Presented by
Simon Chan (1)
Celine Ho (9)
Agnes Ng (21)
Shung Tak Catholic English College
May, 1999
What is Energetics?
•The study of energy change during a chemical reaction is known as
energetics.
The total energy stored in a substance is called the ‘heat
content’ or enthalpy, with symbol H.
△H
= Enthalpy of products – Enthalpy of reactants
△H = Hp – Hr
Heat may be absorbed or evolved. Reactions are therefore
divided into two types: endothermic and exothermic.
Endothermic Reaction

It is a reaction
that absorbed
heat from the
surroundings
and the
enthalpy change
of the reaction
is positive.
Endothermic Reaction
Exothermic Reaction
It is a reaction
that releases
heat to the
surroundings
and enthalpy
change of the
reaction is
negative.
Exothermic Reaction
Applications:
 Combustion
e.g. alkane burns in oxygen with a non-smoky flame and gives out
large amount of heat energy.
CnH
2n2
( 3 n  1)

O 2  nCO
2
2
 ( n  1) H 2 O
Combustion of methane:
CH 4 ( g )  2O2  CO 2 ( g )  H 2 O (l )
•Instant Heat Lunch Box
Some lunch boxes make use of the reaction between iron powder and
oxygen to heat up the content.
4Fe(s)+3O2(g)2Fe2O3(s)
 Hot Packs
It is widely used to release muscular pain.
H=-285.9 kJ mol-1

A hot pack is a heat storage device.

A hot pack, containing a safe food grade salt.,
can heat instantly to a safe temperature
Why a Hot Pack Is Used??
 The advantages of applying hot packs on
painful areas to relieve muscular pains have
been recommended for years all over the
world.
 It helps in the healing process.
Medical Uses of a Hot Pack
 In most situations, a
hot pack may be
stored at the patients
bed side to keep them warm. It is
simple and safe enough to use,
even by the patients
themselves.
The Principle of the Hot Packs
Principle:


If a saturated solution remains undisturbed as it is cooled, it
often passes the point at which it would normally crystallize,
reaching a supersaturated state.
It is an equilibrium that the solution turns to crystals and at the
same time The crystals turn back to solution.
Crystal(s)
Solution(aq)
Chain Reaction

If there is a change in physical condition, e.g. pressure, the
equilibrium position will shift and chain reaction will occur.
In fact, there are two types of hot packs --non-reusable hot pack and reusable hot pack
Non-reusable hot pack --adding chemicals
together and releasing
heat

Reusable hot pack---by
the crystallization of
supersaturated chemical.
Our Choice
The reusable hot pack is more preferable as:
•it can be reused until the container breaks;
•The cost-efficiency ratio is much higher;
•Also, it is more environmental friendly.
Preparation of the Hot Pack Using
Supersaturated Sodium Thiosulphate
Pentahydrate Solution (Na2S2O3
·5H2O)?
Thiosulphate ion (S2O32-)
i). 1-2 grams of sodium thiosulphate pentahydrate is
put into a small test tube.
The test tube is put into a hot water bath
(near boiling).
ii). The contents are swirled periodically until all the crystals
are converted into liquid.
iii). The test tube is allowed to cool to room
temperature.
iv) The test tube is connected to a syringe. Pump in air
and disturbs the equilibrium of the system to initiate
chain reaction and heat is released.
v). To reuse the chemicals, we can
simply heat the crystals until the
system completely becomes
liquid.
Then, allow them to cool and the
pack is ready to be reused.
Enthalpy Change
pressure
Na 2 S 2 O3 (aq)  5 H 2 O (l ) 

 Na 2 S 2 O3  5 H 2 O ( s )
H= -50 kJ mol-1
The crystal formed in the above reaction may tend to
expand and break the test tube. To avoid this problem, we
can reheat the test tube to melt the contents to liquid.
The hot solution can be
quickly cooled in an ice
bath if one is in a hurry.
The hot pack
can be
reused until
the bag
breaks.
Our Design:
Air in
Tube
Sodium
Thiosulphate
Container
Air bag
Mechanism
Press here !
Reaction Started
Data Extracted From:
Following Web sites
 Chemical Sciences.
Solubility Product
Constants

REDOX CHEMICALS
PTY LTD.

Passmores
Comprehensive School

Virginia Tech
Chemistry Department

UCSD, Chem-Biochem,
Undergraduate
Teaching Labs

JCE Software

Microsoft Encarta
Science Lab

Thermo-Pad
