Transcript Chapter 8

Chapter 8
Chemical Reactions
Chapter 8
Section 1:
Writing and Balancing
Chemical Reactions
Describing a Chemical Reaction
Indications of a Chemical Reaction
– Evolution of heat, light, and/or sound
– Production of a gas
– Formation of a precipitate
– Color change
Chemical Equations
• Reactants – the substances that exist before a chemical
change (or reaction) takes place.
• Products – the new substance(s) that are formed during
the chemical changes.
• CHEMICAL EQUATION indicates the reactants and products of
a reaction.
REACTANTS  PRODUCTS
Chemical Equations
aluminum oxide
reactants
Depict the kind of reactants
and products product
and their relative amounts in a reaction.
4 Al(s) + 3 O2(g)
2 Al2O3(s)
The letters (s), (g), and (l) are the
physical states of compounds.
The numbers in the front are called
stoichiometric coefficients.
Chemical Equations
4 Al(s) + 3 O2(g)
aluminum oxide
sandpaper
2 Al2O3(s)
4 g Al + 3 g O2 yield 2 g Al2O3
This equation means:
4 Al atoms + 3 O2 molecules yield 2 molecules of Al2O3
or
4 Al moles + 3 O2 moles yield 2 moles of Al2O3
4 mol Al@27g/mol
108 g
3 mol O2@32g/mol
+
96 g
2 mol Al2O3@102g/mol
=
204 g
Meaning of Chemical Formula
Chemical
Symbol
Meaning
Composition
H2O
One molecule
of water:
Two H atoms and one O atom
2 H2O
Two molecules
of water:
Four H atoms and two O atoms
H 2 O2
One molecule
of hydrogen
peroxide:
Two H atoms and two O atoms
Word Equations
• A WORD EQUATION describes chemical change using the
names of the reactants and products.
Write the word equation for the reaction of methane gas with oxygen gas to
form carbon dioxide and water.
methane
+
oxygen
carbon dioxide
Reactant
CH4
+
+
water
Product
2 O2
CO2
+
2 H2O
Showing Phases in
Chemical Equations
H2O(s)
H2O(l)
H2O(g)
Solid Phase – the substance is relatively rigid and has a definite
volume and shape. NaCl(s)
Liquid Phase – the substance has a definite volume, but is able to
change shape by flowing. H2O(l)
Gaseous Phase – the substance has no definite volume or shape, and
it shows little response to gravity. Cl2(g)
Additional Symbols Used in Chemical Equations
“Yields”; indicates result of reaction
Used to indicate a reversible reaction
(s)
A reactant or product in the solid state;
also used to indicate a precipitate
Alternative to (s), but used only to indicate a precipitate
(l)
A reactant or product in the liquid state
(aq)
A reactant or product in an aqueous solution
(dissolved in water)
(g)
A reactant or product in the gaseous state
Additional Symbols Used in Chemical Equations
Alternative to (g), but used only to indicate a gaseous product
D
2 atm
pressure
Reactants are heated
Pressure at which reaction is carried out, in this case 2 atm
Pressure at which reaction is carried out exceeds normal
atmospheric pressure
0 oC
Temperature at which reaction is carried out, in this case 0 oC
MnO2
Formula of catalyst, in this case manganese (IV) oxide,
used to alter the rate of the reaction
Chemical Equations
Because of the principle of the conservation of matter,
An equation must be balanced.
It must have the same number of atoms
of the same kind on both sides.
Lavoisier, 1788
Unbalanced and Balanced Equations
H
Cl
Cl
Cl
H
H
Cl
H
H
Cl
Cl
Cl
H
H
H2 + Cl2  HCl (unbalanced)
reactants
H
Cl
H2 + Cl2  2 HCl (balanced)
reactants
products
2
1
2
1
H
Cl
products
2
2
2
2
Balancing Chemical Equations
Balanced Equation – one in which the number of atoms of
each element as a reactant is equal to the number of atoms of
that element as a product
What is the relationship between conservation of mass and
the fact that a balanced equation will always have the same
number of atoms of each element on both sides of an equation?
Determine whether the following equation is balanced.
2 Na + H2O  2 NaOH + H2
2 Na + 2 H2O  2 NaOH + H2
Balancing Chemical Equations
• Write a word equation for the reaction.
• Write the correct formulas for all reactants
and products.
• Determine the coefficients that make the
equation balanced.
Write a balanced equation for the reaction between chlorine
and sodium bromide to produce bromine and sodium chloride.
1) Write a word equation for the reaction.
chlorine + sodium bromide  bromine + sodium chloride
2) Write the correct formulas for all reactants and products.
Cl2
+
NaBr

