Problem 6 - Chemistry: Principles and Reactions

Question

On the basis of your experience, predict which of the following reactions are spontaneous. (a) \(\mathrm{CO}_{2}(s) \longrightarrow \mathrm{CO}_{2}(g)\) at \(25^{\circ} \mathrm{C}\) (b) \(\mathrm{NaCl}(s) \longrightarrow \mathrm{NaCl}(l)\) at \(25^{\circ} \mathrm{C}\) (c) \(2 \mathrm{NaCl}(s) \longrightarrow 2 \mathrm{Na}(s)+\mathrm{Cl}_{2}(g)\) (d) \(\mathrm{CO}_{2}(g) \longrightarrow \mathrm{C}(s)+\mathrm{O}_{2}(g)\)

Step-by-Step Solution

Answer

Also, which ones are non-spontaneous and why?

Step 1: Reaction (a) Analysis

In this reaction, we have solid carbon dioxide (dry ice) converting into gaseous carbon dioxide. Intuitively, we can expect an increase in disorder (higher entropy) as the gas phase is more disordered than the solid phase.

Step 2: Reaction (a) Predict and Explain Spontaneity

Based on our experience, we can predict that reaction (a) will be spontaneous at 25°C. This is because there is an increase in entropy as the solid converts into gas, which generally promotes spontaneity in a reaction.

Step 3: Reaction (b) Analysis

In this reaction, we have solid sodium chloride converting into liquid sodium chloride. Going from a solid state to a liquid state usually leads to an increase in disorder (entropy), but we need to keep in mind that this change occurs at 25°C.

Step 4: Reaction (b) Predict and Explain Spontaneity

It's necessary to consider the fact that the melting point of sodium chloride is around 801°C. Since the required temperature for this particular reaction is 25°C, we can predict that reaction (b) is non-spontaneous under these conditions.

Step 5: Reaction (c) Analysis

In this reaction, we have solid sodium chloride breaking down into solid sodium and gaseous chlorine. It is important to note that sodium chloride is considered a stable compound with strong ionic bonds, resulting in low likelihood of easily breaking apart.

Step 6: Reaction (c) Predict and Explain Spontaneity

Given the strong ionic bonds of sodium chloride, the overall stability of the compound, and the need to break those bonds to form the products, we can predict that reaction (c) is non-spontaneous under normal conditions.

Step 7: Reaction (d) Analysis

In this reaction, we have gaseous carbon dioxide breaking down into solid carbon and gaseous oxygen. Similar to reaction (c), carbon dioxide is a stable compound, and breaking it down would require a significant amount of energy.

Step 8: Reaction (d) Predict and Explain Spontaneity

Considering the stability of carbon dioxide and the energy required to break it down into its components, we can predict that reaction (d) is non-spontaneous under normal conditions.

Key Concepts

Spontaneity

A chemical reaction is called spontaneous if it can proceed without any external influence once started. Spontaneity is influenced by several factors:

• Enthalpy change (abla H): The heat absorbed or released.
• Entropy change (abla S): The change in disorder or randomness.
• Temperature: Affects the balance between enthalpy and entropy.

To predict spontaneity, we use Gibbs Free Energy (abla G):

\[\Delta G = \Delta H - T \Delta S\]
If abla G is negative, the reaction is spontaneous. For example, when carbon dioxide (abla {CO}_2eq s \rightarrow {CO}_2eq g) converts from solid to gas at 25°C, the increase in entropy makes abla G negative, indicating spontaneity.

Entropy

Entropy measures the disorder or randomness in a system. Higher entropy means more disorder. When predicting reaction behavior, consider:

• Phase changes: Solids have lower entropy than liquids, and gases have the highest.
• Mixture versus pure substances: Mixing usually increases entropy.

In reaction (a), solid abla {CO}_2 becomes a gas, increasing the entropy as the gaseous phase is more disorderly. Generally, processes that increase entropy tend to be spontaneous unless other factors intervene, like extreme temperatures or strong bonds.

Phase Changes

Phase changes refer to transitions between states of matter: solid, liquid, and gas. During phase changes:

• Solid to liquid (melting) and liquid to gas (boiling) typically increase entropy.
• The reverse processes, such as freezing, decrease entropy.

For a substance to melt or boil, the ambient temperature must reach its melting or boiling point. Reaction (b) with NaCl doesn't occur spontaneously at 25°C because NaCl's melting point is much higher. Hence, no phase change occurs under those conditions.

Reaction Stability

Some compounds are highly stable due to strong bonds that require significant energy to break. When analyzing reaction stability, consider:

• Bond strength: Stronger bonds mean higher stability.
• Energy requirement: Breaking bonds in stable compounds needs external energy, often rendering the reaction non-spontaneous.

Reactions (c) and (d) involve breaking strong bonds in stable compounds like abla {NaCl} and abla {CO}_2. As these reactions demand a high energy input to proceed, they are non-spontaneous under normal conditions, ensuring the integrity and stability of these compounds in typical environments.