1 Energy Changes in Chemical Reaction
Transcript of 1 Energy Changes in Chemical Reaction
-
7/27/2019 1 Energy Changes in Chemical Reaction
1/11
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
-
7/27/2019 1 Energy Changes in Chemical Reaction
2/11
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
-
7/27/2019 1 Energy Changes in Chemical Reaction
3/11
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
-
7/27/2019 1 Energy Changes in Chemical Reaction
4/11
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
-
7/27/2019 1 Energy Changes in Chemical Reaction
5/11
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
-
7/27/2019 1 Energy Changes in Chemical Reaction
6/11
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.
-
7/27/2019 1 Energy Changes in Chemical Reaction
7/11
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.
-
7/27/2019 1 Energy Changes in Chemical Reaction
8/11
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/ -
7/27/2019 1 Energy Changes in Chemical Reaction
9/11
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
-
7/27/2019 1 Energy Changes in Chemical Reaction
10/11
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
-
7/27/2019 1 Energy Changes in Chemical Reaction
11/11
(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