Exothermic Reactions
Hot or cold
From your syllabus…
Lesson Objectives
We are learning to:Identify exothermic and endothermic reactions
What I’m Looking For:You must be able to be able to assign thermic
reactions as +ΔH or –ΔH
You should be able to construct a simple energy
level diagram for endo- and exo- thermic reaction
You could be able to calculate the energy
change in a reaction using bond enthalpies
Exothermic Reactions
 Magnesium + Hydrochloric acid
25o C
45o C
magnesium
Gets hot
Hydrochloric
acid
Heat
energy
given
out
Exothermic Reactions
 If heat is given out this energy must have come
from chemical energy in the starting materials
(reactants).
45
25o C
Reactants convert chemical
energy to heat energy.
The temperature rises.
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.
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
Progress of reaction (time)
Products now have
less chemical energy
than reactants.
Energy Level Diagram for an
Exothermic
Exothermic
Reaction
Reaction
2.
H is how
much energy
is given out
Energy / kJ
reactants
H=negative
products
Progress of reaction
H is negative
because the
products have
less energy than
the reactants.
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
Activity
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.
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
Endothermic Reactions
 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.
o oC
5
25
C
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.
Energy Level Diagram for an
Endothermic Process
Energy / kJ)
products
H=+
reactants
Progress of reaction
This is how
much energy
is taken in
This is positive
because the
products have
more energy
than the
reactants.
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
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
Activity
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.
Activity
Are these endothermic or exothermic?
1. A red glow spread throughout the mixture
exo
and the temperature rose.
2. The mixture bubbled vigorously but the
endo
temperature dropped 150C.
3. Hydrazine and hydrogen peroxide react
so explosively and powerfully that they
exo
are used to power rockets into space.
4. The decaying grass in the compost
maker was considerably above the
outside temperature.
exo
Activity
Sketch the two energy diagrams and label
exothermic and endothermic as appropriate.
H=products
Progress of reaction
products
Energy / kJ
Energy / kJ)
reactants
H=+
reactants
Progress of reaction
Breaking chemical bonds
 Most chemicals will decompose (break up) if we
heat them strongly enough.
 This indicates that breaking chemical bonds
requires energy – is an endothermic process.
Energy needed to
overcome the
bonding between
the atoms
Energy in chemicals
Heat taken in
Energy needed
Making chemical bonds
 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.
Energy given out as
bonds form between
atoms
Energy in chemicals
Heat given out
Energy given out
Changes to chemical bonds
Endothermic Reactions
 In most chemical reactions some existing bonds
are broken (endothermic)
Energy taken
in as old
bonds break
Energy given
out as new
bonds form
Overall
endothermic
in this case
H
Energy in chemicals
• But new bonds are made (exothermic)
reactants
products
Changes to chemical bonds
Exothermic Reactions
 Again some existing bonds are broken
(endothermic)
Energy taken in
as old bonds
break
reactants
Overall
exothermic –
in this case
Energy given out
as new bonds
form
H
Energy in chemicals
• And new bonds are formed (exothermic)
products
Summary – Bond Changes
reactants
products
H
Bonds form
Endo
Bonds break
Bond forming
products
Energy in chemicals
reactants
Exo
H
Bonds break
Energy in chemicals
 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.
Calculating bond energy
changes
Bond energies are measured in kJ/mole.
 For example, the bond energy of the O=O
bond is 498 kJ/mole. This means that 498
kJ is needed to break a mole of these
bonds, but 498 kJ is given out when a
mole of these bonds is made.
There are three steps involved in
calculating a bond energy change:
1. Add up the bond energies for all the
bonds in the reactants. This is the energy
in.
2. Add up the bond energies for all the
bonds in the products. This is the energy
out.
3. The energy change = energy in energy out
If the answer is negative, it is an
exothermic reaction.
If the answer is positive, it is an
endothermic reaction.
Bond
Bond energy in
kJ/mole
H-H
I-I
H-I
436
151
298
Another example
Word eq:
Methane + oxygen  carbon dioxide +
water vapour
Symbol eq:
Bond energy
Bond
Draw the bonds
in kJ/mole
Now draw a
pathway diagram
C-H
435
O=O
498
C=O
803
O-H
464
The answer…
Symbol eq: CH4 + 2O2  CO2 + 2H2O
 (4 x 435) + (2 x 498)  (2 x 803) + (4 x 464)
1740 + 996  1606 + 1856
2736  3462
Bond
= -726 kJ/mol
Bond
energy in
kJ/mole
So it is exothermic
C-H
435
(as it is negative)
O=O
498
C=O
O-H
803
464
The Pathway diagram
/kJ
-726 kJ/mol
Progress of reaction
One more example, work out the the
overall energy change and draw a
pathway diagram
Word eq: nitrogen + hydrogen  ammonia
Symbol eq:
Bond
Draw
Bond
Now draw a
pathway diagram
N=N
H-H
N-H
energy in
kJ/mole
944
436
388
The answer
Symbol eq: N2 + 3H2  2NH3
944 + (3 x 436)  (6 x 388)
2252  2328
= -76 kJ/mol
Bond
N=N
H-H
N-H
Bond
energy in
kJ/mole
944
436
388