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

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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.


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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)


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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)


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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.


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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


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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


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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


Metal + halogenMetal + halogen2 Al + 3 Br2 Al + 3 Br22 ---> 2AlBr ---> 2AlBr33

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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.



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



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?



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



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



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?



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)



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 >


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



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



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)



Identifying Redox Reactions

What are the two classes of chemical reactions?



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.



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?



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.



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.



Redox reactions are identified by changes in oxidation number.

BIG IDEABIG IDEA

Reactions



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.

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