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CHEMICAL ENERGETICS
Objectives:
•Energy changes during a chemical reaction-

exothermic and endothermic reactions
•Explain what is meant by enthalpy change
•Enthalpy changes of a reaction
•Activation energy draw energy profile diagrams for

exothermic and endothermic reactions

Chemical Energetics
▪ This is the branch of chemistry that deals

with energy changes during physical and
chemical reactions
...


Energy locked in chemical bonds
▪ When bonds are broken, energy is

absorbed
...


▪ Making or breaking strong bonds

(ionic, covalent, metallic) involve
large energy changes
...


Energy changes during
chemical reactions
During a chemical reaction:
1
...


2
...


Energy changes during
chemical reactions
REACTANTS
Bonds broken
Energy absorbed

PRODUCTS
Bonds formed
Energy released

Energy changes-Endothermic
reactions
▪ The prefix “endo” means “in”
▪ If the chemical reaction absorbs heat, causing

the container and its content to cool down,

the reaction is said to be endothermic
...
e
...
e
...

▪ Photosynthesis
▪ Thermal decomposition of compounds

Energy changes-Exothermic
reactions
▪ The prefix “exo” means “out”
▪ If the chemical reaction produces heat,

causing the container and its contents to
warm up, the reaction is said to be
exothermic
...
e
...
e
...
The heat liberated or absorbed
during a chemical reaction is called the
ENTHALPY (heat) of reaction
...

▪ Enthalpy is given the symbol H

Change in enthalpy
▪ Change in enthalpy is given the symbol ∆H
▪ Change in enthalpy is the name given to a

change in heat energy content measured at
constant pressure
...


Change in enthalpy

(contd)

▪ The enthalpy change is defined by the

following equation:
energy content
of product
-

energy content
of reactants

Change in enthalpy
i
...


(contd)

∆H = H product – H reactants

H product

+ve

and H reactants denotes the total energy

content or enthalpy of the products and reactants
...


Energy profile diagrams (Contd)
NB The amount of activation energy does not
affect the overall energy change for the
reaction and is not included in the calculation
of ∆H
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The following should be
included on the diagram:
▪ Formulae of the reactants
▪ Formulae of products
▪ Arrows indicating the activation energy and

∆H
▪ Value for ∆H

Energy profile diagram for the formation
of ammonia

Question
▪ Explain what is meant by an

exothermic reaction and an
endothermic reaction
...


Question (contd
...
Indicate on the
diagram what effect adding a catalyst

would have
...
8kj

END



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