This document discusses various concepts in thermochemistry including: - Thermochemistry deals with thermal changes that accompany chemical and physical transformations. It aims to determine energy absorbed or released during reactions. - Reactions require energy to overcome activation energy barriers to start, regardless of whether they are exothermic or endothermic. Exothermic reactions release excess energy to sustain the reaction while endothermic reactions require continuous energy input. - Heat of formation is the energy absorbed or released when forming one mole of a substance from its elements. Hess's law allows calculating reaction energies using heats of formation without direct experimentation. Calorimetry is used to measure energy changes by determining temperature changes of a reaction mixture.

1. PHYSICAL CHEMISTRY
Thermo-chemistry
11-12 Heat of formation, heat of
reaction and heat of solution,
Hess’s law, calorimetry
1

2. THERMOCHEMISTRY
 Thermochemistry is the
branch of chemistry which
deals with the thermal
changes accompanying
chemical and physical
transformations. The aim of
thermochemistry is not only
the determination of energy
emitted or absorbed but
also to develop methods for
calculating these thermal
readjustments without
recourse to experiment.
2

3. THERMOCHEMISTRY
 For practical purpose it
is essential to know
whether the heat is
absorbed or evolved
and how much of it,
because if the heat is
evolved it must be
removed to effect
appropriate reaction
and if it is absorbed,
required amount of
heat must be supplied. 3

4. CHEMICAL REACTIONS
 A reaction always begins
with the breaking a few
bonds and ends with the
formation of a few new
bonds.
4
Since the reaction begins with breaking
of bonds, hence it invariably needs
energy to start. This energy is called
energy of activation.

5. THERMAL CHANGES IN CHEMICAL
REACTIONS
 Energy of activation is required to
start a reaction irrespective of the
fact whether the reaction is
exothermic or endothermic.
 The energy of activation is
actually the barrier between the
reactants and the products and
must be crossed by providing
energy to start the reaction
5

6. EXOTHERMIC REACTIONS
 In exothermic reactions the
total energy of the products is
less than the total energy of
the reactants. Hence, the
difference of energy is
released usually as heat.
 Since more energy is
released than activation
energy, the energy released
is more than enough to
activate more reactant
molecules. Hence once
activated the reaction goes
on by itself and no more
heating is required.
6

7. EXOTHERMIC REACTIONS
7
If the difference of
activation energy and the
total energy released is
less, the reaction is less
exothermic
If the difference of
activation energy and the
total energy released is
more, the reaction is more
exothermic

8. ENDOTHERMIC REACTIONS
 In endothermic reactions the total
energy of the products is greater
than the total energy of the
reactants. Hence, the difference
of energy is absorbed by the
system.
 Since the energy of products is
greater than the reactants, the
energy released after activation is
less than the activation energy
and is not sufficient to activate
more reactant molecules. That is
why endothermic reactions
require continuous supply of
energy (heat) for propagation of
reaction and the reaction stops as
and when the supply of heat is
interrupted. 8

9. ENDOTHERMIC REACTIONS
9
If the difference of activation
energy and the total energy
release is less, the reaction is
less endothermic
If the difference of
activation energy and the
total energy release is
more, the reaction is more
endothermic

10. HEAT OF FORMATION
 The heat of formation is the amount of heat absorbed or
evolved when a mole of a substance is formed from it’s
elements. For example if 2 grams of hydrogen are burned in
oxygen to form liquid water, 68.320 Kilo Calories of heat are
evolved.
H2 + ½ O2 = H2O ΔH = -68.320 Kcal
 The heat associated with chemical reaction not only depends
on whether the reaction is carried out at constant pressure but
also the amounts of substances. Hence if the amount of
hydrogen in the above reaction is doubled (i.e. 4 grams) the
amount of heat evolved will also be doubled, i.e. 68.320 x 2 =
136.640
10

11. HEAT OF FORMATION
H2 + ½ O2 = H2O
 In the above reaction:
 Whether the work will be done or not?
 If work is done, will it be negative (-) or positive (+)?
11

12. HEAT OF REACTION
 All chemical reactions are accompanied by
absorption or evolution of heat. This thermal
change is called heat of reaction.
 Tables of standard enthalpies of a large number of
substances are available and simply substituting
the values of enthalpies in a reaction, the heat of
reaction can be calculated.
 For example sodium carbonate reacts with HCl,
producing NaCl, CO2 and H2O; the heat of this
reaction can be calculated by the use of standard
enthalpies as follows:
12

13. HEAT OF REACTION
 By subtracting the total energy of reactants from
the total energy of products the heat of reaction
is obtained.
13

14. HEAT OF REACTION AT CONSTANT PRESSURE
 If a reaction is carried out at constant pressure
the heat of reaction will be:
qp = ΔE + PΔV
= (Ep-Er) + P(Vp-Vr)
= (Ep+PVp) – (Er+PVr)
=Hp - Hr
= ΔH
ΔH = HH2O (l) – [HH2 (g) +H1/2 O2 (g)]
14

15. HEAT OF REACTION AT CONSTANT VOLUME
 At constant volume ΔV = 0, hence, heat of reaction
will be:
qv = ΔE = Ep - Er
15

16. HEAT OF SOLUTION
 Solution of one substance in another is
accompanied by absorption or evolution of heat and
this thermal effect is called heat of solution of the
substance.
 Per mole of dissolved substance, the heat of
solution at any given temperature and pressure
depends upon the amount of solvent in which
solution takes place. The grater the dilution the
greater the enthalpy of solution. Hence for heat of
solution it is essential to specify the number of
moles of solvent per mole of solute.
16

