Limiting Reactant (Determines Yield)
The reactant that is completely consumed first in a reaction. It limits the amount of product that can be formed and dictates the theoretical yield.
Excess Reactant (Leftover Amount)
The reactant remaining in the reaction mixture after the limiting reactant has been completely used up.
The Sandwich Analogy for Limiting Reactants
\(2\text{ slices of bread} + 1\text{ filling} \rightarrow 1\text{ sandwich}\).
If you have 10 slices of bread but only 3 fillings → the filling is the limiting reactant. You can only make 3 sandwiches, with 4 slices of bread left in excess.
Percentage Yield
\( \% \text{ Yield} = \frac{\text{Actual Yield}}{\text{Theoretical Yield}} \times 100 \)
Theoretical Yield is the maximum you could make. Actual Yield is what you actually get (always less, due to losses).
Determining the Limiting Reactant
Problem: \(5.00\text{ g}\) of \(\text{Mg}\) reacts with \(10.0\text{ g}\) of \(\text{HCl}\). Find the limiting reactant.
\(\text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2\)
1. Calculate Moles of each:
- \(n(\text{Mg}) = \dfrac{5.00}{24.31} = 0.206\text{ mol}\)
- \(n(\text{HCl}) = \dfrac{10.0}{36.46} = 0.274\text{ mol}\)
2. Check Stoichiometric Requirement: \(0.206\text{ mol Mg}\) requires \(0.206 \times 2 = 0.412\text{ mol HCl}\).
3. Conclusion: We only have \(0.274\text{ mol HCl}\), so \(\text{HCl}\) is the limiting reactant (\(\text{Mg}\) is in excess).
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