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Applied Physics and Chemistry

Applied Physics and Chemistry. Limiting Reagents and Percent Yield Lecture 1. Limiting Reagent. Picture making sandwiches: 2 pieces bread + 3 slices meat + 1 slice cheese  sandwich What if you had 20 slices of bread, 24 slices of meat and 12 slices of cheese?

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Applied Physics and Chemistry

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  1. Applied Physics and Chemistry Limiting Reagents and Percent Yield Lecture 1

  2. Limiting Reagent • Picture making sandwiches: • 2 pieces bread + 3 slices meat + 1 slice cheese  sandwich • What if you had 20 slices of bread, 24 slices of meat and 12 slices of cheese? • How many sandwiches could you make? • What would be left over?

  3. Limiting reagent • Definition: • The reactant that runs out first and limits the amount of products that can be formed • How do we know what’s limiting? • Stoichiometry!

  4. Calculations • Requires ability to determine moles from mass and mass from moles! • Requires balanced equation!

  5. Example • Suppose we have 25 kg of nitrogen gas and 5 kg hydrogen gas. We mix them and heat to react and form ammonia. What amount of ammonia can we form? • First: write and balance the equation • N2 + H2 NH3 • N2 + 3H2  2NH3 • Next: Find the moles from grams! • 25 kg N2 = 25 000 g • 5 kg H2 = 5000 g

  6. Example continued • 25 000 g N2 : find moles! • 892 moles N2 • 5 000 g H2 : find moles! • 2480 moles H2 • Next: Determine which reactant is limiting by using mole ratios • 892 moles N2 x 3 moles H2 = 2680 moles H2 • 1 mole N2 • Do we have that much H2? • No! H2 is limiting reagent! • N2 is in excess.

  7. Example continued • Next, use the LIMITING REAGENT to find the amount of product formed. • 2480 mole H2 x 2 mole NH3 = 1650 mole NH3 • 3 mole H2 • Then convert moles of NH3 to mass! • 1650 mole NH3 x 17.03 g NH3 = 28 100 g NH3 • 1 mole NH3

  8. Steps! • Write and balance the equation • Convert masses to moles • Use mole ratios to determine which reactant is limiting • Use limiting reagent to determine the amount of product (moles) • Convert from moles of product to mass

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