Topic: 6 Energetics (Pages: 150--153)
Chemical energetics: Energy from chemicals or
from chemical reactions.
• Enthalpy”(H)” : A thermodynamic quantity equivalent to the total
heat content of a system.
Something that can be measured
In all chemical reactions, there will be an Interrelationship between
heat, temperature and motion of the atoms or molecules in a
system.
The increase or decrease in the heat content will bring about energy
changes in the system.
SO WHAT IS A SYSTEM?
What is the system and surroundings in
thermochemistry?
• System and surroundings
• System → The reactants and
products in the reaction mixture.
• You will focus on the heat changes
between the reactants and products.
• Object (chemical reaction) that is
being focused on
• Surroundings→ The air, apparatus
and everything else other than the
system. Everything outside the
system
Understanding the concept of enthalpy
Enthalpy of a system
Heat content
of a system
Reactants
Reactants start
with a certain
heat content
+
Products in a chemical reaction
Products end with a
certain heat content
You are interested in studying about the changes in heat
content (Whether heat is absorbed or released)
between reactants and products which is =enthalpy
change
Enthalpy change:
• It represents a change in the heat content of a system.
• (Example heat energy can be lost or gained in a chemical reaction by
the system). UNIT=KILOJOULES
• At this point let us watch two videos that show how heat content of a
system(chemical reaction) can change?
• https://www.youtube.com/watch?v=iTqGm-q02Bc -• Exothermic reaction
Exothermic reaction: “Exo” is a Greek prefix means “out”.
Heat is released into the environment
Heat energy in the system has decreased.
Reactant and products are both considered as the system. Heat has been released out. So
the energy of the products in the system is less than the reactants
< 0, It is negative: System is losing energy. Net energy is released
Example: --- 100KJ
Energy diagram for an exothermic reaction:
Draw with me
Endothermic reaction: Heat is sucked(absorbed)
into the system from the environment.
• https://www.youtube.com/watch?v=w_PUydkKKoQ
• The System(products) are going to have more heat energy than the
reactants.
•
= +ve
Energy diagram for an endothermic reaction:
Draw with me
Example: + 100KJ
How to denote standard enthalpy change of any
reaction?(irrespective of whether it is a endothermic
or exothermic reaction)
Standard enthalpy change :
Changes in the heat energy
= enthalpy change measured under standard condition.
Pressure= 100 kPa
Temperature =
Concentration=1mol/dm3 for solution
Take it down into your copybooks
1. Standard enthalpy change of a reaction
• Definition: It is the enthalpy change of a reaction which occurs when
equation quantities of materials react under standard conditions with
everything in its standard state
Whether the reactants and products are
in the S,L,G states(stable form) under
standard conditions
100kpa--- pressure
298K--- temperature
In case of solution the
concentration ----1m/dm3
So now let us pick an equation and write its
standard enthalpy change of reaction
Let us write the standard enthalpy change for the formation of water:
Take the standard states of hydrogen and oxygen which is gaseous state
Attempt Q:2 on page 153
2H2(g)
+
O2(g)
2H2O (l)
Solve the checkpoint on page 151
Solution
Ionization
energy
Worked out examples on Page 152 and 153
Why are some reactions
endothermic and other
reactions exothermic?
Reaction involves= breaking old bonds and making new bonds
Energy has to be put in to
move the two magnets apart
If these two magnets are close enough then they will work by
themselves and stick to each other . No energy has to be given
Into the copy book write the knowledge that you got
from the previous slide
• 1. Chemical reaction involves:
Starting materials:
Reactants
• Reactants : Breaking of old bonds occur
Energy has to be absorbed for breaking bond
Ending materials: Products
Products: Formation of new bonds
Energy will be released
Endothermic: enthalpy change =+ve
Exothermic : enthalpy change=-ve
Move to page 160—6F
How to calculate enthalpy change from bond enthalpy or bond energy
Why bond energy is only defined for the
gaseous state of molecules?
Cl(g)—Cl(g)
In the liquid state or solid state
intermolecular forces of attraction is also
present. During bond energy you want to
measure the energy needed to break the
covalent bond which is only possible in the
gaseous phase.
Bond enthalpy
Energy that is required to break 1 mole of bond of the same type in the
gaseous form
431
Page 158
Do not memorize these values. It will given to
you in the exam paper
Check list to take care while solving problems
on bond enthalpy
• 1. Are all the substances in the gaseous phase.
WARNING: Do not use this energy if the materials are in the solid
or liquid state
• 2. Are all the required bond energy values given
• 3. Add all the bond energies of the reactants ( bonds broken)
• 4. Add all the bond energies of the products ( bonds formed)
• 5.
Making and breaking bonds
Energy
out
Energy in
So what happens in an
exothermic reaction?
REACTANTS  PRODUCTS
More energy is given OUT when new bonds are
formed than is taken in when the old bonds are
broken.
Making and breaking bonds
Energy out
Energy
In
So what happens in an
endothermic reaction?
REACTANTS  PRODUCTS
More energy has to be put IN (absorbed) to
break the old bonds than is released when the
new bonds are formed.
Calculate
energies
Example2:
Combustion of
methane
for a reaction using bond
H
H C H
H
In methane
carbon is bonded
with four
hydrogen atoms
Making and breaking bonds
H
H C H
H
Heat energy is
required to break
chemical bonds
during a reaction.
