Limiting Reactant

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Limiting Reactant
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Stoichiometry Refresher

• Recall
• The mole ratio can be used to determine the
  quantity (moles) or mass of reactants consumed
  and products formed
• However, usually there is one reactant which will
  be consumed entirely and the other reactant is
  left over (in excess)

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• Excess Reactant The reactant that is NOT
  completely consumed in a chemical reaction (i.e.
  it is in excess)
• Limiting Reactant The reactant that is
  completely consumed in a chemical reaction. It
  limits how much product is formed.

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BBQ Analogy

• You are having a BBQ and want to make hot dogs.
• You buy 2 packs of wieners and 2 packs of buns
• There are 10 wieners per pack and 8 buns per pack
  (Marketing ploy??)
• How many hot dogs can you make?
• Write the BCE
• 1 wiener 1 bun ? 1 hot dog
• You can make 16 hot dogs and there will be 4
  wieners left over (excess reactant)
• You are limited by the number of buns (limiting
  reactant)

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Solving Limiting Reactant Problems

1. Write the BCE
2. Set up a stoich table
3. Convert reactants into moles
4. Use mole ratios from BCE to calculate the moles
   of product formed using each reactant
5. The reactant that produces the least amount of
   product is the limiting reactant. Use this amount
   to do any further calculations

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• E.g. What mass of magnesium oxide is produced
  when 6.73 g of magnesium reacts with 8.15 g of
  oxygen

2 Mg(s) O2(g) ? 2 MgO(s)
MM 24.31 g/mol 32.00 g/mol 40.31 g/mol
n 6.73 g X 1 mol 24.31 g 0.2769 mol 8.15 g X 1 mol 32.00 g 0.2547 mol Using Mg 0.2769 molMg X 2 molMgO 2 molMg 0.2769 molMgO Using O2 0.2547 molO2 X 2 molMgO 1 molO2 0.5094 molMgO 0.2769 mol lt 0.5094 mol ? Mg limits
m 6.73 g 8.15 g 0.2769 mol X 40.31 g mol 11.2 g
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Practice!

• Worksheet B (no limiting reactant)
• Worksheet C
• P. 231 1-4
• P. 235 5-10