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Electrochemical & Voltaic Cells

Objectives:

Key Terms:

Notes: (21-1&2)

Voltage is the measure of energy available to move electrons.    It requires a gradient (more energy on one side than the other) and a conductor.  The flow continues until the circuit reaches a common potential (equal charge) and there is no longer a flow of charge.  

(www.saburchill.com/physics/ images/0189.jpg)

In the picture to the left, the tank on the top represents the electric potential energy (V).  The charge is flowing down a gradient from an area of greater GPE to lower GPE.  This system will eventually run out of energy as its supply in the top tank dwindles.

Sometimes called potential difference (electrical PE)

Measured in volts (V) (1Joule/coulomb)

The higher the voltage, the more energy each electron carries

In order for there to be voltage there must be a continuous replacement of charge. (battery, power plant)

the reaction is driven by the cell (electron) potential

is the difference in the amount of electrons (charge) within each cell connected by a conductor

measured in volts (V)

voltage is the measure of the potential size or force of the charge (also known as potential difference)

can be determined by the difference in the half-cell reduction potentials

measured at standard conditions - 1M, 25oC, and 1 atm

reduction (cathode) potential is represented by a positive number

oxidation (anode) potential is represented by a negative number

the greater the absolute value between the two the greater the voltage that is generated

 

In this system a pump has been installed to maintain the gradient.  In this way the system recycles energy with only small losses due to efficiency (WO/WI).  This is how a battery works.

(www.saburchill.com/physics/ images/0189.jpg)

Voltaic Cells

A voltaic cell operates as oxidation and reduction reactions spontaneously occur between two different materials (usually metals).

Provides a steady electric current from chemical energy.

Wet Cell:

Also called a voltaic cell and is another type of electrochemical cell.

  Consists of an electrolyte solution, a zinc rod, and a copper rod, conductor, and a salt bridge

 

   

(library.tedankara.k12.tr/chemistry/ vol1/redox/trans88.jpg)

The zinc is broken (oxidized) down to release ions (Zn2+) in the electrolyte solution (ZnSO4)  

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

is called the anode

producer of negatively charged electrons

The excess electrons then flow from the anode (negative zinc terminal) through the conductor to the cathode (positive copper terminal) and then back through the electrolyte solution to the zinc terminal.  

The salt bridge is created to separate the two metals

differentially allows ions and electrons to travel from one compartment to another while keeping the metals separate

separates the two half-cells of the battery

each half-cell contains ~ the same ratio of + & - ions as long as the circuit is connected

The copper is built up (reduced) absorbing ions from the electrolyte solution (CuSO4)  

Cu2+(aq) + 2e- g Cu(s) 

is called the cathode

absorbs negatively charged electrons creating a more positive charge

Commonly used as car batteries

Dry Cell:

(naio.kcc.hawaii.edu/chemistry/ everyday_battery_fig1.gif)

The dry cell consists of a zinc can, electrolyte paste, a cardboard salt bridge and a carbon rod in the center.

A chemical reaction takes place between the zinc can and the paste.

Electrons flow from the negative (zinc) terminal to the positive (carbon) terminal

Electrons are then pumped back to the negative terminal

The difference in the amount of electrons at the terminals creates electron pressure or what is called an electrochemical gradient.

A series of one or more dry cells connected to one another is called a battery.  

 

 

Last modified: May 06, 2003