Download Reactions Balancing Chemical Equations uses Law of conservation

Reactions
Chapter 8
Combustion
Decomposition
Combination
Chapter 9 Aqueous Reactions
Exchange reactions (Metathesis)
•  Formation of a precipitate
•  Formation of a gas
•  Formation of a week or nonelectrolyte
Single Displacement Reactions
•  Redox reactions
•  Oxidation numbers
Mary J. Bojan
Chem 110
1
Balancing Chemical Equations uses
Law of conservation of mass:
matter cannot be lost in any chemical reaction
Mary J. Bojan
Chem 110
2
Patterns of reactivity:
these are a way of keeping reactions organized.
They help us to predict products (and reactants).
• (Complete) Combustion reactions (Chapter 8)
C3H8(g) + 5O2 (g) ! 3CO2 (g) + 4H2O(g)
• Combination reactions (Chapter 8)
2Mg(s) + O2(!) ! 2MgO(s)
• Decomposition reactions (Chapter 8)
PbCO3(s) !PbO(s) + CO2(g)
Know how to balance a chemical reaction.
Mary J. Bojan
Chem 110
3
Write balanced reactions for
The combustion of ethanol CH3CH2OH
The decomposition of calcium carbonate CaCO3
The combination of oxygen and hydrogen to form water.
Mary J. Bojan
Chem 110
4
These are common patterns of reactivity for
aqueous solutions
• Exchange reactions (Chapter 9)
Precipitation
Pb(NO3)2(aq) + 2KI(aq) ! PbI(s) ! + 2KNO3(aq)
Neutralization
NaOH(aq) + HCl(aq) ! NaCl(aq) + H2O(!)
Gas formation
2HCl(aq) + Na2S(aq) ! H2S(g) " + 2NaCl(aq)
• Single displacement reactions (Chapter 9)
2Ca(s) + O2(g) ! 2CaO(s)
Know how to balance a chemical reaction
Mary J. Bojan
Chem 110
5
Double Displacement is where we exchange
positive ions to get products
AD + XZ ! AZ + XD
Called Exchange reaction
Example:
Reactants: Pb(NO3)2 + KI
Balance the reaction.
Does anything happen? TRY!
Do you have to do the reaction to know?
Mary J. Bojan
Chem 110
6
Metathesis reactions (or double displacement)
requires a driving force
(otherwise nothing happens)
Given reactants, exchange positive ions to get
products
Driving Forces
1.
2.
3.
Precipitate forms
Weak or nonelectrolyte forms
Gas forms
How do you know what is happening?
Net Ionic Equation
Mary J. Bojan
Chem 110
7
Formation of a precipitate is a driving force.
Precipitation Reaction
Mix silver nitrate and sodium chloride.
What happens?
Molecular Equation
Ionic Equation
Spectator ions:
Net Ionic Equation
Mary J. Bojan
Chem 110
8
SOLUBILITY RULES FOR COMMON IONIC
COMPOUNDS IN WATER
1.  Almost all ammonium and alkali metal salts are soluble.
2.  Most nitrates, acetates, chlorides, bromides, and
sulfates are soluble.
Exceptions: silver halides
sulfates of Ca,Ba,Pb,Ag
3.  Most sulfides, carbonates, phosphates and hydroxides
are insoluble.
Exceptions: alkali salts
ammonium salts.
(See #1 above.)
See Table on your data sheet.
Mary J. Bojan
Chem 110
9
Formation of Weak or Nonelectrolyte is a driving force
(neutralization reaction)
Acid + base ! salt + water
Molecular Equation
Ionic Equation
Spectator ions:
Net Ionic Equation
Mary J. Bojan
Chem 110
10
Practice
What is the net ionic equation for the reaction between sodium
hydroxide and acetic acid?
What is the net ionic equation for the reaction between sodium
chloride and potassium iodide?
Mary J. Bojan
Chem 110
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Gas formation is a driving force.
