This document discusses reversible chemical reactions and chemical equilibrium. It defines key terms like activation energy, exothermic and endothermic reactions, and how factors like temperature, concentration, and catalysts affect the rate and direction of reversible reactions. Specifically, it explains that at chemical equilibrium, the rates of the forward and reverse reactions are equal and application of Le Chatelier's principle describes how the system responds to changes to relieve stress.

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CHEMISTRY DEPARTMENTCHEMISTRY DEPARTMENT
CHEM-111CHEM-111
General ChemistryGeneral Chemistry
Unit five( d)Unit five( d)
Reversible ReactionsReversible Reactions

2. Energy in Chemical Reaction
Activation Energy
When molecules collide, bond between
atoms are broken and new bonds are
formed.
The energy needed to break apart those
bonds is called the activation energy.
If energy of a collision is less than the
activation energy, the molecules bounce
apart without reacting.

3. Progress of reaction Progress of reaction
Energy
Energy
---------------
---------------------
-------
---------
Exothermic reaction Endothermic reaction
Activation
energy
Energy of
reactant
Energy of
products
Heat of
reaction
(released)
Activation
energy
Energy of
reactant
Energy of
products
Heat of
reaction
(absorbed)
Exothermic and Endothermic Reaction
Exothermic Reaction
Energy of Product is lower than Energy of Reactant
Endothermic Reaction
Energy of Product is higher than Energy of Reactant

4. Rate of Reaction
The rate or speed of reaction is measured by
the amount of reactant used up, or the
amount of product formed, in a certain period
of time.
Rate of reaction can be affected by :
1. Change in the temperature
At higher temperature kinetic energy of
reactants increases so they move faster and
collide often
2. The amount of reactants in the container
There are more collisions in reactants if amount
is more
3. The addition of catalyst
Catalyst lowers the activation energy

5. -----------------
----------------------------
-----------------
Products
Reactants
Activation
energy
Activation
energy for
catalyzed
reaction
Energy
Progress of reaction
Hb + O2 HbO2
Hemoglobin Oxygen Oxyhemoglobin
if patient having difficulty breathing may be
given a breathing mixture with a higher oxygen
content than the atmosphere.
The Rate of Reaction Increases When Reactants are
Added
Patient can breathe more easily

6. Reversible Reactions
When a reaction occurs both forward and reverse
direction it is called reversible reaction
(In many reactions the products interact and revert
back into the reactants)
2SO2(g) + O2 ( g) 2 SO3(g)
COCl2 CO (g) + Cl2
N2(g) + O2(g) 2NO(g)
Chemical Equilibrium
At Chemical Equilibrium Rate of Forward Reaction
Becomes Equal to Rate of Backward Reaction

7. SO2 + O2
2SO2 + O2 2SO3 SO3
initially(at equilibrium)
Chemical Equilibrium
initially
The same reaction mixture is obtained whether the reaction
starts with the reactants or with just the products
Sample-I Sample-II
At equilibrium both samples have same proportions of reactants
and products.

8. Reactants
Product
Reactants
SO2
O2
SO2
SO3
O2
Reaction
Chemical Equilibrium
The equilibrium favors the formation of product SO3
The reaction of SO2 and O2
Amount in
1 liter
Initial contents Equilibrium contents
2SO2(g) + O2 ( g) 2 SO3(g)

9. Reactant
Reactant
Reaction
CO Cl2
COCl2COCl2
Product
Chemical Equilibrium
At equilibrium, the reaction favors the reactant
because the reaction mixture at equilibrium contains
mostly COCl2
COCl2 CO + Cl2
Initial contents Equilibrium contents
Amount in
1 liter

10. Le Chatelier’s principle
The rate of forward and reversed
reaction will change to relieve the
stress
Effect of Changes on Equilibrium
Factor Change (stress) Reaction Favored to
Remove Stress
Concentration Add more reactant Forward
Remove reactant Reverse
Add product Reverse
Remove product Forward
Temperature (T) Raise T of endothermic reaction Forward
Lower T of endothermic reaction Reverse
Raise T of exothermic reaction Reverse
Lower T of exothermic reaction Forward

11. 1. Classify the following as exothermic or endothermic
reactions:
a. C3H8 + 5O2 3CO2 + 4H2O + 531 Kcal
b. 2Na + Cl2 2NaCl + 196 Kcal
c. PCl5 + 16 Kcal PCl3 + Cl2
d. Ca(OH)2 + 15.6 Kcal CaO + H2O
Exercise