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Identify the reduction half-reaction for this reaction:

[tex]\[ Mg (s) + ZnCl_2 (aq) \rightarrow MgCl_2 (aq) + Zn (s) \][/tex]

A. [tex]\[ Zn (s) \rightarrow Zn^{2+} + 2 e^{-} \][/tex]

B. [tex]\[ Mg (s) \rightarrow Mg^{2+} + 2 e^{-} \][/tex]

C. [tex]\[ Mg^{2+} + 2 e^{-} \rightarrow Mg (s) \][/tex]

D. [tex]\[ Zn^{2+} + 2 e^{-} \rightarrow Zn (s) \][/tex]



Answer :

GinnyAnswer
To identify the reduction half-reaction for the given chemical reaction, we need to understand the concepts of oxidation and reduction. In a redox reaction, oxidation involves the loss of electrons, while reduction involves the gain of electrons. A reduction half-reaction shows the species that is gaining electrons.

The given full reaction is:
[tex]\[ \text{Mg (s)} + \text{ZnCl}_2 (\text{aq}) \rightarrow \text{MgCl}_2 (\text{aq}) + \text{Zn (s)} \][/tex]

We need to determine the species that undergoes reduction.

1. Oxidation States Assignment:
- Magnesium (Mg) in its elemental form (s) has an oxidation state of 0.
- Zinc (Zn) in \(\text{ZnCl}_2\) is in the +2 oxidation state.
- Chlorine (Cl) in \(\text{ZnCl}_2\) and \(\text{MgCl}_2\) is in the -1 oxidation state.
- Magnesium (Mg) in \(\text{MgCl}_2\) is in the +2 oxidation state.
- Zinc (Zn) in its elemental form (s) has an oxidation state of 0.

2. Determine the Species Being Reduced:
- Magnesium (Mg) starts in the 0 oxidation state and ends up in the +2 oxidation state. Hence, magnesium is oxidized (it loses electrons).
- Zinc (Zn) starts in the +2 oxidation state in \(\text{ZnCl}_2\) and ends up in the 0 oxidation state. Hence, zinc is reduced (it gains electrons).

3. Write the Reduction Half-Reaction:
Since zinc is gaining electrons, the reduction half-reaction involves the conversion of zinc ions (\( \text{Zn}^{2+} \)) to elemental zinc (Zn).

Therefore, the reduction half-reaction is:
[tex]\[ \text{Zn}^{2+} + 2e^- \rightarrow \text{Zn (s)} \][/tex]

From the given options:
- A. \( \text{Zn (s)} \rightarrow \text{Zn}^{2+} + 2e^- \) (This is an oxidation reaction)
- B. \( \text{Mg (s)} \rightarrow \text{Mg}^{2+} + 2e^- \) (This is an oxidation reaction)
- C. \( \text{Mg}^{2+} + 2e^- \rightarrow \text{Mg (s)} \) (This is a reduction reaction but not relevant to this problem)
- D. \( \text{Zn}^{2+} + 2e^- \rightarrow \text{Zn (s)} \) (This is the correct reduction reaction)

Thus, the correct answer is:
[tex]\[ \boxed{4} \][/tex]
Which corresponds to option D: [tex]\( \text{Zn}^{2+} + 2e^- \rightarrow \text{Zn (s)} \)[/tex].