Balancing Reactions and Limiting Reagents

_P₄+_O₂ →  _P₂O₃

• 3 P₄+1 O₂ → 2 P₂O₃
• 1 P₄+1 O₂ → 2 P₂O₃
• 1 P₄+ 3 O₂ → 2 P₂O₃
• 1 P₄+ 2 O₂ → 3 P₂O₃

_AlBr₃ + _K₂SO₄ → _KBr + _Al₂(SO₄)₃

• 6 AlBr₃ + 1 K₂SO₄ → 6 KBr + 1 Al₂(SO₄)₃
• 2 AlBr₃ + 3 K₂SO₄ → 6 KBr + 1 Al₂(SO₄)₃
• 1 AlBr₃ + 1 K₂SO₄ → 2 KBr + 3 Al₂(SO₄)₃
• 2 AlBr₃ + 3 K₂SO₄ → 1 KBr + 2 Al₂(SO₄)₃

_Al + _HCl →_H₂ + _AlCl₃

• 2 Al + 2 HCl → 2 H₂ + 6 AlCl₃
• 3 Al + 3 HCl → 6 H₂ + 2 AlCl₃
• 1 Al + 1 HCl → 3 H₂ + 1 AlCl₃
• 2 Al + 6 HCl → 3 H₂ + 2 AlCl₃

_K + _Cl₂ →_KCl

• 2 K + 16 Cl₂ → 8 KCl
• 2 K + 1 Cl₂ → 2 KCl
• 3 K + 4 Cl₂ → 3 KCl
• 1 K + 1 Cl₂ →1 KCl

Balance this equation:  H₂ + N₂ → NH₃

•  H₂ + 4N₂ → NH₃
•  H₂ + 3N₂ → 2NH₃
•  3H₂ + N₂ → 2NH₃
•  2H₂ + 2N₂ → 2NH₃

You need 2 pieces of bread, 1 tablespoon of peanut butter and 2 tablespoons of jelly to make a sandwich.  If you have 10 pieces of bread, 4 tablespoons of peanut butter and 20 tablespoons of jelly, what is the limiting reactant?

• bread
• jelly
• peanut butter
• sandwich

Identify the limiting reagent when 6.00 moles HCl combines with 5.00 moles Mg to form MgCl₂.
 
1 Mg +  2 HCl --> 1 MgCl₂  +  1 H₂   
 

• Mg
• H₂
• MgCl₂
• HCl

When does a chemical reaction stop?

• When the lab is finished.
• When the excess reactant is used. up
• When the limiting reactant is used up.
• Chemical reactions never stop.

When 12 moles of O₂ reacts with 6 moles of C₁₀H₈, what is the limiting reactant?  
1 C₁₀H₈ + 12 O₂ --> 10 CO₂ + 4 H₂O

• Oxygen
• C₁₀H₈
• Water
• Carbon Dioxide

Why is it important to identify the limiting reagent?

• The limiting reagent speeds up the reaction.

• The limiting reagent controls the amount of product formed.

• If there is no limiting reagent, the reaction will not occur.

• No stoichiometry calculations can be done without a limiting reagent.