redox reactions & electrochemistry. what is redox? many of the reactions you have performed this...

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REDOX REACTIONS & ELECTROCHEMISTRY

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Page 1: REDOX REACTIONS & ELECTROCHEMISTRY. What is REDOX? Many of the reactions you have performed this year have been REDOX reactions even though you may not

REDOX REACTIONS&

ELECTROCHEMISTRY

Page 2: REDOX REACTIONS & ELECTROCHEMISTRY. What is REDOX? Many of the reactions you have performed this year have been REDOX reactions even though you may not

What is REDOX?

Many of the reactions you have performed this year have been REDOX reactions even though you may not have known it.

So, now you may well ask,what are they?

REDOX reactions are those in which electrons are transferred from one species to another (i.e. atom, ion, etc.).

Electron transfer involves, you guessed it, production of electricity.

Page 3: REDOX REACTIONS & ELECTROCHEMISTRY. What is REDOX? Many of the reactions you have performed this year have been REDOX reactions even though you may not

What does “REDOX” stand for?

RED = reduction, which occurs when a species (ion or atom) gains an electron

OX = oxidation, which

occurs when a

species (ion or atom)

loses an electron

Page 4: REDOX REACTIONS & ELECTROCHEMISTRY. What is REDOX? Many of the reactions you have performed this year have been REDOX reactions even though you may not

How can I remember what REDOX means?

Remember LEO the Lion says GER

LEO ~ lose electrons = OXIDATION

GER ~ gain electrons = REDUCTION

LEO - GER

Page 5: REDOX REACTIONS & ELECTROCHEMISTRY. What is REDOX? Many of the reactions you have performed this year have been REDOX reactions even though you may not

Now for some examples of REDOX REACTIONS....

#1 Fe + 2HCl → H

2 + FeCl

2

#2 H

2O

2 + Ni2+ → H

2O + Ni3+

#3 H

2 + Cl

2 → 2HCl

Note in #1 that the oxidation number (charge) for the iron (Fe) has gone from zero to 2+.

Note in #1 that the oxidation number (charge)for the hydrogen (H) has gone from 1+ to zero.

Your task is to determine the oxidation number changes in #2 and #3.

Page 6: REDOX REACTIONS & ELECTROCHEMISTRY. What is REDOX? Many of the reactions you have performed this year have been REDOX reactions even though you may not

Look again at the reaction in #1

Fe + 2HCl → H2 + FeCl

2

Let's “dissect” this reaction.

The entire reaction consists of two “half” reactions

Fe → Fe2+ + 2eThe iron atom has LOST two electrons,

therefore,it has oxidized (LEO).

2H1+ + 2e → H2The hydrogen ions have GAINED two

electrons, therefore,they have reduced (GER).

NOTE: The chlorine ions have not changed; they are “spectator” ions.

Page 7: REDOX REACTIONS & ELECTROCHEMISTRY. What is REDOX? Many of the reactions you have performed this year have been REDOX reactions even though you may not

Just as all reactions must balance, so too the number of electrons

transferred must balance.

Fe + 2HCl + 2e → H2 + FeCl

2 + 2e

There can be no REDUCTION without OXIDATION. You can't have one without the other.

There is no net change in the number of electrons in a REDOX reaction.

NOTE: The species that accepts the electrons is the “oxidizing agent.” Why? Because is is facilitating the

oxidation of some other species.

NOTE: The species that donates the electrons is the “reducing agent.” Why? Because is is facilitating the

reduction of some other species.

Page 8: REDOX REACTIONS & ELECTROCHEMISTRY. What is REDOX? Many of the reactions you have performed this year have been REDOX reactions even though you may not

Where do I find REDOX reactions? Are they only in the lab? No!

Fire is a REDOX reaction. Rusting is a REDOX reaction.

Page 9: REDOX REACTIONS & ELECTROCHEMISTRY. What is REDOX? Many of the reactions you have performed this year have been REDOX reactions even though you may not

Why is the Statue of Liberty Green?Because of a REDOX reaction.

The Statue of Liberty was colored when it was first made.

The statue reacted with water and carbon dioxide in the air to form a green “patina”

The copper carbonate and copper hydroxide are both insoluble in water.

2Cu(s)

+ H2O

(g) + CO

2(g) + O

2(g) → CuCO

3(s) + Cu(OH)

2(s)

Page 10: REDOX REACTIONS & ELECTROCHEMISTRY. What is REDOX? Many of the reactions you have performed this year have been REDOX reactions even though you may not

Balancing REDOX reactions

It is possible to balance REDOX reactions and half reactions through a series of steps that are found on p. 90 of Masterson and Hurley. You will not be expected t o balance these equations here, but expect to do lots of them if you take chemistry in college. You should be able to tell whether a half reaction is oxidation (LEO) or reduction (GER).

Page 11: REDOX REACTIONS & ELECTROCHEMISTRY. What is REDOX? Many of the reactions you have performed this year have been REDOX reactions even though you may not

ELECTROCHEMISTRY

Electrochemistry is the study of the interconversion between electrical and chemical energy.

All the reactions in electrochemistry are REDOX reactions.

Electrochemistry takes place in one of two types of electrochemical cells.

VOLTAIC ELECTROLYTIC

Page 12: REDOX REACTIONS & ELECTROCHEMISTRY. What is REDOX? Many of the reactions you have performed this year have been REDOX reactions even though you may not

Voltaic cell ~ a spontaneous reaction that generates electrical energy.

The cell must be designed so that oxidation occurs at one electrode (anode) and reduction occurs at the other electrode (cathode).

Notice the “salt bridge” connecting the two halves and completing the circuit.

Page 13: REDOX REACTIONS & ELECTROCHEMISTRY. What is REDOX? Many of the reactions you have performed this year have been REDOX reactions even though you may not

Electrolytic cell ~a non-spontaneous redox reaction made to occur by pumping electricity into the system

The electron pump (electrical source) pushes electrons into the system at the cathode and removes them from the anode.

Reduction occurs at the cathode (- electrode) because it attracts (+) cation

Oxidation occurs at the anode (+ electrode) because it attracts (-) anions

The REDOX reaction that occurs in an electrolytic cell is called “electrolysis” which means to break up with electricity

Page 14: REDOX REACTIONS & ELECTROCHEMISTRY. What is REDOX? Many of the reactions you have performed this year have been REDOX reactions even though you may not

Electroplating is one use of electrolytic cells

People often “bronze” their children’s baby shoes or otherwise have them coated with a metallic coating for posterity. The shoes are covered with a graphite paste which acts as the cathode and dipped in a metallic ion solution.