Question

Predict whether each of the following oxides is acidic, basic, amphoteric, or neutral: (a) \(\mathrm{NO}_{2}\), (b) \(\mathrm{CO}_{2}\), (c) \(\mathrm{Al}_{2} \mathrm{O}_{3}\), (d) \(\mathrm{CaO}\).

Short answer

(a) \(\mathrm{NO}_{2}\) is acidic as Nitrogen is a non-metal. (b) \(\mathrm{CO}_{2}\) is acidic due to Carbon being a non-metal. (c) \(\mathrm{Al}_{2} \mathrm{O}_{3}\) is amphoteric since Aluminum is a metalloid. (d) \(\mathrm{CaO}\) is basic because Calcium is a metal.

Step-by-step solution

(a) Identifying the type of oxide for \(\mathrm{NO}_{2}\)

: Nitrogen is a non-metal and it forms the oxide \(\mathrm{NO}_{2}\), which is likely to be acidic.

(b) Identifying the type of oxide for \(\mathrm{CO}_{2}\)

: Carbon is also a non-metal, and when it forms the oxide \(\mathrm{CO}_{2}\), it also has acidic properties.

(c) Identifying the type of oxide for \(\mathrm{Al}_{2} \mathrm{O}_{3}\)

: Aluminum is a metalloid, and when it forms the oxide \(\mathrm{Al}_{2} \mathrm{O}_{3}\), it can exhibit both acidic and basic properties. Aluminum sits on the border between metals and non-metals, making this oxide amphoteric.

(d) Identifying the type of oxide for \(\mathrm{CaO}\)

: Calcium is a metal and thus forms a basic oxide when it reacts with oxygen, resulting in \(\mathrm{CaO}\). To summarize the results: (a) \(\mathrm{NO}_{2}\) - Acidic (b) \(\mathrm{CO}_{2}\) - Acidic (c) \(\mathrm{Al}_{2} \mathrm{O}_{3}\) - Amphoteric (d) \(\mathrm{CaO}\) - Basic

Key concepts

Acidic Oxides

Acidic oxides are oxides of non-metals. These oxides react with water to form an acidic solution, and they can also react with bases to form salts and water. An excellent way to identify acidic oxides is to check whether they are oxides of non-metal elements. For instance, both nitrogen dioxide \(\mathrm{NO}_2\) and carbon dioxide \(\mathrm{CO}_2\) are acidic oxides because they are formed by the non-metal elements nitrogen and carbon, respectively. When dissolved in water, \(\mathrm{NO}_2\) forms nitric acid \(\mathrm{HNO}_3\), and \(\mathrm{CO}_2\) forms carbonic acid \(\mathrm{H}_2\mathrm{CO}_3\). Acidic oxides play an essential role in environmental chemistry, particularly relating to phenomena such as acid rain.

Basic Oxides

Basic oxides are typically formed from the elements that are metals. These compounds react with water to generate hydroxides, which are basic in nature. For example, calcium oxide \(\mathrm{CaO}\) is a basic oxide. When it reacts with water, it produces calcium hydroxide \(\mathrm{Ca(OH)_2}\), a strong base. Basic oxides can also react with acids to produce salts and water. Usually, the metals from the alkali and alkaline earth groups in the periodic table form strong basic oxides. As metals are inclined to donate electrons, their oxides often show a considerable tendency to neutralize acids.

Amphoteric Oxides

Amphoteric oxides are unique as they possess both acidic and basic properties. This dual nature allows them to react with both acids and bases, forming salts in the process. A common example of an amphoteric oxide is aluminum oxide \(\mathrm{Al_2O_3}\), which can react with hydrochloric acid to form aluminum chloride \(\mathrm{AlCl_3}\), and with sodium hydroxide to produce sodium aluminate \(\mathrm{NaAlO_2}\). The presence of amphoteric oxides is often associated with elements that straddle the line between metals and non-metals, like aluminum, which is a metalloid. Exploring amphoteric oxides provides insights into the versatile nature of some chemical compounds in reacting with different reagents.

Neutral Oxides

Neutral oxides are quite fascinating as they do not display acidic or basic properties. These oxides neither react with acids nor with bases. So, in essence, they do not participate in acid-base reactions to a noticeable extent. Some well-known neutral oxides include nitrous oxide \(\mathrm{N_2O}\) and carbon monoxide \(\mathrm{CO}\). They are characterized by their chemical neutrality and inability to form salts in reactions with acids or bases, distinguishing them from other types of oxides. Understanding neutral oxides is crucial for chemistry students, as it highlights the spectrum of oxide reactivity.