To determine which reactions will not take place in the specified direction under standard conditions, we will analyze the given half-reactions and their standard reduction potentials.
### Given Data:
- \( \text{Ni}^{2+} + 2e^- \rightarrow \text{Ni} \) \( E^\circ = 0.25 \, \text{V} \)
- \( \text{Cu}^{2+} + 2e^- \rightarrow \text{Cu} \) \( E^\circ = 0.34 \, \text{V} \)
- \( \text{Ag}^+ + e^- \rightarrow \text{Ag} \) \( E^\circ = 0.80 \, \text{V} \)
- \( \text{Zn}^{2+} + 2e^- \rightarrow \text{Zn} \) \( E^\circ = -0.76 \, \text{V} \)
### Analyzing Each Reaction:
1. **Reaction A:**
\[
\text{Ni}^{2+} + \text{Cu} \rightarrow \text{Ni} + \text{Cu}^{2+}
\]
- Here, Ni is trying to displace Cu.
- Since \( E^\circ_{\text{Cu}} (0.34 \, \text{V}) > E^\circ_{\text{Ni}} (0.25 \, \text{V}) \), Cu cannot be displaced by Ni.
- **Conclusion:** This reaction will not take place.
2. **Reaction B:**
\[
\text{Cu} + 2\text{Ag}^+ \rightarrow \text{Cu}^{2+} + 2\text{Ag}
\]
- Here, Ag is trying to displace Cu.
- Since \( E^\circ_{\text{Ag}} (0.80 \, \text{V}) > E^\circ_{\text{Cu}} (0.34 \, \text{V}) \), Ag can displace Cu.
- **Conclusion:** This reaction will take place.
3. **Reaction C:**
\[
\text{Cu} + 2\text{H}^+ \rightarrow \text{Cu}^{2+} + \text{H}_2
\]
- Here, Cu is trying to displace H.
- Since \( E^\circ_{\text{H}^+} (0.00 \, \text{V}) > E^\circ_{\text{Cu}} (0.34 \, \text{V}) \), Cu cannot displace H.
- **Conclusion:** This reaction will not take place.
4. **Reaction D:**
\[
\text{Zn} + 2\text{H}^+ \rightarrow \text{Zn}^{2+} + \text{H}_2
\]
- Here, Zn is trying to displace H.
- Since \( E^\circ_{\text{Zn}} (-0.76 \, \text{V}) < E^\circ_{\text{H}^+} (0.00 \, \text{V}) \), Zn can displace H.
- **Conclusion:** This reaction will take place.
### Summary of Results:
- **Reactions that will not take place:** A and C.
- **Reactions that will take place:** B and D.
### Final Answer:
The reactions that will not take place in the specified direction are **Reaction A** and **Reaction C**.
