Problem 33 - Chemistry and Chemical Reactivity

Question

What feature causes the following reactions to be product-favored? (a) \(\mathrm{CuCl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{S}(\mathrm{aq}) \longrightarrow \mathrm{CuS}(\mathrm{s})+2 \mathrm{HCl}(\mathrm{aq})\) (b) \(\mathrm{H}_{3} \mathrm{PO}_{4}(\mathrm{aq})+3 \mathrm{KOH}(\mathrm{aq}) \longrightarrow 3 \mathrm{H}_{2} \mathrm{O}(\ell)+\mathrm{K}_{3} \mathrm{PO}_{4}(\mathrm{aq})\)

Step-by-Step Solution

Answer

Reaction (a) is favored due to the formation of a solid precipitate, CuS. Reaction (b) is favored due to the formation of stable water from a neutralization reaction.

Step 1: Identify the Type of Reaction

For both reactions, determine if they are precipitation, acid-base, or a different type of reaction. This will help identify why the reaction is favored.

Step 2: Analyze Reaction (a) for Driving Force

Reaction (a) involves \( \mathrm{CuCl}_{2}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{S}(\mathrm{aq}) \longrightarrow \mathrm{CuS}(\mathrm{s})+2 \mathrm{HCl}(\mathrm{aq}) \). Identify that a solid, \( \mathrm{CuS}(\mathrm{s}) \), is formed. The formation of a precipitate indicates a precipitation reaction is the driving force.

Step 3: Analyze Reaction (b) for Driving Force

Reaction (b) involves \( \mathrm{H}_{3} \mathrm{PO}_{4}(\mathrm{aq})+3 \mathrm{KOH}(\mathrm{aq}) \longrightarrow 3 \mathrm{H}_{2} \mathrm{O}(\ell)+\mathrm{K}_{3} \mathrm{PO}_{4}(\mathrm{aq}) \). This is a neutralization reaction with the formation of water, a common product-favored outcome due to its stability.

Key Concepts

Precipitation Reactions

In the world of chemical reactions, precipitation reactions are a type where two aqueous solutions combine to form an insoluble solid known as a precipitate. The solid product settles out of the solution, where it can often be seen as a powder or cloudiness. This process is driven by the principles of solubility, where the newly formed compound is not easily dissolved in the solution, thus creating the solid.
Understanding precipitation reactions is essential because they are common in nature and many industrial processes.

• These reactions help in purifying chemicals by removing unwanted ions from a solution.
• They are crucial in estimating the presence of certain ions and compounds in qualitative analysis.
• In environmental science, they assist in cleaning up waste and residues.

In reaction (a), we observe the formation of copper sulfide (\(\text{CuS}\mathrm{(s)}\)) as the solid precipitate. This formation indicates a strong driving force for the product because the precipitate removes itself from the equilibrium of the reactants.

Acid-Base Reactions

Acid-base reactions, also known as neutralization reactions, occur between an acid and a base to produce a salt and water. These reactions are fundamental and happen in various facets of life—from digestion in our stomachs to the chemical reactions we exploit to clean and disinfect.
In reaction (b), the combination of phosphoric acid,\(\text{H}_3\text{PO}_4\mathrm{(aq)}\), and potassium hydroxide,\(\text{KOH}\mathrm{(aq)}\), results in the formation of water,\(\text{H}_2\text{O}(\ell)\), and potassium phosphate,\(\text{K}_3\text{PO}_4\mathrm{(aq)}\).The formation of water is significant because:

• It is a highly stable compound.
• It signifies the completion of the neutralization process.
• It often results in a reduction of enthalpy, making the reaction favorable.

Reaction favorability here is enhanced by the stability and bonding of water molecules, which drives the reaction to completion.

Reaction Favorability

Reaction favorability refers to how likely a reaction will proceed in a particular direction to form products. Several factors influence this favorability, including the formation of stable products, the decrease in free energy, and the enthalpy change.
In both reactions discussed, we see different driving forces leading to reaction favorability:

• Formation of a Precipitate: In reaction (a), the creation of copper sulfide,\(\text{CuS}(\mathrm{s})\), shifts the reaction toward the product side by removing it from the reaction mixture.
• Stable Products: In reaction (b), the formation of water increases reaction favorability due to water's intrinsic stability and its low energy state.

Understanding these aspects helps in predicting and manipulating chemical reactions, whether in a laboratory setting or in natural processes.
Thus, whether by creating new compounds in a precipitation reaction or stabilizing products in an acid-base reaction, reaction favorability is all about reaching a lower energy state of the system, driving the chemical changes forward.