Problem 107 - Chemistry The Molecular Science

Question

Determine which of these are redox reactions, which are acid-base reactions, and which are gas-forming reactions. Identify the oxidizing and reducing agents in each of the redox reactions. Identify the acid and base in each acidbase reaction. (a) \(\mathrm{NaOH}(\mathrm{aq})+\mathrm{H}_{3} \mathrm{PO}_{4}(\mathrm{aq}) \longrightarrow \mathrm{NaH}_{2} \mathrm{PO}_{4}(\mathrm{aq})+\mathrm{H}_{2} \mathrm{O}(\ell)\) (b) \(\mathrm{NH}_{3}(\mathrm{~g})+\mathrm{CO}_{2}(\mathrm{~g})+\mathrm{H}_{2} \mathrm{O}(\ell) \longrightarrow \mathrm{NH}_{4} \mathrm{HCO}_{3}(\mathrm{aq})\) (c) \(\mathrm{TiCl}_{4}(\mathrm{~g})+2 \mathrm{Mg}(\ell) \longrightarrow \mathrm{Ti}(\mathrm{s})+2 \mathrm{MgCl}_{2}(\ell)\) (d) \(\mathrm{NaCl}(\mathrm{s})+\mathrm{NaHSO}_{4}(\mathrm{aq}) \longrightarrow \mathrm{HCl}(\mathrm{g})+\mathrm{Na}_{2} \mathrm{SO}_{4}(\mathrm{aq})\)

Step-by-Step Solution

Answer

(a) Acid-base reaction; (b) Gas-forming reaction; (c) Redox reaction (oxidizing agent: TiCl4, reducing agent: Mg); (d) Gas-forming.

Step 1: Analyze Reaction (a)

The reaction given is \( \mathrm{NaOH} + \mathrm{H}_3\mathrm{PO}_4 \rightarrow \mathrm{NaH}_2\mathrm{PO}_4 + \mathrm{H}_2\mathrm{O} \). This is a typical reaction between a base (\(\mathrm{NaOH} \)) and an acid (\( \mathrm{H}_3\mathrm{PO}_4 \)), resulting in the formation of a salt (\( \mathrm{NaH}_2\mathrm{PO}_4 \)) and water. Therefore, this reaction is classified as an acid-base reaction.

Step 2: Analyze Reaction (b)

The reaction \( \mathrm{NH}_3 + \mathrm{CO}_2 + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{NH}_4\mathrm{HCO}_3 \) involves ammonia, carbon dioxide, and water forming ammonium bicarbonate. This is not a redox reaction as there are no changes in oxidation states, nor is it an acid-base reaction in the traditional sense since no clear acid or base is neutralized. Instead, the formation of a gas product indirectly makes it more inclined towards a gas-forming reaction.

Step 3: Analyze Reaction (c)

In the reaction \( \mathrm{TiCl}_4 + 2 \mathrm{Mg} \rightarrow \mathrm{Ti} + 2 \mathrm{MgCl}_2 \), we observe a change in oxidation states: Titanium goes from \(+4\) in \(\mathrm{TiCl}_4\) to 0 in \(\mathrm{Ti}\), and magnesium goes from 0 to \(+2\) in \(\mathrm{MgCl}_2\). Titanium is reduced and magnesium is oxidized, indicating this is a redox reaction. The oxidizing agent is \(\mathrm{TiCl}_4\) is being, and the reducing agent is \( \mathrm{Mg} \).

Step 4: Analyze Reaction (d)

The reaction \( \mathrm{NaCl} + \mathrm{NaHSO}_4 \rightarrow \mathrm{HCl} + \mathrm{Na}_2\mathrm{SO}_4 \) leads to the production of \(\mathrm{HCl}\), which is a gas. This suggests a gas-forming reaction is present. Although it's tempting to consider other reaction types, the predominant feature is the generation of gas.

Key Concepts

Redox Reactions

Redox reactions are processes where oxidation and reduction occur simultaneously. In these reactions, the oxidation state of elements changes. One species loses electrons (oxidation), while another gains electrons (reduction). A perfect example from our exercise is reaction (c): \[ \mathrm{TiCl}_4 + 2 \mathrm{Mg} \rightarrow \mathrm{Ti} + 2 \mathrm{MgCl}_2 \]Here, titanium goes from an oxidation state of +4 in \(\mathrm{TiCl}_4\) to 0 in \(\mathrm{Ti}\), signifying reduction. Conversely, magnesium starts at 0 and is oxidized to +2 in \(\mathrm{MgCl}_2\). This electron exchange characterizes it as a redox reaction.

• Oxidation: Loss of electrons—magnesium is oxidized.
• Reduction: Gain of electrons—titanium is reduced.

Understanding redox reactions involves knowing the changes in oxidation states and identifying the substances that gain or lose electrons.

Acid-Base Reactions

Acid-base reactions involve an acid and a base reacting to form water and a salt. These reactions are vital in numerous chemical processes. An excellent example from the exercise is reaction (a):\[ \mathrm{NaOH} + \mathrm{H}_3\mathrm{PO}_4 \rightarrow \mathrm{NaH}_2\mathrm{PO}_4 + \mathrm{H}_2\mathrm{O} \]In this reaction, \(\mathrm{NaOH}\) acts as the base, and \(\mathrm{H}_3\mathrm{PO}_4\) is the acid. The two react to form one mole of water and a salt (\(\mathrm{NaH}_2\mathrm{PO}_4\)).

• Acid: Donates protons—\(\mathrm{H}_3\mathrm{PO}_4\).
• Base: Accepts protons—\(\mathrm{NaOH}\).

By looking at the reactants, we can identify the acid-base pair based on their ability to donate or accept protons.

Gas-Forming Reactions

Gas-forming reactions are characterized by the production of a gaseous product. These reactions can often be identified by the visible release of gas bubbles.In the exercise, reaction (d) demonstrates this type:\[ \mathrm{NaCl} + \mathrm{NaHSO}_4 \rightarrow \mathrm{HCl} + \mathrm{Na}_2\mathrm{SO}_4 \]The formation of \(\mathrm{HCl}\) gas is the highlight of this reaction, clearly marking it as a gas-forming process. Although it is possible for these reactions to involve other mechanisms, the generation of gas is a key indicator.Similarly, in reaction (b):\[ \mathrm{NH}_3 + \mathrm{CO}_2 + \mathrm{H}_2\mathrm{O} \rightarrow \mathrm{NH}_4\mathrm{HCO}_3 \]Although not immediately obvious as a gas-forming reaction, the involvement of gaseous reactants and products may indirectly suggest this category. Gas-forming reactions are intriguing due to their visual and sometimes immediate results.

Oxidizing and Reducing Agents

In redox reactions, identifying oxidizing and reducing agents is crucial. The oxidizing agent gains electrons and is reduced, while the reducing agent loses electrons and is oxidized.In reaction (c):\[ \mathrm{TiCl}_4 + 2 \mathrm{Mg} \rightarrow \mathrm{Ti} + 2 \mathrm{MgCl}_2 \]Identify:

• Oxidizing Agent: \(\mathrm{TiCl}_4\) — it gains electrons, reducing titanium from +4 to 0.
• Reducing Agent: \(\mathrm{Mg}\) — it loses electrons, oxidizing magnesium from 0 to +2.

The flow of electrons from the reducing agent to the oxidizing agent is what drives the redox process. Knowledge of these agents helps predict and balance redox reactions effectively.