# Determine if reactions are redox

1. Oct 14, 2015

### terryds

1. The problem statement, all variables and given/known data

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

2. Relevant equations
Redox reactions are the ones which involves changes in oxidation numbers

3. 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

2. Oct 14, 2015

### Staff: Mentor

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

3. Oct 14, 2015

### terryds

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