This is Part 2 of a two-part introduction to electrochemical cells. Make sure you have seen Part 1 before you view this video.

In Part 2, we’ll explain what happens in the salt bridge, and why it is necessary for the cell to function. We will also review what we’ve learned in Parts 1 and 2.

In order to understand what the function of the salt bridge is, we’ll see what would happen if we didn’t have the salt bridge. + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn 2+ Zn Cu Zn

As positive ions, or cations are formed at the anode (click) and enter the solution, + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn 2+ Zn Cu Zn 2+ Zn

An excess of positive charge would build up in the zinc nitrate solution. Notice we have a total of 4 positive charges and 2 negative charges on the ions depicted in this solution. + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn 2+ Zn Cu Zn 2+ An Excess of + Charge

And looking at the cathode on the left, as copper cations leave the solution and attach to the metal, + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn 2+ Zn Cu Zn 2+ An Excess of + Charge

Forming copper atoms + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn 2+ Zn Cu Zn 2+ Cu An Excess of + Charge

A deficiency of positive charge would develop in the copper(II) nitrate solution. Notice we have a total of 4 positive charges and 6 negative charges on the ions depicted in this solution. + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn 2+ Zn Cu Zn 2+ Cu A Deficiency of + Charge An Excess of + Charge

Alternately, we could also take the viewpoint of negative charge. (click) We could say that the solution around the zinc anode (click) now has a deficiency of negative charge, + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn 2+ Zn Cu Zn 2+ Cu A Deficiency of – Charge

And the solution around the copper cathode (click) now has an excess of negative charge + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn 2+ Zn Cu Zn 2+ Cu An Excess of – Charge A Deficiency of – Charge

The fact is that unbalanced charges cannot really exist in solutions. In every solution, the total positive charge of the cations must equal the total negative charge of the anions + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn 2+ Zn Cu Zn 2+ Cu Unbalanced charges cannot exist in solutions

So the process we described here could not really work with the cell right now. + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn 2+ Zn Cu Zn 2+ Cu

We’ll drop a salt bridge into this cell between the two beakers. + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn 2+ Zn Cu Zn 2+ Cu cotton salt bridge (KNO 3(aq) ) K+K+ K+K+ K+K+ K+K+ K+K+ K+K+

Some of the excess positive charge in the solution around the anode + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn 2+ Zn Cu Zn 2+ Cu cotton salt bridge (KNO 3(aq) ) K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ An Excess of + Charge

Would be relieved by a gradual migration of cations away from the anode, toward the cathode, through the salt bridge. + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn Cu Zn 2+ Cu cotton salt bridge (KNO 3(aq) ) K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ An Excess of + Charge Zn 2+

Similarly, some of the excess negative charge in the solution around the cathode + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn Cu Zn 2+ Cu cotton salt bridge (KNO 3(aq) ) K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ An Excess of – Charge Zn 2+ K+K+

Would be relieved by a gradual migration of anions away from the cathode, toward the anode, through the salt bridge. + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn Cu Zn 2+ Cu cotton salt bridge (KNO 3(aq) ) K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ An Excess of – Charge Zn 2+ K+K+

So if we consider just the ions in the salt bridge, we can picture cations moving toward the cathode, and anions moving toward the anode. + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn Cu cotton salt bridge (KNO 3(aq) ) K+K+ K+K+ K+K+ K+K+ K+K+ K+K+

And if we also consider the wires, we can picture electrons moving from the anode toward the cathode through the wires, and the light bulb. So cations and electrons move toward the cathode and anions move toward the anode. + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn Cu cotton salt bridge (KNO 3(aq) ) K+K+ K+K+ K+K+ K+K+ K+K+ K+K+ e–e–

If we consider just Zinc cations that came from the zinc electrode, (click) they move into the salt bridge as they migrate toward the cathode, just like the other cations. + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn Cu cotton salt bridge (KNO 3(aq) ) Zn 2+

Now the copper cations that were already in the copper(II) nitrate solution around the cathode, are cations too, so they will not enter the salt bridge, but will (click) also move toward the cathode + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn Cu cotton salt bridge (KNO 3(aq) )

Where they will be reduced to neutral copper atoms. + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn Cu cotton salt bridge (KNO 3(aq) ) 2e – Cu

So we can summarize by saying (click) that all cations move toward the cathode, + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn Cu cotton salt bridge (KNO 3(aq) ) Cu All cations move toward the cathode

And all anions move toward the anode, + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn Cu cotton salt bridge (KNO 3(aq) ) Cu All anions move toward the anode

And electrons move from the anode toward the cathode through the wires. + – Zn  Zn e – (oxidation) ANODE Cu e –  Cu (reduction) CATHODE Zn Cu cotton salt bridge (KNO 3(aq) ) Cu Electrons move from the anode toward the cathode.

Here, we’ll summarize the main things you need to remember about electrochemical cells containing two metals and solutions containing their cations.  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

Electrochemical cells are also known as Voltaic Cells. You may see this term in some textbooks.  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

Oxidation always occurs at the anode. One way to remember this is Oxidation and Anode both start with vowels.  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

For example, in the cell we’ve shown here, zinc metal atoms are oxidized to zinc ions.  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

Reduction occurs at the cathode. Both Reduction And Cathode start with consonants.  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

For example, in the cell we’ve shown here, copper (2+) cations are reduced to copper metal atoms at the cathode.  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

If the anode is not made of an inert metal, it will lose mass as its atoms oxidize into ions.  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

For example, this half-reaction tells us that zinc metal oxidizes to zinc ions, thus dissolving into the solution.  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

The cathode gains mass as cations are reduced to become metal atoms, which adhere to its surface.  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

For example, in this cell copper (2+) ions in solution are reduced to copper metal atoms, which stick to the surface of the copper electrode, thus increasing its mass  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

Electrons move from the Anode to the Cathode in the wires. From A to C.  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy A  C In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

This makes sense because electrons are lost by the anode  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy A  C In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

And gained by cations at the cathode.  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy A  C In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

Cations move toward the cathode in the salt bridge (Both words start with “Cat”..  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

This makes sense because cations are produced at the anode,  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

And used up at the cathode.  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

Anions move toward the Anode in the salt bridge. Both Anions and Anode, start with “an”…  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

It is important to remember that reactions in an electrochemical cell are always spontaneous. In another type of cell we’ll look at later, an electrolytic cell, the reactions are nonspontaneous.  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

In all electrochemical cells, chemical energy is transformed into electrical energy.  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:

It is good to study and be familiar with all these facts about electrochemical, or voltaic cells. In Simple Electrochemical Cells (Voltaic Cells) with Two Metals and Solutions Containing their Cations:  OXIDATION occurs at the ANODE (e.g. Zn  Zn e – )  REDUCTION occurs at the CATHODE (e.g. Cu e –  Cu)  The ANODE LOSES MASS as it is oxidized to ions  The CATHODE GAINS MASS as cations are reduced to metal atoms  ELECTRONS move from the ANODE  the CATHODE in the wires  CATIONS move toward the CATHODE in the salt bridge  ANIONS move toward the ANODE in the salt bridge  The HALF-REACTIONS are SPONTANEOUS  CHEMICAL energy is transformed to ELECTRICAL energy