Disproportionation is a special type of redox reaction where a single element in one oxidation state is simultaneously oxidized and reduced. Let's take chlorine in water as an example. When chlorine gas reacts with water, it forms hypochlorous acid and hydrochloric acid. In this reaction, chlorine in the zero oxidation state is both oxidized to plus one in hypochlorous acid and reduced to minus one in hydrochloric acid.
To balance a disproportionation reaction, we follow a systematic approach. First, identify the element that's changing its oxidation state in both directions. In our example, chlorine is that element. Next, write separate half-reactions for the oxidation and reduction processes. For chlorine in water, the oxidation half-reaction shows chlorine becoming hypochlorous acid, while the reduction half-reaction shows chlorine becoming hydrochloric acid. The subsequent steps involve balancing atoms, adding water molecules, hydrogen ions or hydroxide ions depending on whether the solution is acidic or basic, and balancing charges with electrons. Finally, we equalize electron transfer between the half-reactions, combine them, and simplify to get the balanced equation.
Let's balance the half-reactions for our chlorine example in acidic solution. For the oxidation half-reaction, we start with chlorine becoming hypochlorous acid. First, we balance chlorine atoms by using half a mole of Cl₂. Next, we balance oxygen by adding water to the reactant side. Then, we balance hydrogen by adding H⁺ to the product side. Finally, we balance the charge by adding an electron to the product side, since oxidation involves losing electrons. For the reduction half-reaction, we follow similar steps: balance chlorine with half a mole of Cl₂, add H⁺ to the reactant side to balance hydrogen, and add an electron to the reactant side since reduction involves gaining electrons. In basic solutions, we would use hydroxide ions instead of hydrogen ions to balance hydrogen atoms.
Now we'll combine the half-reactions to get the complete balanced equation. First, we need to equalize electron transfer between the half-reactions. In our example, both half-reactions already involve one electron, so no adjustment is needed. Next, we add the two half-reactions together. This gives us half a mole of Cl₂ plus water plus another half mole of Cl₂ plus H⁺ plus an electron on the reactant side, and HClO plus H⁺ plus an electron plus HCl on the product side. We then cancel out common terms that appear on both sides, specifically the H⁺ ions and electrons. After simplification, we get Cl₂ plus H₂O yields HClO plus HCl. This is our final balanced equation, which correctly represents the disproportionation of chlorine in water. You can verify that both atoms and charge are balanced on both sides.
To summarize what we've learned about balancing disproportionation reactions: First, disproportionation is a special type of redox reaction where a single element in one oxidation state is simultaneously oxidized and reduced. The ion-electron method, or half-reaction method, provides a systematic approach to balancing these reactions. Remember that different steps are required depending on whether the reaction occurs in acidic or basic solution. Always verify your final equation by checking that both atoms and charges are balanced on both sides. Finally, this method isn't limited to disproportionation reactions—it works for balancing any redox reaction, making it an essential tool in chemistry.