Stoichiometry and Limiting Reagents

Stoichiometry Examples

• Interpreting stoichiometry with examples like:
  • $C<em>2H</em>4(g) + 3O<em>2(g) \rightarrow 2CO</em>2(g) + 2H_2O(l)$
  • $2Na<em>3PO</em>4(aq) + 3Ba(NO<em>3)</em>2(aq) \rightarrow Ba<em>3(PO</em>4)<em>2(aq) + 6NaNO</em>3(aq)$
• Using a pancake recipe analogy to understand mole ratios in chemical reactions.

Pancakes Analogy

• Recipe: 1 cup flour + 2 eggs + 0.5 tspn baking powder yields 5 pancakes.
• The recipe provides numerical relationships (mole ratios) between ingredients (reactants) and product (pancakes).
• Example: Determining the amount of flour and baking powder needed for a specific number of eggs.
• Adjusting the recipe to make a desired number of pancakes requires proportional adjustments to all ingredients.

Limiting Reagent

• Reactions don't always use exact mole ratios for complete reaction.
• Limiting Reagent: The reactant that is completely consumed and determines the amount of product formed.
• Excess Reagent: The reactant that is leftover after the reaction is complete.

Theoretical Yield

• Maximum amount of product that can be made based on the amount of limiting reagent.

Percentage Yield

• Relates actual yield (amount of product actually obtained) to theoretical yield.
• Formula: $Percentage Yield = (Actual Yield / Theoretical Yield) \times 100 \%$.

Determining Limiting Reagent, Theoretical Yield, and Percentage Yield

1. Write a balanced equation.
2. Find the number of moles of all reactants.
3. Determine the limiting reagent using mole ratios from the balanced equation.
4. Calculate the theoretical yield (moles of product) using mole ratios.
5. Convert moles of product to mass.
6. Calculate the percentage yield.

Example: Methanol Decomposition

• $CH<em>3OH(l) \rightarrow 2H</em>2(g) + CO(g)$
• If 125 g of methanol decomposes, the theoretical yield of hydrogen is calculated.
• If only 13.6 g of hydrogen is obtained, the percentage yield is calculated.

Example: Thermite Reaction

• $Fe<em>2O</em>3(s) + 2Al(s) \rightarrow 2Fe(s) + Al<em>2O</em>3(s)$
• With 50.0 g each of $Fe<em>2O</em>3$ and Al, identify the limiting reagent.

Complex Mole Ratios

• Example: Oxidation of $NH_3$ to NO
  • $4NH<em>3(g) + 5O</em>2(g) \rightarrow 4NO(g) + 6H_2O(l)$
• Determining if reagents are mixed in correct mole ratios when given equal masses of reactants.