Post on 04-Jan-2016
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Stoichiometry
Defined
• Stoichiometry –The study of quantitative (measurable) relationships that exist in chemical formulas and chemical reactions.
• You must have:• 1. The correct formulas – “criss-cross” or
prefixes• 2. Balanced Equations – Five types,
verify law or conservation of mass
Analysis of a chemical reaction/equation
• Consider:
• N2H4 + 2H2O2 → N2 + 4H2O
• Same as saying…• 1mole N2H4 + 2moles H2O2 → 1mole N2 + 4 moles H2O
• The mole ratio is the “recipe” for the reaction.
Mole-Mole Problems
• Factor-Label Method – problem solving method based upon treating units in calculations as if they were algebraic factors.
• N2H4 + 2H2O2 → N2 + 4H2O
• Ex.1.4 moles of N2H4 gives how many moles of N2?
• 1.4 moles N2H4 x mole N2 = moles N2
• mole N2H4
Another Example
• 3Zn + 2H3PO4 → Zn3(PO4)2 + 3H2
• How many moles of Zn3(PO4)2 will be produced from 2.18 moles of H3PO4?
• 2.18 moles H3PO4 x moles Zn3(PO4)2
• mole H3PO4
• 1.09 moles Zn3(PO4)2
Verifying the Law of Conservation of Matter
• 2H2 + O2 → 2H2O
• Calculate the mass of the reactants and the mass of the products
• 2 mol H2 x g H2 + 1 mol O2 x g O2 = 36.04g
• 1 mol H2 1 mol O2
• Or 2 mol H2O x g H2O = 36.04g
• 1 mol H2O
Mass – Mass Problems
• Steps • Find molar mass of both substances• change mass to moles of given• use molar bridge (moles of given moles of
unknown)• change moles to mass of unknown• ***Remember, coefficients in the balanced
equation represent the relative number of moles of reactants and products!***
Mass-Mass Problems
• What mass of water is produced from 1.5 grams of glucose?
• C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
• 1st. change mass to moles using molar mass of glucose. (180 g/mol)
• 2nd. Use molar bridge to change from moles of given to moles of unknown.
• 3rd change to grams of water
The example of Mass-Mass
• C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
• 1.5g C6H12O6 to g of water.
• 0.9g H2O
Another example
• Ex. What mass of aluminum oxide is produced when 2.3 g of aluminum reacts with iron (III) oxide?
• 4.3g Al2O3
Mass-Volume problems
• 1st mass of given
• 2nd moles of given (molar mass)
• 3rd moles of unknown (molar ratio)
• 4th volume of gaseous unknown
• If I have 125 g of Al2O3 how many L of Al do I have @ STP?
Another Example
• Find the mass of aluminum required to produce 1.32L of H2 gas @ STP
• 2Al + 3 H2SO4 → Al2(SO4)3 + 3 H2
•
Volume-volume problems
• Same as mole-mole just using volumes instead!
• If I have 15.5 L of N2 gas, how many L of H2 will react?
• N2 + 3 H2 → 2 NH3
11-3 Limiting Reactants and Percent Yield
• When chemicals combine, they are usually in nonstoichiometric proportions. That means there will be a limiting reactant.
• Limiting Reactant will be completely used up in the reaction.
• The other, leftover amount is said to be in excess.
• **The quantities of products formed in a reaction are always determined by the quantity of limiting reactant. **
LR continued
• To find the limiting Reactant:
• 1st solve 2 separate mass-mass problems.
• 2nd see which number is smaller; this is your limiting reactant.
Example of LR
• Ex. 3.5 g of Cu is added to 6.0 g silver nitrate. Find the limiting reactant.
• (Note: you can calculate the mass of either product, use the easier one to find the molar mass!)
• Cu + 2 AgNO3 → Cu(NO3)2 + 2 Ag
• You can also determine how much of the substance in excess is left over.
• 1st determine the LR and then use the amount of the LR and use that to find the mass of the other substance.
• Ex. 2.0 g of NaHCO3 reacts with 0.5 g of H3C6H5O7 (citric acid) which is the LR and how much of the other will be left over? What volume of CO2 will be produced?
example
• 3 NaHCO3 + H3C6H5O7 → 3 CO2 + 3 H2O + Na3C6H5O7
Percent yield
• Percent yield = actual yield x 100
• Expected
• Actual = what you got in lab
• Expected = what it was supposed to be
Example
• A piece of copper with a mass of 5.00g is placed in a solution of AgNO3 . The silver metal produced has a mass of 15.2g. What is the percent yield for this rxn?
• 1st calculate the mass of the expected silver.
• Cu + 2AgNO3 2 Ag + Cu(NO3)2
Example continued