Lecture03222

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Information about Lecture03222

Published on July 22, 2008

Author: uladzimir

Source: slideshare.net

Description

a supplemental resource for students

Chemical Bonding: Energy and Chemical Change Lecture 3

 

 

Endothermic and exothermic processes

There are endothermic and exothermic reactions. However, why are these reactions endothermic and those exothermic?

Internal energy of the system: Kinetic component: Molecules moving through space Molecules rotating Molecules vibrating Electrons moving within the atoms. Potential component: Electrostatic forces between the vibrating atoms nucleus and electrons electrons nuclei and shared electron pair.

Kinetic component:

Molecules moving through space

Molecules rotating

Molecules vibrating

Electrons moving within the atoms.

Potential component:

Electrostatic forces between

the vibrating atoms

nucleus and electrons

electrons

nuclei and shared electron pair.

The energy released or absorbed during a chemical change is due to differences between the reactant bond energies and the product bond energies.

Germain Henri Hess (1802-1850), Russian scientist

Germain Henri Hess (1802-1850), Russian scientist

The heat of reaction: ΔH 0 rxn = Σ(ΔH 0 f products ) - Σ(ΔH 0 f reactants )

Let us rethink it: ΔH 0 rxn = Σ(ΔH 0 reactant bonds broken ) + + Σ(ΔH 0 product bonds formed )

In an exothermic reaction, ΔH< 0 Σ(ΔH 0 reactant bonds broken )+Σ(ΔH 0 product bonds formed ) < 0 , or Σ(ΔH 0 reactant bonds broken ) < Σ(ΔH 0 product bonds formed )

In an endothermic reaction, ΔH> 0 Σ(ΔH 0 reactant bonds broken )+Σ(ΔH 0 product bonds formed ) > 0 , or Σ(ΔH 0 reactant bonds broken ) > Σ(ΔH 0 product bonds formed )

A sample problem on using bond energies to calculate ΔH 0 rxn .

A fuel is a material that reacts with atmospheric oxygen to release energy and is available at a reasonable cost.

Fuel “parade”:

Fuels with more weak bonds yield more energy than fuels with fewer weak bonds.

Fuel for our engines:

THE END

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