Balancing Redox Equations: Balancing Redox Equations: following the electronsfollowing the electrons

Review: Oxidation and reductionReview: Oxidation and reduction

Oxidation numbersOxidation numbers

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• The total number of electrons is kept constantThe total number of electrons is kept constant

Oxidizing agents oxidize and are Oxidizing agents oxidize and are themselves reducedthemselves reduced

Reducing agents reduce and are Reducing agents reduce and are themselves oxidizedthemselves oxidized

Nuggets of redox processesNuggets of redox processes

Where there is oxidation there is Where there is oxidation there is alwaysalways reductionreduction

Oxidizing agentOxidizing agent Reducing agentReducing agent

Is Is itselfitself reduced reduced Is Is itselfitself oxidized oxidized

GainsGains electrons electrons LosesLoses electrons electrons

Causes oxidationCauses oxidation Causes reductionCauses reduction

Oxidation numbers reviewOxidation numbers review

Metals are more 'cation-like' Metals are more 'cation-like' Have positive oxidation Have positive oxidation

numbersnumbers

Nonmetals are 'anion-like' Nonmetals are 'anion-like' Have negative oxidation Have negative oxidation

numbers.numbers.

Oxidation number is the number of electrons Oxidation number is the number of electrons gained or lost by the element in making a gained or lost by the element in making a compoundcompound

Predicting oxidation numbersPredicting oxidation numbers

Oxidation number of atoms in element is zero in all casesOxidation number of atoms in element is zero in all cases Oxidation number of element in monatomic ion is equal Oxidation number of element in monatomic ion is equal

to the chargeto the charge sum of the oxidation numbers in a compound is zero sum of the oxidation numbers in a compound is zero sum of oxidation numbers in polyatomic ion is equal to sum of oxidation numbers in polyatomic ion is equal to

the chargethe charge F has oxidation number –1F has oxidation number –1 H has oxidn no. +1; except in metal hydrides where it is –H has oxidn no. +1; except in metal hydrides where it is –

11 Oxygen is Oxygen is usually usually –2. Except:–2. Except:

O is –1 in hydrogen peroxide, and other peroxidesO is –1 in hydrogen peroxide, and other peroxides O is –1/2 in superoxides KOO is –1/2 in superoxides KO22

In OFIn OF22 O is +2 O is +2

Position of element in periodic table Position of element in periodic table determines oxidation number determines oxidation number

G1A is +1G1A is +1 G2A is +2G2A is +2 G3A is +3 (some rare exceptions)G3A is +3 (some rare exceptions) G5A are –3 in compounds with metals, H or with NHG5A are –3 in compounds with metals, H or with NH4+4+. .

Exceptions are in compounds to the right; in which case Exceptions are in compounds to the right; in which case use rules 3 and 4.use rules 3 and 4.

G6A below O are –2 in binary compounds with metals, G6A below O are –2 in binary compounds with metals, H or NHH or NH4+4+. When they are combined with O or with a . When they are combined with O or with a lighter halogen, use rules 3 and 4.lighter halogen, use rules 3 and 4.

G7A elements are –1 in binary compounds with metals, G7A elements are –1 in binary compounds with metals, H or NHH or NH4+4+ or with a heavier halogen. When combined or with a heavier halogen. When combined with O or a lighter halogen, use rules 3 and 4.with O or a lighter halogen, use rules 3 and 4.

Redox equationsRedox equations

Net ionic equations summarize the Net ionic equations summarize the essentials of a reaction without including all essentials of a reaction without including all the particles presentthe particles present

Redox equations are a subset which involve Redox equations are a subset which involve electron transferelectron transfer

Without being given all the information, Without being given all the information, balancing redox equations involves balancing redox equations involves balancing electron flowbalancing electron flow

Balancing redox equations: Balancing redox equations: systematic methodssystematic methods

Oxidation number method – tracking Oxidation number method – tracking changes in the oxidation numberschanges in the oxidation numbers

Half-reaction method – tracking changes in Half-reaction method – tracking changes in the flow of electronsthe flow of electrons

Same principles, different emphasisSame principles, different emphasis We will examine the half-reaction methodWe will examine the half-reaction method

The Half-Reaction methodThe Half-Reaction method

Any redox process can be written as the Any redox process can be written as the sum of two half reactions: one for the sum of two half reactions: one for the oxidation and one for the reductionoxidation and one for the reduction

Six habits of the redox equation Six habits of the redox equation balancerbalancer

STEP 1: the unbalanced equationSTEP 1: the unbalanced equation

Dichromate ion reacts with chloride ion to Dichromate ion reacts with chloride ion to produce chlorine and chromium (III)produce chlorine and chromium (III)

)()()()( 232

72 aqClaqCraqClaqOCr

STEP 2: identify the oxidized and STEP 2: identify the oxidized 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

STEP 3: Balance the half reactionsSTEP 3: Balance the half reactions

OxidationOxidation

ReductionReduction

)()(2 2 aqClaqCl

)(2)( 3272 aqCraqOCr

Material balance with HMaterial balance with H22O and HO and H+ + or OHor OH--

Strategy: add HStrategy: add H22O to the side that lacks for O to the side that lacks for O and add HO and add H+ + (the reaction is in acid (the reaction is in acid solution) to the other sidesolution) to the other side

In basic solution we add OHIn basic solution we add OH-- and H and H22O O instead of Hinstead of H22O and HO and H++ respectively respectively

Test equation for both atoms and chargesTest 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

STEP 4: Material balanceSTEP 4: Material balance

Add HAdd H22O to the side lacking O and add HO to the side lacking O and add H++ to to

the other side (for reactions in acid solution)the other side (for reactions in acid solution) Oxidation reaction – unchangedOxidation reaction – unchanged

Reduction reactionReduction reaction

)()(2 2 aqClaqCl

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

72 lOHaqCraqOCraqH

STEP 5: Balance half-reactions for STEP 5: Balance half-reactions for charge by addition of electronscharge by addition of electrons

Balance Balance chargescharges on both sides of each half- on both sides of each half-reactionreaction

2 x -1 = 2 x -1 = 22 x -1 x -1

14 x +1 + -2 + 14 x +1 + -2 + 66 x -1 = 2 x +3 x -1 = 2 x +3

eaqClaqCl 2)()(2 2

)(7)(26)()(14 232

72 lOHaqCreaqOCraqH

STEP 5 cont: Multiply by factors to STEP 5 cont: Multiply by factors to balance total electronsbalance total electrons

Overall change in electrons must be zeroOverall change in electrons must be zero Multiply the oxidation half reaction by Multiply the oxidation half reaction by 33

3 x 2 = 63 x 2 = 6

)(7)(26)()(14 232

72 lOHaqCreaqOCraqH

eaqClaqCl 2)()(23 2

STEP 6: Add half reactions and STEP 6: Add half reactions and eliminate common itemseliminate common items

++

==

Electrons cancel both sidesElectrons cancel both sides

Atoms Atoms andand charges balance charges balance

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

72 lOHaqClaqCraqClaqOCraqH

eaqClaqCl 6)(3)(6 2

)(7)(26)()(14 232

72 lOHaqCreaqOCraqH

Balanced molecular equationBalanced molecular equation

Add in the spectators: there will always be Add in the spectators: there will always be space. space.

Reagents were KReagents were K22CrCr22OO77, NaCl and H, NaCl and H22SOSO44

Net ionic equationNet ionic equation

Balanced molecular equationBalanced molecular equation

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

72 lOHaqClaqCraqClaqOCraqH

)(7)()(3)()()(6)()(7 242234272242 lOHaqSOKaqClaqSOCraqNaClaqOCrKaqSOH