April 10, 2002
Announcements – delay with Exam 3 FR – back on Friday.
Capa 5 handed out today - due Sunday after next exam.
Ex. 3 What concentration of Cu2+ is necessary to initiate
precipitation from 0.0015 M KOH? Ksp Cu(OH)2 = 1.6 x 10-19
(Precipitation initiates at equilibrium)
Ksp = [Cu2+][OH-]2
Cu(OH)2 (s)
?
Cu2+ (aq)
x
+
2OH- (aq)
0.0015 M
Ksp = (x)(0.0015)2 = 1.6 x 10-19
x
= 7.1 x 10-14 M
ends Ch. 20. Begin Ch. 21, electrochemistry.
April 10, 2002
Electrochemistry Handout (1)
- chemical changes produced by ___electrical current__ or
- production of ____electricity__ by chemical reactions
- always ___redox___ reactions (1/2 reactions)
LEO – lose electrons oxidation M à M+ + eGER – gain electrons reduction X + e- à XElectrode – surface where redox reaction occurs
- electrode where ___oxidation___ occurs is the ___anode__
- electrode where ____reduction__ occurs is the ___cathode__
an ox
red cat
2 types of electrochemical cells
1) ___electrolytic_ cell, in which electrical energy causes
____nonspontaneous____ chemical reactions to occur
2) __voltaic/galvanic_ cell, in which ___spontaneous____
chemical reactions produce electrical energy
April 10, 2002
Electrolytic Cell
Battery, ammeter, e- flow, liquid media w/ions, electrodes – anode, cathode. ½ reactions.
Sign conventions for electrolytic cell. Ion migration – electrolytic and metallic
conduction.. See also Fig21-1, textbook.
April 10, 2002
Fig 21-2, electrolysis of molten sodium chloride.
Commercial way to make Na, very reactive metal
Melt NaCl, use as liquid media (ion conduction) 800ºC.
Graphite electrodes, inert
Apply battery (∆V), current flows
Cathode: reduction
Na+
+
e-
à
Na
Anode: oxidation
Cl-
à
½ Cl2
Na+ + Cl- à
+
½ Cl2
e+
Na (overall reaction,
Nonspontaneous)
Note: porous barrier prevents spontaneous recombination of Cl2 and
Na within the cell – at 800ºC, explosive. (We do electrolysis in same
container because we need electrolytic conduction can occur.)
What if we used aqueous NaCl instead?
Fig 21-3, electrolysis of NaCl(aq)
Green gas still formed at anode – Cl- oxidation:
Anode: oxidation
Cl-
à
½ Cl2
+
e-
But now, at cathode, not sodium metal – a gas is forming.
April 10, 2002
Note: Na reacts violently with water:
Na(s)
+
H2O
à
NaOH
+
½ H2
(+ heat)
Bad combination
Evolved gas at electrode is H2 – but it’s not because Na(s) forms and
then immediately reacts. Instead, H2O is reduced at cathode, directly:
Cathode: reduction
H2O +
e-
à
½ H2
+
OH-
* in an electrochemical cell*
- most easily oxidized species is oxidized
- most easily reduced species is reduced
(we’ll discuss later how to determine what is ‘most easily’ oxidized
or reduced).
Overall reaction:
Cl-
à
½ Cl2
+
e-
H2O +
e-
à
½ H2
+
OH-
Cl- +
H2O à
½ H2
+
½ Cl2
+
OH-
This is the commercial prep for H2, Cl2, and NaOH (evaporate away
water at end, get NaOH(s).)
Next time: electrolysis of Na2SO4.