Download Chapter 5 Chemical Reactions and Quantities

Chapter 5
Chemical Reactions and Quantities
Copyright © 2005 by Pearson Education, Inc.
Publishing as Benjamin Cummings
1
Physical Change
• the identity and
composition of the
substance do not
change.
• the state can change or
the material can be torn
into smaller pieces.
2
Chemical Change
• reacting substances form
new substances with
different compositions and
properties.
• a chemical reaction takes
place.
3
Chemical Reactions
4
Learning Check
Classify each of the following as a
1) physical change or 2) chemical change.
2
A. ____Burning
a candle.
1
B. ____Ice
melting on the street.
2
C. ____Toasting
a marshmallow.
1
D. ____Cutting
a pizza.
2
E. ____Polishing
a silver bowl.
5
Chemical Reaction
• a chemical reaction produces
one or more new substances.
• new properties become visible.
• old bonds are broken and new
bonds are formed.
• atoms in the reactants are
rearranged to form one or more
different substances/products.
• Same elements present.
• Ex: Fe and O2 form rust (Fe2O3).
6
Chemical Equation
materials needed & products formed
7
Burning Charcoal:
C(s) + O2 (g)  CO2 (g)
8
Chemical Equations
A chemical equation gives the chemical formulas of the
reactants on the left of the arrow and the products on the
right.
C(s) + O2 (g)  CO2 (g)
Reactants
Product(s)
9
Symbols used in
chemical
equations show
• the states of the
reactants.
• the states of the
products.
• the reaction
conditions.
10
Chemical Equations are
Balanced
In a balanced
chemical reaction
• atoms are not
gained or lost.
• the number of
reactant atoms
is equal to the
number of
product atoms.
11
Balanced Chemical Equation
12
13
CH4 (g) + O2 (g)  CO2 (g) + H2O (g)
Balanced? How many oxygen atoms on left ? on right?
14
A Balanced Chemical Equation
In a balanced chemical equation,
• there must be the same number of each type of atom on
the reactant side and on the product side of a balanced
equation.
• numbers called coefficients are used in front of one or
more formulas.
S
 Al2S3
Not Balanced
+ 3S
 Al2S3
Balanced
Al +
2Al
2 Al
=
2 Al
3S
=
3S
15
Learning Check
State the number of atoms of each element on the
reactant side and the product side for each of the
following balanced equations.
A.
P4(s) + 6Br2(l)  4 PBr3(g)
4P
4P
12 Br
12 Br
B. 2Al(s) + Fe2O3(s)  2Fe(s) + Al2O3(s)
2 Al
2 Al
2 Fe
2 Fe
3O
3O
16
Learning Check
Determine if each equation is balanced or not.
A. Na(s) + N2(g)  Na3N(s)
No. 2 N on reactant side, 1 N on product side.
1 Na on reactant side, 3 Na on product side.
B. C2H4(g) + H2O(l)  C2H5OH(l)
Yes. 2 C
= 2C
6H
= 6H
1O
= 1O
17
Steps in Balancing an Equation
To balance the following equation,
Fe3O4(s) + H2(g)  Fe(s) + H2O(l)
• work on one element at a time.
• use only coefficients in front of formulas.
• do not change any subscripts.
Fe:
Fe3O4(s) + H2(g)  3Fe(s) + H2O(l)
O:
Fe3O4(s) + H2(g)
 3Fe(s)
+ 4H2O(l)
H:
Fe3O4(s) + 4H2(g)  3Fe(s)
+ 4H2O(l)
18
Balancing Chemical Equations
1. Write the equation with the correct formulas.
NH3(g) + O2(g)
 NO(g) + H2O(g)
2. Determine if the equation is balanced.
No, not all atoms are balanced.
3. Balance with coefficients in front of formulas.
4NH3(g) + 5O2(g)  4NO(g) + 6H2O(g)
4. Check that atoms of each element are equal in reactants
and products.
