Chemistry Lecture 9

Limiting Reactants

• Definition: The limiting reactant is the one that is completely used up first in a reaction.

• Identification Steps:

  • Ensure the chemical equation is balanced.

  • Calculate moles of product from each reactant.

  • The reactant producing the least amount of product is the limiting reactant.

Reaction Examples

• NaOH is limiting in the reaction producing Na2CO3; can produce only 0.925 moles.

• Magnesium is the limiting reactant when reacting with nitrogen to form Mg3N2, yielding 48.43 g Mg3N2.

Excess Reactants

• To find excess reactant, subtract the amount consumed from the initial amount available.

• Example: For nitrogen, leftover = initial - consumed.

Percent Yield

• Formula:
  $Percent ext{ }Yield = \frac{Actual ext{ }Yield}{Theoretical ext{ }Yield} \times 100$

• The limiting reactant determines the theoretical yield; actual yield cannot exceed 100%.

Solution Concentration

• Solution Basics: A solution comprises a solute and a solvent; the solvent is the component in the largest amount.

• Molarity (M): Defined as moles of solute divided by liters of solution.

• Example: 0.500 M NaCl means 0.500 mol NaCl in 1.0 L of solution.

Using Molarity in Calculations

• Molarity serves as a conversion factor between moles of solute and volumes of solution.

• Example calculations include determining moles from grams and vice versa.

Charge Distribution in Water

• Water has an uneven electron distribution leading to partial charges (negative on oxygen side, positive on hydrogen side).

• This allows water to solvate ionic compounds.

Molarity of Ions

• Ionic compounds dissociate in solution:

  • $0.5 M NaCl o 0.5 M Na^+ ext{ and } 0.5 M Cl^-$

  • $0.5 M CaCl_2 o 0.5 M Ca^{2+} ext{ and } 1.0 M Cl^-$

Solution Dilution

• Dilution involves adding solvent to a concentrated solution, following:
  $M<em>1 imes V</em>1 = M<em>2 imes V</em>2$

• Example: To dilute concentrated HCl to a lower concentration, calculate based on volumes and molarities.

Stoichiometric Calculations

• Use molarity to convert between moles and liters during chemical reactions, converting back and forth as needed using stoichiometric coefficients in balanced equations.