Molar Heat of Reaction
Chapter 6
Molar Heat of Dissolution
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Is symbolised with ΔHd
Is the quantity of energy absorbed or
released in the dissolution of one mole of
solute in a solvent
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The enthalpy change of a reaction is also
called the heat of reaction or ΔH
This is a general term for the energy
changes caused by the change of
reactants into products
The “quantity” usually used to describe
this is the mole
It could also be described in kilojoules
per mole (kJ/mol)
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ΔH can be used for both physical and
chemical changes as ΔHp and ