Redox Reactions

Which of the following is an example of disproportionation redox reaction?

A disproportionation reaction is a redox reaction in which the same element is both oxidized and reduced.

Let's analyze the options:

1. 
   – Both elements are oxidized; this is not a disproportionation.
2. 
   – This is a disproportionation reaction: nitrogen in NO₂ is both oxidized to NO₃⁻ and reduced to NO₂⁻.
3. 
   – This is thermal decomposition, not a disproportionation.
4. 
   – This is a simple reaction between a hydride and water.
5. 
   – This is a normal redox reaction, not disproportionation.

Correct Answer:

We are given the reaction: To balance this redox reaction, we need to follow the steps:
1. Balance the Cr atoms: There are 2 chromium atoms on the left (in ), so we need 2 chromium ions on the right, which is already the case (as ).
2. Balance the Oxygen atoms: There are 7 oxygen atoms on the left in , so we need 7 oxygen atoms on the right. These come from water molecules. Therefore, we need 7 water molecules, so .
3. Balance the Hydrogen atoms: There are 14 hydrogen atoms on the right (from 7 molecules), so we need 14 hydrogen ions on the left, so .
4. Balance the Charges: The charge on the left is from , and on the right is .
Thus, we need 6 electrons to balance the charges. Therefore, .
Thus, the balanced reaction is:
Thus, the values of , , and are , , and respectively, so the correct answer is .