Br2
+
NaCl
3) Determine the coefficients that make the equation balance.
Cl2
+
2 NaBr

Br2
+
2 NaCl
Write a balanced equation for the reaction between chlorine
and sodium bromide to produce bromine and sodium chloride.
1) Write a word equation for the reaction.
chlorine + sodium bromide  bromine + sodium chloride
2) Write the correct formulas for all reactants and products.
Cl2
+
NaBr

Br2
+
NaCl
3) Determine the coefficients that make the equation balance.
Cl2
+
2 NaBr

Br2
+
2 NaCl
Balancing Chemical Equations
An important point to remember
2 NO(g) + O2(g)  2NO2(g)
The 2 to the left of NO(g) and NO2(g) refers to the number of molecules
present in the balanced equation.
It is a “multiplier” for every atom in the molecule.
The subscript 2 in O2 (g) and NO2(g) refers to the number
of atoms of this type that are present in each molecules
(or ionic compound).
Guidelines for Balancing Chemical Equations
?
1) polyatomic ions first
2) even / odd (make all even)
3) 2 H-OH vs. H2O
Mg(OH)2
4) single elements last
Example: need 13 oxygen atoms
“
”
Multiply by 13 O2 = 13
2
3X + 13 O2
2Y
+ Z
3X + 13 O2
2Y
+ Z
2
2
2
6X +
13 O2
4Y
+ 2Z
CH4 + 2 O2  CO2 + 2 H2O
Reactants
1 C atom
4 H atoms
4 O atoms
Products
1 C atom
4 H atoms
4 O atoms
Reactants  Products
+
C(s)
+
O2(g)
carbon
CO2(g)
oxygen
carbon dioxide
Reactants
1 carbon atom
2 oxygen atoms
Product
1 carbon atom
2 oxygen atoms
catalyst – speeds up reaction
+
2 H2(g)
+
hydrogen
Pt
2
O2(g)
oxygen
Reactants
2 hydrogen atoms
4
2 oxygen atoms
H2O (l)
water
Product
2 hydrogen atoms
4
1 oxygen atoms
2
Un balanced
“At-Seat Self-quiz”
Directions: Complete the following brief assignment at your seat,
quietly.
1. Balance the following reactions:
a. SO2 + O2  SO3
b. Fe2 O3 + H2  Fe +H2O
c. P + O2  P4 O10
d. Al + N2  AlN
e. Write Reaction a, Reaction c, and Reaction d in words.
2. Express in symbol form:
a. sulfur trioxide gas
b. zinc chloride as a catalyst
c. liquid mercury
d. heat supplied to a chemical reaction
e. solid copper
f. potassium nitrate dissolved in water
3. How is the Law of Conservation of Mass related to the balancing of a
chemical equation?
Chapter 8
Section 2:
Types of Chemical Reactions
Types of Chemical Reactions
Synthesis (Combination) reaction
A + B  AB
AB  A + B
Decomposition reaction
ASingle-replacement
reaction
A + BC  AC + B
element
BDouble-replacement
reaction
Ause
activity series to predict
Bdriving
force…water, gas, or precipitate
compound
element
AB + CD  AD + CB
compound
Combustion reaction (of a hydrocarbon)
compound
compound
compound
compound
CH + O2  CO2 + H2O
Combination Reaction
Direct combination reaction (Synthesis)
2 Na
+
Cl2
Na

