Electrochemistry

Electrochemistry

Electrochemistry involves oxidation reduction

reactions that can be brought about by electricity or used to produce electricity

it is concerned with the conversion of chemical energy to electrical energy, and vise versa

An electrochemical cell is a system which produces electrical energy.

Electrochemistry in Everyday Life

The rusting of iron, the photosynthesis that takes place in the leaves of green plants, and the conversion of foods to energy in the body are all examples of chemical changes that involve the transfer of electrons from one chemical species to another.

When such reactions can be made to cause electrons to flow through a wire or when a flow of electrons makes a redox reaction happen, the processes are referred to as electrochemical changes. The study of these changes is called electrochemistry.

An Electrochemical Cell

Where reduction occurs

voltmeter

Where Oxidation occurs

The voltmeter reveals that e’s are flowing through the wire, from the Zn strip to the Cu strip.

The rxn; Cu2+

(aq) + Zn(s) Cu(s) + Zn2+(aq)

only involves the loss and gain of e’s.

Electrochemical Cell is Split into Two Half-cells

Left half-cell (or half rxn.)

Cu2+(aq) + 2e Cu(s)

Right half-cell (or half rxn.)

Zn(s) Zn2+(aq) + 2e

if the salt bridge is removed, rxn stops like flicking off a switch

REDOX

Oxidation Reaction is a half-rxn in which a species loses e’s

reduction Reaction is a half-rxn in which a species gains e’s

A rxn involving the loss & gain of e’s is called reduction-oxidation rxn or REDOX rxn.

LEO the lion says GER

LEO = Loss of Electrons is Oxidation

GER = Gain of Electrons is Reduction

Look @ rxn. again . . .

Cu2+(aq) + Zn(s) Cu(s) + Zn2+

(aq)

Cu2+(aq) + 2e Cu(s)

Gains e, thus Reduction rxn., Cu2+ is reduced, thus it’s the oxidizing agent

Zn(s) Zn2+(aq) + 2e

Loses e , thus Oxidation rxn., Zn is oxidized, thus it’s the reducing agent

IMPORTANT!!

The oxidizing agent is reduced.

The reducing agent is oxidized.

Sample Questions:1. How can you tell which species

is being oxidized & which is being reduced?

A species being oxidized (loses e) becomes more positively charged (or less negatively charged)

A species being reduced (gains e) becomes more negatively charged (or less positively charged)

2. Indicate the following for questions to come:

i) species oxidizied

ii) Species reduced

iii) oxidizing agent

iv) reducing agent

a) Zn2+ + Mg Zn + Mg2+

Zn2+ + 2e Zn

Zn2+ is reduced, thus it’s an

oxidizing agent

Mg Mg2+ + 2e

Mg is oxidized, thus it’s a

reducing agent

b) H2 + Sn4+ 2H+ + Sn2+

H2 2H+ + 2e

H2 is oxidized, thus it’s a

reducing agent

Sn4+ + 2e Sn2+

Sn4+ is reduced, thus it’s an

oxidizing agent

c) 2Li + F2 2Li+ + 2F

2Li 2Li + + 2e

Li is oxidized, thus it’s a

reducing agent

F2 + 2e 2F

F2 is reduced, thus it’s an oxidizing agent

d) 2Fe2+ + Sn4+ Sn2+ + 2Fe3+

2Fe2+ 2Fe3+ + 2e

Fe2+ is oxidized, thus it’s a

reducing agent

Sn4+ + 2e Sn2+

Sn4+ is reduced, thus it’s an oxidizing agent

Spontaneous Redox Rxns using Reduction Potentials we can predict

whether a rxn. is spontaneous or not imagine a redox rxn. as a competition for e b/t

the 2 half-rxn.’s each half-rxn. has a natural tendency to

proceed as a reduction rxn. the magnitude of the tendency to go through

reduction is expressed by its reduction potential or standard reduction potential @ 25°C, 1 M & 1 atm

the rxn. with the larger reduction potential (i.e. the one with the greater tendency to undergo reduction) acquires e’s from the other half-rxn.

Examples:

Cu: oxidized

Ag+: reduced, thus oxid. agent

Ag+

Cu

Spont.

1.

2.

Zn(s) + Cu2+(aq) Zn2+

(aq) + Cu(s)

Zn: oxidized, thus reducing agent

Cu2+: reduced, thus oxidizing agent

Cu2+

Zn

Spont.

3.

Cu(s) + Zn2+(aq) Cu2+

(aq) + Zn(s)

Cu: oxidized (reducing agent)

Zn2+: reduced (oxidizing agent)

Cu Zn2+

Not Spont.

A rxn. will be spontaneous if & only if there is a reactant to be reduced which is ABOVE a reactant to be oxidized

Basically oxidizing agent must be above reducing agent on chart

Examples:Are the following reactions spontaneous or

not?1. Ni2+ + Ag

2. Zn2+ + Li

3. Sn4+ + Au

4. Sn2+ + Co

5. Cu+ + Sn

6. Al3+ + Ni

Examples:

Which is the stronger oxidizing agent?

1. Cu+ or Sn2+

Cu+

2. Cr2+ or Fe2+

Fe2+

Examples:

Which is the stronger reducing agent?

1. Zn2+ or Ca2+

Ca2+

2. Cr3+ or Cu2+

Cr3+

3. Br2 or I2

I2