Energy change during chemical rcts

Energy Change During
Chemical Reactions
Endothermic and Exothermic
Changes

Exothermic & EndothermicExothermic & EndothermicExothermic & EndothermicExothermic & 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

© Boardworks Ltd 2003
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.

Say whether these processes are exothermic.
1. Charcoal burning
2. A candle burning.
3. A kettle boiling
4. Ice melting
5. 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

• Magnesium + Hydrochloric acid
Gets hot
25o
C 45o
C
magnesium
Hydrochloric
acid
Heat
energy
given
out

45o
C
• Almost immediately the hot reaction products start to
lose heat to the surroundings and eventually they return
to room temperature.
25o
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.

Energy/kJ)
Progress of reaction (time)
Energy Level Diagram for an
Exothermic Reaction
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.

Energy/kJ
Progress of reaction
reactants
products
∆H=negative
Exothermic Reaction 2.
∆H is how
much energy
is given out
∆H is negative
because the
products have
less energy than
the reactants.
Exothermic Reaction

Exothermic reactions
give out energy. There
is a temperature rise
and ∆H is negative.
Exothermic Reaction - Definition
products
Energy/kJ)
reactants
∆H is negative

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
• 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.

Cools
Heat
energy
taken
in as the
mixture
returns
back to
room
temp.
Starts 25°C Cools to 5°C
Ammonium
nitrate
Water
Endothermic reactions cause a decrease in temperature.
Returns to 25°C

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.

products
Energy/kJ)
reactants
∆H=+
Endothermic Process
This is positive
because the
products have
more energy
than the
reactants.
This is how
much energy
is taken in

Endothermic reactions
take in energy. There is
a temperature drop and
∆H is positive.
Endothermic Reaction Definition
∆H=+
products
Energy/kJ
reactants

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 150
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
Activity

Endothermic and exothermic reactionsEndothermic 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 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 Methane
CH4 + 2O2 2H2O + CO2
To burn methane
you have to break
all of these bonds:
And then you
have to make
these ones:

Burning MethaneBurning Methane
CH4 + 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 diagram:Drawing this on an energy diagram:
2734 Kj
3462 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

But…But…
… you don’t need to know about bond
energies or how to calculate energy
changes for the exam!

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

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

Which of the following is an endothermic process?
A. Burning wood
B. Reacting an acid and alkali
C. Reacting magnesium with acid
D. Dissolving ammonium nitrate in water

Which of the following is true for an exothermic
process?
products
Energy/kJ
reactants
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 (+).

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(+).

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

Which of the following is the activation energy:
A. X
B. Y
C. Z

That’s all folks!

Energy change during chemical rcts

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Energy change during chemical rcts