Transcript Document 7682928

Stoichiometry
Stoichiometry: study of the quantitative relations
between amounts of reactants and products.
Goals:
1. Perform stoichiometry calculations.
2. Understand the meaning of limiting reactant.
3. Calculate theoretical and percent yields of a
chemical reaction.
4. Use stoichiometry to analyze a mixture of
compounds or to determine the formula of a
compound.
5. Define and use molarity in solution stoichiometry.
6. Perform calculations for pH and titration
problems.
What is STOICHIOMETRY?
The study of the quantitative aspects of
chemical reactions.
It rests on the principle of the _________
________________.
2 Al(s) + 3 Br2(liq) ------> Al2Br6(s)
You must always begin with a balanced equation
before carrying out a stoichiometry calculation.
Write a Chemical Equation
• Phosphine, PH3 (g), combusts in oxygen
gas to form gaseous water and solid
tetraphosphorus decoxide.
Always check (and REcheck) the balancing
Information from a Balanced
Equation
Equation: 2 H2 (g) + O2 (g)
2 H2O (l)
Molecules: 2 molecules H2 + 1 molecule O2
2 molecules H2O
Mass (amu): 4.0 amu H2 + 32 amu O2
36.0 amu H2O
Amount (mol):
Mass (g):
General Plan for Stoichiometry
Calculations
Students should become familiar with
stoichiometry calculations.
Mole relationships in Chemical
Equations
• Stoichiometric factor – relates the amounts
of any two substances involved in a chemical
reaction, on a mole basis.
C3H8 + 5 O2
propane
3 CO2 + 4 H2O
5 mol of O2 are required to burn ____ mol of C3H8
____ mol of H2O are produced for every 1 mol of C3H8 burned
____ mol of CO2 are produced for every 1 mol of C3H8 burned
If 454 g of NH4NO3 decomposes, how much N2O
and H2O are formed? What is the theoretical yield
of products?
STEP 1 Write the balanced chemical equation.
STEP 2 Convert mass of reactant to moles of reactant.
(454 g) --> moles
STEP 3 Convert moles reactant to moles product.
A. Relate moles using coefficients, write a
STOICHIOMETRIC FACTOR.
STEP 3
B) Multiply moles of reactant by the stoichiometric
factor.
STEP 4 Convert moles product to mass product =
___________________ YIELD
STEP 5 How much N2O is formed?
Total mass of reactants =
total mass of products
STEP 6 Calculate the percent yield.
If you isolated only 131 g of N2O, what is the percent
yield?
Actual yield
% yield =
* 100
Theoretical yield
Students should become familiar with % yield
calculations.
What is a Limiting Reactant?
• In a given reaction, there is not enough
of one reagent to use up the other
reagent completely.
• The reagent in short supply _______
the quantity of product that can be
formed.
Which is the Limiting Reactant?
2 NO (g) + O2 (g) - - -> 2 NO2 (g)
Reactants
• Limiting reactant is
• Excess reactant is
Products
C3H8 +
O2
CO2 +
H2O
• How many grams of CO2 are produced from 50 g of propane?
(theoretical yield)
• How much O2 is required to completely burn the 50 g of propane?
• How much H2O is formed?
50 g C3H8 (1 mol/ 44 g) = 1.14 mol C3H8 * (4 mol H2O / 1 mol C3H8)
• If the actual yield of CO2 was 145 g. What is the % yield of the
reaction?
C3H8 +
O2
CO2 +
H2O
• If you begin with 99.5 g of C3H8 and 211 g of O2, which one is the
limiting reactant?
• Assuming all the limiting reactant has reacted, how much CO2 will
be formed?
Limiting Reactant
(See CD Screen 4.8)
React solid Zn with 0.100 mol HCl (aq)
Zn + 2 HCl ---> ZnCl2 + H2
1
2
3
Rxn 1: Balloon inflates fully, some Zn left
* More than enough Zn to use up the 0.100 mol HCl
Rxn 2: Balloon inflates fully, no Zn left
* Right amount of each (HCl and Zn)
Rxn 3: Balloon does not inflate fully, no Zn left.
* Not enough Zn to use up 0.100 mol HCl
Zn + 2 HCl ---> ZnCl2 + H2
0.10 mol of HCl, need ? mol Zn.
0.10 mol HCl 1 mol Zn
2 mol HCl
= 0.050 mol Zn
1
2
3
mass Zn (g)
mol Zn
mol HCl
mol HCl/mol Zn
Lim Reactant
Rxn 1
7.00
0.107
0.100
Rxn 2
3.27
0.050
0.100
Rxn 3
1.31
0.020
0.100
Mix 5.40 g of Al with 8.10 g of Cl2. What
mass of Al2Cl6 can form?
