1 energy changes in chemical reaction

7/27/2019 1 Energy Changes in Chemical Reaction

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Thermochemistry

Energy Changes in Chemical Reaction

Definition:

Poem;

P.K.EXOTHERMIC

ENDOTHERMIC

Very importantBond breaking REQUIRES energyBond formation RELEASES energy

Thermochemistry is the study of changes in heatenergy which take place during chemical reaction

Heat energy, we shalllater studyIs involved in reactions ofthermochemistry,

A chemical reaction that GIVES OUT/RELEASES heat to the surroundings

A chemical reaction that ABSORBS heat

from the surroundings


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EXOTHERMIC REACTIONEnergy Profile Diagram

Study the diagram carefully.So that you will be able to draw the Energy Level Diagram.

What is activation energy?

Activation energy is the energy barrier that must beovercome by the colliding particles of the reactants in order

for reaction to occur / to become the products

H = The change in the amount of heat in a chemicalreaction is called the heat of reaction.

Energy

Reaction path

Energyreleases

during bondformation

Energyrequiresduring

bondbreaking

Reactants

Products

Energy Requires < Energy Releases

X

Y

Z

Activation Energy H = ve


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What we can say about exothermic reaction?

The conclusion is;- Bond breaking requires less energy than bond formation,- So, the energy released to form the bond is higher /

greater than the energy absorbed to break the bonds.- Then, excess heat energy is released to the surroundings.- Thus, during the reaction temperature of the mixture

inreases,- The total energy of product is lower than the total energy

of reactant.

Energy Level Diagram for Exothermic Reaction

Simply mean like this;Project = rm100 000 [energy requires]Complete = rm150 000 [energy releases]

Loss = rm50 000 [ H, heat changes]Saving = less rm50 000 [negative]

Energy

Reactants

Products

H = (negative)

H = Hproducts

Hreactants

H = ve


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ENDOTHERMIC REACTION

Energy Profile Diagram

Study the diagram carefully.

What we can say about endothermic reaction?

The conclusion is- Bond breaking requires more energy than bond formation,- So, the energy released to form the bond is lower / less

than the energy absorbed to break the bonds.- Then, heat energy is absorbed from the surroundings.- Thus, during the reaction temperature of the mixture

decreases,- The total energy of product is higher than the total energy

of reactant.

Energy

Reaction path

Energyreleasesduring bondformation

Energyrequiresduring

bondbreaking

Reactants

Products

Energy Required > Energy Released

Q

P

R

Activation Energy

H = +ve


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Energy Level Diagram for Endothermic Reaction

Simply mean like this;Project = rm100 000 [energy requires]

Complete = rm50 000 [energy releases]Profit = rm50 000 [energy heat changes]

Saving = up to rm50 000 [positive]

Example of exothermic reaction

Most of the chemical reaction is exothermic such as

- neutralization

- combustion- acid and metal

[Tip: better to memorize endothermic reaction, because notmany reaction is endothermic]

Energy

Reactants

Products

H = + (positive)

H = Hproduct

Hreactants

H = +ve


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Example of endothermic reaction1. Salt dissolves in water

NH4Cl (s) NH4+ (aq) + Cl- (aq)

KNO3 (s) K+ (aq) + NO3

- (aq)

2. Salt crystallizationCuSO4.5H2O (s) Cu

2+ (aq) + SO42- (aq) + 5H2O (l)

3. Thermal decomposition

ZnCO3 (s) ZnO(s) + CO2 (g)

2Mg(NO3)2 (s) 2MgO

(s) + 4NO2 (g) + O2 (g)

4. Salt dissociation

NH4Cl (s) NH4+ (aq) + Cl- (aq)

CaCO3 (s) CaO(s) + CO2 (g)

5. Reaction between acid with sodium hydrogen carbonate

and potassium hydrogen carbonate;HCl(aq) + NaHCO3(aq) NaCl(aq) + H2O(l) + CO2(g)

HCl(aq) + KHCO3(aq) KCl(aq) + H2O(l) + CO2(g)

6. photosynthesis

6CO2 + 6H2O C6H12O6 + 6O2

7. process of melting, evaporation and boiling.


