ELECTROCHEMISTRY - WHAT IS IT?
● The branch of chemistry that deals with the
relationship between electricity (flow of electrons)
and chemical reactions.
● Reactions that involve transfer of
electrons (reduction and oxidation, called redox
)
● Spontaneous reactions - galvanic cells or
batteries
● Nonspontaneous reactions -
Electrochemical cells
● Systems utilizing a redox reaction to produce
or use electrical energy
Galvanic cell (Voltaic )
● Spontaneous
● Produces electricity - use activity difference
between two metals
● Converts chemical energy to electrical energy
● Composed of two half cells
● Voltage generated
Electrolytic Cell
● Non- spontaneous
● Redox reaction forced to occur with external
power source
● Converts electrical energy to chemical
energy
● Uses electricity to break ionic compounds
(electrolysis)
Electrochem Terminology
● Species means atom, ion or molecule
● Oxidation number - same as the charge
or the valency
● Example → Mg2+,
○ Charge is 2+
○ Valency is 2+
○ Oxidation number is 2+
Electrochem Terminology
●
Oxidation - a species undergoing
oxidation
○ loses electrons
○ Has an increase in the oxidation
number
○ Charge becomes more positive
Electrochem Terminology
Which equation represents the oxidation of a neutral atom of A?
1.
2.
3.
4.
A+ → A- + e
A → A+ + e
A → A- + e
A- → A+ + e
Answer
2
Reactant A can be a Neutral atom, a cation or anion or even a molecule
1.
2.
3.
4.
Anion
Anion
Neutral
Cation
A2- → A- + e
A- → A + e
A → A+ + e
A+ → A2+ + e
Electrochem Terminology
● Reduction
– a species undergoing reduction
○ gains electrons
○ Has a decrease in oxidation
number
○ Charge becomes more negative
Electrochem Terminology
Which equation represents the reduction of a neutral
atom of X?
Electrochem Terminology
● Oxidizing agent –
○ The species being reduced
○ Gains electrons
○ Causes the other one to be oxidized
Electrochem Terminology
● Reducing agent –
○ The species being oxidized
○ loses electrons
○ causes the other one to be reduced
Electrochem Terminology
Reducing and Oxidizing agent
Oxidation/reduction & Oxidizing/and Reducing
agent
Electrochem Terminology
● REDOX - Short for Oxidation and Reduction
● Both reactions are occuring at the same time
● Each reaction is called a HALF - REACTION
Redox reactions
never show
electrons!
Half-rxn’s always
have electrons
Reduction Half-rxn:
Oxidation Half-rxn:
Learning Check
Identify the species being oxidized and
reduced in each of the following
reactions:
oxidized
reduced
a. Cr+ + Sn4+ → Cr3+ + Sn2+
A.
Cr+
Sn4+
b. 3 Hg2+ + 2 Fe (s) → 3 Hg2 + 2 Fe3+
B.
Fe (s)
Hg2+
C.
As
Cl2
c. 2 As (s) + 3 Cl2 (g) → 2 AsCl3
Learning Check
Would you use an oxidizing agent or reducing
agent in order for the following reactions to occur?
a. O2- → O2
a.
Oxidizing agent
b. Cl2 → Cl-
B. reducing agent
c. Mn2+ → Mn4+
C. oxidizing agent
d. Zn → Zn2+
D. oxidizing agent
Learning Check
Question - In the reaction:
2Fe2+ + Cl2 → 2Fe3+ + 2Cl- …. Identify the following
The Oxidizing Agent: _____________________
The species being oxidized:________________
The reducing agent:______________________
The species being reduced:________________
The species gaining electrons:______________
The species losing electrons:_______________
The product of oxidation___________________
The product of reduction___________________
Learning Check
1. Redox rxn: Pb2+(aq) + Zn(s) → Pb(s) + Zn2+(aq
a. The Pb2+ (loses/gains) ________ 2 electrons.
b. What is the reduction half reaction?
______________________
c. What is the oxidation half reaction?_______________________
d.
e.
In oxidation reactions, e’s are _______ and are found on the ___________ side.
In reduction reactions, e’s are _______ and are found on the ___________ side.
2. Given
the redox reaction: F2(g) + Sn2+(aq) → 2F-(aq + Sn4+(aq)
a. Write the oxidation half-rxn:_____________________________
ANSWERS
Question 1
a. Gains
b. Pb2+(aq) + 2e- → Pb(s)
c. Zn(aq) → Zn2+(S)+ 2e
d. Lost , right
e. Gained, left
Question 2.
a.Sn2+ → Sn4+ + 2e
b. F2 + 2e → 2F-