1 redox reactions day 1 review oxidation numbers reactions batteriesbatteries
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REDOX REACTIONS
REDOX REACTIONS
Day 1 ReviewDay 1 ReviewOxidation numbers
REACTIONSREACTIONS
BatteriesBatteriesBatteriesBatteries
20.1 The Meaning of 20.1 The Meaning of Oxidation and ReductionOxidation and Reduction
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What Are Oxidation and What Are Oxidation and Reduction?Reduction?
What Are Oxidation and Reduction?
What happens to a substance that undergoes oxidation?
What happens to a substance that undergoes reduction?
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Electron Transfer Electron Transfer ReactionsReactions
Electron Transfer Electron Transfer ReactionsReactions
• 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.
20.1 The Meaning of 20.1 The Meaning of Oxidation and ReductionOxidation and Reduction
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Oxygen and Redox
When methane (CH4)burns in air, it oxidizes: reacts with oxygen
• One oxide of carbon is carbon dioxide, CO2.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
20.1 The Meaning of 20.1 The Meaning of Oxidation and ReductionOxidation and Reduction
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Not all oxidation involves burning.
• metals oxidizes to form compounds such as iron(III) oxide (Fe2O3).
4Fe(s) + 3O2(g) → 2Fe2O3(s)
20.1 The Meaning of 20.1 The Meaning of Oxidation and ReductionOxidation and Reduction
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• Common liquid household bleach contains sodium hypochlorite (NaClO), a substance that 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.
20.1 The Meaning of 20.1 The Meaning of Oxidation and ReductionOxidation and Reduction
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A substance that undergoes oxidation gains oxygen. A substance that undergoes
reduction loses oxygen.
• Reactions that involve the processes of oxidation and reduction are called
oxidation-reduction reactions.
• Oxidation-reduction reactions are also known as
redox reactions.
Original definition
20.1 The Meaning of 20.1 The Meaning of Oxidation and ReductionOxidation and Reduction
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What Are Oxidation and What Are Oxidation and Reduction?Reduction?MODERN REDOX DEFINITION
• Oxidation is now defined to mean complete or partial loss of electrons
• Reduction is now defined to mean complete or partial gain of electrons
OxidationLoss of
electrons
ReductionGain of
electrons
20.1 The Meaning of 20.1 The Meaning of Oxidation and ReductionOxidation and Reduction
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What Are Oxidation and What Are Oxidation and Reduction?Reduction?
Redox Reactions During a reaction between a metal and a nonmetal, electrons are transferred from
atoms of the metal to atoms of the nonmetal.
Mg(s) + S(s) MgS(s)heat
metal nonmetal
lose gain
IONIC
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In all reactions if something has been oxidized then something has
also been reduced
REDOX REACTIONSREDOX REACTIONS
Cu(s) + 2 Ag+(aq) Cu2+(aq) + 2 Ag(s)
11REDOX Reactions:REDOX Reactions:
Metals Lose electronsMetals Lose electronsREDOX Reactions:REDOX Reactions:
Metals Lose electronsMetals Lose electrons
HNO3 is the reduced
2 K + 2 H2O --> 2 KOH + H2
Metals (K) are oxidized
Metals (Cu) are “loser”:oxidized
Cu + HNO3 --> Cu2+ + NO2
hydrogen is reduced
12Examples of Redox Examples of Redox ReactionsReactions
Examples of Redox Examples of Redox ReactionsReactions
Metal + acidMetal + acidZn + HClZn + HClZn = loses electronsZn = loses electronsHH++ = gains electrons = gains electronsCl- = spectatorCl- = spectator
13Examples of Redox Examples of Redox ReactionsReactions
Examples of Redox Examples of Redox ReactionsReactions
Metal + halogenMetal + halogen2 Al + 3 Br2 Al + 3 Br22 ---> 2AlBr ---> 2AlBr33
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Examples of Redox Examples of Redox ReactionsReactions
Examples of Redox Examples of Redox ReactionsReactions
Metal (Mg) + OxygenMetal (Mg) + Oxygen
Nonmetal (S) + OxygenNonmetal (S) + Oxygen
20.2 Oxidation Numbers >20.2 Oxidation Numbers >
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Assigning Oxidation NumbersAssigning Oxidation Numbers
oxidation numbers
How do we determine which element has gained and which has lost
electrons?
An oxidation number is a positive or negative number assigned to an atom to indicate its degree of
oxidation or reduction.
20.2 Oxidation Numbers >20.2 Oxidation Numbers >
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Assigning Oxidation NumbersAssigning Oxidation NumbersAssigning Oxidation Numbers
What is the general rule for assigning oxidation
numbers?
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The electric charge an element APPEARS to have when electrons
are counted by some arbitrary rules::
The electric charge an element APPEARS to have when electrons
are counted by some arbitrary rules::
OXIDATION NUMBERSOXIDATION NUMBERS
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+
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OXIDATION NUMBERSOXIDATION NUMBERSOXIDATION NUMBERSOXIDATION 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
20.2 Oxidation Numbers >20.2 Oxidation Numbers >
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K2Cr2O7
+1 +6 –2
CrK(SO4)2
+3 +1 +6 –2
What is the oxidation number of chromium in each compound?
