Problem 73 - Chemistry The Molecular Science

Question

Which of these substances has greater solubility at \(\mathrm{pH}=2\) than at \(\mathrm{pH}=7 ?\) (a) \(\mathrm{Cu}(\mathrm{OH})_{2}\) (b) \(\mathrm{CuSO}_{4}\) (c) \(\mathrm{CuCO}_{3}\) (d) \(\mathrm{CuS}\) (e) \(\mathrm{Cu}_{3}\left(\mathrm{PO}_{4}\right)_{2}\)

Step-by-Step Solution

Answer

(a) \(\mathrm{Cu(OH)_2}\), (c) \(\mathrm{CuCO_3}\), and (e) \(\mathrm{Cu_3\left(PO_4\right)_2}\) have greater solubility at \(\mathrm{pH}=2\).

Step 1: Analyze Solubility and pH

Solubility of substances can be affected by pH if the compound contains a basic anion that can react with acidic conditions to form water or soluble species. This often applies to hydroxides, carbonates, phosphates, and sulfides.

Step 2: Examine Copper(II) Hydroxide \\(\mathrm{Cu(OH)_2}\\)

At low pH (acidic conditions), \(\mathrm{Cu(OH)_2}\) becomes more soluble because the hydroxide ions \((\mathrm{OH}^-)\) react with \(\mathrm{H^+}\) ions to form water, dissolving more solid into solution.

Step 3: Examine Copper(II) Sulfate \\(\mathrm{CuSO_4}\\)

Copper(II) sulfate is generally soluble in water, and its solubility isn’t significantly affected by pH change since sulfate \(\mathrm{(SO_4^{2-})}\) doesn't react appreciably with \(\mathrm{H^+}\) at either pH 2 or 7.

Step 4: Examine Copper(II) Carbonate \\(\mathrm{CuCO_3}\\)

In acidic conditions (pH = 2), \(\mathrm{CuCO_3}\) reacts with \(\mathrm{H^+}\) to form soluble \(\mathrm{Cu^{2+}}\) and carbon dioxide gas, increasing its solubility compared to neutral conditions.

Step 5: Examine Copper(II) Sulfide \\(\mathrm{CuS}\\)

Copper(II) sulfide is generally insoluble in water, and acidic conditions do not significantly change its solubility.

Step 6: Examine Copper(II) Phosphate \\(\mathrm{Cu_3\left(PO_4\right)_2}\\)

Similar to hydroxides and carbonates, phosphates become more soluble in acidic conditions where the \(\mathrm{H^+}\) ions can react with phosphate ions \(\mathrm{(PO_4^{3-})}\) to form \(\mathrm{H_3PO_4}\) or \(\mathrm{H_2PO_4^-}\).

Step 7: Conclusion and Answer

Substances whose solubility increases in acidic conditions due to the reaction of \(\mathrm{H^+}\) ions with the anions include \(\mathrm{Cu(OH)_2}\), \(\mathrm{CuCO_3}\), and \(\mathrm{Cu_3\left(PO_4\right)_2}\). Thus, at least three substances have greater solubility at \(\mathrm{pH}=2\) than at \(\mathrm{pH}=7\).

Key Concepts

Chemical reactions with acidic conditions

When substances are exposed to acidic conditions, chemical reactions may occur that can affect their solubility.In acidic solutions, the presence of excess \(\mathrm{H^+}\) ions can lead to the formation of more soluble products.Substances containing basic anions such as hydroxides \(\mathrm{(OH^-)}\), carbonates \(\mathrm{(CO_3^{2-})}\), and phosphates \(\mathrm{(PO_4^{3-})}\) are particularly reactive in acidic conditions.This is because these basic anions readily react with \(\mathrm{H^+}\), forming water or other typically more soluble species.For instance, hydroxide ions will combine with \(\mathrm{H^+}\) ions to form water, effectively dissolving the solid as more of it reacts.

• Hydroxides: React with \(\mathrm{H^+}\) to form water, increasing solubility.
• Carbonates: React with \(\mathrm{H^+}\) to give \(\mathrm{CO_2}\) gas and water, enhancing solubility.
• Phosphates: React with \(\mathrm{H^+}\) to form species like \(\mathrm{H_2PO_4^-}\), dissolving the compound.

Thus, understanding these reaction pathways helps predict changes in solubility under acidic conditions.

Basic anions and solubility

Basic anions are negatively charged species that can interact favorably with \(\mathrm{H^+}\) ions in solutions.Anions such as hydroxide, carbonate, and phosphate have a strong affinity for binding with \(\mathrm{H^+}\) ions.This interaction results in the formation of neutral or less charged molecules, often leading to an increase in solubility.For example, in copper(II) compounds like \(\mathrm{Cu(OH)_2}\), \(\mathrm{CuCO_3}\), and \(\mathrm{Cu_3(PO_4)_2}\), the basic anions play a pivotal role.These compounds react in acidic environments, breaking down into more soluble forms because:

• Hydroxide ions from \(\mathrm{Cu(OH)_2}\) react to form water, enhancing dissolution.
• Carbonate ions in \(\mathrm{CuCO_3}\) form \(\mathrm{CO_2}\) gas, increasing solubility.
• Phosphate ions from \(\mathrm{Cu_3(PO_4)_2}\) form new soluble phosphate species.

Therefore, the presence and type of basic anion are key determinants in the solubility changes of a compound.

pH effect on solubility

pH is a measure of acidity or basicity of a solution, and it significantly impacts the solubility of certain substances.For compounds with basic anions, a lower pH (more acidic) environment often means increased solubility.This is due to the reaction between the anions and the \(\mathrm{H^+}\) ions in solution.Consider \(\mathrm{Cu(OH)_2}\), \(\mathrm{CuCO_3}\), and \(\mathrm{Cu_3(PO_4)_2}\) as examples:

• At \(\mathrm{pH} = 2\), the acidic environment provides abundant \(\mathrm{H^+}\), which react with the anions, enhancing solubility through product formation.
• At \(\mathrm{pH} = 7\), the solution is neutral, leading to less reaction with \(\mathrm{H^+}\) and thus, lesser solubility.

This understanding is crucial when determining the solubility behavior of substances in different pH environments, such as in the context of copper compounds in acidic solutions.