Final answer:
Without the balanced chemical equation and the stoichiometric coefficients, there is not enough information to determine which substance is the limiting reactant when mixing 2 units of A with 6 units of B.
Step-by-step explanation:
To determine which reactant is the limiting reactant when mixing 2 units of A with 6 units of B, we must compare the molar amounts of each reactant provided to their coefficients in the balanced chemical equation. However, since the generic equation and the stoichiometric coefficients are not provided in the question, there is not enough information given to conclude whether substance A or B will be the limiting reactant. The limiting reactant is the one which will run out first during the chemical reaction, limiting the amount of product formed, while any remaining reactant is considered the excess reactant.