Need help with oxidation number

In this equation, mn has been reduced from +7 to +2 but for o it has been oxidised but I don't quite get it because o is -2 in 4h2o. Why are h2o2 and 5o2 the only one considered not 4h2o ? Thanks

Reply 1

JakeLivingstone

oxygen is always -2 but mn is an element that's oxidation number can differ from reaction to reaction

Reply 2

Pigster

Your equation is all wrong, I think you're trying to combine MnO4- -> Mn2+ - the reduction of mangante(VII) with H2O2 -> H2O + O2 - the disproportion of H2O2 (i.e. the reaction catalysed by MnO2).

You want the overall equation for the reactions: MnO4- ->Mn2+ vs H2O2 -> O2 (no water)

Reply 3

coconut64

Original post by Pigster

Hi, this is a question from an OCR exam paper. In the mark scheme it only says that o is oxidized but it didn't mention that the oxidation number is -2 in 4h2o. Thanks

Reply 4

coconut64

Original post by JakeLivingstone

oxygen is always -2 but mn is an element that's oxidation number can differ from reaction to reaction

I don't quite get it because o is -2 in 4h2o. Why are h2o2 and 5o2 the only one considered not 4h2o for oxidation ? Thanks

Reply 5

Pigster

Since O in H2O is in its lowest oxidation state, that is where it is most reduced.

O2 = 0
H2O2 = -1
H2O = -2

Getting to H2O must be reduction (for the O)

Reply 6

MattRay94

Original post by coconut64

In this equation, mn has been reduced from +7 to +2 but for o it has been oxidised but I don't quite get it because o is -2 in 4h2o. Why are h2o2 and 5o2 the only one considered not 4h2o ? Thanks

A few things: firstly if you check your equation it doesn't balance - you need 8 H₂O on the right hand side. Also it's important to appreciate that oxidation states are not additive - you don't add up the oxidation states of elements on each side of the equation. It is better to think of only certain components as playing a role in the redox reaction, and other components exist as acid/base components to balance the equation. Here it is the oxygen in peroxide that gets oxidised - the oxide ions of permanganate and water are known as spectator ions and only serve to balance the equations.

I have shown below a way to work out the redox reaction. Firstly you should determine which of the reactant species are being reduced and which are being oxidised. Here MnO₄^- is being reduced into Mn²⁺ (+7 -> +2), whilst the oxygen in hydrogen peroxide is being oxidised to oxygen (-1 -> O).

Write out these as two 'half equations'...

REDUCTION: MnO₄^- -> Mn²⁺

OXIDATION: H₂O₂ -> O₂

Now balance for oxygen atoms by adding water to the appropriate sides. Only the first reaction requires that here, as the number of oxygens on each side of the equation balance in the second equation...

RED: MnO₄^- -> Mn²⁺ + 4H₂O

OX: H₂O₂ -> O₂

Now balance for hydrogen atoms by adding H⁺ to the appropriate sides...

RED: MnO₄^- + 8H⁺ -> Mn²⁺ + 4H₂O

OX: H₂O₂ -> O₂ + 2H⁺

Finally balance for charges on each side of the equation by addition of e^-...

RED: MnO₄^- + 8H⁺ + 5e^- -> Mn²⁺ + 4H₂O

OX: H₂O₂ -> O₂ + 2H⁺ + 2e^-

Now we need the numbers of electrons in each half equation to be equal, so multiply the first equation through by 2 and the second by 5 so each involve 10 electrons...

RED: 2MnO₄^- + 16H⁺ + 10e^- -> 2Mn²⁺ + 8H₂O

OX: 5H₂O₂ -> 5O₂ + 10H⁺ + 10e^-

Add these two equations...

REDOX: 2MnO₄^- + 16H⁺ + 10e^- + 5H₂O₂ -> 2Mn²⁺ + 8H₂O + 5O₂ + 10H⁺ + 10e^-

Cancelling excess H⁺ and cancelling the e^- gives...

REDOX: 2MnO₄^- + 6H⁺ + 5H₂O₂ -> 2Mn²⁺ + 8H₂O + 5O₂

It should now be easy to see that no atoms in H⁺ and H₂O are involved in the core Redox process - we simply include them as acid/base components to balance the reaction. Mn is reduced, and O in peroxide is oxidised.

Hope this helps :smile:

Reply 7

MattRay94

Original post by JakeLivingstone

oxygen is always -2 but mn is an element that's oxidation number can differ from reaction to reaction

Oxygen is present in the -2 oxidation state in the oxide ion. It can be present in the 0, -1 or -2 oxidation state. The above reagent - peroxide - is such a case where it exists in the -1 oxidation state as the ions involved are 2 x H⁺ and 1 x O₂^2-. Therefore the oxygen atom in peroxide bears -2/2 = -1 oxidation state.

Molecular oxygen is the 0 oxidation state as it is neutral.