[peggysue32]

I am doing grade 12 chemistry through correspondence (SCH4U) and I have no idea how to do one question.  My text book tells me one way, and it seems as though my learning guide contradicts it.

I need to balance equations by the HALF-CELL method.  Show half-cell reactions and identify them as oxidation or reduction.

How do I go about doing this?  What information do I need for each element?

One question i have is

    2-           -        +              2+          2-
SO3  +  MnO4  +  H   <==> Mn     +  SO4   +  H2O(l)

Thank you in advance, for any help

[Albert]

Half cell method means you have to separate SO₃^-- and KMnO₄. Write for both a chemical equation, where you show how SO₃^-- becomes SO₄^2- and KMnO₄ Mn²⁺.

Once you have done it, make sure the number of electrons it the two half-cells is the same by muliplying each equation, if necessary.

Then, you add the two (multiplied) equations and you will have your balanced equation.

[peggysue32]

Can you please give an example of what you are talking about?  I am so lost!

thanks

[xiankai]

half cell method -- is essentially the same as a redox reaction.

there are always 2 reactions in a redox reaction, namely the reduction reaction and oxidation reaction.

reduction can be defined as the gain of electrons in an atom, or the decrease in oxidation state of an atom.

oxidation can be defined as the loss of electrons in an atom, or the increase in oxidation state of an atom.

an example is the oxidation of iron in iron (II) sulphate by hydrogen peroxide.

the equation at first glance, is

H2O2 + FeSO4 --> H2O + Fe2(SO4)3

(1) if u seperate the reduction and oxidation reactions u'll get

H2O2 --> H2O

Fe2+ --> Fe3+

note that SO4- is not counted. it is a spectator ion and hence is not reduced or oxidised.

(2) now to balance them. first consider that all the atoms are balanced. (except for hydrogen and oxygen)

no balancing needed here, but u know what to do next time.

(3) balance oxygen by adding H2O

H2O2 --> 2H2O

(4) balance hydrogen by adding H+

2H+ + H2O2 --> 2H2O

(5) balance charges by adding electrons.

2H+ + 2e- + H2O2 --> 2H2O

Fe2+ --> Fe3+ + e-

VOILA! both half reactions are balanced.

now to combine them. remember, charge conversation requires charges on both sides to be equal. this can be done by ensuring the number of electrons on both sides of the equation are equal;
overall electron gain = overall electron loss

so we get

2H+ + 2e- + H2O2 --> 2H2O

2Fe2+ --> 2Fe3+ + 2e-

combining now...

2H+ + 2e- + 2Fe2+ + H2O2 -->  2H2O + 2Fe3+ + 2e-

removing electrons...

2H+ + 2Fe2+ + H2O2 --> 2H2O + 2Fe3+

if u want u can add the SO42- which will give us..

H2SO4 + 2FeSO4 + H2O2 --> 2H2O + Fe2(SO4)3

and there u have it, u're done!

NOTE: the above method is for reactions taking place in acidic medium. for basic and neutral mediums, , different methods are used.