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oxidation & reduction watch

    • Thread Starter

    The half equation for decomposition of hydrogen peroxide is :
    H2O2 --> 2H+ + O2 + 2e- (oxidation)
    It is oxidised to oxygen by potassium manganate.
    Write the half equation for the reduction of MnO4- to Mn2+ ions in acidic solution.
    The mark scheme answer says : MnO4- + 8H+ + 5e- --> Mn2+ + 4H2O.

    I dont understand how that answer is obtained. where did 5 electrons come from? Where did the 8 Hydrogen ions come from. This is WJEC summer 2014 them 5 question 3d.

    Can anyone please help me on this?


    Everything needs to be balanced - the charges, the number of each element on each side of the equation. To balance the oxygen you add water to one side, to balance hydrogen, you add H+ to one side - this is the reason you need to acidity solutions when doing a redox reaction/titration (the acid is the source of the H+).
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