Redox Reactions

                      Any chemical reaction in which the oxidation states of the atoms are changed is an oxidation-reduction reaction.This reactions are also known as redox reactions. Redox reactions involve the transfer of electrons between species. Industrial process of electroplating, Extraction of aluminium and sodium metals, manufactures of caustic soda, etc are also based upon the redox reactions. Redox reactions are also form the basis of electrochemical and electrolytic cells. The electron transfer system in cells and oxidation of glucose in the human body are example of redox reaction.
                      The term redox comes from two concepts involved with electron transfer, reduction and oxidation. It can be explained in simple terms.

• Oxidation is a process of addition of oxygen  ( or ) removal of hydrogen ( or ) loss of electron ( or ) increase in oxidation state by a molecule, atom or ion.
• Reduction is a process of removal of oxygen ( or ) addition of hydrogen ( or ) Gain of electrons ( or ) decrease in oxidation state by a molecule, atom or ion.                                 

Example of Oxidation-Reduction Reaction : ( Ionic Equation )
               The reaction between copper and silver in which copper is oxidized and silver is reduced. The ionic equation for this reaction is,

                              Cu (s) +2 Ag ⁺ →Cu²⁺ (aq)  +  2Ag (s)

The over all reaction may be written as two half-reactions.

Oxidation Reaction :

                              Cu (s) →Cu²⁺ + 2e^-

    ^{The above half reaction mention the solid copper is oxidized ( losing electron ) to form a copper ion with plus 2 charge.}

^{Reduction Reaction :}

                                   2Ag⁺(aq)  +  2e^-  →  2Ag (s) 

Here, two silver ions are reduced to form a solid silver.

When adding the reactions together the electrons are canceled.

      Cu (s)  →  Cu²⁺ + 2e^-

     2Ag⁺(aq)  +  2e^-  →  2Ag (s

--------------------------------------------------                             

Cu (s) +2 Ag ⁺ →Cu²⁺ (aq)  +  2Ag (s)  

Note :
Copper metal is immersed in a solution of silver nitrate. The solution gradually acquires the blue color characteristic of the hydrated Cu^{2+ while the copper becomes coated with glittering silver crystals.}

<sup>(The above figure is galvanic cell reaction. Its also referred as voltaic cell or Daniel cell.)

One more example for redox reaction :
    The reaction between hydrogen and fluorine in which hydrogen is being oxidised and fluorine is being reduced.</sup>

                                     H₂ + F₂  →  2HF

^{The over all reaction may be wrotten as two half reactions} 

           H₂    →   2H⁺  +  2e^- ( oxidation )

           F₂ + 2e^-     →   2F^-   ( Reduction )

Adding the reactions together the electrons are canceled.

                            H₂    →   2H⁺  +  2e^- ( oxidation )

                    F₂ + 2e^-     →   2F^-   ( Reduction )

               -------------------------------------------------------               

                      H₂  +  F₂  →   2H⁺  +  2F^-

^{Ions combine to form a hydrogen fluoride.} 

                        2H⁺  +  2F^-    → 2HF

The overall reaction is,

               H₂ + F₂  →  2HF

Cannizzaro Reaction

                             The cannizzaro reaction is a redox reaction. Two molecules ( aliphatic and aromatic aldehydes ) of an aldehyde are reacted to produce a alcohol and carboxylic acid in the presence of strong base.

   a  -   Keto aldehydes give the product of an intramolecular disproportionation in excellent yields.

Mechanism :

NaOH Na ⁺    +  OH ^–   

 

^{Nucleophilic addition of OH- to aldehyde.}

The Crossed cannizzaro reaction, uses formaldehyde as reducing agent.

Stoichiometry Equations And Balancing Chemical Equation

                  Stoichiometry is a branch of chemistry that deals with the relative quantities of reactants and products in chemical reactions. It is the study of the relationship between the number of mole of the reactants and product of a chemical reaction. Stoichiometric equation is a short scientific representation of a chemical reaction. The quantities are expressed as grams or moles.

Rules For Writing Stoichiometric Equations:
                 It takes practice to be able to write balanced equations. Three step process.

1. Write the unbalanced equation.
2. Balance the equation.
3. Indicate the state of matter of the reactant and product.

• Chemical formulas of reactants are listed on the left hand side of the arrow with a positive sign between them.
• Chemical formula of the products are listed on the right hand side of the equations.More than one product , a positive sign is placed between them.
• Reactants and products are separated by  putting an arrow between them to show the directions of the reaction.
• The equation thus obtained above all three steps is called Skeleton equation. For example the chemical reaction between iron and oxygen producing Fe₂O_{3 is represented by the equation as} 

                                  Fe + O₂ → Fe₂O_3.