•  Direct production of a gas CO2, H2S, NO2, SO2
•  Production of weak acid which decomposes.
Example:
Molecular Equation
NaHCO3(aq) + HCl(aq) ! H2CO3(aq) + NaCl(aq)
!
CO2(g) + H2O(l)
Ionic Equation
Na+(aq) + HCO3!(aq) + H+(aq) + Cl!(aq) ! H2CO3 (aq)+ Na+(aq) + Cl!(aq)
!
+
Na (aq) + Cl! (aq) + CO2(g) + H2O(l)
Net Ionic Equation
H+(aq) + HCO3"(aq) ! CO2(g)+ H2O(l)
Mary J. Bojan
Chem 110
12
Single Displacement reactions involve
Oxidation and Reduction
Zn(s) + CuSO4(aq) ! ZnSO4(aq) +Cu(s)
Ionic equation:
Net ionic equation
What is oxidized? What is reduced?
oxidation
reduction
Mary J. Bojan
Chem 110
13
Rules for determining Oxidation States
1. 
2. 
3. 
4. 
5. 
Oxidation state of atom in elemental form is zero.
e.g. Cl2 O2 P4 C(s) S8
The oxidation number of a monatomic ion equals its charge.
Some elements have “common” oxidation numbers that can be used as
reference in determining the oxidation numbers of other atoms in the
compound.
Alkali metals
+1
Alkaline earth metals
+2
Fluorine
–1
H
usually +1
Hydrides: metal-H compounds (–1)
O
usually –2
peroxides (–1) & superoxides possible
Cl, Br, I almost always
–1
Sum of oxidation numbers is equal to overall charge of molecule or ion:
•  For a neutral compound the sum of oxidation numbers equals zero.
•  For a polyatomic ion, the sum of the oxidation numbers is equal to the
charge on the ion.
Shared electrons are assigned to the more electronegative atom of the pair:
more electronegative atom will have a negative oxidation number.
Mary J. Bojan
Chem 110
14
Activity Series
Table 9.6
Will the metal element displace the atom from its compound in
a reaction?
Na(s) + H2O(l) !
Au(s) + H2O(g) !
Zn(s) + AgNO3(aq) !
Active metals (groups IA, IIA, Al) very reactive!
Inactive (inert metals): jewelry metals Do not react readily
Other metals in between
Mary J. Bojan
Chem 110
15
Activity Series Table 9.6
Active
Inert
Potassium
K ! K+ + e "
Barium
Ba ! Ba2+ + 2e"
Calcium
Ca ! Ca2+ + 2e"
Sodium
Na ! Na+ + e"
Magnesium
Mg ! Mg2+ + 2e"
Aluminum
Al ! Al3+ + 3e"
Manganese
Mn ! Mn2+ + 2e"
Zinc
Zn ! Zn2+ + 2e"
Chromium
Cr ! Cr3+ + 3e"
Iron
Fe ! Fe2+ + 2e"
Cadmium
Cd ! Cd2+ + 2e"
Cobalt
Co ! Co2+ + 2e"
Nickel
Ni ! Ni2+ + 2e"
Tin
Sn ! Sn2+ + 2e"
Lead
Pb ! Pb2+ + 2e"
Hydrogen
H2 ! 2H+ + 2e"
Copper
Cu ! Cu2+ + 2e"
Silver
Ag ! Ag+ + e"
Mercury
Hg ! Hg2+ + 2e"
Platinum
Mary J. Bojan
Oxidation half reactions
Li ! Li+ + e"
Gold
Increasing ease of oxidation
Element
Lithium
Pt ! Pt2+ + 2e"
Chem 110Au ! Au3+ + 3e"
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Reactions: Stoichiometry
• 
• 
• 
• 
Solving problems involving Chemical reactions
Limiting Reactants
Solution reactions
Gas phase reactions
Mary J. Bojan
Chem 110
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Basics: before you start!
I will assume you know:
•  Avogadro’s number
memorize it!
