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Energy Diagrams
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ENTHALPY
OBJECTIVES Exothermic changes cause heat to be
released to the surroundings
Endothermic changes cause absorption of heat from the surroundings.
A potential energy diagram can be used to show the energy pathway for a reaction.
The enthalpy change is the energy difference between products and reactants.
Enthalpy DiagramsEnthalpy DiagramsExothermic reactions
energy
course of reaction
Energy Level DiagramsEnergy Level DiagramsExothermic reactions
energy
course of reaction
reactants
Energy Level DiagramsEnergy Level DiagramsExothermic reactions
energy
course of reaction
reactants
products
Energy Level DiagramsEnergy Level DiagramsExothermic reactions
energy
course of reaction
reactants
products
energy given out
∆H is negative
Energy Level DiagramsEnergy Level DiagramsEndothermic reactions
energy
course of reaction
Energy Level DiagramsEnergy Level DiagramsEndothermic reactions
energy
course of reaction
reactants
Energy Level DiagramsEnergy Level DiagramsEndothermic reactions
energy
course of reaction
reactants
products
Energy Level DiagramsEnergy Level DiagramsEndothermic 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
∆∆HHHow 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
C
H
H
H H
O O
O O
C
H
H
H H
O O
O O
Energy absorbed to break bondsEnergy absorbed to break bonds
C
H
H
H H
O O
O O
C H
H
H
H O
O
O
O
Energy released when new bonds are madeEnergy released when new bonds are made
Overall energy changeOverall energy change
==
Energy released on making new bondsEnergy released on making new bonds
--
Energy taken in to break old bondsEnergy taken in to break old bonds
Energy Level DiagramsEnergy Level DiagramsExothermic reactions
energy
course of reaction
methane + oxygen
carbon dioxide + water
Reaction mixtureReaction mixture
Test tubeTest tube
Heat given out to Heat given out to surroundingssurroundings
EXOTHERMICEXOTHERMIC
Heat taken in from Heat taken in from surroundingssurroundings
ENDOTHERMICENDOTHERMIC
TASK Draw energy level diagrams for the
following reactions:1. Photosynthesis
2. Calcium with sulphuric acid
3. Dissolving ammonium nitrate