17. HEAT OF SOLUTION
 Sulphuric acid dissolves
in water with evolution of
heat. If one mole of
H2SO4 is dissolved in
water the heat evolved
depends on the number of
moles of water. It is
evident from results that,
the greater the dilution,
the greater is the ΔH.
Moles of water ΔH (cal)
0.5 -3810
1 -6820
2 -9960
3 -11890
4 -13120
6 -14740
10 -16240
3200 -20050
infinite -22990
17

18. HEAT OF SOLUTION
 The dilution of H2SO4 with water given by
equation:
H2SO4 (l) + aq = H2SO4 (aq)
The heat of solution, ΔH, for this process is
given by:
ΔH = H – (n1H1 + n2H2)
Where H1 and H2 are the enthalpies of two
pure solution constituents and H is the
enthalpy of solution.
18

19. HESS’S LAW OF HEAT SUMMATION
 As we have seen that E and H are functions of the
state of system and consequently ΔE and ΔH must
be true quantities, independent of path.
 It follows from this that the heat absorbed or
evolved in a chemical reaction is independent of the
particular manner in which the reaction is carried
out. This generalization of the statement is called
Hess’s law of heat summation.
 This principle makes it possible to calculate the
heats of many reactions which cannot be directly
measured.
19

20. HESS’S LAW OF HEAT SUMMATION
 If we intend to determine the enthalpy ΔH of the
following reaction:
2C (s)+ 2H2 (g) + O2 (g) = CH3COOH ΔH25 ºC = ?
 It is not possible to determine the enthalpy of this
reaction as the reaction does not occur in this
manner. However we can determine the enthalpy
of this reaction by a different way using Hess’s law.
20

21. HESS’S LAW OF HEAT SUMMATION
 Using the available calorimetric data for the following
reactions and Hess’s law we can calculate ΔH of acetic
acid.
21

22. HESS’S LAW OF HEAT SUMMATION
22

23. MEASUREMENT OF THERMAL CHANGES -
CALORIMETRY
 Heat changes involved in a reaction, are
determined using the instrument named
calorimeter. There are different types of
calorimeters, however, all essentially consists of an
insulated chamber filled with definite amount of
water. The actual reaction is carried out in a
separate chamber which is immersed in water. The
thermal changes occurring in the reaction chamber
directly bring about thermal changes in the water,
which are then measured and calculated.
23

24. CALORIMETRY
 The calorimeter is essentially an
insulated chamber filled with definite
amount of water in which reaction
chamber is immersed. The water in the
insulated chamber
 In an exothermic reaction, the heat
evolved is transferred to water and the
rise in temperature of water is
measured. The data then collected is
used to calculate the amount of heat
evolved in the reaction.
 In case of an endothermic reaction, the
heat is absorbed and the temperature
of water is lowered, which is measured
and corresponding heat change is
calculated. 24

25. CALORIMETRY
 Various types of calorimeters are used
to meet different requirements. The
one shown in the figure is called bomb
calorimeter. It consists of an outer
insulated housing and an inner
container containing pure water. The
reaction chamber, called bomb, is a
sealed vessel immersed in water. The
water is circulated using the stirrer to
keep the temperature homogenous
though out its body. The sample resting
on a boat in the reaction chamber is
ignited electrically and the heat changes
thus occurring are read from the
changes occurring in water.
 Since the process takes place at
constant volume, the reaction vessel is
specially constructed to withstand high
pressure.
25

26. CALORIMETRY
 The simple calorimeter used
in student laboratories is the
styrofoam coffee cup. The
coffee cup is covered by the
lid and a thermometer
inserted through the lid
reads the temperature
changes.
 Styrofoam is a good thermal
insulator over a short time
and provides good results
for students experimantal
work.
26

27. CALORIMETRY – CALCULATING THERMAL
DATA
Heat flow is calculated by using the
equation:
q = C x m x ΔT
Where “q” is the heat flowing through
system boundary, “C” is the specific
heat, “m” is the mass of system and
“ΔT” is rise or fall of temperature.
27

28. PROBLEM 1
 consider a chemical reaction which occurs in 200
grams of water with an initial temperature of
25.0°C. The reaction is allowed to proceed in the
coffee cup calorimeter. As a result of the reaction,
the temperature of the water changes to 31.0°C.
Calculate the enthalpy change (ΔH) of this reaction
in Joules.
N.B. The specific heat of water is = 4.18 J/g,˚C
28

29. PROBLEM 1 - SOLUTION
 The change in temperature of water is used to calculate the heat
evolved or absorbed in the reaction. Use is made of the following
equation:
q = C x m x ΔT
while q = ΔH
 where C is the specific heat capacity, m is the mass and ΔT is the
change in temperature of water.
C = 4.18 J/g ˚C
M = 200 g
ΔT = (31.0 – 25.0) = 6.0 ˚C
qwater = 4.18 J/g,°C x 200 g x (31.0°C - 25.0°C)
= 5.016 kJ
 Since we know that the reaction is exothermic hence the sign for ΔH
will be negative, therefore ΔH for the reaction will be:
ΔH = - 5.016 kJ
29

30. PROBLEM 2
When 7.1 g of NH4NO3 was added to
100 g of water at 18.2 ºC, the
temperature of solution dropped to
12.8 ºC. Calculate the enthalpy
change, ΔH of solution for 1 mole of
NH4NO3.
(specific heat capacity of water =
4.184 J g-1K-1)
30