Honors Chemistry
Chapter 21 Notes – Electrochemistry
(Teacher edition)
Suggested Chapter 21 problem set: 52, 58, 68, 74, 72
21.1Electrochemical Cells
Two Branches of Electrochemistry:
Electrochemistry involves the relation of flow of electrons (current) to chemical changes.
Electrochemistry studies the conversion between chemical and electrical energy.
The 2 main branches are: (both are tied to redox reactions)
Electrolysis: converting electrical energy to chemical energy (electroplating,
charging battery).
Electrochemical cell: converting chemical energy to electrical energy (using a
battery).
Half Reactions and Half Reaction Equations:
Example:
2Na + Cl2 2NaCl
half reaction equations:
2Na 2Na+ + 2e-(oxidation: loss of electrons)
Cl2 + 2e- 2Cl-1(reduction: gain of electrons)
Electric Current: The flow of electric charge (movement of electrons).
in metals – metallic conduction
in solution – ionic conduction – positive and negative ions move along a path
Electrons move away from each other (repulsion) and are attracted to a positive charge.
Direct current: electrons move in one direction only (as opposed to alternating current).
Electrolysis – Current Through an Electrolyte:
Diagram:
Current passes into 1 plate: e- are added from this plate to a chemical in solution.
This is reduction.
Molecules give up electrons at the other plate. This is oxidation.
Electrolysis: electric current causes a redox reaction in a water solution of electrolytes or a
liquid electrolyte.
Cathode: negative plate where reduction occurs.
Anode: positive plate where oxidation occurs.
21.3Electrolytic Cells
Electrolysis – Molten Sodium Chloride:2NaCl(l) 2Na(l) + Cl2(g)
Half reactions:2Na+ + 2 e- 2Na(l)
2Cl- Cl2(g) + 2e-
Electrolysis of Water:2H2O(l) 2H2(g) + O2(g)
cathode reaction:
electrons added
4H2O + 4e- 2H2 + 4OH-1 (basic)
anode reaction:2H2O O2 + 4H+1 + 4e- (acidic)
Add the two reactions above and cancel appropriately:
6H2O 2H2 + O2 + 4H+1 + 4OH-1
2H2O(l) 2H2(g) + O2(g
H2SO4 is added to increase the pace of the reaction (electrolyte). HCl cannot be added as Cl-1 could be oxidized to form Cl2 gas. A ternary acid must be used (HNO3, HClO4, etc.).
Electrolysis of Salt Water:
2H2O + 2e- H2 + 2OH-1
2Cl-1 Cl2 + 2e-
Na is not as easily reduced as water (really Hydrogen).
Water is not as easily oxidized as Cl-1 (really Oxygen)think electronegativity
ionic equation:
2H2O + 2Cl-1 H2 + Cl2 + 2OH-1
molecular equation:
2H2O + 2NaCl(aq) H2 + Cl2 + 2NaOH(aq)
Na+ is spectator ion
Electroplating – coating a cheap metal with an expensive metal
Show diagram: see previous diagram on page 2
Imagine that the anode is a piece of silver metal and that the cathode is a spoon
made up of some other cheap metal.
anode reaction (silver metal plate):AgAg++e-
cathode reaction (spoon) :Ag++e-Ag
21.2 Half-Cells and Cell Potentials - We will skip everything about types of cells – this
information is not necessary to do problems.
To figure out which happens, oxidation or reduction, for two sets of chemicals – choose
the higher, more positive value.
Ex1:2H+ + 2e- H2 (0.0 V)
Or…
Ag+ + e- Ag (+0.80 V)
So…
Ag+ + e- Ag
And…
H2 2H+ + 2e-
Ex2: Cu and Hydrogen (Cu reduction potential = +.52, H = 0.0)
So… Cu gets reduced, Hydrogen gets oxidized.
To find the total Eo (electromotive force) for the reaction: just add the Eo for each
reaction, but switch sign if it’s an oxidation.
Ex3:
Pb+2+2 e-Pb(-0.13 V)
Sn+4+2 e-Sn+2(+0.15 V)
So…
Sn+4+2 e-Sn+2(+0.15 V)
PbPb+2+2 e-(+0.13 V)
+0.28 V
a positive value means the reaction happens
Ex4:
Br2+2 e-2 Br-1(+1.09 V)
Cl2+2 e-2 Cl-1(+1.36 V)
So…
Cl2+2 e-2 Cl-1(+1.36 V)
2 Br-1Br2+2 e-(-1.09 V)
+0.25 Vdoes happen
This makes sense:
2 NaBr + Cl2 2NaCl(Cl is more active than Br)
2 NaCl + Br2 NR(Br can’t replace Cl)
Ex5:
Al + K+1 does this happen?
Al+3+3 e- Al (-1.66 V)
K+1+ e-K(-2.93 V)
So…
Because Al is more easily reduced, this reaction doesn’t happen.
Al+KCl NR
This makes sense. Remember that K is higher on the activity series than Al.
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