A Energy Changes in Chemical Reactions

8/8/2019 A Energy Changes in Chemical Reactions

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CHAPTER 4CHAPTER 4THERMOCHEMISTRYTHERMOCHEMISTRY

A. Energy Changes in ChemicalReaction

B. Heat of Precipitation

C. Heat of Displacement

D. Heat of Neutralization

E. Heat of Combustion


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A_Energy ChangesA_Energy Changesin Chemicalin ChemicalReactionsReactions


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Learning Objectives

After this lesson, you should be able to: State what exothermic and endothermic reactions are

Identify exothermic and endothermic reactions

Give examples of exothermic and endothermic reactions

Construct energy level diagram for both endothermicand exothermic reactions

Interpret energy level diagram

Interrelate energy change with formation and breakingof bonds

Descibe the application of knowledge of exothermic andendothermic reactions in everyday life.


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Energy Changes in Chemical

Reactions In most chemical reactions, there is an energy change. Generally, heat energy is either given out or absorbed

during chemical reactions.

Thermochemistry is a study of changes in heat energywhich take place during chemical reactions.

Energy Changes Example

Chemical Energy Heat Energy

Light Energy

Combustion of chemical

substance in the airHeat Energy Kinetic Energy

Electrical Energy

Steam moves the dynamo to

produce electricity

Electrical Energy Chemical

Energy

Electrolysis


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Unit for energy is Joule (J)

1 kJ = 1000 J

There are two types of chemical Reactions

Exothermic Reactions

Endothermic Reactions


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Exothermic Reaction

An exothermic reaction is a chemicalreaction that gives out heat to thesurroundings.

In an exothermic reaction chemical energy is converted to heat

energy

heat is given out to the surroundings

temperature of surroundings increases

surroundings includes the containerand any solvent the reaction takes

place.


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Examples ofExothermic ReactionsExamples ofExothermic Reactions

involving physical changeinvolving physical change

1. Condensation process

2. Freezing process3. Dissolving of alkalis and acids in water

(especially concentrated acids)

4. Dissolving of anhydrous salts in water


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Examples ofExothermic ReactionsExamples ofExothermic Reactions

involving chemical changeinvolving chemical change

1. Combustion of fuels

2. Oxidation of food in the respiration process

3. The rusting of Iron4. Dissolving soluble bases(metal oxides) in water

5. Neutralization Reactions

6. Reaction between acids and metals or carbonates

7. Displacement reaction8. Haber Process in manufacture of ammonia

9. Contact Process in the production of sulphur trioxide


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Endothermic ReactionEndothermic Reaction

An Endothermic reaction is a chemicalreaction that absorbs heat from thesurroundings.

In endothermic reaction

heat energy is converted to chemicalenergy

heat energy is being absorbed fromthe surroundings

the temperature of surroundingdecreases

Heat absorbed in the reaction isabsorbed from surrounding such ascontainer, so container becomecool.


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Examples ofEndothermic ReactionsExamples ofEndothermic Reactions

involving physical changeinvolving physical change

1. The dissolving of ammonium salts in

water

2. The dissolving of crystalline sallts such as

CuSO4.5H2O, MgSO4.7H2O and

Na2CO3.10H2O

3. Melting process

4. Evaporation and boiling process


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Examples ofEndothermic ReactionsExamples ofEndothermic Reactions

involving chemical changeinvolving chemical change1. The reaction between acids and sodium or

potassium hydrogen carbonate( NaHCO3 ,

KHCO3)2. Thermal decomposition of salts

ZnCO3 ZnO + CO2

3. Thermal dissociation of salts NH4Cl' NH3 + HCl

4. Photosynthesis


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Exothermic &

Endothermic Reaction

During a chemical reaction, heat energy isgiven out or absorbed.

These energy can be measured

These heat is called the heat of reaction,(H.

(H is negative when heat is given out. (H is positive when heat is absorbed during

the reaction


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Energy Level DiagramEnergy Level Diagram

In an exothermic reaction:

The reactants lose heat energy to formproducts.