___Al + ____Cl2 ---> Al2Cl6
Mass
product
Mass
reactant
Moles
reactant
Stoichiometric
factor
Moles
product
Mix 5.40 g of Al with 8.10 g of Cl2. What
mass of Al2Cl6 can form?
___ Al + ___Cl2 ---> Al2Cl6
STEP 1 FIND THE LIMITING REAGENT.
Compare actual mole ratio of reactants to
theoretical mole ratio.
Students should become familiar with calculations
using the concept of limiting reagent.
Mix 5.40 g of Al with 8.10 g of Cl2. What
mass of Al2Cl6 can form?
Calculate moles of each reactant.
Find the mole ratio of reactants:
Mix 5.40 g of Al with 8.10 g of Cl2. What
mass of Al2Cl6 can form?
__ Al + ___ Cl2 ---> Al2Cl6
Limiting reactant = _____
BASE ALL CALCULATIONS on LR ____
mass
____
mass
Al2Cl6
Write conversion factor:
moles
____
moles
Al2Cl6
Mix 5.40 g of Al with 8.10 g of Cl2. What
mass of Al2Cl6 can form?
__ Al + __ Cl2 ---> Al2Cl6
STEP 2 CALCULATE THE MASS OF THE
PRODUCT.
Calculate moles of Al2Cl6 expected based on LR.
Mix 5.40 g of Al with 8.10 g of Cl2.
How much of each reactant will remain
when reaction is complete?
__ Al + __ Cl2 ---> Al2Cl6
Chemical Analysis
• An impure sample of the mineral thenardite contains
Na2SO4. A mass of mineral sample weights 0.123 g.
The Na2SO4 in the sample is converted to insoluble
BaSO4 by adding BaCl2. The recovered mass of
BaSO4 is 0.177 g. What is the mass percent of
Na2SO4 in the mineral?
Chemical Analysis
General Plan for Stoichiometry
Calculations
Chemical Analysis
Balanced equation:
Grams to moles
Moles to moles
Moles to grams
Mass %
Combustion Analysis of
Hydrocarbons
Active Figure 4.9
Procedure for Calculating
Empirical Formula
Grams
of each
element
Use
Molar
mass
Moles
of each
element
Calculate
mole ratio
Empirical
Formula
The central part of the calculation is determining the
number of moles of each element in the compound.
Remember: in the mole ratio, divide by smaller
number, then multiply ‘til whole.
What is the empirical formula of a hydrocarbon,
CxHy, if 0.115 g burn and produce 0.379 g of CO2
and 0.1035 g of H2O.
CxHy + some oxygen --->
0.379 g CO2 + 0.1035 g H2O
First, recognize that all C in CO2 and all H in
H2O is from CxHy.
1. Calculate amount (in moles) of C in CO2
2. Calculate amount (in moles) of H in H2O
3. Find ratio of mol H/mol C to find values of x
and y in CxHy.
What is the empirical formula of a hydrocarbon,
CxHy, if 0.115 g burn and produce 0.379 g of CO2
and 0.1035 g of H2O.
1. Calculate amount (moles) of C in CO2
2. Calculate amount (moles) of H in H2O
3. Ratio of mol H/mol C to find values of x and y in CxHy.
Practice
4.67 Titanium (IV) oxide, TiO2, is heated in hydrogen
gas to give water and a new titanium oxide, TixOy. If
1.598 g of TiO2 produces 1.438 g of TixOy, what is the
formula of the new oxide?
TiO2 + H2
1. Calculate amount of Ti:
H2O + TixOy
Practice…
2. Calculate amount of O:
3. Calculate the molar ratio of O to T
How are Reactions in Solution
Quantified?
In solution we need to define the
• SOLVENT
the component whose
physical state is
____________ when
solution forms
• SOLUTE
the other solution component
What is Molarity?
The amount of solute in a solution is
given by its concentration.
Molarity =
Students should become familiar with
calculations using MOLARITY.
Calculate molarity of a solution of 5.00 g of NiCl2•6 H2O
dissolved in enough water to make 250 mL of solution.
STEP 1 Calculate the number of moles of
solute
STEP 2 Calculate the molarity of the solution
How many IONS are in the Solution?
What mass of oxalic acid, H2C2O4, is required to
make 250 mL of a 0.0500 M solution?
STEP 1 Calculate moles of acid required.
STEP 2 Calculate mass of acid required.
Preparing Solutions
• Weigh out a solid solute
and dissolve in a given
quantity of solvent.
• Dilute a concentrated
solution to give one
that is __________
concentrated.
Preparing Solutions
You have 50.0 mL of 3.0 M NaOH and you
want 0.50 M NaOH. What do you do?
Add water to the 3.0 M solution to
lower its concentration to 0.50 M
Dilute the solution!
M = moles/L
Notice that the amount of NaOH (moles of NaOH)
present did not change.
