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Chapter 8:
Chemical Reactions
Chemical Reactions work like
mathematical formulas:
1 2  3
Na   Cl   NaCl
A reaction is balanced when the
left and right side have the same
number of each element
Recall Dalton’s Law of
Conservation of Mass
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Slide 1
Chemical Reactions
Products and Reactants
Back
• Compounds that react on the left and the
Compounds produced on the right.
compounds to react  compounds produced
reactants  products
Read as “yields”
•
Remember,
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reactants react to produce products
Slide 2
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Symbols and Notation PbI 2 ( s )
– compound is a solid
(l) – cmpd is a liquid
(g) – gas
(aq) – aqueous (dissolved in water,
exists as ions)
↓ - a precipitate has formed
• (s)
•
•
•
•
– (solid falling out of the reaction)
• ↑ - a gas is evolved
– (bubbling out of the reaction)
• Various things can be put over the “yields”
sign (→) to indicate special reaction

conditions.

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H 2O(l )
CH 4 ( g )
KI (aq )
PbI 2 
H2 
ca ta lyst
00 C
 
Slide 3
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Reaction Types
• There are five basic reaction types:
• Synthesis (Composition)
– two or more reactants produce a
single product.
• Decomposition
A  B  AB
A B C
– a single reactant breaks down into its
parts.
• Combustion
AB  A  B
– reaction between fuel(A) and oxygen
C  A B
to produce H2O and CO2.
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A  O2  CO2  H 2O
Slide 4
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Reaction Types
• Single Replacement (Displacement)
– One ion kicks one of the ions in the pair out.
AB  D  AD  B
AB  C  CB  A
• Double Replacement (Displacement)
– Ion pairs switch places (partners).
AB  CD  AD  CB
• Can a negative-negative compound exist?
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Slide 5
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Chemical Equations
• We express a chemical reaction with a chemical
equation.
• Shows relative number of products and reactants
required to satisfy the Law of Conservation of Mass.
• Our lab reaction:
Pb( NO3 ) 2 ( aq)  KI ( aq)  PbI 2 ( s )   KNO3( aq)
• Must be written:
Pb( NO3 ) 2( aq)  2KI( aq)  PbI 2( s )  2KNO3( aq)
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Slide 6
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Recall: Diatomic Molecules
• Some elements must
form a pair with itself.
• In chemical reaction,
these elements must be
written as a X2.
H 2O  2H  O ?
2H 2O  2H 2  O2
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Diatomic
Molecules
Hydrogen = H2
Nitrogen = N2
Oxygen = O2
Fluorine = F2
Chlorine = Cl2
Bromine = Br2
Iodine = I2
Slide 7
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Rules for Balancing Chemical Rxns
• Never Change Subscripts
• Balance Groups First
• Balance H2O, O2, and H2
last.
H 2 SO4
• We balance by adding H  O  H O
2
2
2
coefficients.
• Two reactant
2 H 2  O2  2 H 2O
oxygens
• Four product
hydrogens
• Properly balanced 2H 2  O2  2H 2O
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Slide 8
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Balancing Practice
• Balance the following reactions on your own:
4 Fe  3O2  2Fe2O3
Rust
2 Al  Fe2O3  Al2O3  2 Fe
Thermite
+ Energy
2 C2 H 2  5 O2  4 CO2  2 H 2O
Acetylene torch
+ Energy
C6 H12O6  6 O2  6 CO2  6 H 2O
Respiration
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+ Energy
Slide 9
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Mole Ratios
• Mole ratios:
– how many moles of products are produced
with given a number of moles of reactants.
• Here, the mole ratio is 1:1:2 (1H2 : 1Cl2 : 2 HCl)
• This means two moles of HCl will be produced
when one mole of H2 and one mole of Cl2 react.
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Slide 10
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Energy In and Out
• Recall that a reaction can be
Exothermic (releasing energy) or
• Endothermic (absorbing energy)
• Energy Barrier:
– Even exothermic reactions need a
little energy to get started
Energy
Barrier
• Activation Energy
– energy needed to get the
reaction going.
Exothermic
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Slide 11
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Energetic Reactions
• Exothermic Reaction
– energy acts like it is a product.
CH4  2O2  2H 2O  CO2  888 kJ / mol
• Endothermic Reaction
– energy acts like a reactant.
2H 2O(l )  571 kJ / mol  2H 2( g )  O2( g )
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Slide 12
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Activity Series
• Will an element replace
another element in a single
displacement reaction?
• More reactive elements
– placed high on the activity
series.
• Less reactive elements
– placed low on the series.
Activity Series
(reaction with acids and water)
Most Active
Li
Rb
K
React with cold H2O
and acids, replacing
hydrogen
Ba
Ca
Na
Mg
Al
React with acids and
steam, replacing
Mn
hydrogen
Zn
Cr
Fe
Ni
React with acid,
Sn
but not water
Pb
Halogen
H2
React with
Activity
Cu
oxygen
F2
Hg
Cl2
Ag
Mostly
Br2
Pt
Nonreactive
I2
Au
Least Active
When comparing reactions, the elements above will
replace (kick out) elements below in solutions.
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Slide 13
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Predict the Products
• Use the reactants and the reaction type to
determine the products…
Synthesis: ___ Al( s )  ___ O2( g )  __________ ( s )
Decomposition:
Combustion:

___ CuCl3 ______ ( g )  ______ ( s )
___ C2 H 6( g )  ___ O2( g )  ______ ( g )  ______ ( g )
Single Replacement:
Use Cu2+
___ Ag3 PO4( aq)  ___ Cu( s )  ______ ( s )  ______ ( aq)
Double Replacement:
___ MnBr7 ( s )  ___ Na2O( s )  ______ ( s )  ______ ( s )
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Slide 14
Ionic Cell Potentials (Later…)
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• Ionic solutions conduct electricity as charged ions flow
through a solution. (Electrolytes)
• We can measure the current that flows when certain
ions react in solution.
• Similar to the activity series, we can measure cell
potentials and create a table of electricity produced
when metals react with ions.
Table: Page 615
This is due to oxidation reduction:
when the oxidation number of a
metal changes and electrons flow.
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Slide 15