# NEED HELP ASAP! how do I balance these 3 redox equations.?

1) Mno4 -1 + SO3 2- ---> MnO4 2- + SO4 2-

2) Cr2O7 2- + SO3 2- ---> Cr 3+ + SO4 2-

3) Cu + HNO3 --> Cu(NO3)2 + NO2

if you know how to answer any of these, it would be greatly appreciated!

### 1 Answer

- PaulLv 46 years agoBest Answer
Easiest using oxidation numbers

Manganese goes from +7 to +6 a change of -1

Sulphur goes from + 4 to + 6 a change of + 2

so twice as many Manganese atoms must change as Sulphur

2MnO4 - + SO3 2- -> 2 MnO4 2- + SO4 2-

now check Os ; there is one more on RHS so add H2O to LHS and 2H+ to RHS.

2MnO4 - + SO3 2- + H2O -> 2 MnO4 2- + SO4 2- + 2H+

As a check show that charges balance : 4- and 4- : done.

Copper goes from 0 to + 2

Nitrogen goes from +5 to +4 (in NO2, NO3- is still +5)

so 2 Nitrogens change per copper (but there are 2 unchanged in copper nitrate)

Cu + 4 HNO3 -> Cu(NO3)2 + 2NO2

there are 4 Hs and 2Os to account for - that is 2H2O

Cu + 4 HNO3 -> Cu(NO3)2 + 2NO2 + 2H2O

OK ?

Source(s): for oxidation nunbers see eg http://www.chemguide.co.uk/inorganic/redox/oxidnst...