Final answer:
O₂ is the limiting reactant in the reaction between CH₄ and O₂, and CH₄ is in excess when 2.0 moles of each are combined.
Step-by-step explanation:
To determine which reactant is in excess when 2.0 moles of CH₄ are ignited with 2.0 moles of O₂, we need to look at the stoichiometry of the balanced chemical equation. However, the provided equation CH₄ + ₂O₂ → Cl₃ +2H₂O is incorrect. Assuming we are working with the combustion of methane, the correct balanced equation should be CH₄ + 2O₂ → CO₂ + 2H₂O. Based on this equation, 1 mole of CH₄ reacts with 2 moles of O₂. Having 2.0 moles of CH₄ would require 4.0 moles of O₂ to react completely. Since only 2.0 moles of O₂ are provided, O₂ is the limiting reactant, and CH₄ is in excess. Thus, in this scenario, there would be no excess O₂, but there would be excess CH₄ after the reaction.