Endothermic And Exothermic Reactions

Introduction
-All chemical reactions involves △Energy
Some release energy.  exothermic
Some absorb energy. endothermic


Molecules are held together by chemical bonds.
-Add energy to break bonds
-Give off energy to join together

Takes MORE enerygy to break down than it gives off to form bonds→Endothermic

Takes LESS energy to break down than it gives off to form bonds→ Exothermic

Enthalpy, H, heat contained in the system.

Energy Diagrams
-Chart the potential energy of the chemicals as they change from reactants to products
-Reactants start with a certain amount of energym energy is added to start the reactions and then energy is released as the reaction proceeds.
-The relative amounts of energy determine: Endothermic & Exothermic.

Energy of reactants: total E of all reactants in the reaction
Energy of products: total E of all products in the reaction
Energy of activated complex: potentioal E of the "transition state" between reactants & products
Activation Energy: The E must be added to get the reaction to progress.
△H: the change in potential energy during the reaction.
△H= E of reactants - E of products

Potential Energy Diagram

E of reactants ＜ E of products                                               E of reactants  ＞E of products

△H is positive.                                                                        △H is negative.

Endothermic reaction                                                               Exothermic reaction.

The Energy absorption or release can be placed directly in the equation.

Eg. CH4+2O2→CO2+2H2O+812KJ

      Higher E        Lower E

Exothermic reactions have the E term on the RIGHT side(RHS) -△H

Endothermic reactions have the E term on the Left side (LHS)    +△H

Eg.

E of Reactants= 150 KJ

E of Products= 100KJ

E of Activated complex= 300 KJ

△H=?      Ea=?

△H= E products- E reactants

      = 100KJ - 150KJ= -50KJ  ( exothermic)

Ea= E activated complex - E reactants

    = 300KJ - 150KJ= 150KJ

Eg.

E of Reactants （E）=200 KJ
△H（F）                =100 KJ ( Endothermic)
Ea(B)                        =300 KJ
E of activated complex (C) =?      E of Product (G)=?      

E of activated complex= Ea+ E of reactants= 200KJ+300KJ=500KJ
E of product= E of activated complex- E of reactants= 500 KJ- 200KJ= 300KJ