What is the order of Hydrogen ion reduction reaction?? H+ + e- => H

Dear Karan

Lets start with the oxidation half-reaction.

H2O --> O2 + H+ + e-

There are 2 O atoms on the right, so there should be 2 on the left:

2 H2O --> O2 + H+ + e-

Now there are 4 H atoms on the left but only one H+ ion on the right. Correct that situation by placing a coefficient of 4 in front of H+

2 H2O --> O2 + 4 H+ + e-

The oxygen atoms are being oxidized (from an oxidation state of -2 to 0). That means that each oxygen atom has to lose two electrons, and there are two atoms being oxidized, so there are 4 electrons overall being lost.

2 H2O --> O2 + 4 H+ + 4 e-

Now thats done. Lets focus on the reduction half-reaction.

H2O + e- --> H2 + OH-

There are two hydrogen atoms on the left and 3 on the right. If we double the number of OH- ions, that will give us 4 hydrogen atoms on the right side, which means we can simply double the number of water molecules on the left to balance the half-reaction (as far as atoms go)

2 H2O + e- --> H2 + 2 OH-

Hydrogen atoms are being reduced from an oxidation state of +1 to 0. Each hydrogen atom must gain 1 electron, and there are two hydrogen atoms being reduced (the other two retain their oxidation state of +1). Therefore this half-reaction requires 2 electrons:

2 H2O + 2 e- --> H2 + 2 OH-

The reduction half-reaction is balanced by itself, but it isnt balanced with respect to the oxidation half-reaction. The oxidation half-reaction releases 4 electrons; the reduction half-reaction only absorbs 2. If you want them to be balanced with respect to each other, you have to double all the coefficients in the reduction half-reaction.

2 H2O --> O2 + 4 H+ + 4 e-

4 H2O + 4 e- --> 2 H2 + 4 OH-

If we were to recombine the half-reactions wed get:

2 H2O + 4 H2O + 4 e- --> O2 + 4 H+ + 4e- + 2 H2 + 4 OH-

If we were to combine what we could wed get:

6 H2O + 4e- --> O2 + 4 H2O + 4e- + 2 H2

(The 4 H+ ions would combine with the 4 OH- ions to make 4 water molecules on the right side of the equation).

And cancelling things that appear on both sides of the equation:

2 H2O --> O2 + 2 H2

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Thanks

Aman Bansal

Askiitian Expert

Thank you for your generous explanation regarding the reaction mechanism. But what I had asked is still not been answered. I am curious to know with reference to the Kinetics aspect what is the order of this reaction.. Does rate of hydrogen ion reduction depends on its concentration,i.e, is it a zero order or a First order reaction??