Upload
lorena-saber
View
224
Download
0
Tags:
Embed Size (px)
Citation preview
Stoichiometry
L
What is it really?
• Cooking• Method of finding – how many cookies you can make given these
ingredients – how many ingredients you had to make this many
cookies
All goes back to balanced equations
• First write the equation and balance it.• You are observing the combustion of methane, CH4.
Your teacher asks you how many moles of water are expected to form in the complete combustion of 2.50 mol of methane.
CH4 + O2 CO2 + H2O UNBALENCED
CH4 + 2O2 CO2 + 2H2O BALENCED
Now the math
• Remember that the balanced equations shows the relationship of how many moles of each compound is used in this reaction – Like the ingredients on a recipe (3 eggs + ½ cup
water = eggy water)• So the coefficient of one compound to the
next is the mole ratio
Now the math (….continued)
You are observing the combustion of methane, CH4. Your teacher asks you how many moles of water are expected to form in the complete combustion of 2.50 mol of methane.
CH4 + 2O2 CO2 + 2H2OSo if we had 1 mole of methane we would produce 2 moles of water
But since we have 2.50 mol CH4…
The math
You are observing the combustion of methane, CH4. Your teacher asks you how many moles of water are expected to form in the complete combustion of 2.50 mol of methane.
CH4 + 2O2 CO2 + 2H2O
Therefore we would expect to retrieve 5 mols of H2O from the combustion of 2.5 mol of methane
5 mol H2O
Mole ratio
mole to mole
• The synthesis reaction between nitrogen monoxide and oxygen forms nitrogen dioxide.– How many moles of nitrogen monoxide would be
needed to fully react with 3.6 moles of oxygen– How many moles of nitrogen dioxide would be
produced from 3.6 moles of oxygen– How many moles of oxygen would be needed to
produce 4.67 mol nitrogen dioxide?– How many moles of nitrogen monoxide would be
needed to produce 4.67 mol nitrogen dioxide?
Building off that…
• If you can do mol to mol, the rest should be child’s play• Mass to mol– Convert mass to mol, then mol to mol. – The end
• Mass to mass– Convert mass of given to mol, convert mols of given to
moles of wanted, convert mol to mass– The end
• Particles to anything– Use Avagotto’s number (6.022 x 1023 particles/mol)
Mass to mol
• In a reaction between the elements aluminum and chlorine, aluminum chloride is produced. How many grams of AlCl3 will be produced if 2.50 moles of Al react?
Al + Cl2 AlCl3 Unbalanced
2Al + 3Cl2 2AlCl3 Balanced
Mol to mass continued
2Al + 3Cl2 2AlCl3
How many grams of AlCl3 will be produced if 2.50 moles of Al react?
Therefore 333.33 g of AlCl3 will be produced by the reaction of 2.50 mol of Al
2.50 mol AlCl31 mol AlCl3333.33 g AlCl3
Practice
The ammonia (NH3) used to make fertilizers for lawns and gardens is made by reacting nitrogen and hydrogen.
– Determine the mass in grams of NH3 formed from 1.34 moles of nitrogen.
– What is the mass in grams of hydrogen required to react with 1.34 moles of nitrogen?
– How many moles of nitrogen are required to produce 11.7 grams of NH3?
Mass to mass
• When nitrogen and hydrogen react, they form ammonia gas, which has the formula NH3. If 56.0 g of nitrogen are used up in the reaction, how many grams of ammonia will be produced?
N2 + H2 NH3 Unbalanced
N2 + 3H2 2NH3 Balanced
When nitrogen and hydrogen react, they form ammonia gas, which has the formula NH3. If 56.0 g of nitrogen are used up in the reaction, how many grams of ammonia will be produced?
N2 + 3H2 2NH3
• Convert grams to mol• Convert mol to gram
Therefore, 68.11 g of NH3 are produced from 56 g of N2
3.9971 mol NH3 68.11 g NH3
Particles to Particles
• Hydrogen and Oxygen react to produce water. How many particles of hydrogen would be needed to react fully with 1.01 x 1012 particles of oxygen?
