Topic 5 –
Energetics/Thermochemistry
Learning Objectives
To be able to:
• Define exo and
endothermic and
standard enthalpy
change of reaction
• Interpret enthalpy
diagrams
Starter
What does exothermic and endothermic
mean?
What examples can you give me?
Classify each of the following reactions as:
exothermic (gives out energy/heat to surroundings) or
endothermic (takes in energy/heat)
Combustion
of a fuel
Chemical
handwarmers
Neutralising
an acid
Thermal decomposition
of copper carbonate
Chemical
icepacks
Energy in Bonds
• Chemical reactions involve making and breaking bonds
• Energy is required to break bonds and,
• Energy is given out when bonds are formed
Bond Breaking – Exothermic or Endothermic?
Bond Making – Exothermic or Endothermic?
• If more energy is given out than taken in = reaction is exothermic
• If more energy is taken in than given out = reaction is endothermic
Enthalpy
• Energy contained in bonds can be converted into heat and is
known as enthalpy and given the symbol 𝑯
• Impossible to know the actual amount of energy involved in a
bond, so it is the change in that is measured ∆𝑯 in kJ mol-1
Standard Conditions
The amount of heat taken in or given out will depend on several
factors:
• The nature of the reactants and products
• Concentration of reactants
• State – solid, liquid or gas
• The surrounding temperature
• The surrounding pressure
These can be taken into account with:
standard enthalpy change of reaction ∆𝑯θ
• Defined as: the enthalpy change when molar quantities of
reactants in their normal states react to form products in their
normal states under standard conditions (θ) of 100 kPa (1
atmosphere) and 298K
Enthalpy Diagram for an Exothermic Reaction
reactants
Reactants have more
chemical energy.
Energy / kJ
Some of this is lost as
heat which spreads out
into the room.
products
Products now have less
chemical energy than
reactants = more stable
Progress of reaction (time)
∆𝑯 = 𝑯products - 𝑯reactants
exothermic reaction = negative
Enthalpy Diagram for an Endothermic Reaction
Energy / kJ
products
reactants
Products now have more
chemical energy than
reactants= less stable
Reactants have less
chemical energy.
Progress of reaction (time)
∆𝑯 = 𝑯products - 𝑯reactants
endothermic reaction = positive
What is this?
Methane, CH4
When it combusts
in oxygen, what
are the products?
+
+
We need to break the C-H and O=O
bonds to be able to form new bonds
Bond Enthalpy Values (in kJ)
A bond energy value tells you how much energy it takes to break a
particular bond (endothermic) or make a particular bond (exothermic)
C-H
414
NN
945
C-C
346
N-H
391
C-O
358
H-H
436
C=O
804
Cl-Cl
242
O=O
498
H-Cl
431
O-H
463
[see databook table 10]
Bond energies/ kJ per mole
Burning Methane
H H H H
C
O
O
O
2 x 498kJ
2652kJ neded to
break bonds
O
3460kJ released
4 x 414kJ
H
O
as new bonds form
C
H
O
O
Bond
Forming
H
H
O
808kJ
released
Energy in chemicals
Bond
Breaking
Bonds breaking Bonds forming
C-H
414
C=O
804
O=O
498
O-H
463
=1656+ 996 = 2652kJ
O
H
O
C
2 x 804kJ
O
O
H
4 x 463kJ
=1608 +=1852
= 3460kJ
Energy change = 2652kJ -3460kJ
-808kJ
Progress of reaction
H
H
CH4
+
2 O2
 CO2
+
2 H2O
H
H
C
O=O
H
O=C=O
O
H
O=O
O
H
H
Bonds to be broken:
H
H
Bonds to be made:
4 x C-H
4(413) = 1652
2 x C=O
2(804) = 1608
2 x O=O
2(498) = 996
4 x O-H
4(463) = 1852
2652 kJ
This is energy taken in
to break the bonds
3460 kJ
This is energy released
when bonds are made
2652 – 3460 = -808 kJ
Negative sign means energy released so exothermic
CH4(g) + 2 O2(g)  CO2(g) + 2 H2O(l)
ΔHθ = -808 kJ mol-1
One mole of methane gas reacts with
Two moles of oxygen gas
to give
One mole of carbon dioxide gas and
Two moles of liquid water and
Releases 808 kJ of heat energy
Neutralisation of an acid
HCl(aq)
+
NaOH(aq)

NaCl(aq)
+
H2O(l)
ΔH = -57.3 kJ mol-1
Change of State
Na (s)

Na(g)
ΔH = +108 kJ mol-1
C 2H 6 +
O2

CO2
+
H2O
1.
Divide the reaction in two (reactants and products)
2.
Balance the equation
3.
Draw the structures
4.
Count the different bonds on either side
5.
Add the energies of all the bonds that are broken (reactants)
6.
Add the energies of all the bonds that are made (products)
7.
Subtract the value for bonds made from bonds broken
8.
Is the reaction endo- or exothermic?
2 C2H6
H
2x H
+

7 O2
4CO2 + 6H2O
H
C
C
H
H
H
4x O=C=O
7x O=O
O
6x
H
H
Bonds
to be
to be made:
What
isbroken:
the problemBonds
here??????
12 x C-H
12(414) = 4968
8 x C=O
8(804) = 6432
2 x C-C
2(346) = 692
12 x O-H
12(463) = 5556
7 x O=O
7(498) = 3486
11988 kJ
9146 kJ
ΔH = ΣHbonds broken- ΣHbonds formed
9146 - 11988 = -2842 kJ
-2842 kJ for 2 moles of C2H6 so -1412kJ mol-1
Limitations of Bond Enthalpies
• To use this method, all the reactant and products must be in the
gaseous state
• So what is the problem with the previous reaction?
Homework:
• Read pages 140 and 141 of the Course Companion
• Page 41 of the Revision Guide