Unit 7: Stoichiometry

Mole ratio

• To determine this ratio, you need to look at the coefficients in front of each chemical species in a reaction.

• These coefficients can represent moles or molecules.

Stoichiometric calculation

1. Determine the balanced chemical equation.

2. Convert the amount you are given into moles.

3. Convert from moles of the known to moles of the unknown based on the coefficients in the balanced chemical reaction → mole ratio.

4. Convert from moles of the unknown to the units required.

Enthalpy (ΔH) and moles

• If an equation containing a heat term of ΔH is shown, coefficients always stand for moles.

• N₂ + 3H₂ → 2NH₃ + 46.2 kJ (exothermic!)

  • 1 mole of N₂ is used up → 46.2 kJ of heat is released.

  • 3 moles of H₂ are used up → 46.2 kJ of heat is released.

  • 2 moles of NH₃ are used up → 46.2 kJ of heat is released.

Limiting and Excess Reagents

• Limiting reagent: Starting material that gets used up in a reaction.

• Excess reagent: Starting material that is not used up in a reaction.

  → Use the limiting reagent to determine the yield.

• Finding limiting reagent method:

  1. Calculate the amount of product formed by each reactant.

  2. The reactant that produces the lowest yield is the limiting reactant.

Percent yield

• Actual yield: Result from the experiment.

• Theoretical yield: Result from stoichiometry.

TUMS tablet

• Reasons to take TUMS:

  • Treat heartburn

  • Treat indigestion

  • Treat upset stomach caused by too much stomach acid (HCl)

• How TUMS works:

  Reducing the amount of acid in the stomach.
  → Neutralizes the heartburn-causing acid in our esophagus and stomach on contact → fast relief.

• Acid found the stomach: HCl

• Active ingredient in TUMS tablet: CaCO₃