Electrochemistry Ch.19 & 20

Using chemical reactions to produce electricity

Objectives

Distinguish between oxidation and reduction

Identify the path taken by electrons in an electrochemical cell

Describe how current flows in a voltaic cell

Oxidation and Reduction

Any chemical process in which elements undergo changes in oxidation number is an oxidation-reduction reaction.

This name is often shortened to Redox reaction.

Reactions are carried out in electrochemical cells called: half cells

ReductionReaction in which the oxidation state of an element

decreases

When an atom or ion gains one or more electrons

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

The species being reduced is called the oxidizing agent

Reduction takes place at the cathode (+):metal electrode placed in a half cell

OxidationReaction in which the atoms or ions increase in oxidation

state

When an atom or ion losses one or more electrons.

Zn (s) Zn2+ + 2e-

The species being oxidized is called the reducing agent.

Reduction takes place at the anode (-).

Redox Reactions

O I L R I GXIDATION

S OSS

of

e-

EDUCTION

S AIN

of

e-

In a redox reaction,

number of electrons lost = number of electrons gained

Total Redox Reaction of Cu & Zn:

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

Assigning Oxidation Numbers

General Rules

Assign the most electronegative element first with the value it would have as an anion

Next, decide the oxidation number of the cation (same as usual), if an ionic compound. Or determine the o.n. of the left most element. Always leave the middle element last.

The charge the atom would have in a molecule (or anionic compound) if electrons were completely transferred

Assigning Oxidation NumbersGeneral Rules (continued):

The oxidation numbers of all elements present should add up to zero , if a neutral compound.

Or add up to the charge if an ion.

Specific Rules:

Pure elements always have an oxidation number of zero.

Assigning Oxidation NumbersSpecific Rules (continued):

Fluorine always is _____. It’s the most e.n. element.

Oxygen can be +2 or -2, except in perioxide compounds; Then it is -1.

Hydrogen can be +1 or -1 (only -1 with a metal)

Electron Pathway in an Electrochemical Cell

How are electrochemical processes carried out?Both oxidation and reduction must occur in an

electrochemical reaction.

Half-cell: A single electrode (anode or cathode) immersed in a solution of its ions

An electrochemical cell will contain 2 half-cell

Electrons can be transferred from one half-cell to the other through an external connecting wire called a circuit

• Electricity is the movement of electrons along a given path.

• The circuit is a closed loop path, so the movement of electrons through the wire is balanced by the movement of ions in solution.

Electrochemical Cell

19.2

spontaneousredox reaction

anodeoxidation

cathodereduction

• Voltaic cells (galvanic cells) use spontaneous oxidation-reduction reactions to convert chemical energy into electrical energy.

• The most common application of voltaic cells is in batteries.

Batteries

19.6

Leclanché cell

Dry cell

Zn (s) Zn2+ (aq) + 2e-Anode:

Cathode: 2NH4 (aq) + 2MnO2 (s) + 2e- Mn2O3 (s) + 2NH3 (aq) + H2O (l)+

Zn (s) + 2NH4 (aq) + 2MnO2 (s) Zn2+ (aq) + 2NH3 (aq) + H2O (l) + Mn2O3 (s)