Given the balanced chemical equation:
$$Zn + 2HCl -> ZnCl_2 + H_2$$
Let's assume we have initial moles of reactants a for Zn and b for HCl.
For Zn:
Moles of Zn = a
For HCl:
Moles of HCl = b
According to the stoichiometry, 1 mole of Zn reacts with 2 moles of HCl. So, the mole ratio is 1:2.
If a < 2b, it means there are not enough moles of HCl to react completely with Zn. In this case, HCl is the limiting reactant.
If a > 2b, it means there are excess moles of Zn and not all of the Zn will be consumed. In this case, Zn is the limiting reactant.
To determine the limiting reactant without knowing the actual values of a and b, we can compare the mole ratios. If the ratio of moles of Zn to moles of HCl is less than 1:2, then HCl is the limiting reactant. If the ratio is greater than 1:2, then Zn is the limiting reactant.
Therefore, to determine the limiting reactant in the reaction Zn + 2HCl -> ZnCl2 + H2, we need to compare the initial moles of Zn and HCl and check whether the mole ratio is less than or greater than 1:2.