H2+ O2 H2O Unbalanced
2H2 + O2 2H2O Balanced
Hydrogen and Oxygen react to produce water. How many particles of hydrogen would be needed to react fully with 1.01 x 1012 particles of oxygen?
2H2 + O2 2H2O• Convert particles to mol• Convert mol to mol• Convert mol to particles
Therefore we expect to need 2.14 x 1012 particles of H2 to react fully with 1.01 x 1012 particles of O2
1.6772 x 10-12 mol O2
2.14 x 1012
particles of H2
Practice
• Packet!
Percent yield
• How much are you actually getting out of your experiment?
% Yield = Actual yield/theoretical yield x 100
What is the total mass of H2O produced when 384g Cu is completely consumed in the following reaction?Cu + 4HNO3 Cu(NO3)2 + 2H2O + 2NO2
• Convert mass to mol• Convert mol to mol• Convert mol to mass
Therefore we expect to produce 216g of H2O.
What is the percent yield if this procedure was carried out in the lab and only produced 200g of H2O
216g H2O
We expected to produce 216g of H2O.What is the percent yield if this procedure was carried out in the lab and only produced 200g of H2O
• What was the actual yield?– 200g
• What was the theoretical yield?– 216g
• Solve
• Therefore, our % yield is 92.6%
% yield = Actual yield
Theoretical yield
200g216g
92.6% =
Practice• Ethanol (C2H5OH) is produced from the
fermentation of sucrose in the presence of enzymes:
C12H22O11(aq) + H2O(g) C2H4OH(l) + CO2(g)
• determine the theoretical and percent yields of ethanol if 684g sucrose undergoes fermentation and 349g ethanol is obtained
T.Y. = 369g % yield = 94.6%
• Lead (II) oxide is obtained by roasting galena, lead (II) sulfide, in air:
PbS(s) + O2(g) PbO(s) + SO2(g)
• Determine the theoretical and percent yields of PbO if 200.0g PbS is heated and 170.0g PbO is obtained
T.Y. = 186.6g PbO % Yield = 91.10%
Limiting reagent
• The reactant that is completely consumed during a rxn
• Reactants that remain are excess reactants
Reagent
To find the limiting reagent
• Compare the actual mol:mol ratio to the balanced formula’s mol:mol ratio
• Calculations must be based on given amounts of limiting reactants
Fe + 2NiO(OH) + 2H2O Fe(OH)2 + 2Ni(OH)2
• Determine the number of moles of Fe(OH)2 produced if 5.00 mol Fe and 8.00 mol of NiO(OH) react. What is the limiting reactant? What reactant is in excess and by how many moles?
• 1mol Fe::2mol NiO(OH). Therefore,5mol Fe would need 10mol NiO(OH)
• Since we only have 8.00mol NiO(OH), this is our limiting reagent.
Fe + 2NiO(OH) + 2H2O Fe(OH)2 + 2Ni(OH)2
Determine the number of moles of Fe(OH)2 produced if 5.00 mol Fe and 8.00 mol of NiO(OH) react. What is the limiting reactant? What reactant is in excess and by how many moles?
• Since NiO(OH) is the limiting reagent, find how many moles of Fe are needed to completely react with the amount of it we do have
• 4 Mols of Fe are needed
• Therefore 1 mol of Fe is in excess
4mol Fe
5.00mol Fe(provided) - 4mol Fe(needed) = 1mol Fe(excess)
It gets complicated
• If you are given anything other then mol, you need to first convert to mol for BOTH reagents before you can determine the limiting factor!
Limiti
ng
Reag
ents
Zn + 2MnO2 +H2O Zn(OH)2 + Mn2O3
• Determine the limiting reactant if 25.0g Zn and 30.0g MnO2 are used, Determine the mass of Zn(OH)2 produced.
• 0.77g moles of MnO2 required to react with 25.0g of Zn
• 0.17 moles of Zn required to react with 30.0g of Mn(OH)2
• Mn(OH)2 is the limiting reagent – excess of 0.21mol Zn (0.38-0.17 = 0.21)
• Use 30.0g to find the mass of Zn(OH)2
• 17.07g Zn(OH)2