Energy Changes

1. Energy
Changes

2. Thermochemistry
•Chemical reactions usually involve change
of heat due to energy absorbed or evolved
during reaction.
•The study of heat changes of chemical
reactions is known as
THERMOCHEMISTRY

3. Energy
•Energy can come in different forms;
–like heat, light, electrical, nuclear, etc…
•Chemical energy is stored in all chemical
substances.
•With the law of conservation of energy,
energy is never destroyed or created, only
to be transferred.

4. Bond making /
breaking
•Energy is absorbed or released because
there are bonds being broken and there
are bonds being formed when a chemical
reaction occurs

5. Bond Breaking
•When a bond is being broken, heat energy
is absorbed from the surrounding to
break the bond;
•Bond breaking is endothermic.
•The amount of energy absorbed depends
on the strength of the bond being broken
•The stronger bond, more energy will be
required to break the bond.

6. Bond Making
•When a bond is being formed, heat energy is
evolved, given out to the surroundings
•Bond formation is exothermic.
•The amount of heat energy evolved depends on
the strength of the bond being formed
•The stronger the bond that is formed, more
energy will be released.

7. Net change in energy
•When the amount of energy evolved in a
reaction > the amount of energy absorbed,
there will be a net release of energy to the
surroundings.
•The temperature of the surroundings will
increase and the reaction mixture will feel
warm.
•This is known as an exothermic reaction.

8. Net change in energy
•If the amount of energy absorbed in a
reaction > the amount of energy evolved,
there will be a net absorption of energy
from the surroundings.
•The temperature of the surroundings will
drop and the reaction mixture feels cold.
•This is known as an endothermic reaction.

9. summary
Reaction mixture feels coldReaction mixture feels warm
Temperature of surroundings
drops
Temperature of surroundings
increases
Net absorption of heat energy
from surroundings
Net release of heat energy to
surroundings
More energy absorbed than
energy evolved
More energy evolved than
energy absorbed
Strength of bonds broken
stronger than strength of bonds
formed
Strength of bonds formed
greater than strength of bonds
broken
Endothermic ReactionExothermic Reaction

10. Examples of
exothermic and
endothermic reactions:
Thermal decomposition
Dissolving some ionic salts in
water, like ammonium
chloride, potassium nitrate
and copper(II) sulphate etc…
Photosynthesis
Action of light on silver
bromide
Combustion
Respiration
Neutralisation
Dissolving acids
Dissolving alkalis
Rusting
Oxidation of metals
Nuclear
Endothermic reactionsExothermic reactions

11. Enthalpy Change
•The overall energy change during a reaction is
known as the enthalpy change (DH) of the
reaction.
•The unit for enthalpy change is kJ/mol.
•The unit for energy is kJ.
•An exothermic reaction has a negative enthalpy
change,
–indicating more energy is lost than gained in the
reaction.
•An endothermic reaction has a positive enthalpy
change,
–indicating more energy is gained than lost in the
reaction.

12. Activation
Energies
•In a reaction, the reactant molecules must be
able to overcome an energy barrier before they
can turn into products.
•This energy barrier is known as the activation
energy, Ea.
•Hence, activation energy is the minimum amount
of energy required to initiate a chemical reaction.
•Reactions with high activation energy will not
occur spontaneously and may require heating or
addition of catalyst to initiate the chemical
reaction.

13. Exothermic reaction
Energy
Time
Reactants
Products
Activation energy
DH : -ve

14. Endothermic reaction
Energy
Time
Reactants
Products
Activation energy
DH : +ve

15. Energy Profile
Diagram
•An energy profile diagram is a plot of energy
against time to represent the relative amount of
energy of the reactants and the products.
•An exothermic reaction has an overall energy
loss during the reaction.
•Hence the energy content of the reactants is
higher than the energy content of the products.
•An endothermic reaction has an overall energy
gain during the reaction
•Hence the energy content of the products is
higher than the energy content of the reactants.

16. Exothermic reaction
Energy
Time
Reactants
Products
Enthalpy change (DH)
: negative

17. Endothermic reaction
Energy
Time
Reactants
Products
Enthalpy change (DH)
: positive

18. Bond Energy
•Bond energy is the amount of energy required to
break a given bond.
•Likewise, when the same amount of energy is
evolved when that bond is formed.
•Bond energy values can be used to calculate the
enthalpy change of a reaction.
•When a bond is formed, the bond energy is
assigned a negative value, indicating energy is
lost.
•When a bond is broken, the bond energy is
assigned a positive value, indicating energy
gained.

19. Enthalpy Change
•Enthalpy change of a reaction
= Sum of [energy absorbed due to bond
breaking in reactants (assigned positive
value)]
+[energy evolved due to bond formation in
products (assigned negative value)]

20. •An example of enthalpy change calculation
from bond energies:
•Hydrogen reacts with oxygen to form
water.
•2 H
2
(g) + O
2
(g) ® 2 H
2
O (g)

21. Bond energies
463O – H
496O = O
436H – H
Bond energies(kJ/mol)Bond

22. Calculations
•In the reaction,
–2 hydrogen-hydrogen bonds (1 bond in each hydrogen
molecule) and 1 oxygen-oxygen bond are broken,
–while 4 oxygen-hydrogen bonds (2 bonds in each water
molecule) are formed.
•Hence enthalpy change for the above reaction
= 2 (+436) + (+496) + 4 (–463) = –484 kJ/mol.
•Hence, the thermochemical equation of the reaction
between hydrogen and oxygen to form water can be
written as
•2 H
2
(g) + O
2
(g) ® 2 H
2
O (g)DH = -484 kJ/mol