EFFECTS OF ELECTRICITY ON SUBSTANCES

Electricity is a form of energy produced as a result of flow of electrons through materials.

Investigation of electrical conductivity through solid materials

Examples of such materials include: copper wire, zinc wire, plastic, graphite, rubber.

Procedure

Connect the copper wire to the batteries through the bulb and the switch as shown below.

Results

The bulb produced light on complete connection with copper and zinc. Conclusion

Copper and zinc wires conduct electricity and they are called conductors.

A conductor is a substance in solid form which can conduct electricity. Examples include; all metals and graphite (the only non metal that can conduct electricity.

When the above experiment was repeated using rubber and plastics, the bulb did not light indicating that they do not conduct electricity and are referred to as insulators or non conductors.

A non conductor is a substance in solid form that does not conduct electricity. Examples are all non metal except graphite.

Metals conduct electricity because they have delocalized, free or mobile electrons but non metals do not have these delocalized electrons as they are all locked up in bond formation

Investigation of electrical conductivity through liquid substances in solution Examples: ethanol, urea, hydrochloric acid, copper (II) sulphate, ethanoic acid, water, ammonium hydroxide.

Procedure

1. Put the liquid under investigation in an electrolytic cells.
2. Dip two rods in the liquid which can either be a metal or carbon(graphite) called electrodes
3. Connect the electrodes using a conductor to a bulb via a switch to the source of power (the batteries) as shown below.

4. Close the switch.
5. Repeat the experiment with hydrochloric acid, ethanol, ethanoic acid, water, urea, ammonium, ammonium hydroxide.

Results

When ethanol and urea were used there was no light produced indicating that they do not conduct electricity, they are there fore called non electrolytes.
When ammonium hydroxide and ethanoic acid were used, the bulb produced a dim light indicating that they weakly conduct and are there fore weak electrolytes.

When copper sulphate solution and hydrochloric acid were used, the bulb produced bright light indicating that they strongly conduct electricity and are strong electrolytes.

ELECTROLYSIS

This is the decomposition of a substance in solution form or molten form (electrolyte) as a result of passage of electric current. The decomposition of the electrolyte takes place at the electrodes.

Definitions of common terms used in electrolysis

Electrolytes

This is a substance in solution form or molten state that can conduct electricity. Electrolytes can be categorized as strong, weak or non electrolytes.

a) Strong electrolyte

This is a substance in solution form or molten state that ionizes completely and can easily conduct electricity. The electrolyte decomposes fully during electrolysis.
Examples include, all mineral acids, alkalis, ionic compounds.

b) Weak electrolytes

This is a substance in solution form which is only slightly ionized (partially ionized). The electrolyte is only partially decomposed by the electric current. Most of the ions of the electrolyte remain as un ionized ions or molecules.

Examples include:

c) Non electrolyte

Is a substance in solution form or aqueous state that doesn‘t conduct electricity. This substance is not decomposed at the electrodes. Example include: all covalent compounds, like pure water, benzene, methyl benzene, petrol and diesel.

Electrodes

These are rods or plates or poles of conductors at which electrons enter and leave the electrolyte. The electrodes are either cathodes or anodes.

a) Anode

This is the positive electrode at which the electrons leave the electrolyte. Or is the positive electrode at which electrons enter the external circuit. It is normally connected to the positive terminal of the battery.

b) Cathode

This is the negative electrode at which the electrons enter the electrolyte or is the negative electrode at which the electrons leave the external circuit. It is connected to the negative end of the battery.

Simple electrolytic cell

In an electrolytic cell the electrolyte has to be in solution form or molten state as the ions have to be free to move so as to conduct electricity. Salts such as sodium chloride, and lead (II) bromide do not conduct electricity in solid state because the ions are held together by strong electrostatic forces of attraction and are not free and mobile. However, when the salts are melted or dissolved in water, the electrostatic forces are broken down and the ions become free and mobile and so conduct electricity.

IONIC THEORY

Ionic theory was put forward to explain the phenomenon of electrolysis. According to ionic theory, electrolytes are believed to contain electrically charged particles called ions. The ions can be positively charged (cations) and are obtained from metals, hydrogen and ammonium or negatively charged (anions) and are obtained from non
metals e.g. , , , During electrolysis, the cations are attracted to the negative electrode (cathode) and the anions are attracted to the positive electrode (anode)

Examples of electrolytes and the ion produced.

Explanation of electrolysis by ionic theory

When current is not passed through an electrolyte, the ions are wandering randomly
in solution.

When current is passed through the solution, the cathode attracts the to itself and the anode attracts to it self the anions.

When the cations reach the cathode (negative electrode), they stick to it, gain electrons and become ordinary atoms. e.g.

When the anions reach the anode (positive electrode), they give away the electrons and become ordinary atoms.e.g.

The atoms then combine to form molecules

Experiment to show that Lead (II) bromide only conducts electricity in molten form

 Electrolysis of leads (II) bromide

The electrolysis is done using carbon electrodes (graphite) as shown below.

When carbon electrodes are dipped into solid Lead (II) bromide crystals and the circuit completed, the bulb did not light indicating that there was no conduction because the ions responsible for conduction were locked up in the solid crystal. When heated and molten liquid formed, there was conduction as the bulb lit because the ions were free to move and thus conducted.

Ions present in Lead (II) bromide

Lead ions, migrate to the cathode where it gains electrons and it is discharged to form silvery grey solids of lead. Thus equation at the cathode is

Bromine ions migrate towards the anode and are discharged by losing electrons to form red/brown vapor or liquid. Thus, equation at the anode is

Preferential or selective discharge of ions

When two or more ions of similar charges reach the electrode, one is preferentially selected for discharge and the selection depends on the following factors.

