Ionic Theory, Electrovalent and Covalent bonds, Electrolytes, Conductors and non-conductors

CONTENT

Distinguish between:

• Electrovalent and Covalent Compounds.
• Electrolytes and Non-electrolytes.
• Weak and Strong Electrolytes.
• Conductors and Non –conductors.

IONIC THEORY

The ionic theory was first presented by Arrhenius to explain electrolysis. Ionic theory proposed that when an electrolyte is melted or dissolved in water, some if not all, the molecules of the substance dissociate into freely moving charged particles called ions. The process of dissociation into ions is known as ionization.

The metallic ions, ammonium ions, NH₄⁺, and hydrogen ions, H⁺, are positively charged while the nonmetallic ions and hydroxide ions are negatively charged. When electric current is passed through an electrolyte, the free ions lose their random movement. The positive ions become attracted to the cathode (negative electrode) and are known as cations (i.e cathode ions). The negative ions move towards the anode (positive electrode) and are called anions. (i.e anode ions). Therefore, the current through electrolyte is carried by the movement of ions to the electrodes, and not by flow of electrons in the electrolyte.

Arrhenius version of the ionic theory has been modified and replaced by the modern ionic theory. The modern ionic theory propose that an electrolytes consists of oppositely charged ions even in the solid state and such ions are pulled away from one another either as a result of the heat applied when the solid melts or with the help of the solvent molecules when the solid dissolves.

ARRHENIUS THEORY:  NaCl(s) →    Na⁺_(aq)  + Cl^–_(aq)

MODERN THEORY:       Na⁺Cl^–_(s)       →  Na⁺_(aq)         +     Cl^–(_aq)

ELECTROVALENT AND COVALENT COMPOUND

Electrovalent compounds are formed by complete transfer of an electron(s) from a metal to a non-metal and they are made up of ions while covalent compounds are formed by the sharing of electrons between the two atoms combining together and they are made up of molecules.

CHARACTERISTICS OF ELECTROVALENT COMPOUNDS

1. Electrovalent compounds are usually crystalline solids at room temperature.
2. Electrovalent compounds are usually soluble in polar solvents like water but insoluble in non-polar solvents like tetrachloromethane.
3. Electrovalent compounds generally have high melting and boiling points.
4. Electrovalent compounds are good conductors of electricity in the molten state and in aqueous solutions but insulators in the solid state.

CHARACTERISTICS OF COVALENT COMPOUNDS

1. Covalent compounds are usually liquids or gases.
2. Covalent compounds are soluble in non-polar solvents like benzene or carbon tetrachloride and insoluble in polar solvents like water.
3. Covalent compounds generally have low melting and boiling points.
4. Covalent compounds are bad conductors of electricity.

EVALUATION

1. Explain the ionic theory.
2. Distinguish between electrovalent compounds and covalent compounds.

ELECTROLYTES AND NON-ELECTROLYTES   

An electrolyte is any salt or molecule that ionizes when dissolved in solution and the solution can conduct electricity while a non-electrolyte does not dissociate in solution and does not conduct electricity. Electrolytes can conduct electricity because when a salt dissolves, its dissociated ions can move freely in solution, allowing the flow of -charges.

Examples of electrolytes are: molten or aqueous sodium chlorine, mineral acids like: tetraoxosulphate (VI) acid, hydrochloric acid, potassium hydroxide etc.

Examples of non-electrolytes are: ethanol, benzene, sugar, urea, etc.

WEAK AND STRONG ELECTROLYTES

Strong electrolyte is one which ionizes completely in solution and conduct electric current readily example: tetraoxosulphate (VI) acid, sodium hydroxide, sodium chloride etc; while weak electrolyte is one which ionizes partially in solution and does not conduct electric current readily example ethanoicacid, aqueous ammonia, water and so on.

The conductivity of strong electrolytes decreases slightly with increasing concentration; however the conductivity of weak electrolytes increases with decreasing concentration.

CONDUCTORS AND NON–CONDUCTORS

Any substance that allows the passage of electricity through it is called a conductor; while any substance that does not allow the passage of electricity is known as non-conductor or insulator.

Conductors are mostly metals. Glass, porcelain, plastic, rubber and diamond are examples of non–conductive materials

GENERAL EVALUATION/REVISION

1. Distinguish between conductor and non-conductor giving examples.
2. Differentiate between strong electrolyte and weak electrolyte giving examples.
3. State THREE differences between electrovalent and covalent compounds.
4. Write the electronic configuration of the following atom/ion: S^2-, Al³⁺, Fe, Cl and Ar.
5. Two isotopes of the element Z with mass numbers 18 and 20 are in the ratio of 1:2. Determine the relative atomic mass of Z.

READING ASSIGNMENT

New School Chemistry for Senior Secondary School (6^th edition) by O. Y. Ababio, pages 200-201

WEEKEND ASSIGNMENT

SECTION A: Write the correct option ONLY

1. Which of the following liquids is a good conductor of electricity? A. Methylbenzene B. Deionized water C. Mercury D. Sucrose solution
2. Potassium chloride cannot conduct an electric current in the solid state because it A. does not contain mobile ions. B. is very soluble in water. C. is an electrovalent compound D. is a neutral salt.
3. What happens to the conductivity of a strong electrolyte as its concentration reduces? It A. increases B. decreases C. is unaffected D. reduces to zero
4. The current carriers that are responsible for the conductance of electrolytes are A. hydrated electrons B. hydrated ions C. electrons D. ions
5. The current carriers that are responsible for the conductance of conductors are A. hydrated electrons B. hydrated ions C. electrons D. ions

SECTION B

1. Differentiate between strong electrolyte and weak electrolyte giving examples.
2. State THREE differences between electrovalent and covalent compounds.