Transcript Chapter 7

Chapter 10
Chemical
Reactions
1
All Chemical Reactions
have two parts
 Reactants - the substances you start
with
 Products- the substances you end up
with
 The reactants turn into the products.
 Reactants  Products
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In a Chemical Reaction
The way atoms are joined is changed
 Atoms aren’t created or destroyed.
 Can be described several ways
1. In a sentence
Copper reacts with chlorine to form
copper (II) chloride.
2. In a word equation
Copper + chlorine  copper (II) chloride
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Symbols Used in Equations
the arrow separates the reactants
from the products
 The arrow = “reacts to form”
 The plus sign = “and”
 (s) after the formula = solid
 (g) after the formula = gas
 (l) after the formula = liquid
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Symbols Used in Equations
(aq) after the formula = dissolved in
water, an aqueous solution.
 used after a product indicates a gas
(same as (g))
 used after a product indicates a
solid (same as (s))
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Symbols Used in Equations
indicates a reversible

reaction

heat
  ,    shows that heat
is supplied to the reaction
Pt
   is used to indicate a
catalyst is supplied, in this case,
platinum.
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What is a Catalyst?
A substance that speeds up a
reaction without being changed by
the reaction.
 Enzymes are biological or protein
catalysts.
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Everyday Chemical Reactions
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Diatomic Elements
There are 7 elements that never want
to be alone.
 They form diatomic molecules.
 H2 , N2 , O2 , F2 , Cl2 , Br2 , and I2
 The –ogens and the –ines
 1 and 7 pattern on the periodic table
 Dr. Brinclhof
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1 + 7 Pattern of Diatomics
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Third Type - Skeleton Equation
Uses formulas and symbols to
describe a reaction
 Skeleton doesn’t indicate how many
of each compound is used
 Change this word equation into a
skeleton equation - Iron(s) +
chlorine(g)  iron(III) chloride(s)
 Fe(s) + Cl2(g)  FeCl3(s)
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Convert These to Skeleton Equations
Solid iron (III) sulfide reacts with
gaseous hydrogen chloride to form
solid iron (II) chloride and hydrogen
sulfide gas.
 Nitric acid dissolved in water reacts
with solid sodium carbonate to form
liquid water and carbon dioxide gas
and sodium nitrate dissolved in
water.
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From Skeleton to Word Eq.
Fe(s) + O2(g)  Fe2O3(s)
 Cu(s) + AgNO3(aq) 
Ag(s) + Cu(NO3)2(aq)
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Balancing Chemical Equations
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Balanced Equation
Atoms can’t be created or destroyed
 All the atoms we start with, we must
end up with
 A balanced equation has the same
number of each element on both
sides of the equation.
 Inventory polyatomics as individual
atoms
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C
+
O
O

O C
O
C + O2  CO2
 This equation is already balanced
 What if it isn’t already?
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C
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+
O
O

C
O
C + O2  CO
We need one more oxygen in the
products.
Can’t change the formula, because they
follow the Law of Definite Proportions.
C
+
O
O

C
O
C
O
Must be used to make another CO
 But where did the other C come
from?
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C
+
C
O
O

Must have started with two C
 2 C + O2  2 CO
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C
O
C
O
Rules for Balancing
 Write the correct chemical formulas
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5.
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for all the reactants and products
Count the number of atoms of each
type appearing on both sides
Balance the elements one at a time
by adding coefficients (the numbers
in front) Save H and O until last!
Check to make sure it is balanced.
Simplify coefficients if possible
Never, Never, Never
Change a subscript to balance an
equation.
 If you change the formula you are
describing a different reaction.