Cl
2 NaCl

Cl
Na
General form: A
element or
compound
+
B
element or
compound

AB
compound
Combination Reaction
Direct combination reaction (Synthesis)
2 Na
+
Cl2
Na

2 NaCl
Cl
Na+ Cl -
Cl
Cl - Na+
Na
General form: A
element or
compound
+
B
element or
compound

AB
compound
Decomposition Reaction
Decomposition reaction
2 H2 O
2 H2
+
O2
H
O
H
+
H
O
H
General form: AB
compound
A
+
B
two or more elements
or compounds
Decomposition Reactions
Hydrogen Peroxide
2 H2O2
2 H2O
+
O2
Electrolysis of water
2 H2O
electricity
2 H2
+
Nitrogen triiodide
2 NI3
N2
+ 3 I2
General Form
AB
A + B
O2
Nitrogen triiodide
2 NI3
N2
+ 3 I2
Single and Double Replacement
Reactions
Single-replacement reaction
Mg
+
CuSO4
General form:
A
+ BC

MgSO4

AC
+
+
Cu
B
Double-replacement reaction
CaCO3
+
General form:
AB
+
2 HCl

CaCl2
+
H2CO3
CD

AD
+
CB
Single Replacement Reaction
Example
POW!
Activity Series
(use with single-replacement)
Printable
Version
of
Activity
Series
Element Reactivity
Ca
Foiled again –
Aluminum loses to Calcium
Li
Rb
K
Ba
Ca
Na
Mg
Al
Mn
Zn
Cr
Fe
Ni
Sn
Pb
H2
Cu
Hg
Ag
Pt
Au
Halogen Reactivity
F2
Cl2
Br2
I2
Single-Replacement Reactions
Activity Series
“Magic blue-earth”
Fe
+
FeCl2
CuCl2
+
Cu
Can Fe replace Cu? Yes
Zinc in nitric acid
Zn
+ 2 HNO3
Zn(NO3)2
+ H2
Yes
Can Zn replace H?
NO REACTION
MgCl2
+
MgBr2
Br2
Can Br replace Cl?
+
No
General Form
A
+
BC
AC + B
Cl2
Li
Rb
K
Ba
Ca
Na
Mg
Al
Mn
Zn
Cr
Fe
Ni
Sn
Pb
H2
Cu
Hg
Ag
Pt
Au
F2
Cl2
Br2
I2
Double Replacement Reaction
K2CO3 (aq)
Potassium carbonate
+
BaCl2 (aq)
Barium chloride
2 KCl (aq)
Potassium chloride
+
BaCO3 (s)
Barium carbonate
lead (II) chloride + potassium iodide
Pb2+
K1+
Cl1-
Pb Cl2 (aq)
+
I1-
K I (aq)
Copyright © 2007 Pearson Benjamin Cummings. All rights reserved.
potassium chloride + lead (II) iodide
Pb2+
Cl1-
KCl (aq)
K1+
+
PbI2
I1(s)
“At-Seat Self-quiz”
Directions: Complete the following brief assignment at your seat,
quietly.
1. Balance each equation. Also, identify what type of reaction best
describes the chemical equation.
a.
b.
c.
d.
e.
f.
g.
h.
2.
Pb(NO3)2 + K2CrO4  PbCrO4 + KNO3
Cl2 + KI  KCl + I2
C3H6 + O2  CO2 + H2O
Al(OH)3  Al2 O3 + H2O
Li + O2  Li2O
HCl + Fe2O3  FeCl3 + H2O
MgCO3  MgO + CO2
Ba(CN)2 + H2SO4  BaSO4 + HCN
Predict the products of the following reactions, then complete them by
balancing.
a. CaI2 + Hg(NO3)2 
b. Al + Cl2 
c. Ag + HCl 
d. C2H2 + O2 
e. MgCl2 
f. H2O2
