Consider the following generic chemical equation
2A + 4B �¨ 3C
What is the limiting reactant when 3 moles of A and 4 moles of B are allowed to react?
3 moles A x (3 moles C/2 moles A) = 4.5 moles C.
4 moles B x (3 moles C/4 moles B) = 3 moles C.
Both answers can't be correct; the correct one in limiting reagent problems is ALWAYS the smaller value.
To determine the limiting reactant, we need to compare the mole-to-mole ratio of reactants to the actual mole ratio given. In this case, the mole ratio between A and B is 2:4, which simplifies to 1:2.
Given that we have 3 moles of A and 4 moles of B, let's compare the mole ratio:
- For A, with 3 moles, the mole ratio becomes 3:2 (multiplying by 2/2).
- For B, with 4 moles, the mole ratio remains the same at 4:2.
Now, we can see that for the 3 moles of A, we would need only 2 moles of B to react completely. However, we have more than enough B, as we have 4 moles, which is greater than the required 2 moles.
Therefore, the limiting reactant is A, as it will be completely consumed before all of B is used up.
To determine the limiting reactant, we need to compare the moles of each reactant, A and B, with the stoichiometric coefficients in the balanced chemical equation.
First, let's calculate the number of moles of each reactant:
Moles of A = 3 moles
Moles of B = 4 moles
Next, we need to compare the ratio of the moles of each reactant with the stoichiometric coefficients from the balanced equation:
Moles of A / Stoichiometric coefficient of A = 3 / 2 = 1.5
Moles of B / Stoichiometric coefficient of B = 4 / 4 = 1
By comparing the ratios, we can see that the stoichiometric ratio for reactant A is 1.5, while the stoichiometric ratio for reactant B is 1.
Since the stoichiometric ratio for reactant A is higher than the stoichiometric ratio for reactant B, A is in excess and B is the limiting reactant.