Download Stoichiometry

Q: Why do chemists call helium, curium and
barium the medical elements?
A: Because if you can't helium or curium, you
barium!
Stoichiometry
Ch. 12
The optimist sees the glass half full.
The pessimist sees the glass half
empty.
The chemist see the glass completely
full, half in the liquid state and half in
the vapor state.
Stoichiometry
• Chemists use balanced chemical equations as
a basis to calculate how much reactant is
needed or product is formed in a reaction
• Stoichiometry = the calculation of quantities
in chemical reactions
• It allows chemists to tally the amts. of
reactants and products using ratios of moles
or representative particles
Why are chemists great for solving
problems? They have all the solutions.
Interpreting Chemical Equations
• Number of Atoms
• N2(g) + 3H2(g) → 2NH3(g)
– 2 atoms of nitrogen and 6 of H on each side
• Number of Molecules
– 1 molecules nitrogen reacts w/3 molecules of hydrogen to make 2 molecules
of ammonia
• Moles
– Coefficients are moles
– 1 mole nitrogen, 3 moles hydrogen, 2 moles ammonia
– MOST IMPORTANT
• Mass
– Mass of 1 mol N2 (28 g) + mass of 3 mol H2 (6 g) equals the mass of 2 mol NH3
(34 g)
• Volume
– 22.4 L N2 reacts w/ 67.2L (22.4 x 3) H2 to form 44.8 L (22.4 x 2) NH3
• C.P. 12.1 pg. 358
• P.P. 3-4 pg. 358
Q: Why was the mole of oxygen molecules
excited when he walked out of the singles bar?
A: He got Avogadro's number!
Molar Ratios
• Molar ratio = a conversion factor derived from
the coefficients of a balanced chemical
equation interpreted in terms of moles
Q: What did one ion say to the other?
A: I've got my ion you.
Q: What did the bartender say when oxygen,
hydrogen, sulfur, sodium, and phosphorous
walked into his bar?
A: OH SNaP!
Mole-Mole Problems
• Mole-Mole problems = convert from moles of
one substance to moles of another
• 1. Write out and balance chem. equation
• 2. Determine molar ratios indicated by equation
• 3. Set up a dimensional analysis problem to
convert the # of moles from one substance to
moles of another
– Ex: How many moles of N2O & H2O are produced from
2.25 moles NH4NO3?
–*
– C.P. 12.2, pp 11-12 pg. 360
Solving Stoichiometry Problems
• HOW TO SOLVE
• Quantity of given convert to moles moles of given
Find molar ratio moles of unknown convert to desired units
quantity of unknown
• 3 Types
– 1. Mass-Mass
– 2. Mass-Volume
– 3. Volume-Volume
A small piece of sodium that lived in a test
tube fell in love with a Bunsen burner. "Oh
Bunsen, my flame," the sodium pined. "I melt
whenever I see you," The Bunsen burner
replied, "It's just a phase you're going
through."
Q: What do you call a clown who's in jail?
A: A silicon.
Stoichiometry Island Diagram
Known
Mass
Volume
1 mole = 22.4 L @ STP
Substance A
Unknown
Substance B
Mass
Use coefficients
from balanced
chemical equation
Mole
Mole
1 mole = 22.4 L @ STP
Volume
(gases)
(gases)
Particles
Particles
Stoichiometry Island Diagram
Mass-Mass Problems
• Mass-Mass problem = you are given the mass
of one substance and asked to find the mass
of another substance involved in the same
reaction
• Ex: Glucose is burned in oxygen to produce
carbon dioxide and water. What mass of
water is produced from 1.5 grams of glucose?
*
• SP 12.3, PP 13-14 pg. 361
• SP 12.4, PP 15-16 pg. 364
Mass-Volume Problems
• Mass-Volume Problem = you are given the
mass of one substance and asked to find the
volume of another substance
• Ex: An air bag contains 125 g of sodium azide.
What volume of nitrogen gas is produced at
STP? 2NaN3 → 2Na + 3N2
*
Old chemists never die, they just stop reacting.
Volume-Volume Problems
• Volume-Volume problems = you are given a
volume and asked to find a volume
• ***Coefficients represent the ratio of moles and
volume of gases both
• Ex: Ammonia synthesis N2 + 3H2 → 2NH3
Calculate volume of hydrogen that reacts w/ 15.5
L of nitrogen
*
• SP 12.5, PP 17-18 pg. 365
• SP 12.6, PP 19-20 pg. 366
Limiting Reactants(Reagents)
• Background: To mix pink paint you need a 3:1
ratio of white to red. You have 6 L of white & 1.5
L of red. How much pink paint can you make?
– 1.5 red + 4.5 white = 6.0 pink w/ 1.5 L white left
– You are limited by the amt. of red paint you have
– Most often reactants are available in
nonstoichiometric proportions
• Limiting Reactant(reagent) = reactant that
determines (limits) the amt. of product that can
be formed by a chem. Reaction
– Will be completely used up
– Others will have some unchanged or leftover (excess)
Limiting Reactants cont…
• The quantities of products formed in a reaction
are always determined by the quantity of the
limiting reactant
– Whether or not the reaction is limited depends upon
the molar ratios
• Identifying Limiting Reactants
• 1) write balanced chem. equation
• 2) calculate amt. of product formed by each
amt. of reactant
• 3) smaller answer indicates limiting reactant &
amt. of product
Method #2
• Begin by writing a correctly balanced chemical equation
• Write down all quantitative values under equation (include
units)
• Convert ALL reactants to units of moles
• Divide by the coefficient in front of each reactant
• The smallest value is the limiting reactant
Limiting Reactant Problems
• Ex: Determine limiting reactant in which 3.5 g
copper added to a solution containing 6.0 g
silver nitrate
– Cu +2AgNO3 → Cu(NO3)2 + 2Ag
• 3.5 g Cu(1 mol Cu/63.5 g Cu)(2 mol Ag/1 mol Cu)(108 g
Ag/1 mol Ag) = 12 g Ag ***We can choose either
product but Ag is easier molar mass***
• 6.0 g AgNO3(1 mol AgNO3/169.9 g AgNO3)(2 mol Ag/2
mol AgNO3)(108 g Ag/1 mol Ag) = 3.8 g Ag
• AgNO3 leads to the smallest amt. of Ag so AgNO3 is our
limiting reactant
Finding the Excess Reagent
• A 2.00 g sample of ammonia is mixed with 4.00 g of
oxygen. Which is the limiting reactant and how much excess
reactant remains after the reaction has stopped?
