Problem 187 - A Complete Resource Book in Chemistry for JEE Main

Question

Select correct statement: [2002] (a) when a covalent bond is formed, transfer of electrons takes place (b) pure \(\mathrm{H}_{2} \mathrm{O}\) does not contain any ion (c) a bond is formed when attractive forces overcome repulsive forces (d) HF is less polar than \(\mathrm{HBr}\)

Step-by-Step Solution

Answer

Statement (c) is correct.

Step 1: Understanding Covalent Bond Formation

Covalent bonds involve the sharing of electrons between atoms, not the transfer of electrons. This eliminates statement (a).

Step 2: Assessing Pure Water's Ion Content

Pure water does indeed contain ions, albeit in very small amounts due to self-ionization into \( ext{H}^+\) and \( ext{OH}^-\) ions, disqualifying statement (b).

Step 3: Analyzing Bond Formation Forces

A bond forms when the attractive forces between atoms surpass the repulsive forces, making statement (c) a valid description of bond formation.

Step 4: Comparing Polarity of HF and HBr

HF is more polar than HBr because the electronegativity difference between H and F is greater than that between H and Br. This makes statement (d) incorrect.

Key Concepts

Covalent Bond

Covalent bonds are a fascinating aspect of chemistry where atoms join together by sharing electrons. Unlike in ionic bonds, where electrons are transferred from one atom to another, covalent bonds involve a mutual sharing. This sharing usually happens between non-metal atoms. It allows each atom to achieve a stable electron configuration similar to noble gases.
This concept is critical because the properties of many substances, like water and carbon dioxide, are dictated by the presence of covalent bonds. The beauty of covalent bonding lies in its ability to form molecules of intricate structure. Examples include the vast variety of organic compounds, where carbon commonly forms four covalent bonds in its most stable state.

• This bond type leads to the formation of molecules rather than ions.
• It often results in substances with low melting and boiling points due to the weaker forces holding the molecules together.
• Covalent bonding is responsible for the diversity in chemical reactions and life-forming molecules.

Ionization of Water

The ionization of water is an intriguing natural process where water, \(\mathrm{H}_2\mathrm{O}\), partially dissociates into hydrogen ions \(\mathrm{H}^+\) and hydroxide ions \(\mathrm{OH}^-\). Although pure water is mostly neutral, this self-ionization plays a crucial role in its behavior.
Water's slight ionization is why it can act as a conductor of electricity. The degree of ionization can be explained by the presence of water's exceedingly small but non-zero equilibrium constant, typically represented as the water dissociation constant, \(K_w\). It is important to note that ionization is temperature-dependent, showing slight variations with changes in temperature.

• The concentration of \(\mathrm{H}^+\) and \(\mathrm{OH}^-\) ions in pure water at 25°C is \(1 \times 10^{-7}\) M.
• This small concentration ensures that water maintains a pH close to neutral, around 7.
• The self-ionization affects the properties of water, impacting chemical reactions that water participates in.

Electronegativity

Electronegativity is a fascinating concept that describes the tendency of an atom to attract a bonding pair of electrons. Represented by a scale developed by Linus Pauling, it ranges from around 0.7 to 4.0, with fluorine having the highest electronegativity value of 4.0.
Knowing an atom's electronegativity helps predict the nature of bonds it will form. For example, in a molecule like hydrogen fluoride (HF), the high electronegativity of fluorine compared to hydrogen means electrons are drawn closer to fluorine, resulting in a polar covalent bond.

• Atoms with high electronegativity tend to gain electrons and form negative ions.
• A significant electronegativity difference in bonded atoms leads to polar bonds, enhancing a molecule's polarity.
• Electronegativity influences molecule behavior, impacting properties like boiling points, solubility, and chemical reactivity.