redox reactions 1552

7/24/2019 RedOx Reactions 1552

1/19

Balancing Redox Equations:Balancing Redox Equations:

following the electronsfollowing the electrons

Review: Oxidation and reductionReview: Oxidation and reduction

Oxidation numbersOxidation numbers


2/19

Review: Oxidation - reductionReview: Oxidation - reduction

Oxidation is loss of electronsOxidation is loss of electrons

Reduction is gain of electronsReduction is gain of electrons

Oxidation is always accompanied by reductionOxidation is always accompanied by reduction he total number of electrons is !ept constanthe total number of electrons is !ept constant

Oxidi"ing agents oxidi"e and areOxidi"ing agents oxidi"e and are

themselves reducedthemselves reduced Reducing agents reduce and areReducing agents reduce and are

themselves oxidi"edthemselves oxidi"ed


3/19

#uggets of redox processes#uggets of redox processes

$here there is oxidation there is$here there is oxidation there is alwaysalways

reductionreduction

Oxidizing agentOxidizing agent Reducing agentReducing agent

%s%s itselfitselfreducedreduced %s%s itselfitselfoxidi"edoxidi"ed

GainsGainselectronselectrons LosesLoseselectronselectrons

&auses oxidation&auses oxidation &auses reduction&auses reduction


4/19

Oxidation numbers reviewOxidation numbers review

'etals are more (cation-li!e('etals are more (cation-li!e(

)ave positive oxidation)ave positive oxidation

numbersnumbers

#onmetals are (anion-li!e(#onmetals are (anion-li!e( )ave negative oxidation)ave negative oxidation

numbers*numbers*

Oxidation number is the number of electronsOxidation number is the number of electrons

gained or lost by the element in ma!ing agained or lost by the element in ma!ing a

compoundcompound


5/19

+redicting oxidation numbers+redicting oxidation numbers

Oxidation number of atoms in element is "ero in all casesOxidation number of atoms in element is "ero in all cases

Oxidation number of element in monatomic ion is equal toOxidation number of element in monatomic ion is equal tothe chargethe charge

sum of the oxidation numbers in a compound is "erosum of the oxidation numbers in a compound is "ero

sum of oxidation numbers in polyatomic ion is equal to thesum of oxidation numbers in polyatomic ion is equal to thechargecharge

, has oxidation number ., has oxidation number .

) has oxidn no* /.0 except in metal hydrides where it is ) has oxidn no* /.0 except in metal hydrides where it is

.. Oxygen isOxygen is usuallyusuall

y1* Except:1* Except: O is . in hydrogen peroxide2 and other peroxidesO is . in hydrogen peroxide2 and other peroxides

O is .31 in superoxides 4OO is .31 in superoxides 4O11

%n O,%n O,11O is /1O is /1


6/19

+osition of element in periodic table+osition of element in periodic table

determines oxidation numberdetermines oxidation number

5.6 is /.5.6 is /.

516 is /1516 is /1

576 is /7 8some rare exceptions9576 is /7 8some rare exceptions9

56 are 7 in compounds with metals2 ) or with #)56 are 7 in compounds with metals2 ) or with #);/;/

**Exceptions are in compounds to the right0 in which caseExceptions are in compounds to the right0 in which caseuse rules 7 and ;*use rules 7 and ;*

5


7/19

Redox equationsRedox equations

#et ionic equations summari"e the#et ionic equations summari"e the

essentials of a reaction without including allessentials of a reaction without including all

the particles presentthe particles present

Redox equations are a subset which involveRedox equations are a subset which involve

electron transferelectron transfer

$ithout being given all the information2$ithout being given all the information2balancing redox equations involvesbalancing redox equations involves

balancing electron flowbalancing electron flow


8/19

Balancing redox equations:Balancing redox equations:

systematic methodssystematic methods

Oxidation number method trac!ingOxidation number method trac!ing

changes in the oxidation numberschanges in the oxidation numbers

)alf-reaction method trac!ing changes in)alf-reaction method trac!ing changes inthe flow of electronsthe flow of electrons

