CE Chem m.c. Q.8

The answer is B.

The only method is to compare the oxidation numbers of ALL the elements on the both sides.

A is a redox reaction.
NH4+ ion undergoes oxidation, because the O.N. (oxidation number) of N increases from -3 to +1 (in N2O).
NO3- ion undergoes reduction, because the O.N. of N decreases from +5 to +1 (in N2O).

B is NOT a redox reaction.
There is no change in oxidation number.
The O.N. of N in NH3 = that in NH4+ = -3
The O.N. of N in HNO3 = that in NO3- = +5
The O.N. of H in R.H.S. = that in R.H.S. = +1
The O.N. of O in HNO3 = that in NO3- = -2

C is a redox reaction.
N2O4 undergoes oxidation, because the O.N. of N decreases from +4 to +3 (in HNO2).
N2O4 also undergoes reduction, because the O.N. of N increases from +4 to +5 (in HNO2).

D is a redox reaction.
HNO3 undergoes oxidation, because the O.N. of O increases from -2 to 0 (in O2). Note that 10 O atoms do not change their O.N. as the oxidation number of O atoms in H2O and NO2 remains unchanged (O.N. = -2).
HNO3 undergoes reduction, because the O.N. of N decreases from +5 to +4 (in NO2).
=

睇咩element都得，係比較左右兩邊相同element既O.N.，只要有1個element既O.N.有改變就知係redox.
　　　　　　　　　　　　　　　　　　　　
有d reaction：由element變compound
e.g. Na + HCl → NaCl + H2
一睇就知Na既O.N會多左，Na係element所以ON=0，NaCl既Na係ion所以ON一定唔等如0
有O.N.改變一定係redox，可即刻飛左個ANS
　　　　　　　　　　　　　　　　　　　　
e.g. 2Ag2O → 4Ag + O2
compound→element，element Ag is formed from a compound，Ag既O.N.一定有改變，+2變0
　　　　　　　　　　　　　　　　　　　　
如果兩邊都冇element，先睇容易計既。
H既O.N.多數=+1；O既O.N.多數=-2除非在H2O2
同埋睇ions，例如：
NH4+: N=-3 , H=+1
NO3-: N=+5 , O=-2
SO4^2-: S=+6 , O=-2
好似A咁，一個compound分cation同anion睇，有NH4+同NO3-，已知H=+1,O=-2，再睇H2O，就知一定唔關H同O事，所以睇N。
Product有N2O
NH4+中N既O.N.與N2O中N既O.N.比較
N2O中，O既O.N.=-2，計到N既O.N.=+1
而NH4+中N既O.N.=-3
所以可知道到個reaction係redox