Transcript Redox Reactions

1
REDOX
REACTIONS
REACTIONS
Batteries
Day 1 Review
Oxidation numbers
20.1 The Meaning of
Oxidation and Reduction
>
What Are Oxidation and
Reduction?
What Are Oxidation and Reduction?
What happens to a substance
that undergoes oxidation?
What happens to a substance
that undergoes reduction?
2
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
Electron Transfer Reactions
• oxidation-reduction or redox reactions are
Electron transfer reactions.
• Redox reactions can result in the
generation of an electric current.
• Therefore, this field of chemistry is
often called
ELECTROCHEMISTRY.
3
20.1Oxygen
The Meaning
of Redox
and
Oxidation and Reduction
>
When methane (CH4)burns in air, it
oxidizes: reacts with oxygen
• One oxide of carbon is carbon dioxide, CO2.
CH4(g) +
4
2O2(g) →
CO2(g) + 2H2O(g)
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
20.1 The Meaning of
Not and
all Reduction
oxidation
Oxidation
involves
burning.
>
• metals oxidizes to form compounds such as
iron(III) oxide (Fe2O3).
4Fe(s)
5
+
3O2(g)
→
2Fe2O3(s)
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
•20.1
Common
liquid
The Meaning
of household bleach contains
> a substance that
Oxidation
Reduction (NaClO),
sodiumand
hypochlorite
releases oxygen, which oxidizes stains to a
colorless form.
• Hydrogen peroxide (H2O2) also
releases oxygen when it
decomposes. It is both a
bleach and a mild antiseptic
that kills bacteria by oxidizing
them.
6
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
20.1
Meaning ofthat undergoes
A The
substance
Oxidation and Reduction
oxidation
gains
>
oxygen. A substance that undergoes
reduction loses oxygen.
Original definition
• Reactions that involve the processes
of oxidation and reduction are called
oxidation-reduction reactions.
• Oxidation-reduction reactions are
also known as
redox reactions.
7
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
20.1 The Meaning of
What Are Oxidation and
MODERN
REDOXReduction?
DEFINITION
Oxidation
and Reduction
>
Oxidation
Reduction
Loss of
electrons
Gain of
electrons
• Oxidation is now defined to mean
complete or partial loss of electrons
• Reduction is now defined to mean
complete or partial gain of electrons
8
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
What Are Oxidation and
Redox Reactions
> Reduction?
20.1 The Meaning of
Oxidation and Reduction
During a reaction between a metal and a
nonmetal, electrons are transferred from
atoms of the metal to atoms of the nonmetal.
metal
nonmetal
lose
gain
Mg(s)
9
+
S(s)
IONIC
heat
MgS(s)
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
REDOX REACTIONS
Cu(s) + 2 Ag+(aq) g
Cu2+(aq) + 2 Ag(s)
In all reactions if something has
been oxidized then something has
also been reduced
10
REDOX Reactions:
Metals Lose electrons
Metals
(Cu) are
“loser”:
oxidized
11
Metals
(K) are
oxidized
HNO3 is the
reduced
hydrogen is reduced
Cu + HNO3 --> Cu2+ + NO2
2 K + 2 H2O --> 2 KOH + H2
Examples of Redox Reactions
Metal + acid
Zn + HCl
Zn = loses electrons
H+ = gains electrons
Cl- = spectator
12
Examples of Redox Reactions
Metal + halogen
2 Al + 3 Br2 ---> 2AlBr3
13
Examples of Redox Reactions
Nonmetal (S) + Oxygen
Metal (Mg) + Oxygen
14
20.2 Oxidation Numbers >
Assigning Oxidation Numbers
How do we determine
which element
has gained and which has lost
electrons?
oxidation numbers
An oxidation number is a positive
or negative number assigned to
an atom to indicate its degree of
oxidation or reduction.
15
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
20.2 Oxidation Numbers >
Assigning Oxidation Numbers
Assigning Oxidation
Numbers
What is the general rule
for assigning oxidation
numbers?
16
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
OXIDATION NUMBERS
The electric charge an element
APPEARS to have when electrons
are counted by some arbitrary rules:
1. An element has ox. no. = 0.
Zn
O2
I2
S8
2. In simple ions, ox. no. = charge on ion.
-1 for Cl+2 for Mg2+
17
OXIDATION NUMBERS
3. In a compound: O has ox. no. = -2
(except in peroxides: in H2O2, O = -1)
4. In a Molecule: Ox. no. of H = +1
(except when H is associated with a
metal as in NaH where it is -1)
5. Algebraic sum of oxidation numbers
= 0 for a compound
= overall charge for an ion
18
What
is the oxidation
20.2
Oxidation
Numbers > number of chromium in each
compound?
Chromium
Cr
potassium dichromate
(K2Cr2O7) is Orange
+1 +6
chromium(III) potassium
sulfate (CrK(SO4)2) is purple
+3 +1 +6 –2
CrK(SO4)2
–2
K2Cr2O7
19
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
OXIDATION NUMBERS
NH3
N =
ClO-
Cl =
H3PO4
P =
MnO4-
Mn =
Cr2O72-
Cr =
C 3H 8
C =
Oxidation
number of F
in HF?
