Excess and Limiting Reagents

Key Concepts

• Excess Reagents: Reactants not used up when the reaction is complete.
• Limiting Reagents: The reactant that is completely consumed, limiting the amount of product.
• Stoichiometric Mixture: An ideal reactant ratio where no limiting or excess reagents exist.

Learning Objectives

• Determine excess and limiting reagents using stoichiometric calculations.
• Calculate theoretical yields for reactions with limiting reagents.

Important Reaction Insights

• Reaction equations provide ideal stoichiometric relationships among reactants and products.
• In practical scenarios, reactants might not be in stoichiometric mixture proportions, leading to excess and limiting reagents.

Example Problems

Example 1: Combustion of Ethane

• Equation: [2C_2H_6 + 7O_2 \rightarrow 4CO_2 + 6H_2O]
• Calculation: Given 35.0 mol of (O_2), calculate moles of (CO_2) formed:
  • [35.0 \text{ mol } O_2 \times \frac{4 \text{ mol } CO_2}{7 \text{ mol } O_2} = 20.0 \text{ mol } CO_2]
• Discussion: Balanced equations are necessary for identifying limiting reagents.

Example 2: Reaction of Magnesium and Oxygen

• Equation: [2Mg + O_2 \rightarrow 2MgO]
• Scenario: Two moles of (Mg) with five moles of (O_2).
• Conclusion: (Mg) is the limiting reagent, (O_2) is in excess.

Skill-Developing Problems

1. Reactivity with (Cl_2):
   • Most vigorous: Potassium (K)
2. Reaction with (Cl_2):
   • (1 \text{ mol } Cl_2) reacts with (2 \text{ mol } Na)
3. State of Sodium at room temperature: Solid
4. Best Conductor among given options: Sodium metal
5. Positive Ions in (NaCl) solution: Sodium ions
6. Limiting Reagent in equal weights (Na) and (Cl_2): Chlorine
7. Excess Reagent in (H_2) and (O_2): Hydrogen
8. Limiting Reagent in (H_2) and (F_2): Fluorine
9. Thermite Mixture: (Fe_2O_3) as limiting reagent due to 1:2 mass ratio.

Contributors

• Authored by Chung (Peter) Chieh, University of Waterloo.
• Shared under CC BY-NC-SA 4.0 license by LibreTexts.

These notes summarize the concepts of excess and limiting reagents in stoichiometry, providing examples and practice problems to enhance understanding. They highlight the importance of balanced chemical equations in determining reaction dynamics.