Which Reactions Are Redox?

[terryds]

Homework Statement

Determine which is/are redox reaction/s ?
$1. 2HNO_2 + 2HBr \rightarrow 2NO + BR_2 + 2H_2O$
$2. PbO_2 + SO_2 \rightarrow PbO + SO_3$
$3. 2Ag + Cl_2 \rightarrow 2AgCl$
$4. SO_2 + NaOH \rightarrow NaHSO_3$

Homework Equations

Redox reactions are the ones which involves changes in oxidation numbers

The Attempt at a Solution

Since all reactions involves change in oxidation number
1. Br
2. Pb
3. Cl
4. S

I think all the reactions are redox.
But, the answer is just reaction 1, 2, and 3

Please tell me why reaction 4 is not redox reaction

 

[Borek]

What is the oxidation number of S before the reaction? After the reaction?

 

[terryds]

What is the oxidation number of S before the reaction? After the reaction?

Hmm... it's +4 and +4... No change... Thankss... I missed it