Problem 82 - Chemistry and Chemical Reactivity

Question

For each reaction, write an overall, balanced equation and the net ionic equation. (a) the reaction of aqueous lead(II) nitrate and aqueous potassium hydroxide (b) the reaction of aqueous copper(II) nitrate and aqueous sodium carbonate

Step-by-Step Solution

Answer

(a) Overall: Pb(NO₃)₂ + 2KOH → Pb(OH)₂ + 2KNO₃. Net Ionic: Pb²⁺ + 2OH⁻ → Pb(OH)₂. (b) Overall: Cu(NO₃)₂ + Na₂CO₃ → CuCO₃ + 2NaNO₃. Net Ionic: Cu²⁺ + CO₃²⁻ → CuCO₃.

Step 1: Write the Molecular Equation for Reaction (a)

The reactants given are lead(II) nitrate, which is Pb(NO₃)₂, and potassium hydroxide, which is KOH. These react in an aqueous solution to form lead(II) hydroxide, Pb(OH)₂, and potassium nitrate, KNO₃. The molecular equation is: \[ \text{Pb(NO₃)₂ (aq) + 2KOH (aq) \rightarrow Pb(OH)₂ (s) + 2KNO₃ (aq)} \]

Step 2: Write the Complete Ionic Equation for Reaction (a)

For the complete ionic equation, break all aqueous compounds into their ions and keep the solid compound intact: \[ \text{Pb}^{2+} (aq) + 2\text{NO}_3^- (aq) + 2\text{K}^+ (aq) + 2\text{OH}^- (aq) \rightarrow \text{Pb(OH)}_2 (s) + 2\text{K}^+ (aq) + 2\text{NO}_3^- (aq) \]

Step 3: Write the Net Ionic Equation for Reaction (a)

Remove the spectator ions (those ions that appear on both sides of the ionic equation) to find the net ionic equation. For this reaction, the spectator ions are \(\text{K}^+\) and \(\text{NO}_3^-\). The net ionic equation is: \[ \text{Pb}^{2+} (aq) + 2\text{OH}^- (aq) \rightarrow \text{Pb(OH)}_2 (s) \]

Step 4: Write the Molecular Equation for Reaction (b)

Copper(II) nitrate, Cu(NO₃)₂, and sodium carbonate, Na₂CO₃, react in an aqueous solution. The products are copper(II) carbonate, CuCO₃, and sodium nitrate, NaNO₃. The molecular equation is: \[ \text{Cu(NO₃)}_2 (aq) + \text{Na}_2\text{CO}_3 (aq) \rightarrow \text{CuCO}_3 (s) + 2\text{NaNO}_3 (aq) \]

Step 5: Write the Complete Ionic Equation for Reaction (b)

Break all aqueous compounds into their ions and keep the solid compound intact: \[ \text{Cu}^{2+} (aq) + 2\text{NO}_3^- (aq) + 2\text{Na}^+ (aq) + \text{CO}_3^{2-} (aq) \rightarrow \text{CuCO}_3 (s) + 2\text{Na}^+ (aq) + 2\text{NO}_3^- (aq) \]

Step 6: Write the Net Ionic Equation for Reaction (b)

Remove the spectator ions, which for this reaction are \(\text{Na}^+\) and \(\text{NO}_3^-\), to write the net ionic equation. The net ionic equation is: \[ \text{Cu}^{2+} (aq) + \text{CO}_3^{2-} (aq) \rightarrow \text{CuCO}_3 (s) \]

Key Concepts

Molecular Equation

In chemistry, a molecular equation provides a summary of the chemical reaction between compounds in their complete chemical formulas. The idea is to show the products and reactants in their molecular forms. For instance, in the reaction between lead(II) nitrate and potassium hydroxide, the molecular equation succinctly depicts what happens when these two substances interact. When written out, it looks like this:\[ \text{Pb(NO₃)}_2 (aq) + 2\text{KOH} (aq) \rightarrow \text{Pb(OH)}_2 (s) + 2\text{KNO₃} (aq) \]The above shows lead(II) nitrate and potassium hydroxide reacting in aqueous solution to produce lead(II) hydroxide and potassium nitrate. Notice it includes the physical states of the compounds, such as

• \((aq)\) for aqueous solutions
• \((s)\) for solids

Using the molecular equation helps to clearly understand what reactants are being transformed and what products are being formed. It sets the stage for breaking down these compounds further into ionic forms.

Spectator Ions

Spectator ions are ions present in solution that do not take part in the chemical reaction. They remain unchanged in their ionic form, hovering around either side of the reaction.To identify them, one needs to consider the complete ionic equation.For example, in the aqueous reaction between lead(II) nitrate and potassium hydroxide, the complete ionic equation is:\[ \text{Pb}^{2+} (aq) + 2\text{NO}_3^- (aq) + 2\text{K}^+ (aq) + 2\text{OH}^- (aq) \rightarrow \text{Pb(OH)}_2 (s) + 2\text{K}^+ (aq) + 2\text{NO}_3^- (aq) \]Here, notice the potassium ions (\(\text{K}^+\)) and nitrate ions (\(\text{NO}_3^-\)) are present on both sides of the equation.

• These ions are the spectator ions.
• They do not participate in the formation of the solid precipitate.

Spectator ions are excluded when writing the net ionic equation, allowing a focus on the actual chemical change.

Complete Ionic Equation

The complete ionic equation breaks down strong electrolytes into their individual ions. This representation separates the ions in aqueous solutions, providing insight into the nature of each substance in a chemical reaction.In our example, the complete ionic equation for the reaction between copper(II) nitrate and sodium carbonate is constructed by writing each aqueous compound in its ionic form:\[ \text{Cu}^{2+} (aq) + 2\text{NO}_3^- (aq) + 2\text{Na}^+ (aq) + \text{CO}_3^{2-} (aq) \rightarrow \text{CuCO}_3 (s) + 2\text{Na}^+ (aq) + 2\text{NO}_3^- (aq) \]In this equation:

• Copper(II) ions: \(\text{Cu}^{2+} (aq)\)
• Carbonate ions: \(\text{CO}_3^{2-} (aq)\)
• Sodium ions: \(2\text{Na}^+ (aq)\)
• Nitrate ions: \(2\text{NO}_3^- (aq)\)

This equation reveals that when a solid (like \(\text{CuCO}_3\)) is formed, the ions not involved in forming the solid remain as spectator ions, ultimately seen again in the net ionic equation.