Final answer:
KI must be added to the H₂O₂ decomposition reaction to act as a catalyst, speeding up the production of O₂ despite not appearing in the balanced equation. It provides an alternative pathway with a lower activation energy for the reaction, which is naturally slow due to the small equilibrium constant favoring the reactants. Balanced equations show stoichiometry but not reaction kinetics or catalysts needed.
Step-by-step explanation:
The reason KI needs to be added to the decomposition reaction of hydrogen peroxide (H₂O₂) even though it is not present in the balanced equation is that KI acts as a catalyst. This means it speeds up the reaction without being consumed in the process. Hydrogen peroxide decomposes into water (H₂O) and oxygen gas (O₂), as represented by the balanced equation 2H₂O₂(aq) → 2H₂O(l) + O₂(g). However, this reaction is usually very slow unless a catalyst is present. Potassium iodide (KI) improves the reaction rate by providing an alternative pathway for the reaction with a lower activation energy.
In the absence of a catalyst, the decomposition of hydrogen peroxide would take place very slowly because the equilibrium constant for its decomposition is small, meaning that at equilibrium, the reactants are favored over the products. However, when KI is added to the reaction mixture, it facilitates the conversion of hydrogen peroxide into water and oxygen efficiently, thereby increasing the rate at which oxygen gas is produced.
It's important to note that balanced chemical equations represent the stoichiometry of the reaction - the relative amounts of reactants and products in molar terms - but do not indicate the reaction kinetics or the need for catalysts. The use of catalysts like KI in chemical reactions demonstrates the complexity of real-world chemical processes, which involve factors beyond simple stoichiometric ratios.