Excess Reactants

Introduction

• In chemical reactions, one reactant may be used in excess to ensure complete conversion of the other reactant.
• This approach is cost-effective when one reactant is abundant and inexpensive.
• Excess reactants affect the limiting reactant, which determines the quantity of each product formed.
• Balancing economic efficiency and environmental sustainability is crucial when using excess reactants.

Example 1: Calcium Carbonate and Hydrochloric Acid

• Reaction: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂
• Given: 5 g of CaCO₃
• Calculate the moles of CaCO₃: 5 g / 100 g/mol = 0.05 moles
• Determine the volume of CO₂ produced: 0.05 moles x 22.4 L/mol = 1.12 litres

Example 2: Magnesium and Hydrochloric Acid

• Reaction: Mg + 2HCl → MgCl₂ + H₂
• Given: 0.25 g of Mg, 100 cm³ of 0.1 mol/L HCl
• Calculate moles of Mg: 0.25 g / 24.3 g/mol = 0.01 moles
• Calculate moles of HCl: 0.1 mol/L x 0.1 L = 0.01 moles
• Since HCl moles are twice that of Mg, Mg is in excess.
• 0.005 moles of Mg remain unreacted

Key Points

• Using excess reactants ensures complete conversion of the limiting reactant.
• The limiting reactant determines the quantity of each product.
• Balancing economic efficiency and environmental impact is crucial when using excess reactants.