• First, we need to create a balanced equation for the reaction:
4 NH3(g) + 5 O2(g)4 NO(g) + 6 H2O(g)
• Next we can use stoichiometry to calculate how much product
is produced by each reactant. NOTE: It does not matter
which product is chosen, but the same product must be used
for both reactants so that the amounts can be compared.
Cont.
• The reactant that produces the lesser amount
of product in this case is the oxygen, which is
thus the "limiting reactant." Next, to find the
amount of excess reactant, we must calculate
how much of the non-limiting reactant
(ammonia) actually did react with the limiting
reactant (oxygen).
Cont.
• We're not finished yet though. 1.70 g is the
amount of ammonia that reacted, not what is
left over. To find the amount of excess
reactant remaining, subtract the amount that
reacted from the amount in the original
sample.
More Problems
• Ex: The fizz produced when some antacid tablets are
dropped into H2O is created by the production of CO2
during the reaction b/w sodium bicarbonate (NaHCO3)
& citric acid (H3C6H5O7). Suppose 2.0 g of NaHCO3 &
0.5 g of citric acid are present. What is the limiting
reactant and what volume of CO2 will be produced?
– 3NaHCO3 + H3C6H5O7 → 3CO2 + 3H2O + Na3C6H5O7
–*
– Some NaHCO3 is left over. The amt. is determined by
calculating the amt. of NaHCO3 would react w/ 0.5 g
H3C6H5O7 & subtracting the amt. from the starting amt.
–*
• SP 12.7, PP 25-26 pg. 370, SP 12.8, PP 27-28 pg. 371??
Percent Yield
• We learned how much product SHOULD be produced.
This is not always what we get.
• Reasons: 1) some may not react 2) some may be lost in
transferring 3) some may react w/ a side reaction
• Percent yield = the ratio of the actual yield to the
theoretical yield expressed as a percent
– The percent yield is a measure of the efficiency of a
reaction carried out in the laboratory
– Theoretical (expected) yield = max. amt. of product that
should be produced from given amts. of reactants based
on calculations
– Actual yield = amt. of product that is really obtained in the
lab from a chem. reaction
Percent Yield Problems
• Theoretical yield is determined by doing a
limiting reactant problem
• Theoretical yield probs: SP 12.9, PP 29-30 pg.
374
• Percent yield probs: SP 12.10, PP 31-32 pg.
375
Stoichiometry Practice
• How many grams of KClO3 are required to produce 9.00 L of
O2 at STP? (Decomp. KClO3)
• How many grams of silver will be formed from 12.0 g
copper?
– Cu + 2 AgNO3  2 Ag + Cu(NO3)2
• The compound diborane (B2H6) was at one time considered
for use as a rocket fuel. How many grams of liquid oxygen
would a rocket have to carry to burn 10 kg of diborane
completely? (The products are B2O3 and H2O).
• Solid aluminum react with chlorine gas to yield solid
aluminum chloride. If 125 g of both chlorine and aluminum
react, what is the limiting reactant and how much
aluminum chloride is produced.
2 TiO2 + 4 Cl2 + 3 C
CO2 + 2 CO + 2 TiCl4
How many moles of chlorine will react with 4.55 moles of carbon?
x mol Cl2 = 4.55 mol C
4 mol Cl2
3 mol C
= 6.07 mol C
How many grams of titanium (IV) oxide will react with 4.55 moles of carbon?
x g TiO2 = 4.55 mol C
2 mol TiO2
3 mol C
80 g TiO2
1 mol TiO2
= 243 g TiO2
How many molecules of TiCl4 will react with 115 g TiO2?
x molecules TiCl4 = 115 g TiO2
1 mol TiO2
80 g TiO2
2 mol TiCl4
2 mol TiO2
6.02x1023 molecules TiCl4
1 mol TiCl4
= 8.66x1023 molecules TiCl4
Limiting Reactant Problems
1. According to the balanced chemical equation, how many atoms of silver will be produced
from combining 100 g of copper with 200 g of silver nitrate?
2.
At STP, what volume of “laughing gas” (dinitrogen monoxide) will be produced from 50 g
of nitrogen gas and 75 g of oxygen gas?
3.
Carbon monoxide can be combined with hydrogen to produce methanol, CH3OH.
Methanol is used as an industrial solvent, as a reactant in some synthesis reactions,
and as a clean-burning fuel for some racing cars. If you had 152 kg of carbon monoxide
and 24.5 kg of hydrogen gas, how many kilograms of methanol could be produced?
4. How many grams of water will be produced from 50 g of hydrogen and 100 g of oxygen?
Answers:
1. 7.1 x 1023 atoms Ag
2. 40 dm3 N2O
3. 174.3 kg CH3OH
4. 112.5 g H2O
% Yield Practice
• When 45.8 g of K2CO3 react with excess HCl,
46.3 g of KCl are formed. Calculate the
theoretical and % yields of KCl.