Hydroxy compounds, Rules for calculating the oxidation numbers
The oxygenated acids ( acids contain oxygen ) and bases are considered as hydroxy compounds , They can be represented by the general formula ( MOH ) , where M is the element atom , The hydroxy compounds can be ionized by either ways : As an acid , As a base .
As an acid : The compound will be ionized as an acid if : The ( M − O ) bond is stronger than the ( O − H ) bond , ( The attraction force between M+ and O− − is stronger than that between H+ and O− − ) .
MOH ( oxygenated acid ) ⇔ MO− + H+ ( Hydrogen ion )
MOH ( base ) ⇔ M+ + OH − ( hydroxide ion )
If the strength of ( M − O ) bond and the strength of ( O − H ) bond are equal , the substance will be ionized as an acid or a base depending on the reaction medium , this means that it reacts as a base in the acidic medium and as an acid in the basic medium .
In general the attraction forces between each of ( O− −, M+ ) and ( O− −, H+ ) depend on the volume of atom M and the charge of M in the compound .
The basic property of sodium hydroxide compound
Sodium hydroxide is ionized as a base , where sodium atom has a big volume and its ion has only one positive charge , Accordingly the attraction between Na+ and O− − decreases , The ( O − H ) bond is stronger than that of the ( Na − O ) bond , So , OH− ion is produced .
NaOH ( sodium hydroxide ) ⇔ Na+ ( sodium ion ) + OH − ( hydroxide ion )
The hydroxy compounds of the nonmetallic elements as chlorine are ionized as an acid , Because the nonmetallic elements are characterized by the small atomic volume and the big charge which increase its attraction to O− − ion and the ( Cl − O ) bond becomes stronger than the ( O − H ) bond , so , the positive hydrogen ion H+ is produced .
The strength of the oxygenated acids
The oxygenated acids are represented by the following general formula : M On ( OH ) m
The strength of the oxygenated acid increases as the number of nonbonded oxygen atoms ( On ) with hydrogen increases .
Summary of the elements graduation in the periodic table
Trends and periodicity of properties in the periodic table :
Increasing the atomic number ( horizontal ) → in the periodic table :
- Increasing : [ the ionization potential , the electronegativity , the nonmetallic property , The acidic property , The electron affinity ] .
- Decreasing : [ The atomic radius , The metallic property , The basic property ] .
Increasing the atomic number ( vertical ) , downwards ↓ in the periodic table :
- Increasing : [ The atomic radius , The metallic property ( in groups start by a metal ) , The acidic property ( in groups start by a nonmetal ) , The basic property ( in groups start by a metal ) ] .
- Decreasing : [ The ionization potential , The electronegativity , The electron affinity , The nonmetallic property ] .
Oxidation number is a number that refers to the electric charge ( positive or negative ) that the atom or ion would have in the compound , whether it is an ionic or a covalent compound .
The advantage of using oxidation numbers is that they can help us in determining the type of chemical change occurring to an element during the chemical reaction .
The significance of oxidation numbers :
The significance of oxidation numbers differs in ionic compounds from that of the covalent compounds .
In ionic compounds : If the oxidation number is positive , it indicates that the number of electrons that the atom has lost to give positive ion ( cation ) , If the oxidation number is negative , it indicates that the number of electrons that the atom has gained to give negative ion ( anion ) .
In covalent compounds : If the oxidation number is positive , it indicates that the electronic shift in the chemical bond between the atoms is away from the less electronegative atom , If the oxidation number is negative , it indicates that the electronic shift in the chemical bond between the atoms is toward the most electronegative atom .
Rules for calculating the oxidation numbers
- The oxidation number of the element atom in the molecule of similar atoms equals zero , whatever the multiplicity of the molecule atoms , because the electronic shift in the bonds between the atoms are equal .
- The oxidation number for element ion equals the charge ( valence ) of the ion .
- The oxidation number for polyatomic ions ( atomic groups ) equals the charge of group .
- The oxidation number of any metal in Group 1A elements equals +1 , Group 2A elements equals +2 , Group 3A elements equals +3 .
- The oxidation number for chlorine , bromine , iodine ( halogens ) in the most of their compounds equals ( −1 ) , however their other oxidation numbers can be calculated mathematically .
- The oxidation number of oxygen in most of its compounds is −2 , while its oxidation number in : peroxides equals −1 , Superoxides equals −½ , Its compound with fluorine equals +2 .
- The oxidation number of hydrogen in most of its compounds is +1 , except in binary compounds with active metals which are known as active metal hydrides , its oxidation number is −1 , Active metal hydrides are ionic compounds formed from the combination of an active metal with hydrogen in which hydrogen has an oxidation number −1 ( negative ion ) .
- The algebraic sum of the oxidation numbers of the different atoms in the molecule equals zero , In sodium chloride molecule NaCl : The oxidation no. of Na ( + 1 ) + The oxidation no. of Cl ( −1 ) = zero .
- The algebraic sum of the oxidation numbers of the atomic groups forming the molecule equals zero , In the molecule [ NH4 ]+ [ NO2 ]− : The oxidation no. of ammonium group ( + 1 ) + The oxidation no. of nitrate ( −1 ) = zero .
- The algebraic sum of the oxidation numbers of the different atoms in a polyatomic ion equals the charge of ion ( atomic group ) , In hydroxide group OH− : The oxidation no. of oxygen ( −2 ) + The oxidation no. of hydrogen ( +1 ) = −1 .
- Some elements especially the transition elements have several oxidation numbers which can be calculated by knowing the oxidation numbers of other known elements .
Hydrogen gas evolves at the anode ( positive ) in electrolysis of molten sodium hydride , while in electrolysis of acidified water , it evolves at the cathode ( negative ) , Because the oxidation number of hydrogen in NaH molecule equals −1 , while in H2O molecule equals +1 .
How to assign the oxidation number of an unknown element in a given compound or atomic group
Write the oxidation number for each known element above its atom symbol in the compound molecule or atomic group formula .
Multiply the oxidation number of each element by the number of its atoms in the molecule .
Assign the oxidation number of unknown element according to : The algebraic sum of the atoms of the different elements in the molecule equals zero , The algebraic sum of the atoms of the different elements in the atomic group equals the charge of the group .
Calculating the charge of the oxidation number in the oxidation reduction reaction ( redox reaction )
In the chemical reaction :
The metal loses one or more electrons , So , its oxidation number increases , It is oxidized ( oxidation process ) , The nonmetal gains one or more elements , So , its oxidation number decreases , It is reduced ( reduction process ) .
So , we can determine the changes happen to the elements during the oxidation-reduction reactions through the change in their oxidation no. before and after the reaction , where : the oxidizing agent is reduced , The reducing agent is oxidized .