Final answer:
A reaction is considered unexpected when its products differ from those anticipated, possibly due to atypical conditions, catalysts, or incomplete reactions. Spontaneous reactions exemplify reactions with favorable product formation under existing conditions, while adequate collisions between reactants can still result in unforeseen products.
Step-by-step explanation:
A reaction falls under the category of "unexpected" when the products formed are different from those predicted based on typical chemical reaction behavior. These unexpected reactions can occur due to a variety of reasons such as unusual reaction conditions, the presence of a catalyst that alters the reaction path, or an incomplete reaction where reactants do not fully convert into products. Chemists initially try to predict the products of a chemical reaction using well-established guidelines based on reaction types but must remain open to spontaneous reactions and unique outcomes that defy expectations.
In the study of chemistry, it's known that reactions do not always go to completion. A spontaneous reaction, such as combustion, is characterized by a favorable formation of products under given conditions, often exothermic with increased entropy. However, even when reactants collide with the correct orientation and energy, there is no guarantee of product formation, and sometimes what is formed can be surprising, leading to new insights and theories.