Problem 53 - Chemistry The Molecular Science

Question

Classify each reaction as oxidation-reduction, acid-base, or precipitation. (a) \(\mathrm{Zn}(\mathrm{s})+2 \mathrm{NO}_{3}^{-}(\mathrm{aq})+4 \mathrm{H}^{+}(\mathrm{aq}) \longrightarrow\) $$\mathrm{Zn}^{2+}(\mathrm{aq})+2 \mathrm{NO}_{2}(\mathrm{~g})+2 \mathrm{H}_{2} \mathrm{O}(\ell)$$ (b) \(\mathrm{Zn}(\mathrm{OH})_{2}(\mathrm{~s})+\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq}) \longrightarrow \mathrm{ZnSO}_{4}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{O}(\ell)\) (c) \(\mathrm{Ca}(\mathrm{s})+2 \mathrm{H}_{2} \mathrm{O}(\ell) \longrightarrow \mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{~s})+\mathrm{H}_{2}(\mathrm{~g})\)

Step-by-Step Solution

Answer

(a) Oxidation-reduction, (b) Acid-base, (c) Oxidation-reduction.

Step 1: Identify Reaction (a) as Oxidation-Reduction or Not

In reaction (a), identify changes in oxidation states. Zinc (Zn) changes from oxidation state 0 to +2. Nitrogen in nitrate (NO₃⁻) changes from +5 to +4 in NO₂. A change in oxidation states for two elements classifies this as an oxidation-reduction (redox) reaction.

Step 2: Identify Reaction (b) as Acid-Base, Precipitation, or Redox

In reaction (b), identify changes in state or charge. Zinc hydroxide reacts with sulfuric acid, forming zinc sulfate and water. Here, an acid (H₂SO₄) reacts with a base (Zn(OH)₂) to form a salt (ZnSO₄) and water, which is a classical acid-base neutralization reaction.

Step 3: Identify Reaction (c) as Precipitation, Redox, or Acid-Base

In reaction (c), calcium reacts with water, producing calcium hydroxide and hydrogen gas. The oxidation state of calcium changes from 0 to +2. This is accompanied by the release of hydrogen gas, a characteristic of many redox reactions. Calcium oxidizes, while hydrogen is reduced, indicating an oxidation-reduction reaction.

Key Concepts

Oxidation-Reduction Reactions

Oxidation-reduction reactions, also known as redox reactions, involve the transfer of electrons between substances, changing the oxidation states of the elements involved. In simple terms, one substance donates electrons and gets oxidized while the other accepts these electrons and becomes reduced. Let's break down how this works:

• **Oxidation**: This is when an atom or molecule loses electrons, resulting in an increase in its oxidation state.
• **Reduction**: This process involves gaining electrons which decreases the oxidation state of an atom or molecule.

A good example is reaction (a) from the exercise: \[ \mathrm{Zn}(\mathrm{s}) + 2 \mathrm{NO}_{3}^{-}(\mathrm{aq}) + 4 \mathrm{H}^{+}(\mathrm{aq}) \rightarrow \mathrm{Zn}^{2+}(\mathrm{aq}) + 2 \mathrm{NO}_{2}(\mathrm{g}) + 2 \mathrm{H}_{2}\mathrm{O}(\ell) \]In this reaction, zinc (Zn) is oxidized from an oxidation state of 0 to +2, while the nitrogen in nitrate (NO₃⁻) is reduced from +5 to +4. So, it's clear that electrons have been transferred, classifying it as an oxidation-reduction reaction.

Acid-Base Reactions

Acid-base reactions are a key category in chemistry where an acid reacts with a base to form a salt and usually water. This type of reaction is known for its neutralization aspect, where the characteristics of both the acid and the base are neutralized. Here's what happens:

• **Acid**: A substance that can donate protons (H⁺ ions) in a reaction.
• **Base**: A substance that can accept protons.
• **Neutralization**: This is the process where acids and bases cancel out each other's effects, forming water and a salt.

In reaction (b) from the exercise:\[ \mathrm{Zn}(\mathrm{OH})_{2}(\mathrm{s}) + \mathrm{H}_{2}\mathrm{SO}_{4}(\mathrm{aq}) \rightarrow \mathrm{ZnSO}_{4}(\mathrm{aq}) + 2 \mathrm{H}_{2}\mathrm{O}(\ell) \]We see that zinc hydroxide (Zn(OH)₂), a base, reacts with sulfuric acid (H₂SO₄), an acid, to form zinc sulfate (ZnSO₄) and water. This is a classical example of an acid-base reaction where a salt and water are the products.

Precipitation Reactions

Precipitation reactions occur when two soluble salts react in a solution to form an insoluble product, known as a precipitate. These reactions are typical in aqueous solutions where solids emerge from clear liquid solutions. Here's how they work:

• **Reactants**: Typically involve two ionic compounds, often in aqueous solution.
• **Product**: An insoluble solid forms, which is the precipitate.

Although the original exercise doesn't specifically highlight a precipitation reaction in any of the examples given, understanding how these reactions occur helps broaden our comprehension. When observing a reaction where substances in solution form a solid, it's likely a precipitation event. Just be on the lookout for the appearance of a new solid that wasn't previously visible in the solution.