Question

Will the following oxides give acidic, basic, or neutral solutions when dissolved in water? Write reactions to justify your answers. a. \(\mathrm{Li}_{2} \mathrm{O}\) b. \(\mathrm{CO}_{2}\) c. SrO

Short answer

When dissolved in water: a. Lithium oxide (\(\mathrm{Li}_{2} \mathrm{O}\)) will give a basic solution. Reaction: \[\mathrm{Li}_{2} \mathrm{O (s)} + \mathrm{H}_{2} \mathrm{O (l)} \rightarrow 2 \mathrm{LiOH (aq)}\] b. Carbon dioxide (\(\mathrm{CO}_{2}\)) will give an acidic solution. Reaction: \[\mathrm{CO}_{2} \mathrm{(g)} + \mathrm{H}_{2} \mathrm{O (l)} \rightarrow \mathrm{H}_{2} \mathrm{CO}_{3 \mathrm{(aq)}}\] c. Strontium oxide (SrO) will give a basic solution. Reaction: \[\mathrm{SrO (s)} + \mathrm{H}_{2} \mathrm{O (l)} \rightarrow \mathrm{Sr}(\mathrm{OH})_{2} \mathrm{(aq)}\]

Step-by-step solution

Part a: Lithium oxide (\(\mathrm{Li}_{2} \mathrm{O}\))

Lithium oxide (Li2O) is an ionic compound formed by the reaction between lithium and oxygen. As lithium is an alkali metal, its oxide is a basic oxide. Therefore, when lithium oxide dissolves in water, it will give a basic solution. The reaction between Li2O and water is as follows: \[\mathrm{Li}_{2} \mathrm{O (s)} + \mathrm{H}_{2} \mathrm{O (l)} \rightarrow 2 \mathrm{LiOH (aq)}\] The lithium oxide reacts with water to form lithium hydroxide (LiOH), a strong base that increases the pH of the solution.

Part b: Carbon dioxide (\(\mathrm{CO}_{2}\))

Carbon dioxide (CO2) is a non-metal oxide, specifically an acidic oxide. When it dissolves in water, it forms a weak acid known as carbonic acid (H2CO3). The reaction between CO2 and water is as follows: \[\mathrm{CO}_{2} \mathrm{(g)} + \mathrm{H}_{2} \mathrm{O (l)} \rightarrow \mathrm{H}_{2} \mathrm{CO}_{3 \mathrm{(aq)}}\] As the resulting solution contains an acidic species (carbonic acid), the solution will be acidic.

Part c: Strontium oxide (SrO)

Strontium oxide (SrO) is a basic oxide, as strontium is an alkaline earth metal. Thus when it dissolves in water, it forms a basic solution. The reaction between SrO and water is as follows: \[\mathrm{SrO (s)} + \mathrm{H}_{2} \mathrm{O (l)} \rightarrow \mathrm{Sr}(\mathrm{OH})_{2} \mathrm{(aq)}\] The strontium oxide reacts with water to form strontium hydroxide (Sr(OH)2), a strong base, which therefore results in a basic solution.

Key concepts

Basic Oxides

Basic oxides are typically formed when metals react with oxygen. They are characterized by their ability to react with water to form basic solutions, often resulting in metal hydroxides. When we consider lithium oxide \( (\text{Li}_2\text{O}) \), it is a good example of a basic oxide.

In this reaction, lithium, an alkali metal, combines with oxygen to form lithium oxide. When \( \text{Li}_2\text{O} \) dissolves in water, it reacts to produce lithium hydroxide \( (\text{LiOH}) \), which is a strong base. This means that it will increase the pH of the water, making the solution more basic. The general reaction can be written as follows:

• \( \text{Li}_2\text{O (s)} + \text{H}_2\text{O (l)} \rightarrow 2\text{LiOH (aq)} \)

The resulting lithium hydroxide is a compound with a high pH, which explains why the solution is basic. While not all metal oxides are as reactive, alkali metal oxides like those formed by lithium typically result in very reactive and basic solutions when combined with water.

Acidic Oxides

Acidic oxides are formed by nonmetals reacting with oxygen. These oxides generally produce acidic solutions when dissolved in water. Carbon dioxide \( (\text{CO}_2) \), for instance, is a classic example of an acidic oxide. When it comes into contact with water, carbon dioxide reacts to form carbonic acid \( (\text{H}_2\text{CO}_3) \):

• \( \text{CO}_2 \text{(g)} + \text{H}_2\text{O (l)} \rightarrow \text{H}_2\text{CO}_3 \text{(aq)} \)

This result occurs because carbon dioxide, being a non-metallic oxide, reacts with water to yield an acid. Known for being weak and unstable, carbonic acid is responsible for the acidic nature of the solution.

It’s important to note that acidic oxides like \( \text{CO}_2 \) can significantly lower the pH of their environment, which may lead to various environmental effects, such as acid rain and ocean acidification when present in the atmosphere and water bodies, respectively.

Neutral Solutions

Neutral solutions are those where the pH is balanced, neither leaning towards acidic nor basic. While the original exercise doesn't directly result in a neutral solution, understanding the balance is key to numerous chemical reactions and combinations. In other contexts, neutral solutions often feature water as a solvent, due to water's natural pH of 7.

Whenever an oxide interacts with water, the result can be predicted based on the nature of the oxide:

• Basic oxides, like \( \text{Li}_2\text{O} \), tend to drive pH upwards towards basicity.
• Acidic oxides, like \( \text{CO}_2 \), lower the pH, veering towards acidity.

To maintain a neutral solution, it is important to ensure an equal balance of hydrogen ions \( (\text{H}^+) \) and hydroxide ions \( (\text{OH}^-) \). While oxides that directly produce a neutral solution upon hydration are rare, the interplay of acids and bases in solutions can be manipulated to reach neutral states.