Problem 90 - A Complete Resource Book in Chemistry for JEE Main

Question

With respect to both oxygen and ozone which one of the following statements are correct? (1) oxygen has 2 bonds and ozone has 3 bonds (2) oxygen valency is 2 in both (3) oxygen is colourless while ozone is coloured (4) both have similar reactivity with water (a) 1,2 and 3 (b) 2,3 and 4 (c) 2 and 3 (d) 1 and 2

Step-by-Step Solution

Answer

Option (c) 2 and 3 are correct.

Step 1: Understanding the Bond Structures

Oxygen ( O_2 ) is a diatomic molecule with a double bond between the two oxygen atoms. Ozone ( O_3 ) consists of a resonance structure with an arrangement of three atoms, where a pi bond and single bonds are formed. Thus, oxygen effectively has 2 bonds, while ozone has a complex bonding structure involving three atoms not three bonds between each atom, thus the statement is misleading about ozone having 3 distinct single bonds.

Step 2: Examining Valency

The valency of an element refers to its combining power with other atoms when it forms chemical compounds. Oxygen typically has a valency of 2, meaning it usually forms two bonds in compounds, such as H_2O (water). This property is consistent in both O_2 and O_3 .

Step 3: Assessing Color Properties

Oxygen ( O_2 ) as a gas is colorless, while ozone ( O_3 ) is a pale blue gas. This means oxygen does not exhibit color to the naked eye, but ozone can appear colored in its gaseous phase.

Step 4: Reactivity with Water

Oxygen and ozone both react with water, but not to the same extent or with the same results. Ozone is more reactive due to its additional oxygen atom and forms different compounds, implying they do not have similar reactivity with water.

Step 5: Choosing the Correct Statements

Given the analysis of each statement: (1) is not fully correct, (2) is correct, (3) is correct, and (4) is incorrect. Therefore, the option (c) 2 and 3 is correct.

Key Concepts

Molecular Bonding

Molecular bonding lies at the heart of understanding how different substances are held together. In the case of oxygen ( O_2 ), it is a diatomic molecule, meaning it is formed by two oxygen atoms bonded by a double bond. This double bond consists of a sigma bond and a pi bond, which are types of covalent bonds. On the other hand, ozone ( O_3 ) has a more complex structure. It forms a resonance hybrid, a kind of molecule that can be represented by more than one valid Lewis structure. Ozone's bonding involves two single sigma bonds and one pi bond distributed over the three oxygen atoms. While it might seem like each atom is forming three bonds, that's not truly the case. Instead, the electrons are delocalized over the molecule, giving ozone its unique properties.

Valency in Chemistry

Valency in chemistry refers to an element's capacity to bond with other atoms. It's crucial for understanding how molecules are formed. For oxygen, the valency is 2. This is because an oxygen atom has six electrons in its outer shell and needs two more electrons to complete its "octet" or full shell of eight. Thus, it typically forms two bonds in compounds like water ( H_2O ).

In both oxygen ( O_2 ) and ozone ( O_3 ), the valency effectively remains the same. The two bonds that oxygen forms reflect its valency of 2, helping to predict how it will interact in different chemical reactions. Understanding valency not only aids in predicting molecular structures but also in grasping how chemical reactions might proceed.

Chemical Reactivity with Water

Chemical reactivity with water varies significantly between substances, and this is evident when comparing oxygen and ozone. Although both substances consist of oxygen atoms, their reactivity profiles are different due to the variations in their molecular structures.

Oxygen ( O_2 ) is relatively stable and does not react with water directly under normal conditions. It can dissolve in water, aiding in the respiration processes of aquatic life. Conversely, ozone ( O_3 ) is much more reactive. It readily reacts with water to form hydroxyl radicals ( OH^- ) and oxygen ( O_2 ). This reaction process highlights ozone's role in disinfection and oxidation processes, leveraging its higher reactivity. Therefore, while they both interact with water, the nature and extent of these interactions are different, making ozone much more reactive than oxygen in aqueous environments.