ELECTROCHEMICAL CELLS

In redox reactions, there is a chemical reaction and an exchange of electrons between the particles being oxidized and reduced. An electrochemical cell involves a chemical reaction and a flow of electrons.

PARTS OF AN ELECTROCHEMICAL CELLPARTS OF AN ELECTROCHEMICAL CELL

RED CAT - REDuction = CAThode,RED CAT - REDuction = CAThode,AN OX” – OXidation = ANodeAN OX” – OXidation = ANode

electrodes anode - site where oxidation occurs cathode – site where reduction occurs

half cells – separate containers in which oxidation and reduction half reactions occur

Electrons travel from the anode to the cathode (from loss to gain, from oxidation to reduction) across the wire.

U-tube or salt bridge – lets ions travel between half cells to complete the circuit –ONLY for a voltaic cell

TYPES OF ELECTROCHEMICAL TYPES OF ELECTROCHEMICAL CELLSCELLS VOLTAIC (or galvanic) cell (spontaneous reactions)

uses chemical energy to produce electricity ex include lead acid storage battery (automobile battery), and breathalyzers

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Electrochemical Processes – Electrochemical Processes – Table J Table J

What is Happening Here?

1) The oxidized metal (Al, anode) gets smaller as it turns into metal ions and dissolves into the solution. The reduced metal (Ag, cathode) gets larger as it gets coated with the reduced metal ions from solution.

2) The negative (NO3 -1) anions go through the salt bridge from cathode to anode (from the Ag half-cell to the Al half-cell)

3) The positive (Al+3) cations go through the salt bridge from anode to cathode (from the Al half-cell to the Ag half-cell)

4) The salt bridge can contain a solution of any salt, as long as it does not contain Al or Ag. NaCl can be used. When the cell is first used, Na+1

passes into the cathode half-cell, and Cl-1 passes into the anode half-cell. Later, the ions mentioned in 3) and 4) take their place.

Electrode potential (voltage) is positive; when at zero, you have a dead cell or a dead battery Cathode is (+), anode is (-)

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salt bridge

voltmeter

0

Zn metalAnode

Cu metal Cathode

Zn+2

Zn+2

SO4-2 SO4

-2

Cu+2

Cu+2

ELECTROLYTIC CELLELECTROLYTIC CELL (non spontaneous reactions) uses electricity to cause

a chemical reaction ex include: recharging a battery, electrolysis of a salt and electroplating (ie. jewelry)

Electrode potential is negative (negative voltage reading)

The polarities of the electrodes are reversed (Cathode = negative: anode = positive)

Electrolysis of KI

Using Electrolysis in Metal Using Electrolysis in Metal

(Electroplating)(Electroplating) When electroplating, metal being used is on the anode and object being plated is on the cathode

•In an electrolytic cell used for plating, the object to be plated (a bracelet) is the cathode. •The anode is the plating metal, in this case, a piece of silver. •At the anode, silver is oxidized to silver ions. •Silver ions in the electrolyte solution reduce to silver metal. •The silver metal forms a thin coating on the cathode, producing a plated bracelet.

Electrolytic cells requires electrical energy to produce chemical changes. This process is called electrolysis. Signs of the electrodes switch! Still ANOX REDCAT Anode is positive ; cathode is negative

COMPARING ELECTROCHEMICAL CELLS Component Electrolytic cell Voltaic cell

Electrodes Usually a metal or another conductor

Usually a metal or another conductor

Anode (AN OX) Oxidation half reaction takes place

Has a (+) charge

Oxidation half reaction takes place

Has a (-) charge

Cathode (RED CAT) Reduction half reaction takes place

Has a (-) charge

Reduction half reaction takes place

Has a (+) charge

Electrode potential(Voltage)

negative Positive

When voltage is zero, the cell is dead

spontaneous Yes; converts chemical energy to electrical energy

No, requires an outside energy source; converts electrical energy to chemical energy

Voltaic Cells -vs- Electrolytic Cells