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Energy LevelsEnergy Levels
A
Creative Chemistry
PowerPoint Presentation
By Nigel Saunders
Copyright © 2003 Nigel Saunders, all rights reserved
Permission is granted for personal and educational use only. Commercial copying, hiring, lending is prohibited.
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Energy LevelsEnergy Levels
Exothermic reactions
• Energy is given out
• The products have less energy than the reactants
Combustion and neutralisation are exothermic
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Energy Level DiagramsEnergy Level Diagrams
Exothermic reactions
energy
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Energy Level DiagramsEnergy Level Diagrams
Exothermic reactions
energy
course of reaction
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Energy Level DiagramsEnergy Level Diagrams
Exothermic reactions
energy
course of reaction
reactants
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Energy Level DiagramsEnergy Level Diagrams
Exothermic reactions
energy
course of reaction
reactants
products
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Energy Level DiagramsEnergy Level Diagrams
Exothermic reactions
energy
course of reaction
reactants
products
energy given out
∆H is negative
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Energy Level DiagramsEnergy Level Diagrams
Endothermic reactions
• Energy is taken in
• The products have more energy than the reactants
The energy is taken in from the surroundings
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Energy Level DiagramsEnergy Level Diagrams
Endothermic reactions
energy
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Energy Level DiagramsEnergy Level Diagrams
Endothermic reactions
energy
course of reaction
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Energy Level DiagramsEnergy Level Diagrams
Endothermic reactions
energy
course of reaction
reactants
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Energy Level DiagramsEnergy Level Diagrams
Endothermic reactions
energy
course of reaction
reactants
products
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Energy Level DiagramsEnergy Level Diagrams
Endothermic reactions
energy
course of reaction
energy taken in∆H is positivereactants
products
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Summary TableSummary Table
Exothermic reactions
Endothermic reactions
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Summary TableSummary Table
Exothermic reactions
Endothermic reactions
Energy is given out to the surroundings
Energy is taken in from the surroundings
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Summary TableSummary Table
Exothermic reactions
Endothermic reactions
Energy is given out to the surroundings
Energy is taken in from the surroundings
∆H is negative ∆H is positive
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Summary TableSummary Table
Exothermic reactions
Endothermic reactions
Energy is given out to the surroundings
Energy is taken in from the surroundings
∆H is negative ∆H is positive
Products have less energy than reactants
Products have more energy than reactants
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∆∆HH
How much energy is given out or taken in?
• Energy is needed to break chemical bonds
• Energy is given out when bonds are made
∆H is the difference between the energy needed to break the bonds in the reactants, and the energy given out when new bonds are made in the products
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Bond energiesBond energies
The energy needed to break a chemical bond• Different chemical bonds have different
bond energies
Chemical bond Bond energy, kJ/mole
H―H 436
O=O 498
O―H 464
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Working out ∆HWorking out ∆H
Draw an energy level diagram with all the reactants and products on it
energy
course of reaction
2H2 + O2
2H2O
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Working out ∆HWorking out ∆H
Show all the bonds in the reactants
energy
course of reaction
2H2O
H―HH―H
O=O+
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Working out ∆HWorking out ∆H
Show all the bonds in the products
energy
course of reaction
H―HH―H
O=O+
H HO
H HO
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Working out ∆HWorking out ∆H
Show the bond energies for all the bonds
energy
course of reaction
436436
O=O+
H HO
H HO
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Working out ∆HWorking out ∆H
Show the bond energies for all the bonds
energy
course of reaction
436436
498+
H HO
H HO
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Working out ∆HWorking out ∆H
Show the bond energies for all the bonds
energy
course of reaction
436436
498+
H HO
464 464+
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Working out ∆HWorking out ∆H
Show the bond energies for all the bonds
energy
course of reaction
436436
498+
464 464+
464 464+
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Working out ∆HWorking out ∆H
Add the reactants’ bond energies together
energy
course of reaction
464 464+
464 464+
1370
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Working out ∆HWorking out ∆H
Add the products’ bond energies together
energy
course of reaction
1370
1856
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Working out ∆HWorking out ∆H
∆H = energy in ― energy out
energy
course of reaction
1370
1856
13701856
―
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Working out ∆HWorking out ∆H
∆H = energy in ― energy out
energy
course of reaction
1370
1856
13701856-486
―
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Working out ∆HWorking out ∆H
∆H = energy in ― energy out
energy
course of reaction
1370
1856
∆H = -486
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Working out ∆HWorking out ∆H
Summary
• The energy values have units of kJ/mole
• In the exam, you will be given the energy values and all the bonds to make or break
∆H is energy in – energy out
• Energy goes in to break bonds
• Energy goes out when bonds are made