9.5 – Electrolytic Cells
Electrolysis is used to isolate highly reactive metals, such as sodium and potassium, which would not react in a voltaic cell, but require electrical energy for the reaction to occur. The reactions taking place here are non-spontaneous.
9.5.1 – Describe, using a diagram, the essential components of an electrolytic cell
The power source should be a battery or a DC power source. The electrodes are placed in the electrolyte, connected to the power supply by electrical wires. They must not touch each other.
The electrical energy supplied by the power source induces a flow of electrons from the anode to the cathode. However, unlike in voltaic cells, the anode is the positive electrode and the cathode is the negative electrode.
When the current reaches the electrolyte, the electrical energy is carried by ions, which migrate to the electrodes. The redox reactions taking place at the electrodes take the ions of out solution and allow it to continue.
9.5.2 – State that oxidation occurs at the positive electrode (anode) and reduction occurs at the negative electrode (cathode)
The power source causes the polarity of the electrodes to change.
The negative electrode, or the cathode, has excess electrons, which are removed in a reduction reaction. The cations present in solution will act as oxidising agents and accept the electrons.
The positive electrode, or the anode, is deficient in electrons, which are added back in an oxidation reaction. The anions present in solution will act as reducing agents and donate the electrons.
9.5.3 – Describe how current is conducted in an electrolytic cell
The flow of energy in an electrolytic cell has two forms. From the anode to the cathode, the electrons pass along a conductive wire in the form of electrical energy. Therefore, the current is conducted by the movement of electrons in the external part of the circuit.
However, electrical energy cannot move through the electrolyte, so the current moves in a different form. At the cathode, electrons are accepted by the positive ions. The flow of current in the electrolyte is facilitated by ions, or chemical energy– the positive ions flow towards the cathode and the negative ions flow towards the anode.
9.5.4 – Deduce the products of the electrolysis of a molten salt
Since sodium ions and chlorine ions do not react spontaneously, they require energy in order for the reaction to proceed. In the electrolysis of molten sodium chloride, sodium metal and chlorine gas are formed.
In this electrolytic cell, the electrolyte is molten NaCl. The electrical current flows towards the negative electrode, causing the positive Na+ ions to be attracted to it. These ions accept the excess electrons in a reduction reaction, forming sodium metal on the cathode. As the electrical current continues to flow, this in turn creates a flow of Na+ ions towards the negative electrode.
At the positive electrode, electrons are pulled away, making the anode electron deficient. As a result, the negative Cl– ions in the electrolyte are attracted to it. These will donate electrons to the anode in an oxidation reaction, forming chlorine gas in the process.
The general half-equations for these reactions are:
Cathode: M+ + e– → M
Anode: A– → A + e–
Summary of Voltaic and Electrolytic Cells:
^ This comes up in exams alot