What is an 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.
Predicting the Outcome of Reactions - Part 1
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)
Predicting the Outcome of Reactions - Part 2
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))
Predicting the Outcome of Reactions - Part 3
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.
Finding the Cell Potential
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)
Zn/Cu electrochemical cell = 0.34 - (-0.76) = +1.10 V
Mg/Ag electrochemical cell = 0.80 - (-2.38) = +3.18 V