Chromium
potassium dichromate (K2Cr2O7) is Orange
chromium(III) potassium sulfate (CrK(SO4)2) is purpleCr
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OXIDATION NUMBERSOXIDATION NUMBERSOXIDATION NUMBERSOXIDATION NUMBERS
NHNH33 N = N =
ClOClO-- Cl = Cl =
HH33POPO44 P = P =
MnOMnO44- - Mn = Mn =
CrCr22OO772-2- Cr = Cr =
CC33HH88 C = C =
Oxidation Oxidation number of F number of F
in HF?in HF?
20.2 Oxidation Numbers >20.2 Oxidation Numbers >
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Oxidation-Number Changes in Oxidation-Number Changes in Chemical ReactionsChemical Reactions
Oxidation-Number Changes in Chemical Reactions
How are oxidation and reduction defined in terms
of a change in oxidation number?
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1.1. increase in oxidation increase in oxidation numbernumber of same element = of same element =
OXIDATIONOXIDATION
2.2. decrease in oxidation decrease in oxidation numbernumber of same element = of same element =
REDUCTIONREDUCTION
REDOX REACTIONSREDOX REACTIONSREDOX REACTIONSREDOX REACTIONS
20.2 Oxidation Numbers >20.2 Oxidation Numbers >
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This figure illustrates a redox reaction that shows what occurs when a shiny iron nail is dipped into a solution of
copper(II) sulfate.
• The iron reduces Cu2+
ions in solution and is simultaneously oxidized to Fe2+.
•The iron becomes coated with metallic copper.
Cu2+SO4 + Fe → Fe2+SO4 + Cu
20.2 Oxidation Numbers >20.2 Oxidation Numbers >
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Oxidation-Number Changes in Oxidation-Number Changes in Chemical ReactionsChemical Reactions
2AgNO3(aq) + Cu(s) → Cu(NO3)2(aq) + 2Ag(s) +1 +5 –2 0 +2 +5 –2 0
What happens when copper wire is placed in a solution of silver nitrate?
20.2 Oxidation Numbers >20.2 Oxidation Numbers >
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Use 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
Sulfur is oxidized because its oxidation number increases (–2 to 0).
Nitrogen is reduced because its oxidation number decreases (+5 to +2).
2HNO3(aq) + 3H2S(g) → 2NO(g) + 4H2O(l) + 3S(s)
20.2 Oxidation Numbers >20.2 Oxidation Numbers >
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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)
20.3 Describing Redox Equations >20.3 Describing Redox Equations >
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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
20.3 Describing Redox Equations >20.3 Describing Redox Equations >
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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
20.2 Oxidation Numbers >20.2 Oxidation Numbers >
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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)
20.3 Describing Redox Equations >20.3 Describing Redox Equations >
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Identifying Redox Reactions
What are the two classes of chemical reactions?
20.3 Describing Redox Equations >20.3 Describing Redox Equations >
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All chemical reactions can 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.
20.3 Describing Redox Equations >20.3 Describing Redox Equations >
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During an electrical storm, oxygen molecules and
nitrogen molecules in air react to form nitrogen
monoxide.
N2(g) + O2(g) → 2NO(g)
How can you tell if this is a redox reaction?
20.3 Describing Redox Equations >20.3 Describing Redox Equations >
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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.
20.3 Describing Redox Equations >20.3 Describing Redox Equations >
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Identifying Redox Identifying Redox ReactionsReactions
Examples of reactions that are not redox reactions include
double-replacement reactions
and acid-base reactions.
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Review of Terminology Review of Terminology for Redox Reactionsfor Redox Reactions
Review of Terminology Review of Terminology for Redox Reactionsfor Redox Reactions
• OXIDATIONOXIDATION—loss of electron(s) by a —loss of electron(s) by a species; increase in oxidation number.species; increase in oxidation number.
• REDUCTIONREDUCTION—gain of electron(s); —gain of electron(s); decrease in oxidation number.decrease in oxidation number.
• OXIDIZING AGENTOXIDIZING AGENT—electron acceptor; —electron acceptor; species is reduced.species is reduced.
• REDUCING AGENTREDUCING AGENT—electron donor; —electron donor; species is oxidized.species is oxidized.
• OXIDATIONOXIDATION—loss of electron(s) by a —loss of electron(s) by a species; increase in oxidation number.species; increase in oxidation number.
• REDUCTIONREDUCTION—gain of electron(s); —gain of electron(s); decrease in oxidation number.decrease in oxidation number.
• OXIDIZING AGENTOXIDIZING AGENT—electron acceptor; —electron acceptor; species is reduced.species is reduced.
• REDUCING AGENTREDUCING AGENT—electron donor; —electron donor; species is oxidized.species is oxidized.
20.2 Oxidation Numbers >20.2 Oxidation Numbers >
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Redox reactions are identified by changes in oxidation number.
BIG IDEABIG IDEA
Reactions
20.2 Oxidation Numbers >20.2 Oxidation Numbers >
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Oxidation-Number Changes in Oxidation-Number Changes in Chemical ReactionsChemical ReactionsYou can define oxidation and reduction
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.