You have 50.0 mL of 3.0 M NaOH and you
want 0.50 M NaOH. What do you do?
But how much water do we add?
moles of NaOH in ORIGINAL solution =
moles of NaOH in FINAL solution
Amount of NaOH in original sol. =
Amount of NaOH in final sol. must also =
Volume of final solution =
You have 50.0 mL of 3.0 M NaOH and you
want 0.50 M NaOH. What do you do?
Conclusion:
add ______ of water to the 50.0 mL of 3.0 M NaOH
to make ________ of 0.50 M NaOH.
A shortcut:
Cinitial • Vinitial = Cfinal • Vfinal
• Principle of dilution: addition of solvent does not change the
amount of solute in a solution but does change its concentration.
Solution Stoichiometry
• Zinc reacts with acids
to produce H2 gas.
• Have 10.0 g of Zn
• What volume of 2.50 M
HCl is needed to
convert the Zn
completely?
General Plan for Solution
Stoichiometry
M = moles / volume
Moles = M * volume
Zinc reacts with acids to produce H2 gas. If
you have 10.0 g of Zn, what volume of 2.50 M
HCl is needed to convert the Zn completely?
Stoichiometric
factor
Zinc reacts with acids to produce H2 gas. If you have
10.0 g of Zn, what volume of 2.50 M HCl is needed to
convert the Zn completely?
Step 1: Write the balanced equation
Step 2: Calculate amount of Zn in moles
Step 3: Use the stoichiometric factor
Zinc reacts with acids to produce H2 gas. If you have
10.0 g of Zn, what volume of 2.50 M HCl is needed to
convert the Zn completely?
Step 4: Calculate volume of HCl req’d
* Use molarity (M) as conversion factor
What is pH?
• It is a concentration scale.
• pH: a way to express acidity – the concentration
of H+ in solution.
Low pH: ______ [H+]
Acidic solution
Neutral
Basic solution
High pH: ____ [H+]
pH < 7
pH = 7
pH > 7
The pH Scale
pH = log (1/ [H+])
= - log [H+]
In a neutral solution,
[H+] = [OH-] = 1.00 x 10-7 M at 25 oC
pH = - log [H+] = -log (1.00 x 10-7)
= - [0 + (-7)]
= 7
See CD Screen 5.17 for a tutorial
See book Appendix A.3 for more on logs
pH and [H+]
If the [H+] of soda is 1.6 x 10-3 M,
the pH is ____?
pH and [H+]
If the pH of Coke is 3.12, what is the
[H+]?
What is a Titration?
• Titration – procedure in which two reactants
in solution react in the precise proportions
shown by the chemical equation for the
reaction.
• Buret – a calibrated instrument used in a
titration. It is a graduated, long glass tube
calibrated to deliver precise volumes of
solution through a stopcock valve.
• Equivalence point – the point in a titration at
which one reactant has been exactly
consumed by addition of the other reactant.
Acid-Base Titration
• How do we measure the concentration of
an acid in a solution?
1) A measured volume of a solution of an acid
of unknown concentration is transferred to a
flask.
2) A solution of a base of known concentration
is added carefully from a buret until the
reaction of the acid with the base is just
complete.
3) Equivalence point of the titration – the point
at which the acid is just neutralized. At that
point, the number of moles of OH- added
equals the number of moles of H+ that were
in the sample of acid.
4) The equivalence point is determined with an
indicator dye – a substance that changes color
as the reaction is completed (litmus,
phenolphthalein).
What is a Titration?
Phenolphthalein indicator
1.065 g of H2C2O4 (oxalic acid) requires 35.62 mL of
NaOH for titration to an equivalence point. What is the
concentration of the NaOH?
Step 1:
Step 2:
Step 3:
1.065 g of H2C2O4 (oxalic acid) requires 35.62 mL of
NaOH for titration to an equivalence point. What is the
concentration of the NaOH?
Practice
What volume of 0.155 M FeCl3 contains 12.5 g FeCl3?
Apples contain malic acid, H2C4H4O5.
76.80 g of apple requires 34.56 mL of 0.663 M NaOH
for titration. What is weight % of malic acid?
__H2C4H4O5(aq) + __NaOH(aq) --->
__Na2C4H4O5(aq) + __H2O(l)
?
76.80 g of apple requires 34.56 mL of 0.663 M NaOH
for titration. What is weight % of malic acid?
Step 1:
Step 2:
Step 3:
76.80 g of apple requires 34.56 mL of 0.663 M NaOH
for titration. What is weight % of malic acid?
Step 4:
Step 5:
Remember
• Go over all the contents of your
textbook.
• Practice with examples and with
problems at the end of the chapter.
• Practice with OWL tutor.
• Work on your assignment for OWL
Chapters 4 and 5.
• Practice with the quiz on CD of
Chemistry Now.