>ame principles2 different emphasis>ame principles2 different emphasis $e will examine the half-reaction method$e will examine the half-reaction method


9/19

he )alf-Reaction methodhe )alf-Reaction method

6ny redox process can be written as the6ny redox process can be written as the

sum of two half reactions: one for thesum of two half reactions: one for the

oxidation and one for the reductionoxidation and one for the reduction


10/19

>ix habits of the redox equation>ix habits of the redox equation

balancerbalancer


11/19

>E+ .: the unbalanced equation>E+ .: the unbalanced equation

?ichromate ion reacts with chloride ion to?ichromate ion reacts with chloride ion to

produce chlorine and chromium 8%%%9produce chlorine and chromium 8%%%9

)()()()( 232

72 aqClaqCraqClaqOCr ++ +


12/19

>E+ 1: identify the oxidi"ed and>E+ 1: identify the oxidi"ed and

reduced and write the half reactionsreduced and write the half reactions

Oxidation half-reactionOxidation half-reaction

Reduction half-reactionReduction half-reaction

)()( 2 aqClaqCl

)()( 3272 aqCraqOCr +


13/19

>E+ 7: Balance the half reactions>E+ 7: Balance the half reactions

OxidationOxidation

ReductionReduction

)()(2 2 aqClaqCl

)(2)( 32

72 aqCraqOCr +


14/19

'aterial balance with )'aterial balance with )11O and )O and )// or O)or O)--

>trategy: add )>trategy: add )11O to the side that lac!s forO to the side that lac!s for

O and add )O and add )// 8the reaction is in acid8the reaction is in acidsolution9 to the other sidesolution9 to the other side

%n basic solution we add O)%n basic solution we add O) --and )and )11OO

instead of )instead of )11O and )O and )//respectivelyrespectively

est equation for both atoms and chargesest equation for both atoms and charges

)(8)()()(16)(10)( 222

4 522 lOHaqBraqMnaqHaqBraqMnO ++++ ++

)()()(2)( 22

4 5252 aqBraqMnaqBraqMnO ++ +

(16)()()(8)(10)( 22

24 522 aqOHaqBraqMnaqOHaqBraqMnO +

++++


15/19

>E+ ;: 'aterial balance>E+ ;: 'aterial balance

6dd )6dd )11O to the side lac!ing O and add )O to the side lac!ing O and add )//toto

the other side 8for reactions in acid solution9the other side 8for reactions in acid solution9

Oxidation reaction unchangedOxidation reaction unchanged

Reduction reactionReduction reaction

)()(2 2 aqClaqCl

)(7)(2)()(14 232

72 lOHaqCraqOCraqH ++ ++


16/19

>E+ : Balance half-reactions for>E+ : Balance half-reactions for

charge by addition of electronscharge by addition of electrons

BalanceBalance chargeschargeson both sides of eachon both sides of each

half-reactionhalf-reaction

1 x -. @1 x -. @ 11x -.x -.

.; x /. / -1 /.; x /. / -1 /


17/19

>E+ cont: 'ultiply by factors to>E+ cont: 'ultiply by factors to

balance total electronsbalance total electrons

Overall change in electrons must be "eroOverall change in electrons must be "ero

'ultiply the oxidation half reaction by'ultiply the oxidation half reaction by 77

7 x 1 @


18/19

>E+ E+


19/19

Balanced molecular equationBalanced molecular equation

6dd in the spectators: there will always be6dd in the spectators: there will always be

space*space*

Reagents were 4Reagents were 411&r&r11OO==2 #a&l and )2 #a&l and )11>O>O;;

#et ionic equation#et ionic equation

Balanced molecular equationBalanced molecular equation

)(7)(3)(2)(6)()(14 2232

72 lOHaqClaqCraqClaqOCraqH ++++ ++

)(7)()(3)()()(6)()(7242234272242

lOHaqSOKaqClaqSOCraqNaClaqOCrKaqSOH +++++

redox reactions 1552

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