Question

Which two of the following reactions are oxidationreduction reactions? Explain your answer in each case. Classify the remaining reaction. (a) \(\mathrm{Zn}(\mathrm{s})+2 \mathrm{NO}_{3}-(\mathrm{aq})+4 \mathrm{H}_{3} \mathrm{O}^{+}(\mathrm{aq}) \rightarrow \mathrm{Zn}^{2+}(\mathrm{aq})+2 \mathrm{NO}_{2}(\mathrm{g})+6 \mathrm{H}_{2} \mathrm{O}(\ell)\) (b) \(\mathrm{Zn}(\mathrm{OH})_{2}(\mathrm{s})+\mathrm{H}_{2} \mathrm{SO}_{4}(\mathrm{aq}) \rightarrow \mathrm{ZnSO}_{4}(\mathrm{aq})+2 \mathrm{H}_{2} \mathrm{O}(\ell)\) (c) \(\mathrm{Ca}(\mathrm{s})+2 \mathrm{H}_{2} \mathrm{O}(\ell) \rightarrow \mathrm{Ca}(\mathrm{OH})_{2}(\mathrm{s})+\mathrm{H}_{2}(\mathrm{g})\)

Short answer

Reactions (a) and (c) are oxidation-reduction reactions. Reaction (b) is an acid-base reaction.

Step-by-step solution

Identify Oxidation and Reduction in Reaction (a)

For reaction (a), look for atoms that change their oxidation states. Zn goes from 0 in elemental Zn to +2 in Zn²⁺, which means Zn is oxidized. NO₃⁻ in the reactants turns into NO₂ in the products, as N changes from +5 in NO₃⁻ to +4 in NO₂, indicating reduction. Thus, this reaction involves both oxidation and reduction, making it an oxidation-reduction reaction.

Analyze Reaction (b) for Redox Characteristics

In reaction (b), Zn in Zn(OH)₂ changes from an oxidation state of +2 to +2 in ZnSO₄; there is no change. All other elements (H and S) also retain their oxidation states before and after the reaction. Therefore, no oxidation or reduction occurs, classifying this as a non-redox reaction, specifically an acid-base reaction.

Determine Oxidation and Reduction in Reaction (c)

For reaction (c), Ca changes from its elemental state 0 to +2 in Ca(OH)₂, indicating it undergoes oxidation. Hydrogen in H₂O goes from +1 to 0 in H₂, showing reduction. Thus, this reaction too is an oxidation-reduction reaction.

Key concepts

Oxidation States

Understanding oxidation states is pivotal to grasping oxidation-reduction (redox) reactions. Essentially, an oxidation state represents the hypothetical charge of an atom if all bonds to the atom were 100% ionic. It's a convenient way of keeping track of electrons transferred in chemical reactions. To determine oxidation states:

• Elements in their natural form, like \( \text{Fe} \) or \( \text{N}_2 \), have an oxidation state of zero.
• In monoatomic ions, the oxidation state equals the ion's charge, such as \( \text{Na}^+ \) having +1.
• For compounds, the sum of oxidation states equals the total charge on the molecule. For example, in \( \text{H}_2\text{O} \), the sum is zero, with each hydrogen as +1 and oxygen as -2.

In reaction (a), we noted \( \text{Zn} \) changing from 0 to +2 and \( \text{N} \) in \( \text{NO}_3^- \) going from +5 to +4, revealing significant electron movement due to oxidation and reduction.

Redox Reaction Identification

Identifying redox reactions requires recognizing changes in oxidation states. In every redox reaction, one species undergoes oxidation (loses electrons) while another experiences reduction (gains electrons). To identify such changes:

• Check each element's oxidation states before and after the reaction.
• Identify pairs of oxidation and reduction events.

In the given problems, reactions (a) and (c) are redox reactions. In reaction (a), \( \text{Zn} \) is oxidized to \( \text{Zn}^{2+} \), and \( \text{N} \) is reduced from +5 in \( \text{NO}_3^- \) to +4 in \( \text{NO}_2 \). For reaction (c), \( \text{Ca} \) is oxidized from 0 to +2, with \( \text{H}_2 \text{O} \)'s hydrogen being reduced from +1 to 0. These electron transfers confirm the redox nature of these reactions.

Acid-Base Reactions

Acid-base reactions differ from redox reactions as they involve proton transfer rather than electron transfer. They occur between acids (proton donors) and bases (proton acceptors). A typical reaction forms water and a salt. In reaction (b), \( \text{Zn(OH)}_2 \) reacts with \( \text{H}_2\text{SO}_4 \) to form \( \text{ZnSO}_4 \) and water. Here:

• \( \text{H}_2\text{SO}_4 \) donates protons (acid behavior).
• \( \text{Zn(OH)}_2 \), a base, accepts the protons to form water.

This proton exchange defines it as an acid-base reaction rather than a redox reaction. Understanding acid-base reactions helps categorize reactions with no change in oxidation states, as seen in reaction (b).