•  definition of mole
•  Formula weight (molar mass)
•  the following connections (or
conversions)
gram ! mole
gram ! molecules
•  what is meant by:
Use these along with the
empirical formula
balanced chemical
molecular formula
reaction to solve
problems in chemistry
Mary J. Bojan
Chem 110
18
How to solve problems
•  write the balanced chemical reaction (or process).
•  make connections between experimentally measured
properties and the balanced equation.
In other words:
Given information such as mass, volume, pressure and
temperature, how can one determine quantities of
moles/molecules?
Mary J. Bojan
Chem 110
19
IMPORTANT CONNECTIONS
•  Avogadro’s number
Connects molecules (or atoms) to moles
Connects microscopic properties to macroscopic properties.
•  Conservation of Mass
Mass of Products = Mass of reactants
Balance the reaction
•  Balanced Chemical Equation
Connects moles (molecules) of reactants with moles (molecules) of
products (Think mole ratios…)
Related to conservation of Mass
•  Formula weight
Connects mass to moles
Connects a property that can be measured (determined) experimentally
to moles (or molecules) of substance given in the balanced reaction.
•  Empirical Formula
Tells relative number of atoms in a molecule
Obtained from %mass or molecular formula
Mary J. Bojan
Chem 110
20
Problem Solving Template
1. Write balanced chemical equation
2. 
Make a table: Fill in given information
3. 
Note connections between measured
quantities and moles
Mass
Volume
(solutions)
P, V, T
(gases)
M.W.
moles
concentration
ideal gas law
moles
moles
4.  Fill in table until you are able to solve the problem
5. Make sure your answer is REASONABLE
6. TRY things
Mary J. Bojan
Chem 110
21
Sample Problem 1
Methane reacts with water to produce hydrogen gas and carbon
monoxide. If 8.0g of methane reacts with 9.0 g of water, how
many grams of hydrogen gas will be produced?
Mary J. Bojan
Chem 110
22
Sample Problem 1
Methane reacts with water to produce hydrogen gas and carbon
monoxide. If 8.0g of methane reacts with 9.0 g of water, how
many grams of hydrogen gas will be produced?
CH4(g) +
H2O(g) "
3H2(g) +
CO(g)
Mass (g)
FW (g/mol)
Moles
Mary J. Bojan
Chem 110
23
Sample Problem 1
Methane reacts with water to produce hydrogen gas and carbon
monoxide. If 8.0g of methane reacts with 9.0 g of water, how
many grams of hydrogen gas will be produced?
CH4(g) +
H2O(g) "
Mass (g)
8.0
9.0
FW (g/mol)
16.0
18.0
3H2(g) +
2.0
CO(g)
28
Moles
Mary J. Bojan
Chem 110
24
If a clean burning engine burns 1 gallon of gasoline (assume
this is 2600 g of octane), how many kg of CO2 will it produce?
Octane is C8H18
Assume:
• 
• 
• 
• 
Gasoline is all octane
Density of octane = 0.692 g/mL
1 gal = 3.79 L = 3790 mL
3790 mL # 0.692 g/mL = 2622.68 g # 2600 g of octane
Mary J. Bojan
Chem 110
25
If a clean burning engine burns 1 gallon of gasoline (assume
this is 2600 g of octane), how many kg of CO2 will it produce?
Octane is C8H18
2 C8H18(g) +
25 O2(g) #
Mass (g)
2600
excess
FW (g/mol)
114.22
32.0
16 CO2(g) +
18 H2O(g)
???
44.01
18.02
Moles
Mary J. Bojan
Chem 110
26
Empirical formulas are determined by combustion
analysis.
Fuel + O2 ! CO2
+ H 2O
CxHyOz
Approach:
Find moles of C from moles of CO2.
Find moles of H from moles of H2O.
(N !N2 , S !SO2 , Cl ! Cl2 )
If oxygen is present in the fuel, all products and reactants will have O#
Use conservation of mass to find moles of O in sample (fuel).
