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CHEMISTRY DEPARTMENTCHEMISTRY DEPARTMENT

CHEM-111CHEM-111General ChemistryGeneral Chemistry

Unit five( d)Unit five( d)

Reversible ReactionsReversible Reactions

Energy in Chemical Reaction

Activation EnergyWhen 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.

Progress of reaction Progress of reaction

En

erg

y

Ene

rgy

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Exothermic reaction

Endothermic reaction

Activationenergy

Energy of reactant

Energy of products

Heat of reaction (released)

Activationenergy

Energy of

reactant

Energy of products

Heat of reaction (absorbed)

Exothermic and Endothermic ReactionExothermic Reaction

Energy of Product is lower than Energy of Reactant

Endothermic Reaction

Energy of Product is higher than Energy of Reactant

Rate of ReactionThe 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 temperatureAt higher temperature kinetic energy of reactants

increases so they move faster and collide often2. The amount of reactants in the containerThere are more collisions in reactants if amount

is more3. The addition of catalyst

Catalyst lowers the activation energy

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Products

Reactants

Activation energy

Activation energy for catalyzed reaction

Ene

rgy

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

Reversible ReactionsWhen 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

SO2 + O22SO2 + 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.

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)

ReactantReactant

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

Le Chatelier’s principle

The rate of forward and reversed reaction will change to relieve the stress

Effect of Changes on EquilibriumFactor 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

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