GCSE OCR Gateway Organic Chemistry C6.2p Electrochemical Cells

C6.2p To be able to recall that a chemical cell produces a potential difference until the reactants are used up

Electrochemical Cells

Some metals are more reactive than others for example zinc is more reactive than copper.

You will have heard of the reactivity series and you can find out more about it here.

Reactivity is about the gain or loss of electrons from the metal.

Very reactive metals lose electrons easily,  less reactive metal tend to hold on to their outer shell electrons.

Zinc atoms tend to lose their two outer shell electrons

Zn   ⟶     Zn²⁺     +   2e^–

But copper atoms tend to lose electrons less easily than zinc.

Cu   ⟶     Cu²⁺     +   2e^–

So if we place a zinc rod in an aqueous solution of copper sulphate containing blue Cu²⁺ ions, an exchange of electrons takes place with zinc giving its outer shell electrons to the copper ions.

This is called a displacement reaction and I have discussed them here

Here is the equation for that reaction:

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

And you would observe the blue colour fades after a time (Cu²⁺ (aq) is gaining electrons) just as the zinc rod becomes thinner (Zn(s) dissolves as Zn²⁺ (aq), each ion losing 2 electrons in the process).   As well, a brown solid (copper) forms on the zinc since each copper ion has gained 2 electrons from the zinc atom and become a copper atom.

Now if instead you put a zinc strip and a copper strip (both highly polished) in a solution of dilute sulphuric acid nothing happens until you connect them together with a wire, then changes appear at the metal surface: bubbles especially. 

This is called a Voltaic Cell after Alessandro Volta who first built a chemical cell (you can see it in the photo below to the right) and from whom we have the unit of potential difference: the Volt.  

Now connect both metal strips to a voltmeter and it reads a voltage; a potential difference must have been established between the two metals.

(We'll get to the porous vase in a second!!)

This is the basis of all chemical cells, two metals of different receptivity connected electrically produce a potential difference.  

The more reactive metal is releasing electrons and so it is the negative terminal of the cell and the metal receiving the electrons in this case copper is the positive terminal.

Chemical cells produce a potential difference until the reactants (the metals) are used up.  In the copper—zinc cell the potential difference falls as the metal zinc dissolves.

The chemical cell involving copper and zinc was one of the first to be built and is called the Daniell Cell. Here is a picture of typical set up shown in cut away:

And here is the schematic:

If the solution of sulphuric acid is 1M then the cell’s potential difference is 1.1volts.

These were the first electrical cells where chemical energy is converted into electrical energy.  Here is an old picture of one of the original Daniell Cells with some labelling in French, I believe.

The next picture shows how the solutions are kept apart yet electrically connected using a porous pot container.  Ions can pass between the solutions via the holes in the porous pot.

And then here is the set up on the lab bench showing a voltage of 1.09v—must have been a little resistance in the set up to bring the voltage down by 0.01v. 

Other cells are possible as the technology has moved on considerably from the Daniell Cell.  The lithium ion cell in your smart phone is one of the latest incarnations of a chemical cell.

Here is a typical dry cell (dry because not aqueous solutions used in its construction.)