Chemical Reactions involve the interaction between electrons and atomic nuclei during which electrons are transferred from one atom to another or shared between atoms. Reactions involving electron transfer are called oxidation-reduction(redox) reactions.
Redox reactions involving electron-transfer through a path outside the chemical system are called electrochemical reactions. There are two types of electrochemical reactions the first occurs during the process called electrolysis while the other type occurs in a set-up called an electrochemical cell.
Certain substances allow electric current to pass through them, while others do not. Elements can be classified into conductors and non-conductors in relation to conductivity while compounds can be classified into electrolytes and non-electrolytes.
Conductors are elements that allow electric current to pass through them. They are metals such as iron, magnesium, copper, zinc rod, and graphite while non-conductors are elements that do not allow electricity to pass through them and this is because they do not possess mobile electrons in their atom.
Differences Between a Conductor and Electrolyte
You might be tempted to think that a conductor is the same as an electrolyte because they both allow the flow of electric current but this is not true. Below are some of the differences between a conductor and an electrolyte:
| Conductor | Electrolyte |
| A metal or graphite | An ionic or polar compound |
| Conducts electricity in a solid state | Conducts when molten or in solution |
| Electric current is carried by the flow of electrons(electronic conduction) | Electric current is carried by the flow of ions(electrolytic conduction) |
| Transmits heat by conduction | Transmits heat by convention |
| Not decomposed by electricity | Decomposed by electric current |
The Arrhenius Ionic Theory
This theory was propounded by Swedish Chemist Swante Arrhenius in 1889. The postulates of the modern ionic theory are:
- An electrolyte exists as ions, in a molten state, and as hydrated ions, when dissolved in water
- An ion is an atom or group of atoms which carries either a positive or negative charge.
Electrolytes can be categorized into two namely Strong and Weak electrolytes. Strong Electrolytes are compounds that are completely(100%) dissociated into ions in water e.g include strong mineral acids( HCl, and HNO3 ), strong alkalis(NaOH, and KOH), and Soluble ionic salts like NaCl, and CuSO4
Weak electrolytes are compounds that are only dissociated into ions in water e.g weak acids(CH3COOH, H2CO3, and H2SO4), and weak bases like aqueous ammonia and lime water.
Electrochemical Reactions
Electrolysis
Electrolysis is the process whereby chemical decomposition occurs when an electric current is passed through an electrolyte. Hence, during electrolysis, electric current is used to bring about an electrochemical reaction.
Electrolysis occurs in an electrolytic cell, which is made up of a pair of electrodes dipping into an electrolyte held in a suitable container. Electrodes are poles or plates through which the current enters and leaves the electrolytes. They can be metals or graphite(non-metal.).
The anode is the electrode through which the current enters the electrolyte and it is usually connected by wire to the positive pole of the source of the electric current(the battery). The cathode is the electrode through which the current leaves the electrolyte and it is connected to the negative pole of the battery.
Electrolysis Of A Molten Compound
We would use Molten Sodium Chloride to describe the electrolysis of a Molten Compound. Sodium chloride is a typical ionic compound. In the solid state, the ions in the crystals are not mobile, but in molten state, the electrolyte produces mobile Na+ as the only positive ion, and Cl– as the only negative ions.
NaCl(s) + heat → Na+(I) + Cl–(I)
When the circuit is completed in the electrolytic cells, electrons move along the wire from the negative pole of the battery to the cathode. The cathode becomes electron-rich; hence negatively charged. Consequently, positive ions , Na+ migrate to the cathode. Positive ions are called cations(cathode ions). The anode which is attached to the positive pole of the battery becomes electron deficient, and hence positively charged. Consequently, negative ions, Cl– migrate to the anode.
Half Cell Reactions & Half Cell Equations
In electrolysis, the reaction occurring at each electrode is called half-cell reaction; one represents oxidation while the other represents reduction. Since both reactions occur at the same time, there must be a balance in the number of electrons lost to the anode(oxidation) and the number gained at the cathode(reduction); oxidation and reduction processes are complementary.
In the electrolysis of molten sodium Chloride, 2 moles of electrons are lost to the anode, in order to liberate one mole of chlorine; therefore, 2 moles of electrons must be gained at the cathode, so that 2 moles of sodium atoms are deposited.
The overall reaction in the cell is obtained by the addition of the two half-cell equations:
At anode: 2Cl–(i) → Cl2(g) + 2e– (Oxidation)
At Cathode: 2Na+(i) + 2e–→Na(s) (Reduction)
Net Equation: 2Na+(i) + 2Cl–(i) → 2Na(s) + Cl2(g)
Industrial Applications Of Electrolysis
Some of the industrial applications of electrolysis are outlined below:
- Refining(Purification) Of Metals
- Extraction Of Metals
- Production of Gases
- Electroplating
- Manufacturing Of Compounds