Which of the following combustion reactions is incomplete? A) CH₄ plus O₂ reacts to form C plus 2H₂O. B) C plus O₂ react to form CO₂. C) 2C₂H₂ plus 5O₂ react to form 4CO₂ plus 2H₂O. D) C₂H₄ plus 3O₂ react to form 2CO₂ plus 2H₂O. Or E) 2C₂H₆ plus 7O₂ react to form 4CO₂ plus 6H₂O.
The first thing we should do is satisfy ourselves that all the options correspond to combustion reactions, i.e., reactions with oxygen. In all five cases, one of the reactants is oxygen. Therefore, we’re looking at combustion reactions. But what is an incomplete combustion reaction?
An incomplete combustion reaction is one where the products could still react with oxygen. We can boil this down further when we have carbon as part of the reactants. If we produce carbon or carbon monoxide as part of a combustion reaction, we know the reaction is incomplete. We know this because carbon or carbon monoxide can still react with oxygen to produce carbon dioxide. Carbon dioxide will not react with oxygen any further. Therefore, the combustion reaction is complete.
Therefore, for this question, we are looking for equations with carbon or carbon monoxide as products. The only reaction equation that contains either carbon or carbon monoxide is reaction A. Reactions B, C, D, and E are all examples of complete combustion reactions because the only products are carbon dioxide and water.
Reaction A is incomplete because carbon is a product and is therefore the correct answer.