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Example 1:

Mg + H2SO4 MgSO4 + H2 H = 467 kJ

Energy level diagram

Exercises

Based from the following equations, construct and explainenergy level diagram for the reaction.

(1) CaCO3 CaO + CO2 H = + 178 kJ

(2) 2H2 + O2 2H2O H = 572 kJ

(3) Zn + CuSO4 ZnSO4 + Cu H = 190 kJ

(4) H2 + I2 2HI H = + 53 kJ

Energy

Mg + H2SO

4

MgSO4 + H2

H = - 467 kJ

Explanation:

The reaction is an exothermic reactionTemperature of mixture is increases

Total energy of 1 mole Mg and 1 mole H2SO

4is

higher than 1 mole of MgSO4 and 1 mole H2 by 467kJWhen 1 mole Mg reacts with 1 mole H

2SO

4to form 1

mole of MgSO4

and 1 mole H2

, 467 kJ of heat is

released/produced.


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Effective Practice pg 148 no. 1, 2, 3 & 4

Example 2:

2HgO 2Hg + O2

H = +182 kJEnergy level diagram;

Kamal Ariffin Bin SaaimSMKDBL

http://www.kemhawk.webs.com/

Energy

2HgO

2Hg + O2

H = + 182

Explanation:

The reaction is an endothermic reactionTemperature of mixture is decreases

Total energy of 2 mole HgO is lower than 2 mole ofHg and 1 mole O

2by 182 kJ

When 2 mole HgO decompose to form 2 mole of Hg

and 1 mole O2, 182 kJ of heat is absorbed
http://www.kemhawk.webs.com/http://www.kemhawk.webs.com/


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exampleThe diagram shows an energy profile diagram.

Based on the above energy profile diagram, the amount of activation energy is..A (Y X) kJ mol-1

B X kJ mol-1C (X Y) kJ mol-1D Y kJ mol-1

25 The reaction between nitrogen and oxygen can be represented by the following equation:

N2 (g) + O2(g) 2NO(g) H = +181 kJ

Which of the following energy level diagrams represent the above reaction?

A B

C D

Energy

Reactants

X kJ mol-1

Y kJ mol-1

Products

2NO(g)

Energy

Energy

N2(g) + O

2(g)

2NO(g)

H = +181 kJ

N2(g) + O

2(g)

2NO(g)

H = +181 kJ

Energy

2NO(g)

N2(g) + O

2(g)

H = +181 kJ

Energy

N2(g) + O

2(g)

H = +181 kJ


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6 A pupil carried out an experiment to determine the value of heat of neutralization.Diagram 6 shows the set up of the apparatus used in the experiment.

The following data was obtained;

a) Why was a polystyrene cup used in this experiment?

..[1 mark

(b) Given that the specific heat capacity of the solution is 4.2 Jg-1oC-1 and thedensity of the solution is 1.0 gcm-3.

(i) Calculate the change of heat in the experiment.

(ii) Calculate the heat of displacement in the experiment.

[3 marks

1

Initial temperature of hydrochloric acid = 28oCInitial temperature of sodium hydroxide solution = 28oCHighest temperature of the mixture of product = 41oC

100 cm3of 2.0 mol dm-3

Sodium hydroxide solution

100 cm3of 2.0 moldm-3hydrochloric acid

Thermometer

DIAGRAM 6

Polystyrene cup


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(c) Draw the energy level diagram for the reaction.

[2 marks

(d) Based on the experiment, what is meant by theheat of neutralisation?

.......

[1 mark

(e) The pupil repeats the experiment by replacing hydrochloric acid withethanoic acid. All the other conditions remain unchanged.(i) Predict the value of the heat of neutralisation?

.........[1 mark]

(ii) Explain why?

........

....

........

....................................................................................................................[2 marks

1

1 energy changes in chemical reaction

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