Presented by Presented by Simon Chan (1) Simon Chan (1)

Celine Ho (9)Celine Ho (9)

Agnes Ng (21)Agnes Ng (21) Shung Tak Catholic English Shung Tak Catholic English CollegeCollege

May, 1999

•The study of energy change during a chemical reaction is known as energetics.

•The study of energy change during a chemical reaction is known as energetics.

What is Energetics?What is 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.

It is a reaction that absorbed absorbed heatheat from the surroundings and the enthalpy change of the reaction is positive.

Endothermic Endothermic ReactionReaction

R e a c t i o n C o o r d i n a ti o n

Ent

halp

y (k

Jmol

-1)

r e a c t a n t

p r o d u c t

△ H = p o s it i v e

Endothermic ReactionEndothermic Reaction

Exothermic Reaction

It is a reaction that releases heat to the surroundings and enthalpy change of the reaction is negative.

product

Reaction Coordination

Ent

halp

y (k

Jmol

-1) reactant

△ H= negative

Exothermic Reaction

Applications:Applications: CombustionCombustion

e.g. alkane burns in oxygen with a non-smoky flame and gives out large amount of heat energy.

Combustion of methane:

)()(2)( 2224 lOHgCOOgCH

OHnnCOOn

HC nn 22222 )1(2

)13(

Some lunch boxes make use of the reaction betweeSome lunch boxes make use of the reaction between iron powder and oxygen to heat up the content.n iron powder and oxygen to heat up the content.

4Fe(s)+3O2(g)2Fe2O3(s) H=-285.9 kJ mol-1

•Instant Heat Lunch Instant Heat Lunch BoxBox

Hot PacksHot Packs It is widely used to release muscular pain.It is widely used to release muscular pain.

A hot pack is a heat storage A hot pack is a heat storage device.device.

A hot pack, containing a safe A hot pack, containing a safe food grade salt., can heat food grade salt., can heat instantly to a safe temperatureinstantly to a safe temperature

Why a Why a Hot PackHot Pack Is Is Used??Used??

Why a Why a Hot PackHot Pack Is Is Used??Used??

The advantages of applying The advantages of applying hot packs on painful areas to hot packs on painful areas to relieve muscular pains have relieve muscular pains have been recommended for years been recommended for years all over the world.all over the world.

It helps in the healing It helps in the healing

process.process.

Medical Uses of a Hot PackMedical Uses of a Hot PackMedical Uses of a Hot PackMedical 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 Principle of the the Hot PacksHot Packs

Principle: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 ReactionChain Reaction If there is a change in physical If there is a change in physical

condition, e.g. pressure, the condition, e.g. pressure, the equilibrium position will shift and equilibrium position will shift and chain reaction will occur.chain reaction will occur.

In fact, there are two In fact, there are two types of hot packs --- non-types of hot packs --- non-reusable hot pack and reusable hot pack and reusable hot pack reusable hot pack

Reusable hot Reusable hot pack---by the pack---by the crystallization of crystallization of supersaturated supersaturated chemical.chemical.

Reusable hot Reusable hot pack---by the pack---by the crystallization of crystallization of supersaturated supersaturated chemical.chemical.

Non-reusable hot pack ---adding chemicals together and releasing heat

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.

Our ChoiceOur Choice

Preparation of the Hot Preparation of the Hot Pack Using Pack Using

Supersaturated Supersaturated Sodium Thiosulphate Sodium Thiosulphate

Pentahydrate Solution Pentahydrate Solution (Na2S2O3 ·5H2O)?

Thiosulphate ion (S2O32-)

ii).). 1-2 grams of sodium 1-2 grams of sodium thiosulphate pentahydrate is thiosulphate pentahydrate is put into a small test tube. 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)iv) The test tube is connected The test tube is connected to a syringe. Pump in air and to a syringe. Pump in air and disturbs the equilibrium of the disturbs the equilibrium of the system to initiate chain system to initiate chain reaction and heat is released.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 ChangeEnthalpy Change

Reaction Coordination

)(5)( 2322 lOHaqOSNa

)(5 2322 sOHOSNa

Enthalpy Level diagram for the crystallization of supersaturated sodium thiosulphate (Na2S2O3)

En

thal

py (kJ

mol-1)

)(5)(5)( 23222322 sOHOSNalOHaqOSNa pressure

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 The hot pack can be can be reused until reused until the bag the bag breaks.breaks.

Our Design:Air in

Air bag

Tube

ContainerSodium

Thiosulphate

Press here !

Reaction Started

Mechanism

Data Extracted From:Data Extracted From:Following Web sitesFollowing Web sites Chemical Sciences. Chemical Sciences.

Solubility Product Solubility Product ConstantsConstants

Passmores Passmores Comprehensive SchoolComprehensive School

UCSD, Chem-Biochem, UCSD, Chem-Biochem, Undergraduate Teaching Undergraduate Teaching LabsLabs

Science LabScience Lab

REDOX CHEMICALS REDOX CHEMICALS PTY LTD.PTY LTD.

Virginia Tech Virginia Tech Chemistry DepartmentChemistry Department

JCE SoftwareJCE Software

Microsoft EncartaMicrosoft Encarta

Thermo-PadThermo-Pad