Final answer:
Excess Mg used in a lab to reduce copper ions remains unreacted after all copper ions are reduced. It becomes the excess reactant and can be calculated by subtracting the reacted Mg mass from the initial Mg mass. Excess Mg may also react with water or oxygen to form other compounds if the conditions allow.
Step-by-step explanation:
When Mg is used in excess to reduce copper ions in a lab, the unused Mg remains as it is once all the copper ions are fully reduced. According to the balanced chemical equation Mg(s) + Cu(NO₃)₂ (aq) → Mg(NO₃)₂ (aq) + Cu(s), magnesium (Mg) acts as a reducing agent, and when it is in excess, not all of the Mg will react. Eventually, there will be no more copper ions to react with, making the leftover magnesium the excess reactant. To determine the amount of remaining excess Mg, one would subtract the mass of Mg that reacted from the initial mass of Mg. If the reaction were to continue, the excess Mg could also react with other substances, such as water or atmospheric oxygen, potentially forming Mg(OH)₂ or MgO respectively.