H2O is a different compound than H2O2
 Never put a coefficient in the middle of
a formula
i.e. 2NaCl is okay, Na2Cl is not.
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Example
H2 + O2  H2O
Make a table to keep track of where you
are at
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Example
H2 + O2  H2O
R
P
2 H 2
2 O 1
Need twice as much O in the product
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Example
H2 + O2 
R
P
2 H 2
2 O 1
Changes the O
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2 H2O
Example
H2 + O2 
2 H2O
R
P
2 H 2
2 O 1 2
Also changes the H
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Example
H2 + O2 
2 H2O
R
P
2 H 2 4
2 O 1 2
Need twice as much H in the reactant
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Example
2 H2 + O2 
2 H2O
R
P
2 H 2 4
2 O 1 2
Recount
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Example
2 H2 + O2 
2 H2O
R
P
4 2 H 2 4
2 O 1 2
The equation is balanced, has the same
number of each kind of atom on both sides
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Example
2 H2 + O2 
2 H2O
R
P
4 2 H 2 4
2 O 1 2
This is the answer
Not this
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Techniques
If an atom appears more than once on
a side, balance it last.
 If you fix everything except one
element, and it is even on one side and
odd on the other, double the first
number, then move on from there. If
this is the last element to balance, then
double all the coefficients.
 C4H10 + O2  CO2 + H2O
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Balance These Examples
CH4 + O2  CO2 + H2O
 AgNO3 + Cu  Cu(NO3)2 + Ag
 Mg + N2  Mg3N2

P + O2  P4O10
 Na + H2O  H2 + NaOH
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Try to Balance These
a) Pb(NO3)2 + K2CrO4  PbCrO4 + KNO3
b) MnO2 + HCl  MnCl2 + H2O+ Cl2
c) C3H6 + O2 CO2 +H2O
d) Zn(OH)2 + H3PO4  Zn3(PO4)2 + H2O
e) CO + Fe2O3 Fe + CO2
f) CS2 + Cl2 CCl4 +S2Cl2
g) CH4 + Br2  CH3Br + HBr
h) Ba(CN)2 + H2SO4  BaSO4 + HCN
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Five Types of Reactions
And Predicting the Products
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Types of Reactions
There are millions of reactions.
 Can’t remember them all
 Fall into several categories.
 We will learn 5 types. Movie
 Will be able to predict the products.
 For some, we will be able to predict
whether they will happen at all.
 Will recognize them by the reactants
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#1 Synthesis Reactions
Synthesis or Combination Reaction –
To put together
 2 elements, or compounds combine
to make one compound.
 A + B AB is the general formula
 Ca + O2 CaO

SO3 + H2O  H2SO4
 We can predict the products if there
are only two elements.
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#2 Decomposition Reactions
decompose = break apart
 one reactant breaks apart into two or
more elements or compounds.
 AB A + B is the general formula
electricity
 NaCl   
 Na + Cl2

 CaCO3   CaO + CO2
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#2 Decomposition Reactions
You can predict the products if it is a
binary compound
 Made up of only two elements
 Falls apart into its elements
electricity

 H2O   

 HgO  
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#2 Decomposition Reactions
If the compound has more than two
elements, you must be given one of
the products
 The other product will be from the
missing pieces

