For the redox reaction, #MnO_4^(-) + C_2O_4^(2-) + H^(+) -> Mn^(2+) + CO_2 +H_2O# The correct coefficients of the reactants for the balanced reaction are respectively, #MnO_4^-# , #C_2O_4^(2-)# , #H^+# ? :-

(1) 2,5,16
(2) 16,3,12
(3) 15,16,12
(4) 2,16,5

3 Answers
Feb 6, 2018

This is a lot of work for a multiple choice question.... I make it #(1)#...

Explanation:

Permanganate, #Mn(VII)#, is REDUCED to #Mn^(2+)#:

#MnO_4^(-) +8H^+ + 5e^(-) rarr Mn^(2+)+4H_2O(l)# #(i)#

Oxalate anion, #C(+III)#, is oxidized to #CO_2#:

#C_2O_4^(2-)rarr2CO_2+2e^(-)# #(ii)#

And we take #2xx(i)+5xx(ii)#

#2MnO_4^(-) +16H^+ +10e^(-)+5C_2O_4^(2-)rarr10CO_2+10e^(-)+5Mn^(2+)+8H_2O(l)#

..to give finally....

#underbrace(2MnO_4^(-))_"bright purple" +5C_2O_4^(2-)+16H^+rarrunderbrace(2Mn^(2+))_"colourless"+10CO_2+8H_2O(l)#

An examiner is being lazy if they set a question like this....

Feb 6, 2018

(1)

Explanation:

at first you must find the elements that change their oxidation number end calculate these numbers before and after the reaction:

Mn goes from +7 to +2 and it gain 5 electrons:
#MnO_4^(-) + 5 e + 8 H^+= Mn^(2+) + 4 H_2O#
look that you must balance the atoms of oxygen with water and the atoms of Hydrogen with ion#H^+#

while C goes from + 3 to +4 and it lost 1 electron for each atoms
#C_2O_4^(2-) = 2CO_2 + 2 e#

but you must balance the number of electrons in the two reactions, so you mustmoltiplicate for 2 the first and for 5 the second reaction:
#2MnO_4^(-) + 10 e + 16 H^+= 2Mn^(2+) + 8 H_2O#
#5C_2O_4^(2-) =10CO_2 + 10 e#

now you summ the two reactions and you have:
#2MnO_4^(-) + 10 e + 16 H^+5C_2O_4^(2-) =10CO_2 + 10 e+ 2Mn^(2+) + 8 H_2O#

semplifying the electrons, you get
#2MnO_4^(-) + 16 H^+5C_2O_4^(2-) =10CO_2 + 2Mn^(2+) + 8 H_2O#
that is what you were looking for

Feb 6, 2018

(1) 2,5,16

Explanation:

Because of the #H_2# on the right, the number of #H^+# ions on the left must be even. (4) is eliminated.

Looking at the equation, there must be a significantly smaller number of #MnO_4# ions than #H^+# ions (otherwise there would be far too many oxygen atoms for the carbon and hydrogen atoms to balance them out in the form of #CO_2# and #H_2O#), so (1) is the only choice left.

One can definitely try to balance the equation, but in this case it is unnecessary.