Find out total number of compounds which on heating undergo redox reactions.
`PbCl_(4),Mg(NO_(3))_(2),HgC_(2)O_(4),Ag_(2)CO_(3),Pb(CN)_(4),Al(OH)_(3),Cu(CN)_(2)`

Explanation

To determine the total number of compounds that undergo redox reactions upon heating from the given list, we will analyze each compound one by one. A redox reaction involves both oxidation (loss of electrons) and reduction (gain of electrons).

### Step-by-Step Solution:

1. **Analyze PbCl4:**
- Decomposes to PbCl2 + Cl2.
- Oxidation states: Pb (from +4 to +2) undergoes reduction, Cl (from -1 to 0) undergoes oxidation.
- **Conclusion:** This is a redox reaction.

2. **Analyze Mg(NO3)2:**
- Decomposes to MgO + NO2 + O2.
- Oxidation states: Mg (remains +2), N (from +5 to +4) undergoes reduction, O (from -2 to 0) undergoes oxidation.
- **Conclusion:** This is a redox reaction.

3. **Analyze HgC2O4:**
- Decomposes to Hg + CO2.
- Oxidation states: Hg (from +2 to 0) undergoes reduction, C (from +3 to +4) undergoes oxidation.
- **Conclusion:** This is a redox reaction.

4. **Analyze Ag2CO3:**
- Decomposes to Ag + CO2 + O2.
- Oxidation states: Ag (from +1 to 0) undergoes reduction, O (from -2 to 0) undergoes oxidation.
- **Conclusion:** This is a redox reaction.

5. **Analyze Pb(CN)4:**
- Decomposes to Pb(CN)2 + CN2.
- Oxidation states: Pb (from +4 to +2) undergoes reduction, CN (from -1 to 0) undergoes oxidation.
- **Conclusion:** This is a redox reaction.

6. **Analyze Al(OH)3:**
- Decomposes to Al2O3 + H2O.
- Oxidation states: Al remains +3, O remains -2, H remains +1. No change in oxidation states.
- **Conclusion:** This is NOT a redox reaction.

7. **Analyze Cu(CN)2:**
- Decomposes to Cu + CN2.
- Oxidation states: Cu (from +2 to +1) undergoes reduction, CN (from -1 to 0) undergoes oxidation.
- **Conclusion:** This is a redox reaction.

### Summary of Findings:
- Compounds that undergo redox reactions: PbCl4, Mg(NO3)2, HgC2O4, Ag2CO3, Pb(CN)4, Cu(CN)2.
- Total number of compounds undergoing redox reactions: **6**.

### Final Answer:
The total number of compounds which on heating undergo redox reactions is **6**.

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