Transcript Chemical Equations and Stoichiometry

Stoichiometry
Chapter 11
11.1 Stoichiometry

Stoichiometry is the study of quantitative
relationships between the amounts of
reactants used and amounts of products
formed by a chemical reaction.
– 2 slices of bread + 1 slice of ham  1 ham sandwich

How do you figure out a chemical equation’s recipe?
N2 + 3H2  2NH3
This 1:3:2 ratio can symbolize numbers of molecules or moles.
Mole Relationships


Mole Ratio: a ratio between the numbers
of moles of any two of the substances in
a balanced chemical equation.
What are all of the possible mole ratios in the
equation?
– 4Al(s) + 3O2(g) → 2Al2O3(s)
Mole/Mole Stoichiometry

Moles of known x mole ratio = moles of unknown
substance
substance
– Mole ratio = moles of unknown (coefficient)
moles of known (coefficient)
2 slices of bread + 1 slice of ham  1 ham sandwich
If you have 10 slices of bread how many slices of ham are needed?
Mole/Mole Stoichiometry


Find the quantities of reactants and
products in chemical reactions
Use the mole ratio to convert what
you have to what you want to find
moles(coefficient )unknown
molesknown 
 molesunknown
moles(coefficient )known
Mole/Mole Stoichiometry

How many moles of aluminum are needed
to form 3.7 moles of aluminum oxide?
____Al + ____O2 → ____Al2O3

How many moles of Al2O3 are formed when
0.78 moles of O2 reacts with aluminum?
11.2 Calculations
Moles to Mass



Given the number of moles of a reactant or
product, we can calculate the mass of
another reactant or product.
Use the mole ratio to convert from what you
have to what you want to find.
Use the molar mass of what you want to
find to convert from moles to grams.
moles(coef ficient) un known molar mass un known
moles known 

moles(coef ficient) known
1 mole un known
Moles to Mass Practice
1TiO2 + 1C + 2Cl2  1TiCl4 + CO2
.25 mol
?g
2NaCl  2Na + 1Cl2
2.50 mol
?g
Mass to Mass



Given the number of grams of a reactant or
product, we can calculate the number of grams of
another reactant or product.
Use molar mass of what you have to convert it to
moles.
The rest of the process is the same:
– Use mole ratio to convert from what you have to what
you need find.
– Use molar mass of what you need find to convert your
answer from moles to grams.
grams known 
1 mole known
moles(coef ficient) un known molar mass un known


molar mass known
moles(coef ficient)kn own
1 mole un known
Mass to Mass Practice
2NaN3  2Na + 3N2
100.0 g
?g
2SO2 + 1 O2 + 2H2O  2H2SO4
2.50 g
?g
Volume and Particle
Relationships


How many molecules of oxygen are
produced by the decomposition of
6.54g of potassium chlorate?
How many liters of oxygen are
required to burn 3.86L of carbon
monoxide?
Mass Relationships


must obey the law of conservation of
mass (moles aren’t equal, but mass
must be the same on both sides)
What mass of oxygen, O2, is required
to completely combust 454 g of
propane, C3H8? What masses of CO2
and H2O are produced?
C3H8 + 5O2  3CO2 + 4H2O
More Mass Practice…



CaC2 + 2H2O  C2H2 + Ca(OH)2
How many grams of C2H2 are
produced by adding water to 5.00g
CaC2?
How many moles of CaC2 are needed
to react completely with 49.0g H2O?
11.3 Limiting Reactants

A limiting reactant determines the amount
of product formed.
– 1. Start with a balanced chemical equation.
– 2. Identify what you have for each reactant.
– 3. Convert what you have to moles.
– 4. Calculate the ratio of available moles of each
reactant. (what you have)
– 5. Use the balanced chemical equation to
calculate the ratio of moles of each reactant
(what you need)
– 6. For each reactant, do you have more or less
than what you need?
1.
2.
3.
4.
5.
Limiting Reactant
6Na + 1Fe2O3  3Na2O + 2Fe
100.0 g 100.0 g
. 100.0 g Na  1 mole Na  4.35 moles Na
22.99 g Na
1 mole Fe2O3
. 100.0 g Fe2O3  159.7 g Fe O  0.62 moles Fe2O3
2 3
. 4.35 mol Na
6.94 mol Na

( HAVE )
0.62 mol Fe2O 3 1 mol Fe2O3
6 mol Na
( NEED)
1 mol Fe 2 O 3
6.
We have 6.94 mol Na; we need 6 mol. Na is in
excess. We have 1 mol Fe2O3; we need 1 mol.
Fe2O3 will run out first.
Limiting Reactants



C2H4 +3O2  2CO2 + 2H2O
If 2.70mol C2H4 is reacted with 6.30
mol O2; identify the limiting reactant.
Identify the limiting reactant if 2.70
mol C2H4 is reacted with 6.30 mol O2.
Limiting Reactants
Pure silicon, required for computer
chips and solar cells, is made by the
reaction
SiCl4 + 2Mg  Si + 2MgCl2
If you begin with 225 g each of SiCl4
and Mg, which is the limiting reactant
in this reaction? What quantity of Si,
in grams, can be produced?

11.4 Percent Yield

The ratio of the actual yield (formed in
lab) to the theoretical yield (max
amount that could be formed)
actual yield (from an experiment )
percent yield 
100
theoretica l yield (from stoichiome tric calculatio ns)
Percent Yield

When 84.8g of iron (III) oxide reacts
with an excess of carbon monoxide,
54.3g of iron is produced. What is the
percent yield?
Percent Yield
Methanol, CH3OH, can be burned in oxygen to
provide energy, or it can be decomposed to
form hydrogen gas, which can then be used
as a fuel.
CH3OH  2H2 + CO
If 125 g of methanol is decomposed, what is
the theoretical yield of hydrogen? If only
13.6 g of hydrogen is obtained, what is the
percent yield of the gas?