suited for explaining chemical adsorption.

Valence Bond Theory

Valence Bond Theory (VBT) is pivotal in understanding how atoms combine to form molecules. It's a model focusing on the role of electron pairs and atomic orbitals in chemical bonding.
The theory explains the formation of covalent bonds, where electron pairs are shared between two atoms to form a stable molecule.

• VBT uses concepts such as hybridization—where atomic orbitals mix to form new hybrid orbitals facilitating bond formation.
  
• While effective in explaining the bonding structure of molecules, VBT does not explain the color of complexes.
  
• The color effects in complexes are related to the energy transitions within d-orbitals, a topic covered by Crystal Field Theory.

VBT is excellent for conceptualizing the shapes and bonding in molecules but not properties like color.

Crystal Field Theory

Crystal Field Theory (CFT) is essential for understanding the color and magnetism of transition metal complexes. It explores how the distribution of electrons in the d-orbitals of a metal ion is influenced by surrounding ligands.
CFT provides insights that are far beyond the explanations offered by Valence Bond Theory.

• In CFT, ligands surrounding a metal ion are treated as point charges creating an electrostatic field that affects the energies of d-orbitals.
  
• The splitting of d-orbitals occurs due to ligand interactions, which leads to different energy states.
  
• Color arises in these complexes because electrons transition between the split d-orbitals, absorbing specific wavelengths of light.

This splitting, often referred to as "d-orbital splitting", is crucial in explaining why compounds appear colored, making CFT indispensable for studying transition metals.