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KS4 Chemistry
Energy Transfer
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Energy Transfer
Bonds and energy
Activation energy
Exothermic reactions
Summary activities
Endothermic reactions
Contents
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Are these endothermic or exothermic?1. A red glow spread throughout the mixture
and the temperature rose. 2. The mixture bubbled vigorously but the
temperature dropped 15°C.3. Hydrazine and hydrogen peroxide react
so explosively and powerfully that they are used to power rockets into space.
4. The decaying grass in the compost maker was considerably above the outside temperature.
exo
endo
exo
exo
Exothermic or endothermic?
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• Examples include:Burning reactions including the combustion of fuels.Detonation of explosives.Reaction of acids with metals.
Thermit reaction
Magnesium reacting with acid
Exothermic reactions increase in temperature.
Exothermic reactions
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• magnesium + hydrochloric acid
Gets hot
25° C 45° Cmagnesium
hydrochloricacid
Heatenergygivenout
Exothermic reaction: Mg + HCl
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• If heat is given out this energy must have come from chemical energy in the starting materials (reactants).
Reactants convert chemical energy to heat energy.
The temperature rises.
25o C45° C
Exothermic reactions: source of energy
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45o C
• Almost immediately the hot reaction products start to lose heat to the surroundings and eventually they return to room temperature.
25° CChemical energy becomes heat energy.
The reaction mixture gets hotter.
Eventually this heat is lost to the surroundings.
It follows that reaction products have less chemical energy than the reactants had to start with.
Exothermic reactions: energy changes
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Ene
rgy
/ kJ
)
Progress of reaction (time)
reactantsReactants have more chemical energy.
Some of this is lost as heat which spreads out into the room.
productsProducts now have less chemical energy than reactants.
Exothermic energy level diagram
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Ene
rgy
/ kJ
Progress of reaction
reactants
products
H=negative
H (delta H) ishow much energyis given out
H is negativebecause the products have less energy than the reactants.
Exothermic reactions and ΔH
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Exothermic reactions give out energy. There is a temperature rise and H is negative.
products
Ene
rgy
/ kJ
)
Progress of reaction
reactants
H is negative
Definition of an exothermic reaction
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Exothermic reactions summary
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Energy Transfer
Bonds and energy
Activation energy
Exothermic reactions
Summary activities
Endothermic reactions
Contents
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• Endothermic chemical reactions are relatively rare.
• A few reactions that give off gases are highly endothermic - get very cold.
• Dissolving salts in water is another process that is often endothermic.
Endothermic reactions cause a decrease in temperature.
Endothermic reactions
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Cools
Heatenergytakenin as the mixture returns back to room temp.
Starts 25° C Cools to 5° C
ammoniumnitrate
water
Endothermic reactions cause a decrease in temperature.
Returns to 25° C
Endothermic reaction: NH4NO3 + H2O
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• Extra energy is needed in order for endothermic reactions to occur.
• This comes from the thermal energy of the reaction mixture which consequently gets colder.
Reactants convert heat energy into chemical energy as they change into products. The temperature drops.
25o C5° C
Endothermic reactions: source of energy
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25o C
• The cold reaction products start to gain heat from the surroundings and eventually return to room temperature.
5o C The reactants gain energy.25° C
This comes from the substances used in the reaction and the reaction gets cold.
Eventually heat is absorbed from the surroundings and the mixture returns to room temperature.
Overall the chemicals have gained energy.
Endothermic reactions: energy changes
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products
Ene
rgy
/ kJ
)
Progress of reaction
reactants
H=+
This is positivebecause the products have more energy than the reactants.
This is howmuch energyis taken in
Endothermic reactions and ΔH
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Endothermic reactions take in energy. There is a temperature drop and H is positive.
H=+
products
Ene
rgy
/ kJ
Progress of reaction
reactants
Definition of an endothermic reaction
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Endothermic reactions summary
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Sketch the two energy diagrams and label exothermic and endothermic as appropriate.
H = +
products
Ene
rgy
/ kJ
Progress of reaction
reactantsproducts
Ene
rgy
/ kJ
)
Progress of reaction
reactants
H = -
Energy diagrams
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True or false?
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Energy Transfer
Bonds and energy
Activation energy
Exothermic reactions
Summary activities
Endothermic reactions
Contents
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• Most chemicals will decompose (break up) if we heat them strongly enough.
• This indicates that breaking chemical bonds requires energy – is an endothermic process.
Heat taken in
Energy needed to overcome the
bonding between the atoms
En
erg
y in
ch
emic
als
Energy needed
Breaking chemical bonds
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• It is reasonable to assume that bond making will be the opposite of bond breaking
• Energy will be given out in an exothermic process when bonds are formed.
