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© Boardworks Ltd 2005 1 of 44
KS4 Chemistry
Energy Transfer
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Practical Skills
The focus of today’s lesson: • To observe and record your findings of a
series of reactions
• Explain what endothermic and exothermic reactions are
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Experiment
1. Read the instruction sheet carefully 2. Complete each reaction and record your
results either on the sheet or draw a table in your book.
3. Once you have completed your experiment, clear away your equipment
4. Have a go at the past paper question
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Energy Transfer
Bonds and energy
Activation energy
Exothermic reactions
Summary activities
Endothermic reactions
Contents
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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° C magnesium
hydrochloric acid
Heat energy given out
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 C 45° 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° C Chemical 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)
reactants Reactants have more chemical energy.
Some of this is lost as heat which spreads out into the room.
products Products now have less chemical energy than reactants.
Exothermic energy level diagram
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Ene
rgy
/ kJ
Progress of reaction
reactants
products
ΔH=negative
ΔH (delta H) is how much energy is given out
ΔH is negative because 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 ΔH is negative.
products E
nerg
y / k
J)
Progress of reaction
reactants
ΔH is negative
Definition of an exothermic reaction
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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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products E
nerg
y / k
J)
Progress of reaction
reactants
ΔH=+ This is positive because the products have more energy than the reactants.
This is how much energy is taken in
Endothermic reactions and ΔH
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Endothermic reactions take in energy. There is a temperature drop and ΔH is positive.
ΔH=+
products
Ene
rgy
/ kJ
Progress of reaction
reactants
Definition of an endothermic reaction
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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
Ener
gy in
che
mic
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
Ener
gy in
che
mic
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)
Ener
gy in
che
mic
als
reactants products
Energy given out as new bonds form
+ΔH
Overall endothermic in this case
Bonds and endothermic 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.
Ener
gy in
che
mic
als
reactants
products
-ΔH
Bon
ds b
reak
Bon
d fo
rmin
g
Ener
gy in
che
mic
als
reactants
products
+ΔH
Bon
ds b
reak
Bon
ds fo
rm Exo Endo
Summary of bond changes
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• This is an exothermic reaction.
Bond Forming
Bond Breaking
Progress of reaction
Ener
gy in
che
mic
als
O O O O
H
C H H H
O O O O
C H H H H
O C O O
O H H
H H -ΔH
Burning methane
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Energy Transfer
Bonds and energy
Activation energy
Exothermic reactions
Summary activities
Endothermic reactions
Contents
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Ene
rgy
/ kJ)
Progress of reaction
reactants
products
ΔH= -
Ea= + Activation energy
Exothermic reactions and Ea
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Ene
rgy
/ kJ)
Progress of reaction
reactants
products
ΔH = +
Ea= + Activation energy
Endothermic reactions and Ea
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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
activation energy
Petrol + oxygen: activation energy
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• Exothermic reactions: Are common. Give out heat. Have a negative ΔH. 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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Glossary
l activation energy – The amount of energy needed for a reaction to begin.
l bond – A strong force that joins atoms or ions together in molecules and giant lattices.
l delta H – The amount of energy change during a reaction.
l endothermic – A type of reaction that requires energy.
l exothermic – A type of reaction that generates energy.
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