AQA AS Chemistry Revision Notes 2022 updated savemyexams.co.uk
YOUR NOTES
1.6 Energetics ⬇
CONTENTS
Bond Energy
Energy Level Diagrams
Enthalpy Changes
Calorimetry
Hess' Law
Applications of Hess’s Law
Bond Enthalpies
1.6.1 BOND ENERGY
Bond Energy
During a reaction, enthalpy changes take place because bonds are being broken and formed
Energy (in the form of heat) is needed to overcome attractive forces between atoms
Bond breaking is therefore endothermic
Energy is released from the reaction to the surroundings (in the form of heat) when new
bonds are formed
Bond forming is therefore exothermic
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YOUR NOTES
1.6 Energetics ⬇
To break bonds energy is required from the surroundings and to make new bonds energy
is released from the reaction to the surroundings
The sum of the energy in and energy out determines whether the reaction is overall
exothermic or endothermic
If more energy is required to break bonds than energy is released when new bonds are
formed, the reaction is endothermic
If more energy is released when new bonds are formed than energy is required to break
bonds, the reaction is exothermic
In reality, only some bonds in the reactants are broken and then new ones are formed
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YOUR NOTES
1.6 Energetics ⬇
1.6.2 ENERGY LEVEL DIAGRAMS
Energy Level Diagrams
An energy level diagram is a diagram that shows the energies of the reactants, the transition
state(s) and the products of the reaction as the reaction proceeds ( called ‘extent of the
reaction’ below)
The transition state is a stage during the reaction at which chemical bonds are partially
broken and formed
The transition state is very unstable – a molecule in the transition state cannot be isolated
and is higher in energy than the reactants and products
The activation energy (Ea) is the energy needed to reach the transition state
We can define the activation energy as ‘the minimum amount of energy needed for
reactant molecules to have a successful collision and start the reaction’
The energy level diagram for the reaction of hydrogen with chlorine to form hydrogen
chloride gas
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YOUR NOTES
1.6 Energetics ⬇
Exothermic reaction
In an exothermic reaction, the reactants are higher in energy than the products
The reactants are therefore closer in energy to the transition state
This means that exothermic reactions have a lower activation energy compared to
endothermic reactions
The energy level diagram for exothermic reactions. ∆H is negative because the system
(reactants) loses energy to the surroundings.
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REVISION NOTES TOPIC QUESTIONS PAST PAPERS
YOUR NOTES
1.6 Energetics ⬇
CONTENTS
Bond Energy
Energy Level Diagrams
Enthalpy Changes
Calorimetry
Hess' Law
Applications of Hess’s Law
Bond Enthalpies
1.6.1 BOND ENERGY
Bond Energy
During a reaction, enthalpy changes take place because bonds are being broken and formed
Energy (in the form of heat) is needed to overcome attractive forces between atoms
Bond breaking is therefore endothermic
Energy is released from the reaction to the surroundings (in the form of heat) when new
bonds are formed
Bond forming is therefore exothermic
AQA AS Chemistry Resources
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REVISION NOTES TOPIC QUESTIONS PAST PAPERS
, AQA AS Chemistry Revision Notes 2022 updated savemyexams.co.uk
YOUR NOTES
1.6 Energetics ⬇
To break bonds energy is required from the surroundings and to make new bonds energy
is released from the reaction to the surroundings
The sum of the energy in and energy out determines whether the reaction is overall
exothermic or endothermic
If more energy is required to break bonds than energy is released when new bonds are
formed, the reaction is endothermic
If more energy is released when new bonds are formed than energy is required to break
bonds, the reaction is exothermic
In reality, only some bonds in the reactants are broken and then new ones are formed
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, AQA AS Chemistry Revision Notes 2022 updated savemyexams.co.uk
YOUR NOTES
1.6 Energetics ⬇
1.6.2 ENERGY LEVEL DIAGRAMS
Energy Level Diagrams
An energy level diagram is a diagram that shows the energies of the reactants, the transition
state(s) and the products of the reaction as the reaction proceeds ( called ‘extent of the
reaction’ below)
The transition state is a stage during the reaction at which chemical bonds are partially
broken and formed
The transition state is very unstable – a molecule in the transition state cannot be isolated
and is higher in energy than the reactants and products
The activation energy (Ea) is the energy needed to reach the transition state
We can define the activation energy as ‘the minimum amount of energy needed for
reactant molecules to have a successful collision and start the reaction’
The energy level diagram for the reaction of hydrogen with chlorine to form hydrogen
chloride gas
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, AQA AS Chemistry Revision Notes 2022 updated savemyexams.co.uk
YOUR NOTES
1.6 Energetics ⬇
Exothermic reaction
In an exothermic reaction, the reactants are higher in energy than the products
The reactants are therefore closer in energy to the transition state
This means that exothermic reactions have a lower activation energy compared to
endothermic reactions
The energy level diagram for exothermic reactions. ∆H is negative because the system
(reactants) loses energy to the surroundings.
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