Problem 33 - Chemistry The Molecular Science

Question

If aqueous solutions of potassium carbonate and copper(II) nitrate are mixed, a precipitate is formed. Write the complete and net ionic equations for this reaction, and name the precipitate.

Step-by-Step Solution

Answer

Precipitate: copper(II) carbonate. Net ionic equation: \( CO_3^{2-}(aq) + Cu^{2+}(aq) \rightarrow CuCO_3(s) \).

Step 1: Write the Molecular Equation

First, write the balanced molecular equation for the reaction. Potassium carbonate (K_2CO_3) reacts with copper(II) nitrate (Cu(NO_3)_2) to form copper(II) carbonate (CuCO_3), which is insoluble in water, and potassium nitrate (KNO_3), which remains in solution.The balanced molecular equation is:\[K_2CO_3(aq) + Cu(NO_3)_2(aq) \rightarrow CuCO_3(s) + 2KNO_3(aq)\]

Step 2: Dissociate Aqueous Compounds into Ions

Write the complete ionic equation by representing all soluble ionic compounds as dissociated ions. All (aq) labeled compounds dissociate into their corresponding ions, while the solid does not.The complete ionic equation is:\[2K^+(aq) + CO_3^{2-}(aq) + Cu^{2+}(aq) + 2NO_3^-(aq) \rightarrow CuCO_3(s) + 2K^+(aq) + 2NO_3^-(aq)\]

Step 3: Identify and Remove The Spectator Ions

Identify the spectator ions, which are ions that appear on both sides of the complete ionic equation and do not participate in the formation of the precipitate. In this reaction, the spectator ions are potassium ion ( K^+ ) and nitrate ion ( NO_3^- ). After removing the spectator ions, you are left with the net ionic equation:

Step 4: Write the Net Ionic Equation

The net ionic equation is the equation that shows only the ions involved in forming the precipitate.\[CO_3^{2-}(aq) + Cu^{2+}(aq) \rightarrow CuCO_3(s)\]

Step 5: Name the Precipitate

The precipitate formed in this reaction is copper(II) carbonate. It is an insoluble compound that precipitates out of the solution because its solubility product is exceeded.

Key Concepts

Molecular Equation

A molecular equation is a way to represent the overall reaction involving the reactants and products in their complete, non-dissociated forms. In chemical reactions involving ionic compounds in aqueous solutions, the molecular equation provides a general overview by showing the compounds involved without detailing the dissociation into ions. This representation helps chemists understand which substances are initially present and those formed after the reaction.

For instance, when aqueous solutions of potassium carbonate (K_2CO_3) and copper(II) nitrate (Cu(NO_3)_2) are mixed, the molecules of these compounds interact. The reaction leads to the formation of a solid and some ions remain dissolved, as shown in the molecular equation:\[K_2CO_3(aq) + Cu(NO_3)_2(aq) \rightarrow CuCO_3(s) + 2KNO_3(aq)\]

• Potassium carbonate (K_2CO_3) is mixed with copper(II) nitrate (Cu(NO_3)_2).
• This results in copper(II) carbonate (CuCO_3) as a solid precipitate and potassium nitrate (KNO_3) remaining in the solution.

This equation is balanced with respect to both atoms and charge, ensuring the principle of conservation of mass is maintained.

Spectator Ions

In ionic reactions, ions that do not participate in the chemical transformation are known as spectator ions. These ions remain unchanged in the solution before and after the reaction. Identifying these ions is crucial to focus on the actual chemical reaction taking place.

The process of identifying spectator ions involves checking ions that appear on both sides of the complete ionic equation and canceling them out. In the example of potassium carbonate and copper(II) nitrate, after breaking down the molecular equation to form the complete ionic equation:\[2K^+(aq) + CO_3^{2-}(aq) + Cu^{2+}(aq) + 2NO_3^-(aq) \rightarrow CuCO_3(s) + 2K^+(aq) + 2NO_3^-(aq)\]

• Potassium ions (K^+) and nitrate ions (NO_3^-) are present on both sides without undergoing any change.

These ions are therefore considered spectator ions as they do not take part in forming the solid product, copper(II) carbonate (CuCO_3). Removing these ions simplifies the equation to only include the active components.

Precipitation Reaction

A precipitation reaction is a type of chemical reaction where soluble ions in separate solutions are mixed together to form an insoluble compound, known as a precipitate. This reaction is fundamental in understanding how compounds behave in aqueous solutions, especially in contexts like chemistry and environmental sciences.

When potassium carbonate interacts with copper(II) nitrate in solution, a notable precipitation reaction occurs, forming copper(II) carbonate:\[CO_3^{2-}(aq) + Cu^{2+}(aq) \rightarrow CuCO_3(s)\]

• The carbonate ions (CO_3^{2-}) and copper ions (Cu^{2+}) are the key actors in forming the solid precipitate.

This happens because copper(II) carbonate is insoluble in water, meaning it doesn't dissolve and instead settles as a solid. The formation of a precipitate indicates that the solubility product of the compound has been exceeded. This aspect is crucial for predicting the outcome of reactions in solutions and is an essential concept for chemical analysis and synthesis.