Problem 88 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the correct order of acidic strengths of \(\mathrm{CO}_{2}\), \(\mathrm{CuO}, \mathrm{CaO}\) and \(\mathrm{H}_{2} \mathrm{O} .\) (a) \(\mathrm{CaO}<\mathrm{CuO}<\mathrm{H}_{2} \mathrm{O}<\mathrm{CO}_{2}\) (b) \(\mathrm{H}_{2} \mathrm{O}<\mathrm{CuO}<\mathrm{CaO}<\mathrm{CO}_{2}\) (c) \(\mathrm{CaO}<\mathrm{H}_{2} \mathrm{O}<\mathrm{CuO}<\mathrm{CO}_{2}\) (d) \(\mathrm{H}_{2} \mathrm{O}<\mathrm{CO}_{2}<\mathrm{CaO}<\mathrm{CuO}\)

Step-by-Step Solution

Answer

The correct order is (a) CaO < CuO < H2O < CO2.

Step 1: Understand Acidic Strength Trends

Acidic strength is influenced by the nature of the oxide. Usually, non-metal oxides like CO2 are acidic, whereas metal oxides like CuO and CaO can be basic or amphoteric. Amphoteric oxides contain both acidic and basic properties, and water (H2O) can be considered as a very weak acid because it can self-ionize to form hydronium and hydroxide ions.

Step 2: Analyze the Compounds

We categorize the compounds: CO2 is a non-metal oxide and generally acidic. CaO is a metal oxide and typically basic. CuO is also a metal oxide but less basic than CaO and may show amphoteric properties at times. H2O is also considered in its role as a neutral solvent and weak acid.

Step 3: Rank the Compounds

Based on the analysis: CO2 is the most acidic, followed by H2O, which is more acidic than the basic metal oxides. Among the metal oxides, CuO is less basic and sometimes considered amphoteric compared to CaO, which is the most basic.

Step 4: Identify the Correct Order

Therefore, the order of acidic strengths from least to most acidic is: CaO (most basic), CuO, H2O, and CO2 (most acidic). This matches with option (a) "CaO < CuO < H2O < CO2".

Key Concepts

Non-Metal Oxides

Non-metal oxides, such as carbon dioxide (\(\mathrm{CO}_{2}\)), tend to exhibit acidic properties. These oxides react with water to form acids. For instance, carbon dioxide when dissolved in water forms carbonic acid (\(\mathrm{H}_{2}\mathrm{CO}_{3}\)). This reaction showcases their inherent acidity:

• \(\mathrm{CO}_{2} + \mathrm{H}_{2}O \rightarrow \mathrm{H}_{2}\mathrm{CO}_{3}\)

Non-metal oxides are generally acidic because the non-metal element is bonded to oxygen with a high electronegativity difference, allowing them to accept electron pairs, acting as Lewis acids. This characteristic is seen in many non-metal oxides including sulfur dioxide (\(\mathrm{SO}_{2}\)) and nitrogen dioxide (\(\mathrm{NO}_{2}\)). Due to these properties, \(\mathrm{CO}_{2}\), a non-metal oxide, often ranks highest in terms of acidic strength compared to metal oxides.

Metal Oxides

Metal oxides are typically basic and often react with acids to form salts and water. This basicity is due to the metal component in the compound which tends to easily lose electrons and form cations. For example, calcium oxide (\(\mathrm{CaO}\)), also known as quicklime, reacts with water to form calcium hydroxide (\(\mathrm{Ca(OH)}_2\)), a strong base:

• \(\mathrm{CaO} + \mathrm{H}_{2}O \rightarrow \mathrm{Ca(OH)}_2\)

This property of forming a strong base from simple hydration is why \(\mathrm{CaO}\) is considered more basic relative to other metal oxides. Comparatively, copper(II) oxide (\(\mathrm{CuO}\)) is also a metal oxide but has the potential to exhibit amphoteric behavior under certain conditions. In general, the basic nature of an oxide is closely linked to the metal's ability to form positive ions.

Amphoteric Oxides

Amphoteric oxides are quite fascinating as they behave both as an acid and a base. This dual behavior means they can react with both acids and bases, allowing them to neutralize either and form a variety of compounds. An example of an amphoteric oxide is zinc oxide (\(\mathrm{ZnO}\)), although copper(II) oxide (\(\mathrm{CuO}\)) can also display amphoteric properties in specific environments.In acid reactions, these oxides often behave as bases and in reactions with bases, they behave as acids. This flexibility is why \(\mathrm{CuO}\) is noted to be less basic than \(\mathrm{CaO}\) and sometimes gets categorized as amphoteric. Therefore, understanding the context of the reaction conditions is crucial to predicting and explaining the behavior of amphoteric oxides.

Self-Ionization of Water

The self-ionization of water is a critical concept in chemistry because it establishes water as a neutral but weakly acidic compound. In this process, two water molecules react to form hydronium ions (\(\mathrm{H}_{3}\mathrm{O}^{+}\)) and hydroxide ions (\(\mathrm{OH}^{-}\)):

• \(2 \mathrm{H}_{2}\mathrm{O} \rightleftharpoons \mathrm{H}_{3}\mathrm{O}^{+} + \mathrm{OH}^{-}\)

This reaction is at the heart of the pH scale, with pure water having a pH of 7 because the concentrations of \(\mathrm{H}_{3}\mathrm{O}^{+}\) and \(\mathrm{OH}^{-}\) are equal. Although water is neutral in this state, it can exhibit acidic characteristics when reacting with very strong bases or within particular environments.Overall, this phenomenon sets a baseline of weak acidity, distinguishing \(\mathrm{H}_{2}\mathrm{O}\) from other compounds, and helps explain why water is considered slightly more acidic than the more basic metal oxides like \(\mathrm{CaO}\).