20
20.2
Oxidation Numbers >
Oxidation-Number
Oxidation-Number Changes in
Changes
in Chemical
Chemical Reactions
Reactions
How are oxidation and
reduction defined in terms
of a change in oxidation
number?
21
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
REDOX REACTIONS
1.increase in oxidation
number of same element =
OXIDATION
decrease in oxidation
number of same element =
REDUCTION
2.
22
ThisOxidation
figure illustrates
20.2
Numbers a> redox reaction that shows what
occurs when a shiny iron nail is dipped into a solution of
copper(II) sulfate.
Cu2+SO4 + Fe → Fe2+SO4 + Cu
• The iron reduces Cu2+
ions in solution and is
simultaneously
oxidized to Fe2+.
•The iron becomes coated
with metallic copper.
23
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
Oxidation-Number Changes in
copper
wire
is
placed
in
a
Chemical Reactions
solution of silver nitrate?
20.2
Oxidation
Numbers
>
What
happens
when
+1 +5 –2
0
+2
+5 –2
0
2AgNO3(aq) + Cu(s) → Cu(NO3)2(aq) + 2Ag(s)
24
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
20.2
Numbers
>
UseOxidation
changes
in oxidation
number to identify
which atoms are oxidized and which are
reduced in the following reaction.
2HNO3(aq) + 3H2S(g) → 2NO(g) + 4H2O(l) + 3S(s)
+1 +5 –2
+1 –2
+2 –2
+1 –2
0
2HNO3(aq) + 3H2S(g) → 2NO(g) + 4H2O(l) + 3S(s)
Sulfur is oxidized because its oxidation number
increases (–2 to 0).
Nitrogen is reduced because its oxidation number
decreases (+5 to +2).
25
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
20.2 Oxidation Numbers >
Use changes in oxidation number to identify which atoms are
oxidized and which are reduced in the following reactions.
a. Cl2(g) + 2HBr(aq) → 2HCl(aq) + Br2(l)
b. C(s) + O2(g) → CO2(g)
26
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
20.3 Describing Redox Equations >
Which of the following are redox
reactions?
A. NH3 + HCl → NH4Cl
B. SO3 + H2O → H2SO4
C. NaOH + HCl → NaCl + H2O
D. H2S + NHO3 → H2SO4 + NO2 + H2O
27
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
20.3 Describing Redox Equations >
Which of the following are redox
reactions?
A. NH3 + HCl → NH4Cl
B. SO3 + H2O → H2SO4
C. NaOH + HCl → NaCl + H2O
D. H2S + NHO3 → H2SO4 + NO2 + H2O
28
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
20.2 Oxidation Numbers >
Identifying Oxidized and Reduced Atoms
Use changes in oxidation number to identify which atoms
are oxidized and which are reduced in the following
reaction.
Zn(s) + 2MnO2(s) + 2NH4Cl(aq) →
ZnCl2(aq) + Mn2O3(s) + 2NH3(g) + H2O(l)
29
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
20.3 Describing Redox Equations >
Identifying Redox Reactions
What are the two classes of
chemical reactions?
30
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
20.3
Redox
Equationscan
>
AllDescribing
chemical
reactions
be assigned to
one of two classes.
One class of chemical reactions is
oxidation-reduction (redox)
reactions, in which electrons are
transferred from one reacting
species to another.
The other class includes all other
reactions, in which no electron
transfer occurs.
31
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
20.3 Describing Redox Equations >
How can you tell if this is a redox reaction?
N2(g) + O2(g) → 2NO(g)
During an electrical storm,
oxygen molecules and
nitrogen molecules in air
react to form nitrogen
monoxide.
32
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
20.3 Describing
Redox Equations >
single-replacement,
synthesis,
decomposition, and combustion reactions
are redox reactions.
• Potassium metal
reacts violently with
water to produce
hydrogen gas
(which ignites) and
potassium
hydroxide.
33
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
20.3 Describing Redox Equations >
Identifying Redox
Reactions
Examples of reactions that are not
redox reactions include
double-replacement reactions
and acid-base reactions.
34
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
35
Review of Terminology
for Redox Reactions
• OXIDATION—loss of electron(s) by a
species; increase in oxidation number.
• REDUCTION—gain of electron(s);
decrease in oxidation number.
• OXIDIZING AGENT—electron acceptor;
species is reduced.
• REDUCING AGENT—electron donor;
species is oxidized.
20.2 Oxidation Numbers >
BIG IDEA
Reactions
Redox reactions are identified by
changes in oxidation number.
36
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.
Oxidation-Number Changes in
20.2
Oxidation
Numbers
>
You can define oxidation
and
reduction
Chemical
Reactions
in terms of a change in oxidation
number.
An increase in the oxidation
number of an atom or ion
indicates oxidation.
A decrease in the oxidation
number of an atom or ion
indicates reduction.
37
Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.