General rule says that if the reaction takes place in acidic conditions we use water and H + to balance oxygens, and if the reaction takes place in basic conditions - we use OH - and water. We have not only balanced atoms but we are also ready to balance charge by adding two electrons on the left:Īnd the half reaction is ready to be used. Is not balanced, but once we add water and OH -: But we can also use OH - and water to do the trick, for example half reaction: To balance oxygen we can add H + on the side where there is oxygen excess and water on the second, just as we did in the above example. MnO 4 - + 8H + + 5Fe 2+ → Mn 2+ + 4H 2O + 5Fe 3+īalancing hydrogen and oxygen in the half reactions method requires knowledge about the conditions in which reaction takes place. To have five electrons in both equations we have to multiply first equation by 5:Īnd when we add both half reactions we will get Here comes the final trick - we add these half reactions, multiplying them by such coefficients that electrons cancel out (easiest approach is to use just numbers of electrons, although if often means you will have to find lowest coefficients later). To balance charges we have to add 5 electrons on the left:Īt the moment we have two balanced half reactions. Once the atoms are balanced it is time to balance charge with electrons - there is a total +7 charge on the left side of the equation and +2 on the right side. Using simple balancing by inspection we will add two coefficients to balance atoms: What to do with the oxygen? We know that the reaction takes place in acidic conditions (see sulfuric acid in the skeletal reaction at the beginning) - so we can add H + on the left and H 2O on the right: Now we have to balance permanganate reduction half reaction:īefore balancing charges we have to balance atoms. We can use electrons safely, as the final step of balancing will be electron cancellation. To balance charge we will add one electron on the right side: In the case of iron oxidation half reaction atoms are already balanced, but charge is not. We will start equation balancing with balancing these half reactions - using electrons to balance charge. Besides, charges are what is really important in the half reactions method.Īt first glance you can see that in the reaction iron gets oxidized and permanganate gets reduced: Let's try to use half reactions method to balance reaction equation for Fe(II) oxidation with permanganate in the acidic solution:įeSO 4 + KMnO 4 + H 2SO 4 → Fe 2(SO 4) 3 + MnSO 4 + H 2O + K 2SO 4įirst of all, let's concentrate on what is really important - on the net ionic reaction:įe 2+ + MnO 4 - + H + → Fe 3+ + Mn 2+ + H 2OĪll other ions (K + and SO 4 2-) are only spectators and we don't need them to balance the equation. In the case of oxidation numbers method we assume electrons are transferred between atoms (which is only an approximation), in the case of half reactions method we assume there are two systems exchanging electrons (which is much closer to reality, although what we observe may be a multistep process with numerous intermediate reagents). When balancing redox reactions we have always - apart from all the rules pertaining to balancing chemical equations - additional information about electrons moving. » Equation balancing and stoichiometry calculator.
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