8.2 Electrolytes, ionisation and conductivity

Electrolytes, ionisation and conductivity

Conductivity in aqueous solutions, is a measure of the ability of water to conduct an electric current. The more ions there are in the solution, the higher its conductivity.

Conductivity

Conductivity is a measure of a solution's ability to conduct an electric current.

Electrolytes

An electrolyte is a material that increases the conductivity of water when dissolved in it. Electrolytes can be further divided into strong electrolytes and weak electrolytes .

Electrolyte

An electrolyte is a substance that contains free ions and behaves as an electrically conductive medium. Because they generally consist of ions in solution, electrolytes are also known as ionic solutions.

1. Strong electrolytes A strong electrolyte is a material that ionises completely when it is dissolved in water:
   $$AB(s,l,g)\to A^{+}\left(aq\right)+B^{-}\left(aq\right)$$
   This is a chemical change because the original compound has been split into its component ions and bonds have been broken. In a strong electrolyte, we say that the extent of ionisation is high. In other words, the original material dissociates completely so that there is a high concentration of ions in the solution. An example is a solution of potassium nitrate:
   $$KN{O}_3\left(s\right)\to K^{+}\left(aq\right)+N{O}_3^{-}\left(aq\right)$$
2. Weak electrolytes A weak electrolyte is a material that goes into solution and will be surrounded by water molecules when it is added to water. However, not all of the molecules will dissociate into ions. The extent of ionisation of a weak electrolyte is low and therefore the concentration of ions in the solution is also low.
   $$AB(s,l,g)\to AB\left(aq\right)\rightleftharpoons A^{+}\left(aq\right)+B^{-}\left(aq\right)$$
   The following example shows that in the final solution of a weak electrolyte, some of the original compound plus some dissolved ions are present.
   $$C_2{H}_3{O}_{2}H\left(l\right)\to C_2{H}_3{O}_{2}H\rightleftharpoons C_2{H}_3{O}_2^{-}\left(aq\right)+H^{+}\left(aq\right)$$

Non-electrolytes

A non-electrolyte is a material that does not increase the conductivity of water when dissolved in it. The substance goes into solution and becomes surrounded by water molecules, so that the molecules of the chemical become separated from each other. However, although the substance does dissolve, it is not changed in any way and no chemical bonds are broken. The change is a physical change . In the oxygen example below, the reaction is shown to be reversible because oxygen is only partially soluble in water and comes out of solution very easily.

Factors that affect the conductivity of water

The conductivity of water is therefore affected by the following factors:

• The type of substance that dissolves in water. Whether a material is a strong electrolyte (e.g. potassium nitrate, KNO ${}_3$ ), a weak electrolyte (e.g. acetate, CH ₃ COOH) or a non-electrolyte (e.g. sugar, alcohol, oil) will affect the conductivity of water because the concentration of ions in solution will be different in each case.
• The concentration of ions in solution. The higher the concentration of ions in solution, the higher its conductivity will be.
• Temperature. The warmer the solution, the higher the solubility of the material being dissolved and therefore the higher the conductivity as well.

Experiment : electrical conductivity

Aim:

To investigate the electrical conductivities of different substances and solutions.

Apparatus:

Solid salt (NaCl) crystals; different liquids such as distilled water, tap water, seawater, benzene and alcohol; solutions of salts e.g. NaCl, KBr; a solution of an acid (e.g. HCl) and a solution of a base (e.g. NaOH); torch cells; ammeter; conducting wire, crocodile clips and 2 carbon rods.

Method:

Set up the experiment by connecting the circuit as shown in the diagram below. In the diagram, 'X' represents the substance or solution that you will be testing. When you are using the solid crystals, the crocodile clips can be attached directly to each end of the crystal. When you are using solutions, two carbon rods are placed into the liquid and the clips are attached to each of the rods. In each case, complete the circuit and allow the current to flow for about 30 seconds. Observe whether the ammeter shows a reading.

Results:

Record your observations in a table similar to the one below:

What do you notice? Can you explain these observations?

Remember that for electricity to flow, there needs to be a movement of charged particles e.g. ions. With the solid NaCl crystals, there was no flow of electricity recorded on the ammeter. Although the solid is made up of ions, they are held together very tightly within the crystal lattice and therefore no current will flow. Distilled water, benzene and alcohol also don't conduct a current because they are covalent compounds and therefore do not contain ions.

The ammeter should have recorded a current when the salt solutions and the acid and base solutions were connected in the circuit. In solution, salts dissociate into their ions, so that these are free to move in the solution. Acids and bases behave in a similar way and dissociate to form hydronium and oxonium ions. Look at the following examples:

$\mathrm{KBr}\to {\mathrm{K}}^{+}+{\mathrm{Br}}^{-}$

$\mathrm{NaCl}\to {\mathrm{Na}}^{+}+{\mathrm{Cl}}^{-}$

$\mathrm{HCl}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{Cl}}^{-}$

$\mathrm{NaOH}\to {\mathrm{Na}}^{+}+{\mathrm{OH}}^{-}$

Conclusions:

Solutions that contain free-moving ions are able to conduct electricity because of the movement of charged particles. Solutions that do not contain free-moving ions do not conduct electricity.