Energy Changes
Ref: Conceptual Learning Chemistry, 3rd Edn
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SYLLABUS
4 Energy Changes
Candidates should be able to:
(a) describe the term exothermic as a process or chemical reaction which transfers energy, often
in the form of heat, to the surroundings and may be detected by an increase in temperature, e.g.
the reaction between sodium hydroxide and hydrochloric acid
(b) describe the term endothermic as a process or chemical reaction which takes in energy, often
in the form of heat, from the surroundings and may be detected by a decrease in temperature,
e.g. the dissolving of ammonium nitrate in water
Energy Changes
1.
In a chemical reaction, there is energy exchange between the reaction system and the
surroundings.
2.
Principle of energy conservation � Energy cannot be destroyed or created.
● reaction system
(reactants and products)
● surroundings
1 | Energy Changes Science Fun
Refers to
Energy Type
the particles involve in
the reaction
kinetic energy
environment outside the
reaction
heat energy
Physical
measurement
cannot be measured
can be measured by
temperature change
3.
For a reaction to take place and to form new products,
I.
atoms in the reactants must break existing bonds
✔ Enough energy must be absorbed to break old bonds to overcome the Ea.
✔ This energy is taken in / absorbed from the surroundings.
II.
to form new chemical bonds in products
✔ atoms must slow down / release energy to the surroundings so that they
can come close together to forge new bonds.
Illustration on covalent molecules
A quick glance of terminologies on movement of energy to and from the surroundings
Heat Exchange with the
surroundings
Key Term
Temperature change
Heat taken in / absorbed
from the surroundings
Endothermic
Decrease
Heat given out / released to
the surroundings
Exothermic
Increase
Checkpoint
Attempt to write the definition of
1.
Exothermic reaction
2.
Endothermic reaction
Types of Endothermic Reactions
Examples
Thermal Decomposition
CaCO3(s)
Dissolving ammonium nitrate in water (Impt)
NH4NO3(s)
2 | Energy Changes Science Fun
CaO(s) + CO2(g)
NH4NO3(aq)
Synthesis – Photosynthesis in Plants
6CO2 + 6H2O
C6H12O6 + 6O2
Examples of Exothermic
Reactions
(Important)
3 | Energy Changes Science Fun
Check Point
1.
State the change in the temperature of the surroundings whether there is an ‘increase’,
‘decrease’ or ‘remains the same’ for the following type of reactions.
Reaction
Exothermic
Endothermic
2.
4 | Energy Changes Science Fun
Temperature in the surrounding