• The position of the ion in the electrochemical series
• The concentration of the electrolyte
• The nature of electrodes

a) The position of metal or radical in the reactivity series

The ion that is lower in the electrochemical series is selected for discharge in
preference to one above it.

Example: Consider the decomposition of copper(II)sulphate solution

Electrolysis of some electrolytes

Electrolysis of dilute sulphuric acid (electrolysis of water)

The electrolysis is done by use of platinum or carbon rods for both electrodes. During the electrolysis, 2 volumes of hydrogen is produced at the cathode and one volume of oxygen is formed at the anode. Total acidity of the products remains the same as the products formed are elements of water.

Drawing of apparatus

Ions present;

At cathode

Hydrogen ions migrate to the cathode where they are discharged by receiving electrons from the cathode and form atoms. The hydrogen atoms pair up to form hydrogen gas (bubbles of a colorless gas that burns with a pop sound).

Equation at the cathode

At the anode

 Both the sulphate ions and hydroxyl ions migrate to the anode. The hydroxyl ions are discharged in preference to the sulphate ions as it is below the sulphate ions in the electrochemical series.

Equation at the anode

Overall equation

Electrolysis of dilute sodium hydroxide solution using carbon electrodes Setup of the apparatus

Ions present

At the cathode

Hydrogen ions are discharged in preference to sodium ions because it is below it in the electrochemical series. Therefore, at the cathode, bubbles of a colorless gas that burns with a pop sound is produced.

Equation

At the anode

At the anode, the hydroxyl ions are discharged giving off bubbles of a colorless gas that relights a glowing splint.

Equation

Electrolysis of copper (II) Sulphate solution using copper cathode and platinum or graphite anode

Set up of apparatus

Ions present

At the cathode

Both copper ions and hydrogen ions migrate to the cathode but the copper ions being lower than hydrogen in the electrochemical series, it‘s discharged in preference to hydrogen ions. There fore at the cathode, the copper ions gain electrons and are deposited as brown solids of copper.

Equation

At the anode

Both sulphate and hydroxyl ions migrate to the anode. The hydroxyl ions being lower than the sulphate ions in the electrochemical series are discharged forming water and hydrogen as final products.

Equation

Note

The blue color of the copper (II) Sulphate fades away with time as the copper ions which are responsible for the blue color are being discharged and deposited at the cathode as brown copper metal.

The discharge of the hydroxyl ions at the anode disturbs the ionic equilibrium of water; therefore more water ionizes to restore this equilibrium. The excess hydrogen ions produced combines with the undischarged sulphate ions forming sulphuric acid which makes the solution around the anode acidic.

The nature of electrode

Different electrodes for a given electrolyte may cause different products to be formed at the electrodes.

Electrolysis of Copper(II) sulphate solution using copper electrodes

Set up of the apparatus

Ions present

At the cathode

Both copper ions and hydrogen ions migrate to the cathode but copper ions are discharged in preference to hydrogen ions since it‘s below it in the electrochemical series.

Equation

At the anode

Both sulphate and hydroxide ions migrate to the anode but non is discharged. Instead, the copper anode goes into solution i.e. it dissolves to form copper ions. This process is called electrode ionization. Such a process in favored in this case as it requires less energy than the discharge of ions.

Overall equation

Note

1. During the experiment, the anode loses mass and the cathode gains mass. The loss in mass at the anode equals the gain in mass at the cathode. The change in mass at either electrode is proportional to the quantity of electricity passed through the
2. The intensity of the blue color of copper (II) sulphate remains constant as the process is a mere transfer of the copper ions from anode to cathode. i.e. the copper from the anode goes into solution as ions to replace the lost copper ions at the Overall concentration of the electrolyte remains

Electrolysis of dilute sodium chloride using a mercury cathode and graphite/platinum anode

 Ions present

At the anode

Both the chloride and hydroxyl ions migrated to the anode but the hydroxyl ions being lower than the chloride ions in the reactivity series, the hydroxyl ions are discharged by losing electrons in preference to chloride ions. Therefore bubbles of colorless gas that relights a glowing splint is observed at the anode.

Equation

Concentration

 Increase in concentration of ions tends to promote its chance of being discharged. E.g. if concentrated Hydrochloric acid is used.

Electrolysis of concentrated hydrochloric acid using graphite electrodes

Ions present

At the cathode

Hydrogen ions migrate to the cathode where they are discharged forming bubbles of a colorless gas that burns with a pop sound (hydrogen gas).

Equation

At the anode

Both the chloride and the hydroxyl ions migrate to the anode but the hydroxyl ions despite them being lower than chloride ions in the electrochemical series are not discharged. Instead the chloride ions are discharged since they are present in a much higher concentration. Therefore, a green-yellow gas that bleaches damp litmus paper is observed indicating that the gas is chlorine gas.

Equation

Electrolysis of concentrated sodium chloride solution using graphite electrodes

Ions present

At the anode

Both the chloride and hydroxyl ions migrate to the anode but chloride ions being present in a much higher concentration are discharged in preference to the hydroxyl ions. There fore, bubble of a green yellow gas that bleaches damp litmus paper (chlorine) is observed.

Equation

At the cathode

Both sodium and hydrogen ions migrate to the cathode. Hydrogen being lower in the electrochemical series are discharged in preference to sodium ions. Therefore, bubbles of a colorless gas that burns with a pop sound is produced at the cathode.

Equation

END OF UNIT