Heat given out
Energy given out as bonds form between
atoms
En
erg
y in
ch
emic
als
Energy given out
Making chemical bonds
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• In most chemical reactions some existing bonds are broken (endothermic)
Energy taken in as old bonds break
• But new bonds are made (exothermic)
En
erg
y in
ch
emic
als
reactants
products
Energy given out as new bonds form
+H
Overall endothermic in this case
Bonds and endothermic reactions
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• Again some existing bonds are broken (endothermic)
Energy taken in as old bonds break
• And new bonds are formed (exothermic)
En
erg
y in
ch
emic
als
reactants
products
Energy given out as new bonds form
-H
Overall exothermic – in this case
Bonds and exothermic reactions
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• Where the energy from bond forming exceeds that needed for bond breaking the reaction is exothermic.
• Where the energy for bond breaking exceeds that from bond forming the reaction is endothermic.
En
erg
y in
ch
emic
als
reactants
products
-H
Bo
nd
s b
reak
Bo
nd
fo
rmin
g
En
erg
y in
ch
emic
als
reactants
products
+H
Bo
nd
s b
reak
Bo
nd
s fo
rm
Exo Endo
Summary of bond changes
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• This is an exothermic reaction:
Bond Forming
BondBreaking
Progress of reaction
En
erg
y in
ch
emic
als
OO
OO
H
CHHH
O OOO
C H H H H
O C OO
OH H
H H-
H
Burning methane
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Energy Transfer
Bonds and energy
Activation energy
Exothermic reactions
Summary activities
Endothermic reactions
Contents
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Activation energy
Why do all reactions need some energy to start with?
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Ene
rgy
/ kJ
)
Progress of reaction
reactants
products
H= -
Ea= +Activation energy
Exothermic reactions and Ea
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Ene
rgy
/ kJ
)
Progress of reaction
reactants
products
H = +
Ea= +Activation energy
Endothermic reactions and Ea
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Petrol + oxygen
Copy the energy diagram and use it to help you explain why garages can store petrol safely but always have notices about not smoking near the petrol pumps.
Ene
rgy
/ kJ
)
Progress of reaction
petrol + oxygen
carbon dioxide + water
activationenergy
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The reaction is exothermic but requires the activation energy to be provided before the reaction can get underway.This is necessary to break some of the bonds in the oxygen or petrol before new bonds can start forming.
Ene
rgy
/ kJ
)
Progress of reaction
petrol + oxygen
carbon dioxide + water
activationenergy
Petrol + oxygen: activation energy
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• Exothermic reactions:Are common.Give out heat.Have a negative H.Bond forming gives out more energy than bond breaking consumes.Have reactants that contain more chemical energy than the products.
• Endothermic reactions:Bond breaking is endothermic.Bond forming is exothermic.Reactions require an activation energy to help start the bond breaking process.
Summary
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Energy Transfer
Bonds and energy
Activation energy
Exothermic reactions
Summary activities
Endothermic reactions
Contents
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Glossary
activation energy – The amount of energy needed for a reaction to begin.
bond – A strong force that joins atoms or ions together in molecules and giant lattices.
delta H – The amount of energy change during a reaction.
endothermic – A type of reaction that requires energy.
exothermic – A type of reaction that generates energy.
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Anagrams
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Exothermic or endothermic?
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• The formation of nitrogen (IV) oxide (formula NO2) from reaction of nitrogen with oxygen in car engines has a H value of +33.2kJ per mol of nitrogen oxide.
Write a word equation for the reaction.Write a chemical equation for the reaction.Is H positive or negative?Is the reaction exothermic or endothermic?Draw an simple energy diagram for the reaction (not
showing bond breaking and forming.) Which involves the biggest energy change: bond
breaking or bond forming?
Activity 1
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Nitrogen + oxygen nitrogen(IV)oxideN2 + 2O2 2NO2.H positive (+33.2kJ/mol).The reaction is endothermic.Energy diagram Bond breaking involves
the biggest energy change.products
Ene
rgy
/ kJ
Progress of reaction
reactants
Activity 1 answer
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• Hydrogen peroxide decomposes as shown:
1. Calculate energy for bond breaking.
2. Calculate the energy from bond making
3. What is the value of H for the reaction shown
O
H H
O
H H
O O
H
H
O O
H
H
O O
Bond Energy (kJ)
H-O 464
O-O 146
O=O 498
Activity 2
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Bond breaking. (endothermic)4(O-H) + 2(O-O)=1856+292 = +2148kJ
Bond forming: (exothermic)4(O-H) + 1(O=O)=1856+498 = -2354kJ
H = +2148 – 2354 = -206kJ
(Exothermic) O
H H
O
H H
O O
H
H
O O
H
H
O O
Bond Energy (kJ)
H-O 464
O-O 146
O=O 498
Activity 2 answer
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• Most chemical reactions, including exothermic reactions, seem to need an input of energy to get the reaction started.
• This fits completely with what we have already explained:
Before new bonds can be formed we need to break at least some existing chemical bonds.
This requires an energy input –known as the activation activation energy (Eenergy (Eaa or E Eactact))
Once an exothermic reaction is underway it can provide its own activation energy (from the bond forming stage) and so sustains the reaction.
Summary on activation energy
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Multiple-choice quiz