However, if the compound is unable to ionise it does not conduct electricity it is called a non-electrolyte. In general, the extent to which an electrolyte can break up into ions categorises an electrolyte.
This gives a measure of the degree of dissociation (a) of an electrolyte. Based on this degree the electrolytes can be classified as strong or weak electrolyte and non-electrolyte.
Strong Electrolyte
A strong electrolyte, such as a solution of sodium chloride dissociates or ionises completely or almost completely to form free mobile ions in the solution or molten form. The more the availability of free mobile ions in an electrolyte, the greater is its capacity to carry or conduct current i.e. the stronger the electrolyte.
The ability to conduct current can be observed by setting up a cell as shown in figure 4.4. The bulb glows brightly. For e.g., Sodium chloride even in crystalline form consists of ions. But the ions are not mobile so it does not conduct electricity and the bulb does not light. When melted or dissolved in water, it dissociates completely into free, mobile ions. Pure sulphuric acid exists mostly in the form of molecules. But when mixed with water, it almost completely breaks up into free mobile ions.
Weak Electrolyte
A weak electrolyte ionises or dissociates only partially to form free mobile ions. Most of the electrolyte remains as un-ionised molecules. For example in acetic acid, the number of its dissociated ions (the acetate and hydrogen ions) is less compared to the total amount of acetic acid molecules present.
Similarly in ammonium hydroxide the number of its dissociated ions (the ammonium and hydroxyl ions) are less compared to the total amount of the molecules present.
Thus both these compounds are weak electrolytes. When the number of mobile ions is less in an electrolyte, the lesser is its capacity to carry or conduct current i.e. the weaker is the electrolyte. This is observed by setting up the cell as shown in figure 4.5. The bulb glows less brightly.
If one liter of a solution containing one molar mass of sulphuric acid, and one liter of a solution containing one molar mass of citric or acetic acid, are subjected to the same current, then:
- The bulb glows brightly in the case of the sulphuric acid, showing it to be a strong electrolyte
- The bulb glows dimly in the case of the citric or acetic acid, showing that it is a weak electrolyte.
Non-electrolyte
A non-electrolyte does not provide ions in a solution and therefore current does not flow through such solution. The bulb in the given set up does not glow (Fig.4.6). Some examples of non-electrolytes are: alcohol, carbon tetrachloride, carbon disulphide.
Ionisation
The process of conversion of a neutral atom into charged ions to complete its octet is known as ionization. In this process, the neutral atom loses or gains electrons. The particle that loses electrons gains positive charge equal to the number of electrons lost, while the particle that gains electrons gains negative charge equal to the number of electrons gained.
When atoms from metallic elements combine with those from non-metals, they do so by transfer of electrons from one atom to another, forming compounds having “ionic or electrovalent” bonds. The neutral atom that loses an electron becomes a cation and the neutral atom that acquires an electron becomes an anion.
For e.g., when a sodium atom combines with a chlorine atom to form sodium chloride, the sodium atom loses one electron and becomes positively charged ion. The chlorine atom gains the electron and it becomes negatively charged ion.
Electrolytic Dissociation
Electrovalent substances are made up of ions in the solid state. The oppositely charged ions are held together by strong electrostatic force of attraction. Due to these forces the ions cannot move.
However, when these substances are dissolved in water or melted, the ions free themselves from this binding. Thus the break up of an electrovalent compound into free mobile ions when dissolved in water or when melted, is called electrolytic dissociation
Theory of Electrolytic Dissociation
The main ideas of the ionic theory or theory of electrolytic dissociation are as follows:
- On dissolving in water an electrolyte, breaks up into free cations and anions.
- The energy associated with moving charges is called current or electricity.
- The ions carry an electric charge and also allow the flow of electric current through it.
- The flow of electricity is due to the flow of the ions.
- The total number of positive and negative charges of the ions in the compound is equal.
Electrolysis Splits a Compound:
When substances which are made of ions are dissolved in water, or melted material, they can be broken down (decomposed) into simpler substances by passing an electric current through them. This process is called electrolysis. Since it requires an ‘input’ of energy, it is an endothermic process.
During electrolysis:
- Positive metal ions or hydrogen ions move to the negative electrode where the metal lower in the reactivity series gets discharged by gain of electrons (a reduction process).
- They are known as cations because they drift towards the cathode.
- Negative non-metal ions drift to the positive electrode (anode) where again the less reactive ions get discharged from the solution by loss of electrons (oxidation).
They are known as anions because they drift towards the anode. In the electrolyte (solution or melt of free moving ions), Positive metal or hydrogen ions move to the negative electrode (cations attracted to cathode), e.g. in the diagram, sodium ions Na+, move to the -ve electrode, and negatively charged ions move to the positive electrode (anions attracted to anode), e.g. in the diagram, chloride ions Cl-, move to the +ve electrode.
During electrolysis, gases may be given off, or metals dissolve or are deposited at the electrodes.
In summary, the following substances are electrolytes:
- Molten salts
- Solutions of salts in water
- Solutions of acids
- Solutions of alkalis
Metallic conductivity:
- Electrons flow(carry charge)
- It is a property of elements, graphite and alloys
- It takes place in solids and liquids
- No chemical change takes place.
Electrolytic conductivity:
- Ions flow (carry charge)
- It is a property of ionic compounds
- Takes place in liquids (molten salts) and solutions but not solids
- Chemical decomposition takes place.
Electrolysis Circuit
There are two ion movements in the electrolyte flowing in opposite directions.
Positive cations e.g. Na+ attracted to the negative cathode electrode. Negative anions e.g. Cl- attracted to the positive anode electrode.
No electrons flow in the solution.
They only flow in metal wires or carbon (graphite) electrodes of the external circuit. The molten or dissolved materials (electrolytes) are usually acids, alkalis or salts and their electrical conduction is usually accompanied by chemical changes e.g. decomposition. Liquids that conduct must contain freely moving ions to carry the current and complete the circuit.
Electrolysis can’t be performed with an ionic solid. This is because the ions are too tightly held by chemical bonds and can’t flow. When ionically bonded substances are melted or dissolved in water, the ions are free to move about. However some covalent substances dissolve in water and form ions. Hydrogen chloride (HCl) is covalent. However it dissolves in water to form ‘ionic’ hydrochloric acid H+Cl-(aq)