Transcript Energy Change During Chemical Reactions

Energy Change During
Chemical Reactions
Endothermic and Exothermic
Changes
Exothermic & Endothermic
Exothermic reactions
1. An exothermic reaction is one which
releases heat energy to the
surroundings
2. The temperature of the surroundings
increases
Exothermic Reactions
Exothermic reactions increase in temperature.
•
Examples include:
–
–
–
Burning reactions including the combustion
of fuels.
Detonation of explosives.
Reaction of acids with metals.
Magnesium
reacting with
acid
Thermit reaction
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Activity
Say whether these processes are exothermic.
1.
2.
3.
4.
5.
Charcoal burning
A candle burning.
A kettle boiling
Ice melting
A firework exploding
yes
yes
no
no
yes
You have to put heat in for boiling and melting.
You get heat out from all the other processes
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Exothermic Reactions
•
Magnesium + Hydrochloric acid
25o C
45o C
magnesium
Gets hot
Heat
energy
given
out
Hydrochloric
acid
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Exothermic Reactions
•
Almost immediately the hot reaction products start to
lose heat to the surroundings and eventually they return
to room temperature.
25o C
45
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.
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Energy Level Diagram for an
Exothermic Reaction
Energy / kJ)
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.
Progress of reaction (time)
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Energy Level Diagram for an
Exothermic
Exothermic
Reaction
Reaction
2.
H is how
much energy
is given out
Energy / kJ
reactants
H=negative
products
H is negative
because the
products have
less energy than
the reactants.
Progress of reaction
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Exothermic Reaction - Definition
Energy / kJ)
Exothermic reactions
give out energy. There
is a temperature rise
and H is negative.
reactants
H is negative
products
Progress of reaction
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Activity
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Endothermic reactions
1. An endothermic reaction is on which
takes in heat energy from the
surroundings
2. The temperature of the surroundings
decreases
Endothermic Reactions
Endothermic reactions cause a decrease in temperature.
•
•
•
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.
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Endothermic Reactions
Endothermic reactions cause a decrease in temperature.
Ammonium
nitrate
Cools
Water
Starts 25°C
Cools to 5°C
Heat
energy
taken
in as
the
mixture
returns
back to
room
temp.
Returns to 25°C
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Endothermic Reactions
•
The cold reaction products start to gain heat
from the surroundings and eventually return to
room temperature.
o oCC
25
5
The reactants gain energy.
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.
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Energy Level Diagram for an
Endothermic Process
Energy / kJ)
products
H=+
reactants
This is how
much energy
is taken in
This is positive
because the
products have
more energy
than the
reactants.
Progress of reaction
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Endothermic Reaction Definition
Endothermic reactions
take in energy. There is
a temperature drop and
H is positive.
Energy / kJ
products
H=+
reactants
Progress of reaction
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Activity
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
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Endothermic and exothermic reactions
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 diagrams
Energy
level
Activation
energy
Using a catalyst
might lower the
activation energy
Energy given
out by
reaction
Reaction progress
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 Methane
CH4 + 2O2
To burn methane
you have to break
all of these bonds:
And then you
have to make
these ones:
2H2O + CO2
Burning Methane
CH4 + 2O2
2H2O + CO2
Methane
Oxygen
Carbon dioxide
Water
Bond energies
C-H = 435 Kj
O=O = 497 Kj
Total for breaking bonds = 4x435 + 2x497 = 2734 KJ/mol
C=O = 803 Kj
H-O = 464 Kj
Total for making bonds = 2x803 + 4x464 = 3462 KJ/mol
Total energy change = 2734-3462 = -728 KJ/mol
Drawing this on an energy diagram:
2734 Kj
3462 Kj
-728 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.
Bond 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
Activity
•
Hydrogen peroxide decomposes
as shown:
Bond
Energy (kJ)
H-O
464
O-O
146
H
O
O
H
H
O
O
H
O=O
1.
2.
3.
498
Calculate energy for bond
breaking.
Calculate the energy from bond
making
What is the value of H for the
reaction shown
O
O
O
H
H
O
H
H
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Answer
Bond
Energy (kJ)
H-O
464
O-O
146
O=O
498
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)
H
O
O
H
H
O
O
H
O
O
O
H
H
O
H
H
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Activity
Copy the summary using the words from the box to fill
in the gaps:
endothermic
exothermic
lose
common
positive
1. Exothermic reactions are _____.
common
endothermic
2. Reactions that get cold are called _____.
3. Bond forming is an _______
exothermic process.
positive H.
4. Endothermic reactions have a _____
5. In exothermic reactions the chemicals ___
lose
chemical energy.
continued
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Activity
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 activation
______ energy
2. In endothermic reactions bond breaking
more energy than is produced by
requires ___
bond forming.
endothermic process.
3. Bond breaking is an _________
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Which of the following is an endothermic process?
A.
B.
C.
D.
Burning wood
Reacting an acid and alkali
Reacting magnesium with acid
Dissolving ammonium nitrate in water
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Which of the following is true for an exothermic
process?
products
Energy / kJ
A. The reactants lose
energy to the
surroundings
B. The reaction gets cold
C. The energy diagram will
be as shown
D. The reaction will have a
H that is positive (+).
reactants
Progress of reaction
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Which of the following is true when chemical
bonds are broken?
A. The process is exothermic
B. Energy is given out
C. A physical change is occurring
D. 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:
X
Energy / kJ)
A. X
B. Y
C. Z
Z
Y
Progress of reaction
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