Unit 5 Section 2 Electrochemical Series

Electrochemical series: A list of electrode potentials written in order from most negative to most positive.

Electrode potential: the voltage measured when a half-cell is connected to a standard hydrogen electrode

An electrochemical series shows you what's reactive and what's not. The more reactive a metal is, the more it wants to lose electrons to form a positive ion. More reactive metals have more negative standard electrode potentials.

The more reactive a non-metal the more it wants to gain electrons to form a negative ion. More reactive non-metals have more positive standard electrode potentials.

e.g. Will zinc react with aqueous copper ions?

1. Find the two half equations for the redox reaction, and write them both out as reduction reactions:

• Zn,2+(aq) + 2e,- <<<>>> Zn(s) and Cu,2+(aq) + 2e,- <<<>>> Cu(s)

2. Use an electrochemical series to work out which half-equation has the more negative electrode potential:

• Zn,2+/ZN = -0.76 V

Cu,2+/Cu = +0.34 V

3. Put the half equation with the more negative electrode potential on top of the other one:

• Zn,2+(aq) + 2e,- <<<>>> Zn(s)

Cu,2+(aq) + 2e,- <<<>>> Cu(s)

4. Draw on the anti-clockwise arrows:

• Zn,2+(aq) + 2e,- <<<>>> Zn(s) <--- anti-left

Cu,2+(aq) + 2e,- <<<>>> Cu(s) ---> anti-right

5. Look at the non-pointy ends of the arrows to work out if the reaction would happen:

• The substance at the ends of the arrows are Zn(s) and Cu,2+(aq). So zinc will react with aqueous copper(II) ions. (The arrows show that the zinc half-reaction goes backwards and the copper half-reaction goes forwards, so the overall equation is: Zn(s) + Cu,2+(aq) >>> Zn,2+(aq) + Cu(s))

e.g. Will silver react with aqueous magnesium ions?

Following the same steps as before, you get:

Mg,2+(aq) + 2e,- <<<>>> Mg(s) <--- anti-left = -2.38 V

Ag,+(aq) + e,- <<<>>> Ag(s) ---> anti-right = +0.80 V

Here, the substances at the non-pointy ends of the arrows are Mg(s) and Ag,+(aq). So, the magnesium will react with aqueous silver ions, but silver and aqueous magnesium ions will not react.

You can also use the anti-clockwise rule to find cell potentials. To find the cell potential use this formula:

• E*cell = (E*bottom - E*top)

e.g.
Zn/Cu electrochemical cell = 0.34 - (-0.76) = +1.10 V

Mg/Ag electrochemical cell = 0.80 - (-2.38) = +3.18 V