The products formed have less energy

content than the reactants.(H is negative


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Energy Level Diagram

ex

othermic reaction

Reactants

Products

(( H= negativeH= negative

Energy


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Example ofELD

ex

othermic reaction

CH4 + 2O2

CO2 + 2H2O

(( H=H= 900 kJ mol900 kJ mol11

Energy


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Energy Level DiagramEnergy Level Diagram

In an endothermic reaction:

The reactants absorb heat energy from thesurroundings to form the products.

The products formed have higher energy

content than the reactants.(H is positive


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endothermic reaction

Reactants

Products

(( H= positiveH= positive

Energy


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endothermic reaction

CaCO3

CaO + CO2

(( H= + 570 kJmolH= + 570 kJmol--11

Energy


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Energy Change during Formation

and Breaking of Bonds

A chemical reaction involves bond

breaking and bond formation.

Bond breaking always requires energy.

Bond formation always releases energy.


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If bond formation releases more energy than is

required in the bond breaking, the excess energy

is released to the surrounding as heat.

(heat absorbed < heat released)

(( H= negativeH= negative

This happens in an exothermic reaction


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If bond breaking requires more energy

than is releases in the bond formation

(heat absorbed > heat released)

((H

= positiveH

= positive

This happens in an endothermic reaction


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Application ofExothermic and

E

ndothermic ReactionsIce packs and hot packs are used to reduceswelling and pain due to muscle or joint

sprain.

In hospital, cold packs are put on the

foreheads of patients to reduce fever.


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Application ofExothermic and

E

ndothermic Reactions

Solid NaOH (lye) is a common drain

cleaner.

Dissolving lye in water is an exothermic

process and heat liberated may melt the

grease, allowing it to be flushed from a

clogged drainpipe.


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Heat of Reaction

The change in the amount of heat in a

chemical reaction is called the Heat of

Reaction.

Is given the symbol, HH

Unit of Heat of Reaction = kJmol-1


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Determination of the

Heat of Reaction, H The amount of heat released or absorbed in

a reaction can be determined by using

formula:(H= m v c v U

m = mass of solution (g)c = specific heat capacity ( Jg-1C-1)

U = change in temperature of solution( C )


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ASSUMPTIONSWHEN CALCULATINGASSUMPTIONSWHEN CALCULATING

HH==

mcmcUU

***** assumedin any aqueous solution

1cm3 = 1 g

*****No loss of heat energy to the surroundings or

gain of heat energy from the surroundings.

*****the container, the thermometer and all otherapparatus used in the experiment absorb a

negligible amount of heat.


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Example of Calculation of H

Example 1

When 50cm3 of NaOH is mixed with

40cm3 sulphuric acid, the mixture solution

temperature changes from 28C to 43 C .

Calculate the heat change for the reaction.

[ specific heat capacity of water

: 4.2 J g1 C 1

density of solution : 1 g cm3 ]


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Solution:Solution:

Heat of Reaction , H = mcU

m = mass of mixture solutions

= total volume of solution v density of solution

= (50+40)cm3 v 1 g cm3

= 90g

c = 4.2 J g1 C 1

U = change in temperature= (4328) C

= 15 C


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Thus, Heat of reaction= ( 90g ) (4.2 J g1 C 1 )(15 C )

= 5670 J

= 5.67 kJ

Thus,

Heat released in this experiment is 5.67 kJ


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Example 2

A spatula of ammonium chloride powder isadded to 100 cm3 distilled water. Distill waterbecame cold and heat loss is 6.0 kJ. If the

initial temperature of distilled water is 29 C,what is the minimum temperature that can beachieved by water?

[ specific heat of water : 4.2 J g1 C 1

density of solution : 1 g cm3]


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Solution:

Heat of reaction = mcU

m = mass of distilled water= 100 g

c = 4.2 J g1 C 1

Assumed t is the lowest temperature.U = change in temperature

= 29 tC


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Heat of reaction = Heat absorbed

H= mcU

6.0 kJ = (100 g) ( 4.2 J g1 C 1 ) (29 tC )

t = 29 ( 6.0 / 0.42 )

= 14.7 C

42.0/0.642.0

0.6

42.0

0.6

42.0

0.6

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A Energy Changes in Chemical Reactions

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