Problem 19 - Chemistry The Central Science In Si Units

Question

When carbon dioxide dissolves in water, it is in equilibrium with carbonic acid \(\mathrm{H}_{2} \mathrm{CO}_{3},\) which is a weak electrolyte. What solutes are present in aqueous solution of this compound? Write the chemical equation for the ionization of \(\mathrm{H}_{2} \mathrm{CO}_{3}\)

Step-by-Step Solution

Answer

In solution: \( \mathrm{H}^+, \mathrm{HCO}_3^-, \mathrm{CO}_3^{2-} \). Ionization equation: \( \mathrm{H}_2\mathrm{CO}_3 \rightleftharpoons \mathrm{H}^+ + \mathrm{HCO}_3^- \rightleftharpoons \mathrm{H}^+ + \mathrm{CO}_3^{2-} \).

Step 1: Identify the Compounds Involved

The problem involves carbon dioxide \(\mathrm{CO}_2\) dissolving in water. This leads to the formation of carbonic acid \(\mathrm{H}_2\mathrm{CO}_3\). This compound can further dissociate into ions because it is a weak electrolyte.

Step 2: Formation of Carbonic Acid

When \(\mathrm{CO}_2\) dissolves in water, it combines with water to form carbonic acid. The chemical reaction is: \[\mathrm{CO}_2 (g) + \mathrm{H}_2O (l) \rightarrow \mathrm{H}_2\mathrm{CO}_3 (aq)\] This is an important step as it forms the basis for the ionization of \(\mathrm{H}_2\mathrm{CO}_3\).

Step 3: Write the Ionization of Carbonic Acid

Carbonic acid is a weak acid, which means it partially dissociates in water. It dissociates in two steps:1. \[\mathrm{H}_2\mathrm{CO}_3 (aq) \rightleftharpoons \mathrm{H}^+ (aq) + \mathrm{HCO}_3^- (aq)\]2. \[\mathrm{HCO}_3^- (aq) \rightleftharpoons \mathrm{H}^+ (aq) + \mathrm{CO}_3^{2-} (aq)\] This shows the ions present in the solution: \(\mathrm{H}^+\), \(\mathrm{HCO}_3^-\), and \(\mathrm{CO}_3^{2-}\).

Step 4: List the Solutes Present

The solutes in the aqueous solution are the ions produced from the ionization of carbonic acid: \(\mathrm{H}^+\), \(\mathrm{HCO}_3^-\), and \(\mathrm{CO}_3^{2-}\). These ions coexist with the undissociated \(\mathrm{H}_2\mathrm{CO}_3\).

Key Concepts

carbon dioxide

Carbon dioxide, represented by the chemical formula \( \mathrm{CO}_2 \), is a colorless gas that is naturally present in our atmosphere. It plays a key role in Earth's carbon cycle. When \( \mathrm{CO}_2 \) dissolves in water, it can react with water molecules to form carbonic acid \( \mathrm{H}_2\mathrm{CO}_3 \). This reaction is essential because it establishes the initial conditions for the process of ionization.

When you open a bottle of soda, the fizz you notice is largely due to the release of carbon dioxide. This common experience helps to illustrate how \( \mathrm{CO}_2 \) can readily dissolve in liquids. In aquatic environments and even in simple lab experiments, \( \mathrm{CO}_2 \) is a crucial component that sets the stage for chemical changes in water.

The equation \( \mathrm{CO}_2 (g) + \mathrm{H}_2O (l) \rightarrow \mathrm{H}_2\mathrm{CO}_3 (aq) \) represents this transformation and establishes a dynamic between carbon dioxide, water, and the formation of carbonic acid.

chemical equilibrium

Chemical equilibrium is a concept that describes a state in which the forward and reverse reactions occur at the same rate. In the context of carbonic acid ionization, this means that the formation of ions from carbonic acid is balanced by the recombination of these ions back into carbonic acid.

The equation for carbonic acid ionization can be shown in two steps:

• \( \mathrm{H}_2\mathrm{CO}_3 (aq) \rightleftharpoons \mathrm{H}^+ (aq) + \mathrm{HCO}_3^- (aq) \)
• \( \mathrm{HCO}_3^- (aq) \rightleftharpoons \mathrm{H}^+ (aq) + \mathrm{CO}_3^{2-} (aq) \)

This equilibrium allows both ions and undissociated carbonic acid to be present in solution simultaneously.

Understanding chemical equilibrium helps predict the concentrations of different species in solution at any given time. It is crucial in identifying how changes in conditions such as temperature or pressure can shift the equilibrium position, thus altering the concentrations of the reactants and products involved.

weak electrolyte

A weak electrolyte is a substance that does not completely dissociate into ions in solution. Carbonic acid \( \mathrm{H}_2\mathrm{CO}_3 \) is an example of a weak electrolyte. This characteristic means that only a fraction of \( \mathrm{H}_2\mathrm{CO}_3 \) molecules will split into ions when dissolved.

In the ionization of carbonic acid, the weak electrolyte behavior is seen in its partial dissociation. The primary ions produced are \( \mathrm{H}^+ \), \( \mathrm{HCO}_3^- \), and in later steps, \( \mathrm{CO}_3^{2-} \). Conversely, a large number of \( \mathrm{H}_2\mathrm{CO}_3 \) molecules remain intact, contributing little to the overall ion concentration in solution.

The concept of weak electrolytes is significant because it affects the solution's conductivity and the equilibrium position of electrolytic reactions. For example, solutions with weak electrolytes are typically less conductive compared to those with strong electrolytes, where ionization is almost complete.

aqueous solution

An aqueous solution is a mixture where water is the solvent. In such a solution, various solutes can be dissolved, establishing a medium for chemical reactions. When carbon dioxide dissolves in water, it forms an aqueous solution of carbonic acid.

The solutes in an aqueous solution of carbonic acid include:

• Hydrogen ions \( \mathrm{H}^+ \)
• Bicarbonate ions \( \mathrm{HCO}_3^- \)
• Carbonate ions \( \mathrm{CO}_3^{2-} \)

These ions coexist with non-ionized \( \mathrm{H}_2\mathrm{CO}_3 \), providing a dynamic environment where chemical equilibrium can play out.

In the world of chemistry, aqueous solutions are common and pivotal. They are used in countless reactions and processes due to the unique properties of water as a solvent. This characteristic allows for the efficient transport and interaction of ions and molecules, facilitating a wide range of chemical transformations.