Limiting Reactants and Theoretical Yield Notes

Limiting Reactants

• The limiting reactant is the substance that is totally consumed when the chemical reaction goes to completion.
• The theoretical yield of a product is the amount that could potentially be formed from the limiting reactant.
• Key Step: Always use the limiting reactant for calculating theoretical yield.

Calculation Steps

1. Identify the Limiting Reactant:

   • Examine the balanced equation to find which reactant runs out first during the reaction.
   • Example: If you have a balanced equation showing reactants A and B producing products C and D, and reactant A is limiting, then all calculations will use A.

2. Start with the Limiting Reactant:

   • To calculate the yield, start with the mass (in grams) of the limiting reactant.
   • Example: If the limiting reactant is oxygen, write down its mass for calculations.

3. Utilize Molar Mass:

   • Convert grams of the limiting reactant into moles using its molar mass (found on the periodic table).
   • Example: For oxygen, the molar mass is approximately $32 ext{ g/mol}$.

4. Conversion to Desired Product:

   • Use stoichiometry to convert moles of the limiting reactant to moles of the desired product (e.g., $SO_3$).
   • This utilizes the coefficients from the balanced equation.

5. Calculate Actual Yield & Theoretical Yield:

   • The actual yield (given in the problem) vs. theoretical yield (calculated based on the limiting reactant) can determine the efficiency of the reaction.
   • Example: If the problem states the actual yield of $SO_3$ is 1.5 grams then calculate how much could be produced ideally to see the efficiency ratio.

6. Multiple Steps vs. Series of Conversions:

   • It's acceptable to break these steps into smaller segments or write them as one long step as preferred.

Example Calculation Using Stoichiometry

• Molar Mass of Sodium Hydroxide (NaOH):

  • Sodium (Na): $22.99 ext{ g/mol}$
  • Oxygen (O): $16.00 ext{ g/mol}$
  • Hydrogen (H): $1.01 ext{ g/mol}$
  • Thus the Molar Mass of NaOH = $22.99 + 16.00 + 1.01 = 40.00 ext{ g/mol}$

• From the hypothetical reaction, if you need to find out how much of Sodium is there in Sodium Sulfate with the coefficients:

  • 2 moles of Na+ for every mole of Sodium Sulfate (Na2SO4).

By following these steps, you can effectively determine the limiting reactant and calculate the theoretical yield for any chemical reaction.