Problem 55 - Chemistry: An Atoms-Focused Approach

Question

The use of lead(II) carbonate and lead(II) hydroxide as white pigments in paint was discontinued in the United States in 1978 because these compounds dissolved in the stomachs of young children who ingested paint chips. The \(\mathrm{Pb}^{2+}\) ions released when the compounds dissolve interfere with neurotransmissions in the brain, causing neurological disorders. Using net ionic equations, show why lead(II) carbonate and lead(II) hydroxide dissolve in acidic solutions.

Step-by-Step Solution

Answer

Answer: 1. Lead(II) carbonate: \(\mathrm{PbCO_3(s) + 2H^+(aq) \rightarrow Pb^{2+}(aq) + H_2O(l) + CO_2(g)}\) 2. Lead(II) hydroxide: \(\mathrm{Pb(OH)_2(s) + 2H^+(aq) \rightarrow Pb^{2+}(aq) + 2H_2O(l)}\)

Step 1: Reaction of lead(II) carbonate with hydrochloric acid

Write down the balanced molecular equation of lead(II) carbonate reacting with hydrochloric acid, then dissociate all strong electrolytes and identify spectator ions. Chemical reaction: $$\mathrm{PbCO_3(s) + 2HCl(aq) \rightarrow PbCl_2(aq) + H_2O(l) + CO_2(g)}$$ Dissociate strong electrolytes: $$\mathrm{PbCO_3(s) + 2H^+(aq) + 2Cl^-(aq) \rightarrow Pb^{2+}(aq) + 2Cl^-(aq) + H_2O(l) + CO_2(g)}$$ Identify spectator ions (chloride): $$\mathrm{PbCO_3(s) + 2H^+(aq) \rightarrow Pb^{2+}(aq) + H_2O(l) + CO_2(g)}$$

Step 2: Net ionic equation for lead(II) carbonate dissolution

Eliminate spectator ions in the equation from Step 1 to obtain the net ionic equation: $$\mathrm{PbCO_3(s) + 2H^+(aq) \rightarrow Pb^{2+}(aq) + H_2O(l) + CO_2(g)}$$

Step 3: Reaction of lead(II) hydroxide with hydrochloric acid

Write down the balanced molecular equation of lead(II) hydroxide reacting with hydrochloric acid, then dissociate all strong electrolytes and identify spectator ions. Chemical reaction: $$\mathrm{Pb(OH)_2(s) + 2HCl(aq) \rightarrow PbCl_2(aq) + 2H_2O(l)}$$ Dissociate strong electrolytes: $$\mathrm{Pb(OH)_2(s) + 2H^+(aq) + 2Cl^-(aq) \rightarrow Pb^{2+}(aq) + 2Cl^-(aq) + 2H_2O(l)}$$ Identify spectator ions (chloride): $$\mathrm{Pb(OH)_2(s) + 2H^+(aq) \rightarrow Pb^{2+}(aq) + 2H_2O(l)}$$

Step 4: Net ionic equation for lead(II) hydroxide dissolution

Eliminate spectator ions in the equation from Step 3 to obtain the net ionic equation: $$\mathrm{Pb(OH)_2(s) + 2H^+(aq) \rightarrow Pb^{2+}(aq) + 2H_2O(l)}$$ In conclusion, the net ionic equations for the dissolution of lead(II) carbonate and lead(II) hydroxide in acidic solutions are: 1. Lead(II) carbonate: \(\mathrm{PbCO_3(s) + 2H^+(aq) \rightarrow Pb^{2+}(aq) + H_2O(l) + CO_2(g)}\) 2. Lead(II) hydroxide: \(\mathrm{Pb(OH)_2(s) + 2H^+(aq) \rightarrow Pb^{2+}(aq) + 2H_2O(l)}\)

Key Concepts

net ionic equations

Net ionic equations are simplified versions of chemical equations that focus only on the chemical species that participate in the reaction. They exclude spectator ions, which do not change during the course of the reaction.
For instance, in the reaction of lead(II) carbonate with hydrochloric acid, the net ionic equation is:

• \( \mathrm{PbCO_3(s) + 2H^+(aq) \rightarrow Pb^{2+}(aq) + H_2O(l) + CO_2(g)} \)

This version of the equation demonstrates how solid lead(II) carbonate converts into solvable lead ions in the presence of an acidic solution. Discarding spectator ions, such as chloride ions, focuses attention on the core chemical changes. It's essential to balance these equations with respect to both mass and charge, making sure the reaction is fully and accurately portrayed.

acid-base reactions

Acid-base reactions are processes in which an acid reacts with a base, resulting in the formation of water and possibly other products. In this context, hydrochloric acid acts as the acid, donating protons (H⁺), and lead(II) carbonate and lead(II) hydroxide act as bases accepting those protons.
In an acid-base reaction involving lead(II) carbonate or lead(II) hydroxide, the hydrogen ions from the acid facilitate the dissolution of these lead compounds. This dissolution is evident in both chemical reactions:

• Lead(II) carbonate transforms into lead ions, water, and carbon dioxide.
• Lead(II) hydroxide converts directly into lead ions and water.

These reactions are crucial because they demonstrate why lead compounds are soluble in acidic environments, such as within the stomach. They highlight the fundamental principles of chemical reactivity between acids and bases.

neurological disorders

Neurological disorders can arise when lead ions, like those released from dissolved lead(II) carbonate or lead(II) hydroxide, enter the bloodstream. Once in the bloodstream, lead ions can interfere with the normal functioning of the nervous system.
Lead has a particularly adverse impact on the brain, where it can disrupt neurotransmission—the process by which nerve cells communicate with each other. This disruption can lead to cognitive impairments and behavioral issues, especially in children who ingest lead from contaminated sources, such as paint chips.
Prolonged exposure to lead can cause more severe neurological damage, manifesting as learning disabilities, decreased IQ, and other cognitive deficits. Understanding this is critical, as it underscores the importance of removing lead from environments where children are likely to ingest or inhale its compounds.

lead compounds in paint

Lead compounds, specifically lead(II) carbonate and lead(II) hydroxide, were historically used in paint due to their bright white coloring and protective properties. However, these compounds were eventually banned from paints in the United States in 1978.
The primary concern was the dissolution of these compounds in the acidic environment of a child's stomach. When young children ingest paint chips, the acid can dissolve these lead compounds, releasing lead ions.

• This leads to potential lead poisoning.
• It catalyzes gastrointestinal absorption, impacting various bodily functions.

The historical use of these compounds underlines the need for awareness about environmental exposure to hazardous substances. Today's paints are made with safer alternatives that minimize risks to human health, avoiding the neurological and systemic damage associated with lead exposure.