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Energy Change During Chemical Reactions
Endothermic and Exothermic Changes
Exothermic & EndothermicExothermic & EndothermicExothermic & EndothermicExothermic & Endothermic
Exothermic & Endothermic
1. An exothermic reaction is one which releases heat energy to the surroundings
2. The temperature of the surroundings increases
Exothermic reactionsExothermic reactionsExothermic reactionsExothermic reactions
Exothermic reactions
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Exothermic Reactions
• 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.
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Say whether these processes are exothermic.1. Charcoal burning2. A candle burning.3. A kettle boiling4. Ice melting5. A firework exploding
yes
yes
yes
no
no
You have to put heat inin for boiling and melting.
You get heat outout from all the other processes
Activity
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• Magnesium + Hydrochloric acid
Gets hot
25o C 45o Cmagnesium
Hydrochloricacid
Heatenergygivenout
Exothermic Reactions
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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.
25o 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
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Ene
rgy
/ kJ
)
Progress of reaction (time)
Energy Level Diagram for an Exothermic Reaction
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.
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Ene
rgy
/ kJ
Progress of reaction
reactants
products
H=negative
Energy Level Diagram for an Exothermic Reaction 2.
H is howmuch energyis given out
H is negativebecause the products have less energy than the reactants.
Energy Level Diagram for an Exothermic Reaction
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Exothermic reactions give out energy. There is a temperature rise and H is negative.
Exothermic Reaction - Definition
products
Ene
rgy
/ kJ
)
Progress of reaction
reactants
H is negative
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Activity
1. An endothermic reaction is on which takes in heat energy from the surroundings
2. The temperature of the surroundings decreases
Endothermic reactionsEndothermic reactionsEndothermic reactionsEndothermic reactions
Endothermic reactions
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Endothermic Reactions
• 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.
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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 Reactions
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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.25o 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
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products
Ene
rgy
/ kJ
)
Progress of reaction
reactants
H=+
Energy Level Diagram for an Endothermic Process
This is positivebecause the products have more energy than the reactants.
This is howmuch energyis taken in
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Endothermic reactions take in energy. There is a temperature drop and H is positive.
Endothermic Reaction Definition
H=+
products
Ene
rgy
/ kJ
Progress of reaction
reactants
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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 150C.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
Activity
Endothermic and exothermic Endothermic and exothermic reactionsreactions
Step 1: Energy must be SUPPLIED to break bonds:
Step 2: Energy is RELEASED when new bonds are made:
A reaction is EXOTHERMIC if more energy is RELEASED then SUPPLIED. If more energy is SUPPLIED then is
RELEASED then the reaction is ENDOTHERMIC
Energy level diagramsEnergy level diagrams
Energy level
Reaction progress
Activation energy
Energy given out
by reactionUsing a catalyst might lower the
activation energy
Exothermic vs endothermic:Exothermic vs endothermic:
EXOTHERMIC – more energy is given out than is taken in (e.g. burning, respiration)
ENDOTHERMIC – energy is taken in but not necessarily given out (e.g. photosynthesis)
Burning MethaneBurning MethaneCH4 + 2O2 2H2O + CO2
To burn methane you have to break all of these bonds:
And then you have to make these ones:
Burning MethaneBurning MethaneCH4 + 2O2 2H2O + CO2
Methane
Carbon dioxide Water
Oxygen
Bond energiesBond energies
C-H = 435 Kj
O=O = 497 Kj
Total for breaking bonds = 4x435 + 2x497 = 2734 KJ/mol
H-O = 464 KjC=O = 803 Kj
Total for making bonds = 2x803 + 4x464 = 3462 KJ/mol
Total energy change = 2734-3462 = -728 KJ/mol
Drawing this on an energy Drawing this on an energy diagram:diagram:
2734 Kj3462 Kj
More energy is given out (3462) than is given in (2734) – the reaction is EXOTHERMIC. The total (“nett”) energy change is –728 Kj. An endothermic reaction would have a positive energy change.
-728 Kj
Bond energy valuesBond energy values
C-H = 435 KJ/mol
O-H = 464 KJ/mol
O=O = 497 KJ/mol
C=O = 803 KJ/mol
C-O = 360 KJ/mol
C-C = 346 KJ/mol
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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
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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
Answer
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Copy the summary using the words from the box to fill in the gaps:
endothermic lose positive
exothermic common
1. Exothermic reactions are _____.
2. Reactions that get cold are called _____.
3. Bond forming is an _______ process.
4. Endothermic reactions have a _____ H.
5. In exothermic reactions the chemicals ___ chemical energy. continued
common
endothermic
exothermic
positive
lose
Activity
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Copy the summary using the words from the box to fill in the gaps:
more endothermic activation
1. The energy needed to start off a reaction is called the ______ energy
2. In endothermic reactions bond breaking requires ___ energy than is produced by bond forming.
3. Bond breaking is an _________ process.
activation
more
endothermic
Activity
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Which of the following is an endothermic process?
A. Burning woodB. Reacting an acid and alkaliC. Reacting magnesium with acidD. Dissolving ammonium nitrate in water
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Which of the following is true for an exothermic process?
products
Ene
rgy
/ kJ
Progress of reaction
reactants
A. The reactants lose energy to the surroundings
B. The reaction gets coldC. The energy diagram will
be as shownD. The reaction will have a
H that is positive (+).
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Which of the following is true when chemical bonds are broken?
A. The process is exothermicB. Energy is given outC. A physical change is occurringD. The reaction will have a H that is
positive(+).
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Which of the following is true for an exothermic reaction?
A. Bond breaking involves a bigger energy change than bond making
B. Bond making involves a bigger energy change than bond breaking
C. Bond making involves the same energy change as bond breaking
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Which of the following is the activation energy:
A. XB. YC. Z