Lecture on Conservation of Mass and Stoichiometry

Introduction

• The lecture focuses on mass conservation during chemical reactions, stoichiometry, and how to use balanced equations to determine product amounts.
• Antoine Lavoisier proposed the Law of Conservation of Mass in the late 18th century.

Law of Conservation of Mass

• States that in a closed system, mass remains constant over time, irrespective of processes occurring.
• Total mass of substances before a reaction equals the total mass after.
• Example: Burning wood - mass of carbon, hydrogen, and oxygen atoms equals mass of CO2, water vapor, and ashes.

Importance in Chemistry

• Basis for balancing chemical equations and tracking mass changes in reactions.
• Atoms, elements, ions, and compounds can be represented through formula writing.
• Correct representations help track chemical changes accurately.

Balancing Chemical Equations

• Reactants are on the left, products on the right of an equation.
• Coefficients are used to balance mass and atom numbers in reactions.
• Example: Methane combustion - one molecule of methane reacts with two molecules of oxygen to produce one molecule of CO2 and two molecules of water.

Working with Moles

• Practical to use moles instead of individual molecules.
• One mole is equivalent to Avogadro's constant (6.02 x 10^23).
• Balanced equations can be interpreted in terms of moles.

Stoichiometry

• Mole ratios from coefficients allow quantification and tracking of reactants and products.
• Stoichiometric problems involve calculations using mole ratios.

Limiting and Excess Reactants

• Not all reactants are in the precise amounts needed; one is often in excess.
• Limiting reactant is used up first and determines the theoretical yield.
• Example: Reaction of aluminum with copper(II) chloride - aluminum can be limiting.

Determining Limiting Reactants

• Several approaches:
  1. Divide moles of substances by coefficients from the balanced equation.
  2. Calculate theoretical yield of a product from each reactant.
  3. Use mole ratios to check if enough of other reactants are available.

Summary

• Mass is conserved in reactions; total mass of reactants equals products.
• Stoichiometric coefficients represent relative moles of substances.
• Limiting reactant is used up first; excess reactant remains.
• Moles of limiting reactant determine moles of product.