Question

An aqueous solution of sodium carbonate is alkaline because sodium carbonate is a salt of: (a) Weak acid and weak base (b) Weak acid and strong base (c) Strong acid and strong base (d) Strong acid and weak base

Short answer

(b) Weak acid and strong base

Step-by-step solution

Understand the Components of Sodium Carbonate

Sodium carbonate, \( \text{Na}_2\text{CO}_3 \), is composed of a sodium ion \( \text{Na}^+ \) and a carbonate ion \( \text{CO}_3^{2-} \). The sodium ion behaves as a neutral entity in solution and does not affect the acidity or alkalinity.

Identify the Nature of the Carbonate Ion

The carbonate ion, \( \text{CO}_3^{2-} \), comes from carbonic acid (\( \text{H}_2\text{CO}_3 \)), which is a weak acid. This weak acid is partially dissociated in water, producing a basic solution due to the presence of the \( \text{CO}_3^{2-} \).

Match Component Ions to Acid-Base Properties

Sodium (\( \text{Na}^+ \)) comes from sodium hydroxide (\( \text{NaOH} \)), which is a strong base. Carbonate (\( \text{CO}_3^{2-} \)) comes from carbonic acid (\( \text{H}_2\text{CO}_3 \)), which is a weak acid.

Analyze the Resulting Solution

A salt formed from a weak acid and a strong base results in an alkaline solution because the anion \( \text{CO}_3^{2-} \) tends to react with water to form \( \text{OH}^- \) ions, increasing the pH.

Conclusion

Therefore, sodium carbonate is a salt of a weak acid (carbonic acid) and a strong base (sodium hydroxide), making the solution alkaline.

Key concepts

Alkaline Solution

An alkaline solution is one that is basic in nature. This means it has a pH greater than 7. Sodium carbonate in water creates an alkaline solution, due to the chemical properties of its components. When sodium carbonate dissociates in water, it increases the hydroxide ion concentration, making the solution more basic. In essence, an alkaline solution is characterized by:

• Presence of more hydroxide ions (\( \text{OH}^- \)) compared to hydronium ions (\( \text{H}_3\text{O}^+ \))
• A pH exceeding 7
• Capability to neutralize acids

Thus, an aqueous solution of sodium carbonate is a classic example of an alkaline solution.

Weak Acid

A weak acid is a type of acid that does not completely dissociate in solution. Carbonic acid (\( \text{H}_2\text{CO}_3 \)) is an example of a weak acid. This means when dissolved in water, only a small proportion of carbonic acid molecules release protons (\( \text{H}^+ \)) into the solution. Characteristics of weak acids include their partial ionization, which can be represented by reversible reactions. For example:\[\text{H}_2\text{CO}_3 \rightleftharpoons \text{H}^+ + \text{HCO}_3^- \] The presence of weak acids in solution brings about a less pronounced acidity compared to strong acids, making them milder in nature.

Strong Base

A strong base is a substance that fully dissociates into its ions in solution, releasing hydroxide ions (\( \text{OH}^- \)). Sodium hydroxide (\( \text{NaOH} \)) is an exemplar of a strong base. When sodium hydroxide dissolves in water, it completely separates into sodium ions (\( \text{Na}^+ \)) and hydroxide ions (\( \text{OH}^- \)). Tests to characterize strong bases include:

• Full ionization in aqueous solutions
• Elevated pH values when dissolved
• High ability to neutralize acids

In the context of sodium carbonate, the sodium ion originates from this strong base, influencing the overall properties of the resultant solution.

Carbonate Ion

The carbonate ion (\( \text{CO}_3^{2-} \)) plays a crucial role in determining the characteristics of a sodium carbonate solution. This ion is derived from carbonic acid, a weak acid. When present in solution, the carbonate ion contributes to the alkaline nature because it can react with water:\[\text{CO}_3^{2-} + \text{H}_2\text{O} \rightleftharpoons \text{HCO}_3^- + \text{OH}^- \] This equilibrium reaction raises the hydroxide ion concentration, thereby making the solution more alkaline. Properties of carbonate ions include:

• Ability to form equilibrium with bicarbonate ions (\( \text{HCO}_3^- \))
• Contribution to increasing pH in solution
• Commonly resulting from the dissociation of carbonic acid

pH

The pH scale measures the acidity or basicity of a solution. It ranges from 0 (very acidic) to 14 (very basic), with 7 being neutral. In the case of a sodium carbonate solution, the pH is typically above 7, indicating that the solution is basic or alkaline. Understanding pH is fundamental because:

• It helps to identify the nature of solutions as acidic, neutral, or basic
• pH influences chemical reactions and biological processes
• Alkaline solutions, such as that of sodium carbonate, have a pH greater than 7

The pH value is a pivotal indicator used widely in chemistry to formulate conclusions about the solution's properties, like its potential to neutralize acids and its overall reactivity.