A Energy Changes in Chemical Reactions
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Transcript of 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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Energy Level Diagram
endothermic reaction
Reactants
Products
(( H= positiveH= positive
Energy
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Energy Level Diagram
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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Energy Change during Formation
and Breaking of Bonds
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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Energy Change during Formation
and Breaking of Bonds
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