Hi chaocheng,


Yes, you are precisely right. This is a good point to remember since many commits error because of this.

An example is the molecular formula C2H4O2. This can be ethanoic acid or methyl methanoate. These two compounds belong to different homologous series carboxylic acid and ester.

Learning Organic Chemistry


Part 2:


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ELECTROLYSIS OF AQUEOUS SOLUTIONS OF COMPOUNDS


1. An aqueous solution of a compound contains

Cations and anions of the compound,
Hydrogen ions, H+ and hydroxide ions, OH- from water molecules

2. In electrolysis, when more than one type of cation or anion is present in the solution, only one cation and one anion are preferentially discharged.
This is called the selective discharge of ions.

3. The selective discharge of ions depends on three factors:

The position of ions in the reactivity series.
The concentration of the ions.
The nature of the electrode used.

4. Notes on the reactivity series and the selective discharge of ions:

a. The lower the position of the ion in the reactivity series,

The more readily the ion gains or loses electrons,

The higher the tendency of the ion to be selectively discharged.

b. If a solution containing Cu2+ ions and H+ ions is electrolysed, Cu2+ ions will be preferentially discharged to form copper metal. Meanwhile, H+ ions will remain in the solution.
Cu2+ (aq) + 2e- -----> Cu (s)

c. If a solution containing SO42- ions and OH- ions is electrolysed, OH- ions will
be preferentially discharged to form oxygen gas. Meanwhile, SO42- ions will remain in the solution.
4OH- (aq) -----> 2H2O (l) + O2 (g)

5. Electrolysis of water

a. Pure water is a poor conductor of electricity. If a small amount of ionic compound or dilute sulfuric acid is added to water, the solution becomes a good conductor of electricity.

b. When water acidified with dilute sulfuric acid is electrolysed, two volume of hydrogen is produced at the cathode and one volume of oxygen is produced at the anode.
2H2O (l) -----> 2H2 (g) + O2 (g)

6. Electrolysis of dilute sodium chloride solution

a. An aqueous solution of sodium chloride contains four different types of ions:

a.1. Ions from sodium chloride, Na+(aq) and Cl- (aq)
a.2. Ions from water: H+(aq) and OH-(aq)

b. The electrolysis of dilute sodium chloride solution is equivalent to the electrolysis of water. Two volumes of hydrogen are liberated for one volume of oxygen.

c. Since water is being removed during electrolysis, the concentration of sodium chloride solution increases gradually.

7. Effect of concentration on selective discharge of anions

a. The selective discharge at the cathode is determined by the position of the cation in the reactivity series.

b. However, the selective discharge at the anode is influence by two factors:

b.1. The position of the anion in the reactivity series
b.2. The concentration of the anion in the electrolyte.

c. When a solution containing OH- ion and a halide ion (Cl-, Br- or I- ion) is electrolysed, the halide ion is selectively discharged at the anode, if it has a high concentration.
The product of electrolysis at the anode is always oxygen, unless the electrolyte contains a high concentration of halide ions.

8. Electrolysis of concentrated sodium chloride solution

a. At Cathode,

a.1. Na+ and H+ ions are attracted to the cathode.

a.2. H+ ions accept electrons more readily than Na+ ions.

a.3. H+ ions are selectively discharged because H+ is lower than Na+ in the reactivity series.

2H+ (aq) + 2e- -----> H2 (l)

a.4. Na+ ions remain in the solution.

b. At the Anode,

b.1. Cl- and OH- are attracted to the anode.

b.2. Although the OH- ions are more easily discharged, the Cl- ions are selectively discharged because the concentration of Cl- ions is higher.

2 Cl- (aq) ----> Cl2 (g) + 2e-

b.3. OH- ions remain in solution.

c. The overall reaction is:
2NaCl (aq) + 2H2O (l) ----electrolysis—> 2NaOH (aq) + H2 (g) + Cl2 (g)

d. During the electrolysis of concentrated sodium chloride solution, equal volumes of hydrogen and chlorine are liberated at the electrodes.

e. The solution becomes alkaline as H+ ions and Cl- ions are discharged, leaving behind Na+ ions and OH- ions in the solution.

9. Effect of nature of electrodes on selective discharge of ions

a. Inert electrodes are electrodes that do not take part in chemical reactions during electrolysis.
b. When electrolysis is carried out using inert electrodes, such as carbon or platinum electrodes, selective discharge depends on two factors:

b.1. Position of the ion in the reactivity series
b.2. Concentration of the ion

In the electrolysis of an aqueous solution using inert electrodes:

A metal or hydrogen gas is formed at the cathode (negative electrode)

A non-metal (other than hydrogen) is formed at the anode (positive electrode).

c. Electrodes which are not inert undergo chemical reactions during electrolysis. These electrodes are called active electrodes (reactive electrodes).

d. Copper electrode is an active electrode when it is used for the electrolysis of electrolytes containing Cu2+ ions.

More about Electrolysis!


ELECTROLYSIS OF MOLTEN IONIC COMPOUNDS

1. Electrolysis of molten ionic compound produce metals at the cathode and non-metals at the anode.

2. Electrolysis of molten sodium chloride:

When solid sodium chloride is heated strongly, it melts at 801degC.

The molten sodium chloride contains mobile cations, Na+ ions and anions, Cl- ions.

The attached image shows the apparatus set-up for the electrolysis of molten sodium chloride.

The attached image shows the redox reactions at the electrodes during the electrolysis of molten sodium chloride.

During electrolysis of molten sodium chloride, tiny drops of sodium metal float to the surface and produce flashes of yellow light when burnt in air. A pale greenish-yellow gas of chlorine is seen around the anode.
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