Stoichiometry of a Reaction

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Stoichiometry of a Reaction
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Stoichiometry

• Reactants react in specific number (mole) ratios.
• 4 Al(s) 3 O2(g) ? 2Al2O3(s)
• Determination
• Theoretical known products
• Experimental unknown products

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Experimental Stoichiometry Determination

• Vary ratio of reactants
• Constant total moles
• Constant moles of one reactant
• Identify maximum reaction
• Maximum heat transfer
• Maximum product formed

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Reactions to be investigated

• Aqueous solution (occur in water)
• X NaClO(aq)  Y Na2SO3 (aq) ? products
• X NaClO(aq)  Y (???) (aq) ? products
• KI potassium iodide
• Na2S2O3 sodium thiosulfate
• KSCN potassium thiocyanate
• Exothermic (release heat)

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Aqueous Reactants

• In water
• 0.5 moles per liter (0.5M)
• Total of 50 mL of reactant for each trial
• Varying ratios

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Exothermic Reactions

• Heat Released
• Heat transferred to reaction solution
• Insulated from surroundings
• Quantified by observed temperature increase

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Dispense reactants

• Required Quantities from stock in beakers
• (DO NOT take more)
• 300 ml of NaClO
• 160 ml of Na2SO3
• 160 ml of assigned reactant  (KI, Na2S2O3, or
  KSCN)
• Graduated cylinder
• Use 2 different graduated cylinders (share)
• Read bottom of meniscus
• Use eye dropper
• Pour directly from graduated cylinder to
  Sytrofoam cup (modification to page 40)

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React Reactants

• In Styrofoam cup
• Dedicated graduated cylinders
• Rinsed and dried thermometer
• MIX well

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Record Max solution temperature reached

• Observe thermometer continuously (reaction begins
  immediately)
• Keep thermometer in reaction solution

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Work safely

• Eye protection
• Appropriate clothing
• Stabilize Styrofoam cup in beaker
• Do not leave thermometer standing in cup
• Perform reaction involving potassium thiocyanate
  (KSCN) in hood.

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Handle Waste

• Minimize waste by taking recommended quantities
  from stock
• 300 ml of NaClO
• 160 ml of Na2SO3
• 160 ml of assigned reactant  (KI, Na2S2O3, or
  KSCN)
• Collect waste at bench
• Dispose of waste in liquid waste receptacles
• Monitor fluid level in waste receptacles
• Do not allow waste to overflow
• Rinse and dry Styrofoam cups and leave on end of
  bench

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Analyze Data

• Plot mole fraction of NaClO vs change in
  temperature
• Draw two best fit lines
• Identify if and what additional point is needed
• Total volume 50 mL
• Instructor approval
• Identify stoichiometric ratio from intersection
  of drawn lines.

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Example

• Stoichiometric point at mole fraction of A 0.8
• Mole fraction B 1-mole fraction A (2 reactants)
• AB ratio 0.8 to 0.2 equivalent to 4 to 1
• Reaction stoichiometry 4A B ? Products

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Next Week Qualitative Analysis Cations