TYPES OF CHEMICAL REACTIONS PART-2

Exothermic and endothermic reaction-

Exothermic reaction (Exergonic reaction)

Endothermic reaction

A reaction that releases energy in any form (e.g., light, sound, heat).

Heat is absorbed in an endothermic reaction.

The change in enthalpy for an exothermic reaction is constantly negative.

The change in enthalpy for an endothermic reaction is constantly positive.

Reactants → Products + Energy (heat)

Reactants + Energy (heat)→ Products

Exothermic reactions are generally spontaneous.

Endothermic reactions are generally non spontaneous.

  Redox reaction:

 An oxidation-reduction (Redox) reaction is a type of chemical reaction that involves a transfer of electrons between two species. An oxidation-reduction reaction is any chemical reaction in which the oxidation number of a molecule, atom, or ion changes by gaining or losing an electron.

Oxidation: This process involves gain of oxygen or loss of hydrogen.

Reduction: This process involves gain of hydrogen or loss of oxygen.

 

 → Oxidizing Agent

It is the substance which gives oxygen or gains hydrogen.
Or it the substance which is reduced itself and oxidizes other.

→ Reducing Agent
It is the substance which gives hydrogen or gains oxygen.
Or it the substance which is oxidized itself and reduces other.
Oxidation is the process which involves loss of electrons but reduction is the process which involves gain of electrons.

 Examples:

 

  • CuO+H2(g)——-> Cu+ H2O (l)

CuO is reduced to Cu
H isoxidised to H2O
CuO is the oxidising agent
H2 is the reducing agent

  • H2S + Cl2 ———–>  S + 2HCl

H2S  is oxidised to S

Cl2  is reduced to HCl

Clis the oxidising agent 

H2S  is the reducing agent

Corrosion –

  • Corrosion is the process in which metals are eaten up gradually by the action of air, moisture or a chemical (such as an acid) on their surface.
  • Corrosion of iron (commonly known as rusting) occurs in presence of water and air.
  • Alkaline medium decrease or inhibits the rusting of iron.
  • Corrosion is an electrochemical phenomenon.
  • In corrosion, iron acts as an anode, impure surface area acts as cathode and moisture acts as an electrolyte.

Silver

Reacts with H2S in air.

Form silver sulphide (Ag2S)

Black colour coating is formed.

Copper

Reacts with moist carbon dioxide .

Form basic copper carbonate.

(CuCO3.Cu(OH)2)

Green colour coating formed.

Iron

Reacts with moist air.

Form rust.

(Fe2O3.xH2O)

Reddish –brown colour coating is formed.

 Prevention of Rusting: 

For rusting, iron must come in contact with oxygen and water. Rusting is prevented by preventing the reaction between atmospheric moisture and the iron article.

This can be done by:

  • Painting
  • Greasing
  • Galvanization
  • Electroplating
  • Alloying

Rancidity:

  • When fats and oils are oxidized (addition of oxygen), they become rancid and their smell and taste change and this phenomenon is known as rancidity.

Prevention of rancidity:

  • Adding antioxidant to food (Antioxidants are the substance which can prevent the oxidation of compound.)
  • Keeping food in refrigerator - Refrigerator has low temperature , oxidation slow down at low temperature. 
  • Keeping food away from light - In absence of light, oxidation process slow down.
  • Packaging food and oil in nitrogen gas - Nitrogen gas prevent oxygen from reaching the food and prevent oxidation.
  • Food items can also be prevented from getting oxidized by keeping them in air tight packets.