Problem 73 - Chemistry and Chemical Reactivity

Question

Based on your experience and common sense, which of the following processes would you describe as productfavored and which as reactant-favored under standard conditions? (a) \(\mathrm{Hg}(\ell) \longrightarrow \mathrm{Hg}(\mathrm{s})\) (b) \(\mathrm{H}_{2} \mathrm{O}(\mathrm{g}) \longrightarrow \mathrm{H}_{2} \mathrm{O}(\ell)\) (c) \(2 \mathrm{HgO}(\mathrm{s}) \longrightarrow \mathrm{Hg}(\ell)+\mathrm{O}_{2}(\mathrm{g})\) (d) \(\mathrm{C}(\mathrm{s})+\mathrm{O}_{2}(\mathrm{g}) \longrightarrow \mathrm{CO}_{2}(\mathrm{g})\) (e) \(\mathrm{NaCl}(\mathrm{s}) \longrightarrow \mathrm{NaCl}(\mathrm{aq})\) (f) \(\operatorname{CaCO}_{3}(\mathrm{s}) \longrightarrow \mathrm{Ca}^{2+}(\mathrm{aq})+\mathrm{CO}_{3}^{2-}(\mathrm{aq})\)

Step-by-Step Solution

Answer

(a) Product-favored, (b) Product-favored, (c) Reactant-favored, (d) Product-favored, (e) Product-favored, (f) Reactant-favored.

Step 1: Understand Product-Favored and Reactant-Favored Processes

Product-favored processes tend to produce products with lower energy or greater stability than the reactants. Under standard conditions, these processes generally occur spontaneously. Reactant-favored processes do not occur spontaneously and tend to revert back to reactants unless influenced by external conditions.

Step 2: Analyze Each Reaction - (a) Mercury Phase Change

The transformation from \( \mathrm{Hg}(\ell) \longrightarrow \mathrm{Hg}(\mathrm{s}) \) is a phase change from liquid to solid. Generally, the solid state of a substance is more stable under standard conditions due to its lower energy, hence, this process is product-favored.

Step 3: Analyze Each Reaction - (b) Condensation of Water Vapor

The process \( \mathrm{H}_{2} \mathrm{O}(\mathrm{g}) \longrightarrow \mathrm{H}_{2} \mathrm{O}(\ell) \) involves the condensation of water vapor into liquid water. Liquids are often more stable than gases at standard conditions, so this process is product-favored.

Step 4: Analyze Each Reaction - (c) Decomposition of Mercury(II) Oxide

The decomposition \( 2 \mathrm{HgO}(\mathrm{s}) \longrightarrow \mathrm{Hg}(\ell) + \mathrm{O}_{2}(\mathrm{g}) \) is breaking down a solid compound into liquid and gas products. This is generally reactant-favored as it requires energy input to dissociate the solid into separate phases.

Step 5: Analyze Each Reaction - (d) Combustion of Carbon

The reaction \( \mathrm{C}(\mathrm{s}) + \mathrm{O}_{2}(\mathrm{g}) \longrightarrow \mathrm{CO}_{2}(\mathrm{g}) \) is a combustion process. Combustion is usually exothermic and product-favored because it results in the formation of stable products like \( \mathrm{CO}_{2} \).

Step 6: Analyze Each Reaction - (e) Dissolution of Sodium Chloride

The dissolution \( \mathrm{NaCl}(\mathrm{s}) \longrightarrow \mathrm{NaCl}(\mathrm{aq}) \) indicates the dissolving of solid table salt in water, which is product-favored as the ions are stabilized by the surrounding water.

Step 7: Analyze Each Reaction - (f) Dissolution of Calcium Carbonate

The dissolution process \( \mathrm{CaCO}_{3}(\mathrm{s}) \longrightarrow \mathrm{Ca}^{2+}(\mathrm{aq}) + \mathrm{CO}_{3}^{2-}(\mathrm{aq}) \) is generally reactant-favored under standard conditions, as calcium carbonate is sparingly soluble in water.

Key Concepts

Phase Changes

Phase changes involve a substance transitioning from one state of matter to another, such as solid to liquid or liquid to solid. These changes occur when energy is added or removed. For example, when liquid mercury (\( \mathrm{Hg}(\ell) \rightarrow \mathrm{Hg}(\mathrm{s}) \)) solidifies, it undergoes a phase change from liquid to solid. Under standard conditions, solid states are generally more stable because they have lower energy compared to liquids and gases. This process is often termed product-favored since it naturally tends to form the more stable solid phase.

Condensation

Condensation is the process of a gas turning into a liquid. It often happens when the gas cools down. A common example is water vapor condensing into liquid water (\( \mathrm{H}_{2} \mathrm{O}(\mathrm{g}) \rightarrow \mathrm{H}_{2} \mathrm{O}(\ell) \)). This occurs when humid air comes into contact with a cool surface, causing the water vapor to lose energy and change into liquid form. The liquid form is generally more stable than the gaseous form under standard conditions, making condensation a product-favored process.

Combustion

Combustion is a chemical reaction that usually involves a substance reacting with oxygen to produce heat and light. A typical combustion reaction is that of carbon and oxygen (\( \mathrm{C}(\mathrm{s}) + \mathrm{O}_{2}(\mathrm{g}) \rightarrow \mathrm{CO}_{2}(\mathrm{g}) \)). This process is exothermic, meaning it releases energy and forms stable products like carbon dioxide. Because of the energy release and the stability of the products, combustion reactions are generally considered product-favored. They tend to happen spontaneously in the presence of sufficient oxygen.

Dissolution

Dissolution involves the process of a solute dissolving in a solvent, forming a solution. When sodium chloride (table salt) dissolves in water (\( \mathrm{NaCl}(\mathrm{s}) \rightarrow \mathrm{NaCl}(\mathrm{aq}) \)), the solid salt separates into ions, which are stabilized by water molecules. This often results in a more stable system in solution form, making it product-favored. However, not all dissolution processes are equally favored, such as calcium carbonate, which dissolves sparingly in water and tends to remain as a solid.

Decomposition Reactions

Decomposition reactions involve breaking down a compound into simpler substances. The decomposition of mercury(II) oxide (\( 2 \mathrm{HgO}(\mathrm{s}) \rightarrow \mathrm{Hg}(\ell) + \mathrm{O}_{2}(\mathrm{g}) \)) means the compound splits into mercury and oxygen. These reactions often require an input of energy. Because they break apart into less stable states than they started with, decomposition reactions are usually reactant-favored. This means under normal conditions, they don't occur spontaneously but need energy to proceed.