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Limiting Reactants. Substances can only react in a chemical reaction until the reactants are used up. Reactions only occur when reactants are in a specific ratio to one another and in multiples thereof.
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Limiting Reactants • Substances can only react in a chemical reaction until the reactants are used up. • Reactions only occur when reactants are in a specific ratio to one another and in multiples thereof. • If 1 pencil reacts with 4 pieces of paper, then 2 pencils will react with 8 pieces of paper. 3 pencils will react with 12 pieces of paper etc. at a continuous ratio of 1:4. • If I have 32 sheets of paper, how many pencils do I need to use them all up? • If I have 1236 pencils and 5000 sheets of paper, which one will be used up first? • The one used up first is the limiting reactant.
Steps to Solve Limiting Reactants Problems • Identify what you know and what you do not know and write it down. • Convert any masses given to moles. • Mass of substance (g) = Number of Moles Molar Mass of substance (g/mol) • Determine the Mole Ratio needed. • Coefficient of unknown substance in BALANCED chemical equation Coefficient of known substance in BALANCED chemical equation • mol unknown molecule # mol known molecule • # mol comes from BALANCED chemical equation NOT the problem itself. • Use the mole ratio to determine the amount of product the known reactant will produce. • Known Moles X Mole Ratio = Moles of Product • Convert moles of product to grams. • Number of Moles X Total Molar Mass = Mass of substance (g)
Steps to Solve Limiting Reactant Problems …continued • Compare the amount of “Unknown” calculated and compare it to the amount of “Unknown” given in the problem. • If there is more “Unknown” calculated than given, then it is the limiting reactant. • If there is more “Unknown” given than calculated, then it is the excess reactant and the “Known” is the limiting reactant. Extension: If the problem continues to ask specifically how much one of the products in the equation will be formed by the reaction…Steps 1-6 Same as Above and then: • Determine your new Mole Ratio using the Limiting Reactant identified above. • Coefficient of unknown substance in BALANCED chemical equation Coefficient of Limiting Reactant in BALANCED chemical equation • mol unknown molecule # mol Limiting Reactant • # mol comes from BALANCED chemical equation NOT the problem itself. • Use the mole ratio to determine the amount of substance the Limiting Reactant will produce. • Known Moles of Limiting Reactant X Mole Ratio = Moles of Unknown • Convert moles of product to grams. • Number of Moles X Total Molar Mass = Mass of substance (g)
I Limiting Reactants Practice 1. Calcium hydroxide, used to neutralize acid spills, reacts with hydrochloric acid according to the following equation: Ca(OH)2 + 2HCl → CaCl2 + 2H2O. If you have spilled 6.3 mol of HCl and put 2.8 mol of Ca(OH)2 on it, which substance is the limiting reactant?
We Limiting Reactants Practice …continued 2. Aluminum oxidizes according to the following equation: 4Al + 3O2 → 2Al2O3. 0.048 mol of powdered Al is placed into a container containing 0.030 mol O2. What is the limiting reactant?
We Limiting Reactants Practice …continued 3. Chlorine can replace bromine in bromide compounds forming a chloride compound and elemental bromine. 2KBr + Cl2 → 2KCl + Br2 is an example of this reaction. When 0.855 g of Cl2 and 3.205 g of KBr are mixed in solution, which is the limiting reactant? How many grams of Br2 are formed?
You Limiting Reactants Practice …continued 4. A process by which zirconium metal can be produced from the mineral zirconium(IV) orthosilicate, ZrSiO4 , starts by reacting it with chlorine gas to form zirconium(IV) chloride. ZrSiO4 + 2Cl2 → ZrCl4 + SiO2 + O2 What mass of ZrCl4 can be produced if 862 g of ZrSiO4 and 950.0 g of Cl2 are available? (You must first determine the limiting reactant.)