The above equation is not a balanced one. In many cases skeleton equation is not a balanced one.

For example ;

                                SnO₂ + H₂ → Sn + H₂O

            The above equation is not a balanced one. So we must balance the equation by balancing coefficients before the various molecules and to ensure the number of atoms on the left side of the arrow corresponds exactly to the number of elements on the right.

             In this case, two oxygen atoms on the left hand side of the equation and only one atom on the write hand side of the equation. So we putting a coefficient of 2 in front of water.

                                SnO₂ + H₂ → Sn +2 H₂O

              Now check the equation is not a balanced one.Because 2 hydrogen atoms on the left hand side and 4 hydrogen atoms on the right hand side.Add coefficients of 2 for left hand side of the hydrogen.  

                                 SnO₂ + 2H₂ → Sn +2 H₂O

               After balancing and adjusting coefficient, we check the equation is a balanced one. Each side of the equation has 1 Sn atom, 4 Hydrogen atom, 2 Oxygen atom.Finally we specifying the states of matter.

               The final equation also indicates the physical state of each substance. The abbreviation are used in these states are,

Solid     (s)

Liquid    (l)

Gas        (g)

Aqueous Solution (aq) .

In the above equation oxides are solid, hydrogen forms a diatomic gas, ton is a solid and water vapor indicates that water is in the gas phase.

                               SnO₂ (s) + 2H₂ (g) → Sn (s) +2 H₂O(g)

• Important conditions such as temperature, pressure, catalyst etc may be noted above or below arrow of the equation.Upward arrow indicate the gaseous product. Downward arrow indicate the precipitated product.

             

Mole Concept

           Number of particles of the substance is related to the mass of the substance. Mole is a unit of measurement used in chemistry. A chemical mass unit is defined to be 6.023 × 10^{23. atoms , molecules , ions or some other units.For Example in chemical equation ,}

                                           2H₂ + O₂ → 2H₂O
                  The above equation implies that 2 mole of dihydrogen and one mole of dioxygen react to give 2 mole of water. The mole may also be used to express the number of atoms , ions, or some other substances. But the concentration of solution is commonly expressed by its molarity , defined as the number of moles of the dissolved substance per litre of solution.
               
                     The mole may be defined as the amount of the substance that contains as many specified elementary particles as the no.of atoms in 12g of carbon-12 isotope. (i.e ) one mole of an atom consists of Avogadro number of particles.

Example for mole substances :
A mole of oxygen atom contain 6.023 × 10^{23 oxygen atoms.
A mole of oxygen molecule contains} 2 × 6.023 × 10<sup>23 oxygen.

Example for ionic substance :</sup>

A mole of Sodium carbonate ( Na₂CO₃ ) quantity containing 6.023 × 10^{23  ,} Na₂CO_{3 units. Each formula units of sodium carbonate contains} 2 × 6.023 × 10^23,  Na ^{+  ions and}  1 × 6.023 ×10²³  CO₃^{2-   ions.}

Avogadro Number

           In Chemistry Avogadro number is defined as the number of particles ( atom or molecule ) found in one molecule of a substance. It is a number of atom in a 12 g sample of carbon- 12. It has dimensions of reciprocal mol and its value is equal to 6.0221367  × 10^{23 .per mole, which is 6.023} 

× 10^{23. Avogadro number is also known as Avogadro constant.}

Uses of alums.

• Alum is used in fire extinguishers to smother chemical and oil fires.
• Alum is a component of foamite.
• Styptic pencils containing aluminium sulfate are used as astringents to prevent bleeding from small sharing cuts.
• Purification of drinking water as a chemical flocculant.
• Pickling agent to help keep pickles crispness.
• Alum is used as the acidic component of some commercial backing powders.
• Alum is an ingredient in some homemade modeling compounds intended for use by children. (Play Clay )
• Alum is used in tanning.
• Alum is used to fix pigments on a surface.
• Alum is an ingredients in some hair removal waxes.
• Alum is used in skin whitener.

Selenate alum

                            Alums are also known that contain selenium in place of sulfur in the sulfate anion making selenate    SeO₄^{2-   instead . They Known as selenium or selenate alums. The selenium containing alums are strong oxidizing agents, so they used in antiseptics, among other uses.}