Making and breaking bonds
When breaking a chemical bond, energy is put IN
Making and breaking bonds
When breaking a chemical bond, heat energy is put
IN
If heat energy is put in or absorbed to break bonds
, then this must be ENDOTHERMIC
Making and breaking bonds
When breaking a chemical bond, energy is put IN
If energy is put in, then this must be
ENDOTHERMIC
When making a chemical bond, energy is given OUT
Making and breaking bonds
When breaking a chemical bond, energy is put IN
If energy is put in, then this must be
ENDOTHERMIC
When making a chemical bond, energy is given OUT
If heat energy is given out or released, then this
must be EXOTHERMIC
Making and breaking bonds
When breaking a chemical bond, energy is put IN
If energy is put in, then this must be
ENDOTHERMIC
When making a chemical bond, energy is given OUT
If energy is given out, then this must be
EXOTHERMIC
BREAKING bonds is ENDOTHERMIC,
MAKING bonds is EXOTHERMIC
Making and breaking bonds
Energy
out
Energy
In
So what happens in an
endothermic reaction?
REACTANTS  PRODUCTS
More energy has to be put IN (absorbed) to
break the old bonds than is released when the
new bonds are formed.
Making and breaking bonds
Energy
out
Energy
in
So what happens in an
exothermic reaction?
REACTANTS  PRODUCTS
More energy is given OUT when new bonds are
formed than is taken in when the old bonds are
broken.
Making and breaking bonds
Each type of bond (eg. C-C) has a specific bond
energy – it is the same whether the bond is being
made or broken.
BOND
BOND
ENERGY
kJ/mol
C–H
435
C=O
O=O
803
497
O–H
464
YOU
DON’T
NEED TO
LEARN
THESE!
Let us consider this
chemical reaction: Making
and breaking bonds
Methane + Oxygen  Carbon Dioxide + Water
CH4
2O2
CO2
2H2O
Making and breaking bonds
Methane + Oxygen  Carbon Dioxide + Water
CH4
2O2
CO2
2H2O
In methane there are
four C-H bonds. Each
one has a bond energy
of 435 kJ/mol. So to
break methane apart
requires 4 x 435
kJ/mol = 1740 kJ/mol
Making and breaking bonds
Methane + Oxygen  Carbon Dioxide + Water
CH4
2O2
CO2
2H2O
O=O
O=C=O
H-O-H
O=O
H-O-H
Making and breaking bonds
Complete combustion of methane
Methane + Oxygen  Carbon Dioxide + Water
CH4
2O2
CO2
2H2O
O=O
O=C=O
H-O-H
O=O
H-O-H
4 x (C-H)
2 x (O=O)
2 x (C=O)
4 x (O-H)
Making and breaking bonds
Methane + Oxygen  Carbon Dioxide + Water
CH4
2O2
CO2
2H2O
O=O
O=C=O
H-O-H
O=O
H-O-H
4 x (C-H) 2 x (O=O)
4 x 435 + 2 x 497
2 x (C=O) 4 x (O-H)
2 x 803 + 4 x 464
Making and breaking bonds
Methane + Oxygen  Carbon Dioxide + Water
CH4
2O2
CO2
2H2O
O=O
O=C=O
H-O-H
O=O
H-O-H
4 x (C-H) 2 x (O=O)
4 x 435 + 2 x 497
= 1740
+
994
2 x (C=O) 4 x (O-H)
2 x 803 + 4 x 464
1606
+
1856
Making and breaking bonds
Methane + Oxygen  Carbon Dioxide + Water
CH4
2O2
CO2
2H2O
O=O
O=C=O
H-O-H
O=O
H-O-H
4 x (C-H) 2 x (O=O)
4 x 435 + 2 x 497
2 x (C=O) 4 x (O-H)
2 x 803 + 4 x 464
= 1740 + 994
= 2734 kJ/mol
1606
+
1856
= 3462 kJ/mol
Making and breaking bonds
Methane + Oxygen  Carbon Dioxide + Water
CH4
2O2
CO2
2H2O
O=O
O=C=O
H-O-H
O=O
H-O-H
4 x (C-H) 2 x (O=O)
4 x 435 + 2 x 497
2 x (C=O) 4 x (O-H)
2 x 803 + 4 x 464
= 1740 + 994
1606
+
1856
= 2734 kJ/mol
= 3462 kJ/mol
2734 - 3462 = - 728 kJ/mol
Making and breaking bonds
Methane + Oxygen  Carbon Dioxide + Water
CH4
2O2
CO2
2H2O
O=O
O=C=O
H-O-H
O=O
H-O-H
4 x (C-H) 2 x (O=O)
4 x 435 + 2 x 497
2 x (C=O) 4 x (O-H)
2 x 803 + 4 x 464
= 1740 + 994
= 2734 kJ/mol
1606
+
1856
= 3462 kJ/mol
2734 - 3462
= - 728 kJ/mol
The negative sign shows that this is an EXOTHERMIC reaction and heat energy is given OUT.
So ΔH= negative
Calculate the enthalpy change using bond
energies
Ans:=-88KJ/mol
Mean bond enthalpy
07/02/2022
Answers to the checkpoint on page
160
07/02/2022
Check whether all of them are in the gaseous phase
Calculating bond energy
Cross out the common bonds on
L.H.S. and R.H.S. to help you to work
easily
=1
(+612 +464)– (410+347+358)
=39 KJ/mol
Bond energy example:
6G: Checkpoint answers on
page 163
07/02/2022
Question 2
Question 3