Question

The following series of observations is made: (1) a small piece of dry ice \(\left[\mathrm{CO}_{2}(\mathrm{s})\right]\) is added to \(0.005 \mathrm{M}\) \(\mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{aq}) .(2)\) Initially, a white precipitate forms.(3) After a short time the precipitate dissolves. (a) Write chemical equations to explain these observations. (b) If the \(0.005 \mathrm{M} \mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{aq})\) is replaced by 0.005. \(\mathrm{M} \mathrm{CaCl}_{2}(\mathrm{aq}),\) would a precipitate form? Explain. (c) If the \(0.005 \mathrm{M} \mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{aq})\) is replaced by 0.010 \(\mathrm{M} \mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{aq}),\) a precipitate forms but does not re-dissolve. Explain why.

Short answer

The reactions are \(\mathrm{CO}_{2}(\mathrm{s})+\mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{aq}) \rightarrow \mathrm{CaCO}_{3}(\mathrm{s})+\mathrm{H}_{2}\mathrm{O}\) and \(\mathrm{CaCO}_{3}(\mathrm{s}) + \mathrm{CO}_{2}(\mathrm{g}) + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{HCO}_{3})_{2}(\mathrm{aq})\). Calcium chloride does not form a precipitate since it is soluble. There is non-dissolution of precipitate with the higher concentration of calcium hydroxide due to the formation of excess insoluble Calcium Carbonate.

Step-by-step solution

Write Equations for Dry Ice and Calcium Hydroxide Reactions

1. When dry ice \(\left[\mathrm{CO}_{2}(\mathrm{s})\right]\) is added to \(0.005 \mathrm{M}\) calcium hydroxide (\(\mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{aq})\)), it forms a white precipitate of calcium carbonate, and water is also produced. This can be represented by the chemical equation\(\mathrm{CO}_{2}(\mathrm{s})+\mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{aq}) \rightarrow \mathrm{CaCO}_{3}(\mathrm{s})+\mathrm{H}_{2}\mathrm{O}\).2. After a short time, the precipitate dissolves because the carbon dioxide reacts with the water to form carbonic acid, which in turn reacts with the calcium carbonate to form calcium bicarbonate that is soluble. The chemical equation for this is \(\mathrm{CaCO}_{3}(\mathrm{s}) + \mathrm{CO}_{2}(\mathrm{g}) + \mathrm{H}_{2}\mathrm{O} \rightarrow \mathrm{Ca}(\mathrm{HCO}_{3})_{2}(\mathrm{aq})\).

Explain Formation of a Precipitate with Calcium Chloride

If the \(0.005 \mathrm{M} \mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{aq})\) is replaced by \(0.005 \mathrm{M} \mathrm{CaCl}_{2}(\mathrm{aq})\), no precipitate would form. This is because calcium chloride is soluble, and when it reacts with carbon dioxide, no insoluble substance is formed. Thus, no precipitate is observed.

Explain Non-Dissolution of Precipitate with Higher Concentration of Calcium Hydroxide

Replacing the \(0.005 \mathrm{M} \mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{aq})\) with \(0.010 \mathrm{M} \mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{aq})\) causes the formation of a precipitate that does not dissolve. This is due to the increased concentration of Calcium ions in the solution that results in exceeding the solubility product of Calcium Carbonate, which is relatively insoluble.

Key concepts

Precipitate Formation

When chemical reactions occur in a solution, a solid known as a precipitate may form if the product is insoluble in water. In the case of calcium hydroxide and dry ice, the precipitate that forms is calcium carbonate. This happens because carbon dioxide from the dry ice reacts with calcium hydroxide to produce calcium carbonate and water. You may visually identify a precipitate as it appears as a powdery, sometimes crystalline, solid in the liquid. Over time, if the conditions are right, such a precipitate may dissolve. Understanding why a precipitate might form or dissolve involves understanding the solubility of the compounds involved.
For example, calcium carbonate, although initially forming as a solid, can dissolve back into solution if additional chemical reactions take place.

Solubility Product

The solubility product, represented as K_sp, is a crucial concept when discussing when and why precipitates form. It refers to the equilibrium constant for a solid substance dissolving in an aqueous solution. The solubility product helps predict whether a precipitate will form under a set of specific conditions.
For calcium carbonate, the solubility product determines how much of the compound can dissolve before it forms a solid precipitate. When the ion product, which is calculated from the concentrations of the ions present in solution, exceeds the K_sp, the excess ions will combine to form a precipitate. Conversely, if the ion product is less than the K_sp, no precipitate will form, or an existing precipitate could dissolve back into the solution if the overall concentration conditions allow it.

Calcium Hydroxide

Calcium hydroxide, denoted as Ca(OH)_2, is a compound with significant applications and chemical properties. When dissolved in water, it forms a basic solution due to the dissociation of hydroxide ions. In the context of the experiment, calcium hydroxide is used to interact with carbon dioxide from dry ice to form calcium carbonate.
The concentration of calcium hydroxide is essential in this chemical reaction. A low concentration (like 0.005 M) may initially form a precipitate of calcium carbonate that can eventually dissolve if more CO₂ is added. However, increasing its concentration to 0.010 M results in a more substantial formation of calcium carbonate, which surpasses its solubility product limit and does not dissolve. This highlights the importance of solubility and concentration in chemical reactions involving precipitation.

Carbonic Acid Reaction

Carbonic acid plays a pivotal role in the dissolution process of precipitates such as calcium carbonate. It is formed when carbon dioxide reacts with water: CO₂(g) + H₂O(l) → H₂CO₃(aq).
When carbonic acid is present, it can further react with calcium carbonate, resulting in the formation of soluble calcium bicarbonate: CaCO₃(s) + H₂CO₃(aq) → Ca(HCO₃)₂(aq).
This reaction facilitates the dissolution of the precipitate, converting the solid into a soluble form. Thus, the cycle of precipitation and dissolution can be driven by the interplay of carbon dioxide and calcium-based compounds, demonstrating the reversible nature of these chemical processes. Understanding how carbonic acid affects the solubility of carbonates is critical for predicting and explaining the behavior of precipitates in solution.