 NiCO3   NiO +
 H2CO3(aq)H2O +
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#3 Single Replacement
One element replaces another
 Reactants must be an element and a
compound.
 Products will be a different element
and a different compound.
 A + BX AX + B is general formula
 K + NaCl  Na + KCl
 F2 + LiCl  LiF + Cl2
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#3 Single Replacement
Metals replace metals (and hydrogen)
 K + AlN 
 Zn + HCl 
 Think of water as HOH
 Metals replace one of the H, combine
with hydroxide.
 Na + HOH 
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#3 Single Replacement
We can tell whether a reaction will
happen
 Some are more active than other
 More active replaces less active
 There is a list on your handout
 Higher on the list replaces lower
 If the lone element is higher, it
replaces compound- if lower, it doesn’t
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Activity Series
The more active element
will replace the less active
element in a chemical
reaction. The K atom
replaced Al in the
previous example
 The same is true for the
more active halogens
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#3 Single Replacement
H can be replaced in acids by
everything higher – Pb and above
 Only the first 5 (Li - Na) react with water.
 Fe + CuSO4 
 Pb + KCl 
 Al + HCl 
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#3 Single Replacement
What does it mean that Au And Ag are
on the bottom of the list?
 Nonmetals can replace other nonmetals
 Limited to F2 , Cl2 , Br2 , I2
 The order of activity is that on the table.
 Higher replaces lower.
 F2 + HCl 
 Br2 + KCl 
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#4 Double Replacement
Two things replace each other.
 Reactants must be two ionic
compounds or acids.
 Usually in aqueous solution
 AX + BY  AY + BX is general formula
 NaOH + FeCl3 
 Two positive ions (cation) change place
+3 OH- + Na+1Cl-1
 NaOH + FeCl3 Fe
 NaOH + FeCl3 Fe(OH)3 + NaCl
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Complete and Balance
assume all of the reactions take
place.
 CaCl2 + NaOH 
 CuCl2 + K2S 
 KOH + Fe(NO3)3 
 (NH4)2SO4 + BaF2 
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How to Recognize Reaction
Look at the reactants
E+E
Synthesis or Combination
C
Decomposition
 E + C Single replacement
 C + C Double replacement
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Examples
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H2 + O2 
H2O 
Zn + H2SO4 
HgO 
Mg(OH)2 + H2SO3 
KBr +Cl2 
AgNO3 + NaCl 
Synthesis
Decomposition
Single Replacement
Decomposition
Double Replacement
Single Replacement
Double Replacement
#5 Combustion
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A compound typically composed of only C
H and maybe O is reacted with oxygen.
Movie – ethanol popper
If the combustion is complete, the
products will be CO2 and H2O.
If the combustion is incomplete, the
products will be CO and H2O.
Need for oxygen Movie
Review Movie - 5 types of reactions
Examples
C4H10 + O2  (complete)
 C4H10 + O2  (incomplete)
 C6H12O6 + O2  (complete)
 C8H8 +O2  (incomplete)
 Movie – methane mamba
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Distinguish between complete and incomplete
combustion.
Write a balanced equation for the complete
combustion of each of these compounds.
a) acetic acid, HC2H3O2 c) glycerol, C3H8O3
b) decane, C10H22
d) sucrose, C12H22O11
Write a balanced equation for the incomplete
combustion of each of these compounds.
a) glycerol, C3H8O3
c) acetic acid, HC2H3O2
b) glucose, C6H12O6
d) acetylene, C2H2
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Reaction Types and Balancing
Equations Summary
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A Chemical Equation
Describes a reaction
 Must be balanced because of the
Law of Conservation of Mass
 Can only be balanced by changing
the coefficients.
 Has special symbols to indicate
state, and if catalyst or energy is
required.
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Chemical Reactions
Come in 5 main types.
 Can often tell what type they are by the
type of reactants.
 Single Replacement happens based on
the activity series using activity chart.
 Double Replacement typically happens
if the product is a solid, water, or a
gas.
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The Process
Determine the type by looking at the
reactants.
 Put the pieces next to each other
 Use charges to write the formulas
 Use coefficients to balance the
equation.
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Homework
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Section 10.3
Reactions in Aqueous Solution
 OBJECTIVES:
–Write and balance net ionic
equations.
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Net Ionic Equations
 Many
reactions occur in water- that is,
in aqueous solution
 Many ionic compounds “dissociate”,
or separate, into cations and anions
when dissolved in water
 NaCl(s) dissolved in H2O
Na+(aq) + Cl-(aq)
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Net Ionic Equations
 Example:
KI(aq) + AgNO3(aq)  KNO3(aq) + AgI(s)
1. Above the Full balanced equation
2. Below a Complete ionic equation
K+(aq) + I-(aq) + Ag+(aq) + NO3-(aq)
K+(aq) + NO3-(aq) + AgI(s)
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Net Ionic Equations
3. can be simplified by eliminating ions
not directly involved (spectator ions)
K+(aq) + I-(aq) + Ag+(aq) + NO3-(aq)
K+(aq) + NO3-(aq) + AgI(s)
Produces the Net ionic equation
= I-(aq) + Ag+(aq)
AgI(s)
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Predicting the Precipitate
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