THERMOCHEMISTRY F5

8/8/2019 THERMOCHEMISTRY F5

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FORM 5


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A. ENERGY CHANGES IN

CHEMICAL REACTIONSy Allliving thingsdepend on the energyreceived

from sun.

y Plantsstore light energyas chemical energyina chemical reaction called photosynthesis.

y Chemical energyin food is released duringrespiration.

y

Energyplaysanimportant role in chemistry.y In most chemical reactions, there isan energy

change.

yHeat energyis eithergiven out or absorbed

during chemical reactions


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WHAT IS

THERMOCHEMISTRY ???

The studyof changesin

heat energywhichtake place duringchemical reactions


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EXOTHERMIC REACTION

CHEMICAL REACTION THATGIVES OUTHEAT TO THE

SURROUNDINGS


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EXOTHERMIC REACTION

HEAT HEAT

HEAT


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ENDOTHERMIC REACTION

CHEMICAL REACTIONTHAT ABSORBSHEAT

FROM THE

SURROUNDINGS


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ENDOTHERMIC REACTION

HEAT HEAT

HEAT


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EXERCISES

Identify whether these examples areexothermic or endothermic reactions

yFrying an egg

yRusting of iron

yPhotosynthesis

yCooking lemang

yA glass of milo ice

yBurning of wood


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WHAT IS H ???yHEATOF REACTION.

y H IS NEGATIVE WHEN HEAT IS GIVEN OUT

y H IS POSITIVEWHEN HEAT ISABSORBED

y = CHANGE


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ENERGY LEVEL DIAGRAMy Exothermic reaction, reactants lose heat to form

products. The energy content of the products is

lower than the reactants. Hence, H is given anegative sign.

y Endothermic reaction, reaction is given a positivesign. Reactants absorb heat energy from the

surroundings to form products. Thus, energycontent of the products is higher than thereactants.


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DIAGRAMSENERGY

H= negative

Products

Reactants

Reactants

ENERGY

products

H=

positive

Exothermic reaction Endothermic reaction


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STEPS TO CONTRUCT ENERGY

LEVEL DIAGRAMSyText book page 145


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EXERCISE-CONSTRUCT ENERGY

LEVEL DIAGRAMS

yPage 146,learning task 4.2


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ENERGY CHANGE DURING FORMATION

& BREAKING OF BONDSy Exothermic reaction

Bond formation

releases > energythan is required inbond breaking, the

excess energy isreleased to thesurroundings asheat

y Endothermic reaction

The breaking bond

required > energythan the energy thatis released during

the formation ofbonds.


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APPLICATION OF EXORTHERMIC &

ENDOTHERMIC REACTIONSy Instant cold packs used to treat sportsinjuries, have separate compartments of

water & solid ammonium nitrate,NH3NO3 placed ina plastic ofbag.

yFirst-aid chemical hot pack.

yReusable heat pack.yDrain cleaner / lye.


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DIFFERENT TYPES

OF REACTIONSTYPE OFREACTION

HEATOF REACTION

PRECIPITATION HEAT OFPRECIPITATION

DISPLACEMENT HEAT OFDISPLACEMENT

NEUTRALIZATION HEAT OFNEUTRALIZATION

COMBUSTION HEAT OF COMBUSTION


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B. HEAT OF PRECIPITATIONy H= Hproducts Hreactants

yThe change in amount of heat in achemical reaction is called heat ofreaction.

yThe heat of reaction is normallyexpressed in a thermochemicalequation.


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DEFINITION OF HEAT OF

PRECIPITATIONyTHEHEATCHANGE

WHEN ONE MOLE OFAPRECIPITATE IS FORMED

FROMTHEIR IONS INAQUEOUS SOLUTION.


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Exampley Pb(NO3)2(aq) + K2SO4(aq) PbSO4(s)+2KNO3(aq)

y

In this reaction, 50 kJ of heat


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C. HEAT OF DISPLACEMENT


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D. HEAT OF NEUTRALIZATION


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E. HEAT OF COMBUSTION


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THERMOCHEMISTRY F5