An electrochemical reaction is a chemical reaction that produces a voltage, and therefore a ﬂow of electrical current. An electrochemical reaction can also be the reverse of this process, in other words if an electrical current causes a chemical reaction to take place (electrolysis).
Electrochemistry is the branch of chemistry that studies these electrochemical reactions. In this tutorial, we will be looking more closely at diﬀerent types of electrochemical reactions, and how these can be used in diﬀerent ways.
Half-cell reactions in the Zn-Cu cell
The experiment below demonstrated a zinc-copper cell. This was made up of a zinc half cell and a copper half cell.
A half cell is a structure that consists of a conductive electrode surrounded by a conductive electrolyte. For example, a zinc half cell could consist of a zinc metal plate (the electrode) in a zinc sulphate solution (the electrolyte).
Reactions in the Zn-Cu cell
At the copper plate, there was an increase in mass. This means that Cu2+ ions from the copper sulphate solution were deposited onto the plate as atoms of copper metal. The half-reaction that takes place at the copper plate is:
Cu2+ + 2e− → Cu (Reduction half reaction) Another shortened way to represent this copper half-cell is Cu2+/Cu.
At the zinc plate, there was a decrease in mass. This means that some of the zinc goes into solution as Z2+ ions. The electrons remain on the zinc plate, giving it a negative charge. The half-reaction that takes place at the zinc plate is:
Zn → Zn2+ + 2e− (Oxidation half reaction) The shortened way to represent the zinc half-cell is Zn/Zn2+.
The overall reaction is:
Zn + Cu2+ + 2e− → Zn2+ + Cu + 2e− or, if we cancel the electrons: Zn + Cu2+ → Zn2+ + Cu
For this electrochemical cell, the standard notation is: Zn|Zn2+||Cu2+|Cu where
| = a phase boundary (solid/aqueous) || = the salt bridge
In the notation used above, the oxidation half-reaction at the anode is written on the left, and the reduction half-reaction at the cathode is written on the right. In the Zn-Cu electrochemical cell, the direction of current ﬂow in the external circuit is from the zinc electrode (where there has been a build up of electrons) to the copper electrode.
Components of the Zn-Cu cell
In the zinc-copper cell, the copper and zinc plates are called the electrodes. The electrode where oxidation occurs is called the anode, and the electrode where reduction takes place is called the cathode. In the zinc-copper cell, the zinc plate is the anode and the copper plate is the cathode.
An electrode is an electrical conductor that is used to make contact with a metallic part of a circuit. The anode is the electrode where oxidation takes place. The cathode is the electrode where reduction takes place.
The zinc sulphate and copper sulphate solutions are called the electrolyte solutions.
An electrolyte is a substance that contains free ions and which therefore behaves as an electrical conductor.
The U-tube also plays a very important role in the cell. In the Zn/Zn2+ half-cell, there is a build up of positive charge because of the release of electrons through oxidation. In the Cu2+/Cu halfcell, there is a decrease in the positive charge because electrons are gained through reduction. This causes a movement of SO2− 4 ions into the beaker where there are too many positive ions, in order to neutralize the solution. Without this, the ﬂow of electrons in the outer circuit stops completely. The U-tube is called the salt bridge. The salt bridge acts as a transfer medium that allows ions to ﬂow through without allowing the diﬀerent solutions to mix and react.
A salt bridge, in electrochemistry, is a laboratory device that is used to connect the oxidation and reduction half-cells of a galvanic cell.