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Energy LevelsEnergy Levels
Energy LevelsEnergy Levels
Exothermic reactions
• Energy is given out
• The products have less energy than the reactants
Combustion and neutralisation are exothermic
Energy Level DiagramsEnergy Level Diagrams
Exothermic reactions
energy
Energy Level DiagramsEnergy Level Diagrams
Exothermic reactions
energy
course of reaction
Energy Level DiagramsEnergy Level Diagrams
Exothermic reactions
energy
course of reaction
reactants
Energy Level DiagramsEnergy Level Diagrams
Exothermic reactions
energy
course of reaction
reactants
products
Energy Level DiagramsEnergy Level Diagrams
Exothermic reactions
energy
course of reaction
reactants
products
energy given out
∆H is negative
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
Energy Level DiagramsEnergy Level Diagrams
Endothermic reactions
energy
Energy Level DiagramsEnergy Level Diagrams
Endothermic reactions
energy
course of reaction
Energy Level DiagramsEnergy Level Diagrams
Endothermic reactions
energy
course of reaction
reactants
Energy Level DiagramsEnergy Level Diagrams
Endothermic reactions
energy
course of reaction
reactants
products
Energy Level DiagramsEnergy Level Diagrams
Endothermic reactions
energy
course of reaction
energy taken in∆H is positivereactants
products
Summary TableSummary Table
Exothermic reactions
Endothermic reactions
Summary TableSummary Table
Exothermic reactions
Endothermic reactions
Energy is given out to the surroundings
Energy is taken in from the surroundings
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
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
∆∆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
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
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
Working out ∆HWorking out ∆H
Show all the bonds in the reactants
energy
course of reaction
2H2O
H―HH―H
O=O+
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
Working out ∆HWorking out ∆H
Show the bond energies for all the bonds
energy
course of reaction
436436
O=O+
H HO
H HO
Working out ∆HWorking out ∆H
Show the bond energies for all the bonds
energy
course of reaction
436436
498+
H HO
H HO
Working out ∆HWorking out ∆H
Show the bond energies for all the bonds
energy
course of reaction
436436
498+
H HO
464 464+
Working out ∆HWorking out ∆H
Show the bond energies for all the bonds
energy
course of reaction
436436
498+
464 464+
464 464+
Working out ∆HWorking out ∆H
Add the reactants’ bond energies together
energy
course of reaction
464 464+
464 464+
1370
Working out ∆HWorking out ∆H
Add the products’ bond energies together
energy
course of reaction
1370
1856
Working out ∆HWorking out ∆H
∆H = energy in ― energy out
energy
course of reaction
1370
1856
13701856
―
Working out ∆HWorking out ∆H
∆H = energy in ― energy out
energy
course of reaction
1370
1856
13701856-486
―
Working out ∆HWorking out ∆H
∆H = energy in ― energy out
energy
course of reaction
1370
1856
∆H = -486
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
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