4 N (4 x 1 N)
=
4 N (4 x 1 N)
12 H (4 x 3 H)
=
12 H (6 x 2 H)
19
10 O (5 x 2 O)
=
10 O (4 O + 6 O)
Balancing with Polyatomic Ions
MgCl2(aq) + Na3PO4(aq)  NaCl(aq) + Mg3(PO4)2(s)
Balance PO43- as a unit
MgCl2(aq) + 2Na3PO4(aq)  NaCl(aq) + Mg3(PO4)2(s)
2 PO43=
2 PO43-
Balance Mg and Cl
3MgCl2(aq) + 2Na3PO4(aq)  6NaCl(aq) + Mg3(PO4)2(s)
3 Mg2+
=
3 Mg2+
6 Na+
=
6 Na+
20
6 Cl=
6 Cl-
Types of Chemical Reactions (5.4)
Chemical reactions can be generally classified as:
• combination reactions
• decomposition reactions
• single replacement reactions
• double replacement reactions
• combustion reaction
21
2Mg(s) + O2(g)
2MgO(s)
2Na(s) + Cl2(g)
2NaCl(s)
SO3(g) + H2O(l)
H2SO4(aq)
22
2HgO(s)
2KClO3(s)
2Hg(l) + O2(g)
2KCl(s) + 3 O2(g)
23
Learning Check
Classify the following reactions as
1) combination or 2) decomposition.
___A. H2(g) + Br2(g)
2HBr(l)
___B. Al2(CO3)3(s)
Al2O3(s) + 3CO2(g)
___C. 4Al(s) + 3C(s)
Al4C3(s)
24
Replacement
In a single replacement reaction,
• one element takes the place of a different element in a
reacting compound.
Zn(s) + 2HCl(aq)
ZnCl2(aq) + H2(g)
Fe(s) + CuSO4(aq)
FeSO4(aq) + Cu(s)
25
Zn + HCl: a Single Replacement Reaction
26
Double Replacement
In a double replacement,
• two elements in the reactants exchange places.
AgNO3(aq) + NaCl(aq)
AgCl(s) + NaNO3(aq)
ZnS(s)
ZnCl2(aq) + H2S(g)
+ 2HCl(aq)
27
Na2SO4 + BaCl2:
a double replacement reaction
28
Learning Check
Identify each reaction as
1) combination
3) single replacement
A. 3Ba(s) + N2(g)
B. 2Ag(s) + H2S(aq)
C. SiO2(s) + 4HF(aq)
2) decomposition
4) double replacement
1
Ba3N2(s)
3
Ag2S(s) + H2(g)
4
D. PbCl2(aq) + K2SO4(aq)
2
E.
K2CO3(s)
4
SiF4(s) + 2H2O(l)
2KCl(aq) + PbSO4(s)
K2O(aq) + CO2(g)
29
Combustion
Carbon-containing compounds burn in oxygen
(O2) from air, producing CO2 and H2O and energy.
CH4 (g) + 2 O2 (g)  CO2 (g) + 2 H2O (g)
30
31
Oxidation-Reduction Reactions
• provides us with energy from food.
• provides electrical energy in
batteries.
• Example:
occurs when iron rusts.
4Fe(s) + 3O2(g)
2Fe2O3(s)
Think: What other type of reaction can this
reaction between iron and oxygen be
classified?
32
Loss of electrons, gain of electrons
33
Oxidation and Reduction
34
Zn and Cu2+
Zn(s)
Silvery metal
Cu2+(aq) + 2eBlue
Zn2+(aq) + 2e-
oxidation
Cu(s)
orange
reduction
35
Electron Transfer from Zn to Cu2+
Oxidation: electron loss
Reduction: electron gain
36
Learning Check
Identify each of the following as
1) oxidation or 2) reduction.
A.
Sn(s)
Sn4+(aq) + 4e−
Oxidation
B.
Fe3+(aq) + 1e−
Fe2+(aq)
Reduction
C.
Cl2(g) + 2e−
2Cl-(aq)
Reduction
37
Writing Oxidation and Reduction
Reactions
Write the separate oxidation and reduction reactions
for the following equation.
2Cs(s) + F2(g)
2CsF(s)
A cesium atom loses an electron to form cesium ion.
Cs(s)
Cs+(s) + 1e−
oxidation
Fluorine atoms gain electrons to form fluoride ions.
F2(s) + 2e2F−(s)
reduction
38
Learning Check
Identify the substances that are oxidized and reduced in
each of the following reactions.
A.
Mg(s) + 2H+(aq)
Mg2+(aq) + H2(g)
Mg is oxidized Mg(s)
Mg2+(aq) + 2e−
H+ is reduced 2H+ + 2e−
H2
B.
2Al(s) + 3Br2(g)
2AlBr3(s)
Al is oxidized Al
Al3+ + 3e−
Br is reduced Br + e−
Br −
39