Chemistry · Redox Equilibrium · NEET
In basic medium you must NOT leave H+ in the final equation, because a basic solution has almost no free H+. You balance using OH- and H2O. A common way is to first balance using H+ and H2O (the acidic route), then neutralise every H+ by adding an equal number of OH- to both sides. Each H+ + OH- becomes one H2O. This removes all H+ and leaves a clean basic-medium equation.
You add OH- to both sides so you do not change the chemistry, only the form. If you add OH- only on one side, atoms and charge stop matching. Adding the same number of OH- to each side keeps the equation balanced. On the side that had H+, the H+ and OH- merge into water; on the other side the OH- ions stay as OH-. That is how H+ quietly disappears and OH- appears.
Three quick steps. (1) Take the fully balanced acidic equation. (2) Count the H+ ions and add that many OH- ions to BOTH sides. (3) On the side that had H+, combine H+ + OH- into H2O, then cancel any water that now appears on both sides. The result is the basic-medium equation. Only do this AFTER the acidic equation is already balanced for atoms, oxygen, hydrogen, and charge.
In the NCERT oxidation-number method for basic medium, you first equalise the oxidation-number change, then add OH- to make the total ionic charge equal on both sides. Water goes on whichever side is short of hydrogen atoms. In the NCERT MnO4- + Br- example, OH- is added on the right (product side) to fix charge, then one H2O is added on the LEFT to balance hydrogen.
Balance the atoms that change oxidation state, make the electron gain equal the electron loss, add OH- to the side that is more positive to equalise charge, then add H2O to fix hydrogen. Always finish by checking that BOTH atoms and total charge match on the two sides. In an exam this oxidation-number route is faster than the full half-reaction route for most basic-medium questions.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Yes. Balancing in acidic form first (with H+ and H2O) and then neutralising the H+ with OH- always gives the correct basic-medium equation. It is a safe, standard method and is fully accepted in NEET.
NCERT solves basic-medium problems with the oxidation-number method (Problem 7.9: MnO4- + Br- giving MnO2 + BrO3-). There it adds OH- on the product side to equalise charge and H2O on the reactant side to balance hydrogen.
Check two things: every atom is balanced on both sides, and the total ionic charge is equal on both sides. If both match and there is no H+ left, the equation is correct.
Both give the same answer. The oxidation-number method is usually faster for MCQs. The half-reaction method is clearer when you need to show electrons, but it takes a little longer.
