ppt on chemical energetics

1. ENDOTHERMIC & EXOTHERMIC
OBJECTIVES:
To explain the terms endothermic and exothermic
To give examples of reactions for each
STARTER:
What do we mean by the
terms endothermic and
exothermic?
Can you give any examples of
each?
KEY WORDS:
ENDOTHERMIC
EXOTHERMIC
ENTHALPY
THERMOCHEMISTRY

2. Chemical reactions can either release energy to
their surroundings, __________, or energy can be
transferred to them from the surroundings,
____________.
EXOTHERMIC
ENDOTHERMIC
KEY IDEA
Bonds contain energy. You need to put in energy to
break bonds apart in the REACTANTS in a chemical
reaction. Energy is released when new bonds form in the
PRODUCTS in a chemical reaction. When bonds break,
energy is absorbed (endothermic). When bonds form,
energy is released (exothermic).
7.1 ENDOTHERMIC & EXOTHERMIC
http://www.youtube.com/watch?v=p-
27I_osoaw

5. 7.1 ENDOTHERMIC & EXOTHERMIC
EXAMPLES:
Exothermic:
• Neutralising an acid with an alkali
• Burning magnesium
• Adding water to anhydrous copper sulphate
Endothermic:
• Thermal decomposition of limestone
• Photosynthesis
• Heating blue copper sulphate
A reaction endothermic in one
direction is always exothermic
in the other

6. 7.1 ENDOTHERMIC & EXOTHERMIC
QUANTITIES:
• Energy is measured in kilojoules per mole
• kJmol-1
CH4 (g) + 2O2 (g)  CO2 (g) + 2H2O (l)
• Combustion of methane  one mole of methane reacts
with 2 moles of oxygen
• It gives out 890kJ of energy

7. 7.1 ENDOTHERMIC & EXOTHERMIC
INVESTIGATION
Follow the instructions on the handout and then answer the
following questions:
1. The first reaction is between an acid and an alkali, what do we call
this type of reaction?
2. Which gas is given off when sodium hydrogen carbonate reacts with
citric acid?
3. Which type of reaction takes place between copper (II) sulfate and
magnesium?
4. Which reactions are exothermic and which are endothermic?
5. Describe in terms of bond breaking and bond making, why some
reactions are exothermic and some are endothermic.

8. 7.1 ENDOTHERMIC & EXOTHERMIC
INVESTIGATION
1. Heat some blue copper sulphate in a boiling tube
2. Let it cool
3. Add a couple of drops of water and feel the bottom of
the boiling tube
4. Write out an equation for the reaction
5. Which way is endothermic which is exothermic? How do
you know?

9. 7.1 ENDOTHERMIC & EXOTHERMIC
ANSWERS
1. Neutralisation.
2. Carbon dioxide.
3. Displacement reaction – magnesium is higher in the reactivity series
so it displaces copper from its sulphate.
4. Sodium hydrogen carbonate solution + citric acid is endothermic, the
other reactions are exothermic.
5. Reactions involve making and breaking bonds. Energy is required to
break bonds. Energy is released when bonds are made. If more
energy is required to break the bonds in a reaction than that
released by making the bonds then the reaction is endothermic
(heat is absorbed). If less energy is required to break the bonds
than that released by making the bonds then the reaction is
exothermic (heat is produced).

10. I CAN…I AM…
How low can you go?? Write what you can do and what
grade this is  show some proof you can do this!
I CAN… I AM…
Describe the terms endothermic and
exothermic
C
Give examples of endothermic and exothermic
reactions
B
Explain endothermic and exothermic reactions
using ideas of bond energy
A

11. Can you answer this…?
This is the first law of
Thermodynamics. Using
the words ‘system’ for an
explosive and
‘surroundings’ for the air
around the explosive,
explain whether explosives
break this law.
“Energy cannot be made or destroyed.”

12. Reactions that give
out HEAT energy to
the surroundings are
____THERMIC
Reactions that take
in heat energy from
the surroundings are

13. Reactions that give
out HEAT energy to
the surroundings are
EXOTHERMIC
Reactions that take
in heat energy from
the surroundings are
ENDOTHERMIC

14. Exothermic
Reactions
Hot or cold

15. 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

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

17. • 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.
25o C
45o C
Exothermic Reactions

18. 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.
Exothermic Reactions

19. 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.

20. Energy
/
kJ
Progress of reaction
reactants
products
H=negative
Energy Level Diagram for an
Exothermic Reaction 2.
H is how
much energy
is given out
H is negative
because the
products have
less energy than
the reactants.
Energy Level Diagram for an
Exothermic Reaction

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

22. Activity

23. 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.

24. 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
Endothermic Reactions

25. • 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.
25o C
5o C
Endothermic Reactions

26. 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

27. products
Energy
/
kJ)
Progress of reaction
reactants
H=+
Energy Level Diagram for an
Endothermic Process
This is positive
because the
products have
more energy
than the
reactants.
This is how
much energy
is taken in

28. Endothermic reactions
take in energy. There is
a temperature drop and
H is positive.
Endothermic Reaction Definition
H=+
products
Energy
/
kJ
Progress of reaction
reactants

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

30. Activity

31. 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.

32. 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

33. Sketch the two energy diagrams and label
exothermic and endothermic as appropriate.
H=+
products
Energy
/
kJ
Progress of reaction
reactants
products
Energy
/
kJ)
Progress of reaction
reactants
H=-
Activity

34. 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.
Heat taken in
Energy needed to
overcome the
bonding between
the atoms
Energy
in
chemicals
Energy needed

35. 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.
Heat given out
Energy given out as
bonds form between
atoms
Energy
in
chemicals
Energy given out

36. Changes to chemical bonds
Endothermic Reactions
• In most chemical reactions some existing bonds
are broken (endothermic)
Energy taken
in as old
bonds break
• But new bonds are made (exothermic)
Energy
in
chemicals
reactants
products
Energy given
out as new
bonds form
H
Overall
endothermic
in this case

37. Changes to chemical bonds
Exothermic Reactions
• Again some existing bonds are broken
(endothermic)
Energy taken in
as old bonds
break
• And new bonds are formed (exothermic)
Energy
in
chemicals
reactants
products
Energy given out
as new bonds
form
H
Overall
exothermic –
in this case

38. Summary – Bond Changes
• 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.
Energy
in
chemicals
reactants
products
H
Bonds
break
Bond
forming
Energy
in
chemicals
reactants
products
H
Bonds
break
Bonds
form
Exo Endo