Electrochem Homework

3, 15, 17, 23, 27, 29, 35, 37, 41, 44, 49, 51, 53, 59, 63, 89, 91, 97a, 99, 117

Oxidation NumbersOxidation Numbers• # of e- gained or lost when forming a compound• If ion, actual charge• Use the Periodic Table!

General RulesGeneral Rules• Free uncombined state of atoms -> 0

Ex. Na, K, S8 , Cl2• Sum of oxidation #’s in neutral compound is zero• Polyatomic ions, oxidation #’s add up to the

charge on the ion• F has a -1 ox. state

General Rules General Rules ContinuedContinued

• H usually has an ox. state of +1 Exception: NaH where H is -1

• O usually has an ox. State of -2Exceptions: O-1 and O-1/2

• Use the Periodic Table to find the location of the element

ExamplesExamples• HNO3

• NO3-1

New MethodNew Method# of valence electrons

- # of unshared electronsMidway number

- # of bonding electrons assigned to it*

oxidation number

* Higher electronegative atom gets the e-

ExamplesExamples• NH3

• CH3OH

Why do we need a new Why do we need a new

method?method?• SO4

-2 vs. S2O3-2

ElectrochemistryElectrochemistry• Study of the relationship between chemical

change and electrical work

• Application of thermodynamics in everyday applicationsEx. Radios, iPods, calculators

• Battery houses a spontaneous chemical reaction that releases free energy to produce electricity

Electrochemistry cont.Electrochemistry cont.

• 2nd type of reaction:

Nonspontaneous and absorbs free energy from an external source of electricity

Ex. Electroplating, recovery of metals from ores

Electrochemistry cont.Electrochemistry cont.• Amount of energy (electrical) consumed or

produced in chemical reactions can be accurately measured

• All electrochemical reactions involve the transfer of electrons

Redox reactions!• The electrochemical cell contains the

reacting system

Electrochemistry cont.Electrochemistry cont.• Sites of oxidation and reduction separated

physically so that reduction occurs at one electrode and oxidation occurs at the other electrode

Red Cat and An Ox

Examples of Redox Examples of Redox reactionsreactions

4 Al(s) + 3 O2(g) ---> 2 Al2O3(s)

Balancing redox Balancing redox reactionsreactions

1. Write the unbalanced equation2. Assign oxidation numbers3. Choose coefficients that make the transfer of

electrons the same4. Balance the charge5. Balance remaining elements if necessary

Balancing In Acidic Balancing In Acidic MediaMedia

• Add H+ and H2O to balance charge

Ex. Cr2O7-2 + I- Cr+3 + IO3

-

Balancing In Basic Balancing In Basic MediaMedia

• Balance charge by adding OH- and H2O

Ex. Cr2O7-2 + I- Cr+3 + IO3

-

Balance in AcidBalance in Acid• MnO4

- + Br- MnO2 + BrO2

-1

Balance in Base Balance in Base (Drano)(Drano)

• Al + H2O [Al(OH)4]- +

H2

Balance in AcidBalance in Acid Cu + NO3

- Cu+2 + NO

Half Rxn Method for balancing Half Rxn Method for balancing

redox equationsredox equations• Overall reaction split into two half reactions• Separates oxidation and reduction half reactions• Easy to balance redox reactions occurring in

acidic or basic solutions

How?How?1. Divide equation into two half reactions2. Balance the atoms and charges in each half

reaction- e- added to left is reduction - e- added to right is oxidation

3. Multiply by integer so e- lost = e- gained4. Add balanced equations

Ex. Balance in acidic mediaEx. Balance in acidic media

Ex. Cr2O7-2 + I- Cr+3 +

I2

Ex. Balance in basic mediaEx. Balance in basic media

MnO4-1 + C2O4

-2 MnO2 + CO3-2

Breathalyzer reactionBreathalyzer reaction

Cr2O7-2

+ C2H5OH + H+ Cr+3 + C2H4O + H2O

EquivalentsEquivalents• Acids and Bases Just deal with the number of

H+ or OH- being donated

Ex. HCl -> 1 equivalent H3PO4 -> 3 equivalents

NaOH -> 1 equivalentCa(OH)2 -> 2 equivalents

Acid ReactionAcid ReactionH2SO4 + H2O 2 H+ + SO4

-2 1 mol 2 mol 1 mol 98.0 g/mol 2.0 g/mol 96.0

g/mol 2 equivalents

1 equivalent HNO3 – 63.01 g1 equiv. CH3COOH – 60.03 g1 equiv. H2SO4 - 49.04 g1 equivalent of Ca(OH)2 - 37.05 g

NormalityNormality• Molarity = moles of solute Liters of solution

• Normality = equivalents of solute Liters of solution• N = M x #equivalents

Equivalents and RedoxEquivalents and Redox

• Not only do equivalents pertain to # H+ or # OH- but also to electrons

• Equivalent – mass of oxidizing or reducing substance that gains or loses 1 mole of electrons

• 1 mole of electrons equals 1 equivalent of electrons

EquivalentsEquivalents• One equiv. oxidized reacts with one equiv.

reduced 1 eq. ox. = 1 eq. red.• If redox equation is balanced – mole ratios

can be used in stoichiometry• If NOT balanced – equivalents can be used

END!END!