Exothermic and Endothermic Reactions.ppt

exo

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

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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.
25
o
C45
o
C
Exothermic Reactions

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Energy / kJ)
Progress of reaction (time)
Explaining Exothermic Reactions
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.

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Energy / kJ
Progress of reaction
reactants
products
H=negative
His how
much energy
is given out
His given a
negative sign for
exothermic
reactions
Energy of reactants and products in
Exothermic Reactions

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Exothermic reactions
give out energy. There
is a temperature rise
and H is negative.
Definition of an Exothermic Reaction
products
Energy / kJ)
Progress of reaction
reactants
His negative

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Cools
Heat
energy
Is used
up
during
the
reaction
so the
contents
in tube
feel cold
Starts 25°C Cools to 5°C
Ammonium
nitrate
Water
Endothermic reactions cause a decrease in temperature.
Endothermic Reactions

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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.
25
o
C5
o
C
Explaining Endothermic Reactions

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products
Energy / kJ)
Progress of reaction
reactants
H=+
Energy of Reactants and Products in an
Endothermic Process
His given a
positive sign for
endothermic
reactions
This is how
much energy
is taken in

11. © Boardworks Ltd 2003
Endothermic reactions
take in energy. There is
a temperature drop and
H is positive.
Definition of Endothermic Reactions
H=+
products
Energy / 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 15
0
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

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New bonds
form
Old bonds
start to break
Activation energy
•Chemical reactions involve the formation of
bonds between atoms but often before new
bonds can be formed old ones have to be
broken.
•This means that there has to be enough energy
(activation energy)to start breaking the old
bonds before a reaction can occur.
Reactants
Activation
energy
needed to
break existing
bonds
Energy is
given out as
new bonds
form

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Most reactions are exothermic (give out heat) overall but
there is still a need for energy to get the reaction started.
Reaction
Activation energy
Break old bonds
Energy taken in
Form new bonds
Energy given out
Explaining Activation Energy

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Energy / kJ)
Progress of reaction
reactants
products
H= -
Activation Energy and Exothermic
Reactions
Activation energy

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Energy / kJ)
Progress of reaction
reactants
products
H=+
Activation
energy
Activation Energy and Endothermic
Reactions

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Use this diagram
to explain why
garages can store
petrol safely but
always have
notices about not
smoking near the
petrol pumps.
Energy / kJ)
Progress of reaction
Petrol
+
oxygen
Carbon dioxide +
water
ActivationE
nergy
Activity

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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.
Energy / kJ)
Progress of reaction
Petrol
+
oxygen
Carbon dioxide +
water
ActivationE
nergy
Answer

20. © Boardworks Ltd 2003

21. © Boardworks Ltd 2003
Catalysts
For chemical reactions to occur:
•Existing bonds have to begin breaking so that new ones
can be formed.
•The molecules have to collide in such a way that the
reacting parts of the molecules are brought together.
Catalysts can help with either or both of these processes.
A catalyst is a substance that speeds up a
reaction without getting used up in the process.

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Effect of a Catalyst on Activation Energy

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Inside car engines some of the
nitrogen and oxygen from the air
combine to form poisonous nitrogen
oxide. Inside the exhaust system a
catalyst encourages decomposition
back into nitrogen and oxygen.
1.Copy the energy profile for the
uncatalysed reaction and draw in
new lines showing how the presence
of a catalyst will alter the profile.
2NO
2
N
2+ 2O
2