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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: HERculina on September 14, 2011, 07:53:13 pm

Title: Redox unit 2 help!
Post by: HERculina on September 14, 2011, 07:53:13 pm
Hey guys, how do you work out whether these equations are redox by using oxidation no.s? Can u please show oxidation no. Of each atom too :)                                                                                                   1. 2NO + O2 --> 2NO2.           2. K2O + H2O --> 2KOH     3. NaOH + HCl --> NaCl + H2O.       
Title: Re: Redox unit 2 help!
Post by: b^3 on September 14, 2011, 07:58:05 pm
For them to be redox reactions, there must be an increase in oxidation number on one species and a decrease in oxidation number on another, i.e one must be oxidised and another reduced.
For equ 1) 2NO + O2-->2NO2
On the left hand side, O in NO will be -2, making N +2. O in O2 will be 0
On the right hand side, O in NO2 will be -2 so N+2(-2)=0, N=+4
so the O.N of O has decreased from 0 (in O2) to -2 in NO2 - it is being reduced
The O.N of N has increased from +2 in NO to +4 in NO2 - it is being oxidised
So the reaction is a redox reaction.
Equ 2)
in K2O O.N of O is -2 so the O.N of K is K+2(-2)=0, K=+4
in H2O, H is +1 and O is -2
in KOH, H is +1, O is -2 so K is K-2+1=0, K=+1
The Oxidation number of K has decreased from +4 to +1 but there is no increase in O.N in any other species, so this reaction is not a redox reaction
Equ 3) NaOH, O=-2, H=+1 so Na=+1
HCL, H=+1, Cl=-1
NaCl, Na=+1, Cl=-1
H2O, H is +1, O is -2
No oxidation numbers has changed, so it is not a redox reaction.
Title: Re: Redox unit 2 help!
Post by: HERculina on September 14, 2011, 08:05:04 pm
Ok thanks! But you know the oxygen in the first reactant, '2NO', why isnt it used? Like wht was it the oxidation no. Of O in O2 that was considered on that side of the equation :)

Title: Re: Redox unit 2 help!
Post by: b^3 on September 14, 2011, 08:09:12 pm
If you look at the Oxidation Number in 2NO, it is -2, On side of the equation in NO2, it is still -2, it hasn't changed so it is not of interest to us. But be careful because, there the O.N of O didn't change in those two but in O2 to NO2 it has changed from 0 to -2. We are looking at the O.N's that change as that is what is going to tell you if oxidation and reduction half reactions are occuring and thus a redox reaction.
Title: Re: Redox unit 2 help!
Post by: HERculina on September 14, 2011, 08:13:49 pm
Oh ok so u basically ignore all the no.s in front such as the 2 in front o the NO of the compounds and ignore anything that hasnt changeg when you're trying to figure out if it's redox?
Title: Re: Redox unit 2 help!
Post by: REBORN on September 14, 2011, 08:14:36 pm
Yes.

The coefficients are irrelevant. Just as they are in calculating molar mass.
Title: Re: Redox unit 2 help!
Post by: HERculina on September 14, 2011, 08:16:00 pm
Ok thanks :D