Mass of sample = mass of C + mass of H + mass of O
MJ Bojan
Chem 110
27
4 g sample of an alcohol produces 7.65 g of CO2 and 4.70
g of H2O upon combustion.
What is the empirical formula of the alcohol?
CxHyOz(g) +
O2(g) #
CO2(g) +
H2O(g)
Mass (g)
4g
excess
7.65g
4.70g
FW (g/mol)
???
32.0
44.01
18.02
Moles
To get empirical formula need mole ratio( x:y:z) or
Moles C =
Moles H =
Moles O =
Mary J. Bojan
Chem 110
28
You try!
If 36.6 g of C2H5OH reacts with 63.8 g of O2, how many
grams of CO2 will be produced?
1.  26.0g
2.  43.2g
3.  58.5 g
4.  70.4 g
5.  100.4g
Mary J. Bojan
Chem 110
29
How do you know which reactant is the
limiting reactant?
Try to connect chemical reactions to something you know:
making a sandwich.
Example: 1 lb. of bread, $ lb of Ham, % lb. cheese
How many sandwiches can I make?
Bd +
bread
Hm + Ch !
+
ham + cheese =
Reactants
Bd Hm C
sandwich
How many Products?
slices of bread
slices of ham
slices of cheese
What is the limiting reagent?
What is left over?
Mary J. Bojan
Chem 110
30
Review limiting reagent problem
1.  Balance the reaction
2.  Test both reactants.
3.  Different ways to get the limiting reagent.
A.  Determine quantity of product
B.  Use mole ratio of reactants.
If 36.6 g of C2H5OH reacts with 63.8 g of O2, how
many grams of CO2 will be produced?
C2H5OH(g) +
Mass (g)
FW (g/mol)
36.6 g
46.1 g/mol
O2(g) "
CO2(g) +
H2O(g)
63.8g
32.0 g/mol
44.0 g/mol 18.0 g.mol
Moles
Mary J. Bojan
Chem 110
31
Not all reactions go to completion.
Theoretical yield = maximum amount of product that can
be obtained under the experimental conditions.
Actual yield = amount of product obtained.
actual yield
% yield =
! 100%
theoretical yield
Mary J. Bojan
Chem 110
32
A 20.2 g sample of magnesium nitride is reacted with
9.00 g of water and 15.3 g of MgO is recovered.
What is the percent yield of the reaction?
Mg3N2 (s) +
Mass (g)
FW (g/mol)
20.2 g
101. g/mol
3 H2O(!) "
2NH3(aq) +
9.00 g
18.0 g.mol
3MgO(s)
15.3 g
17.0 g/mol
40.3 g/mol
Moles
Mary J. Bojan
Chem 110
33
Problem Solving with solutions: Titration
Molarity is the connection between moles and volume in
solutions.
If 46.0ml of 0.100M NaOH is needed to neutralize 20.0 ml
of a H2SO4 solution what is the concentration of H2SO4?
Mary J. Bojan
Chem 110
34
Problem Solving with solutions: Titration
If 46.0ml of 0.100M NaOH is needed to neutralize 20.0 ml
of a H2SO4 solution what is the concentration of H2SO4?
H2SO4 (aq) +
NaOH(aq) "
Na2SO4 (aq) +
H2O(!)
Molarity (M)
Volume (L)
Moles
Mary J. Bojan
Chem 110
35
Problem Solving with gases
For a gas, PV=nRT is the link between the # of moles
(n) and the experimental quantities P,V, and T of gas.
Example: How many L of N2 at 735 mm Hg and 26°C are
produced from 126g of NaN3 (sodium azide)?
sodium azide
2NaN3(s) "
Mass (g)
126 g
FW (g/mol)
65
gas in an airbag
2 Na (s)
+ 3 N2(g)
P = 735 mmHg
T = 26 °C
V = ???
PV = nRT
Moles
Mary J. Bojan
P,V,T
moles
Chem 110
36