Question

Which reaction is not a redox reaction?
(a) 2Mg + O2 -> 2MgO
(b) Mg + CuSO4 -> Cu + MgSO4
(c) MgCO3 -> MgO + CO2
(d) 2Mg(NO3)2 -> 2MgO + 4NO2 + O2

I know the answer is C but I am not sure why so could someone please explain it to me? Thanks

Answer 1

Redox reaction means a reaction where some Atoms get oxidized while another get reduced
And whenever oxidation occurs reduction also happens and vice versa 'cause their must be a process to acquire the electrons released during oxidation and that process is reduction.
Also, When oxidation occurs the oxidation number of an atom increases while reduction decreases the oxidation number
I suggest u to study about oxidation number at first place if u don't know about it. U could Google it and their are plenty of good sites for that purpose.
If u know about oxidation number all u have to do is check the state of oxidation number of the elements in the reactions in the given choices. And answer should be the instance where oxidation number haven't change 'cause oxidation number changes whenever oxidation or reduction occurs

1st choice
2Mg + O2 -> 2MgO

*oxidation number of an element in it's natural state is 0
so the oxidation no. of both Mg and O2 is 0
* but in MgO oxidation no. of O2 is -2 and Mg is +2
'cause overall value of MgO shoul equal 0
* So here one oxidation no. has increased while other has decrease. So this is
a REDOX reaction

2nd choice
Mg + CuSO4 -> Cu + MgSO4

* Like in the above choice oxidation o. of Mg has increased from 0 to +2
While Oxidation no. of Cu has decreased from +2 to 0
( overall value of CuSO4 = 0 and oxidation no. of SO4 is -2 therfor oxidation no. of Cu should be +2 in the beginning. After the reaction Cu comes to it's natural state where it's oxidation no. is 0 )

* So this one is also a REDOX reaction

3rd choice
MgCO3 -> MgO + CO2

Here the overall value of MgCo3 should be 0
oxidation no. of Mg = +2
oxidation no. of CO3 ion = -2
hence
overall value of CO3 ion = -2
oxidation no. of C =+2
oxidation no. O = -2

After the reaction
As usual overall value of
MgO = 0
which means
oxidation no. of Mg = +2
oxidation no. of O = -2
So here nothing has changed in oxidation number

Also,
overall value of CO2 = 0
which means
oxidation no. of C = +4
oxidation no. of O = -2

here only the oxidation no. of C has changed and therefor it's not a REDOX reaction

4th choice
here (NO3) ion has reduced and oxidized at the same time which we call a diproportionation and u could give it a try and find out the changes in oxidation number


Hope this will help ya out !!!

Answer 2

Thank you very much!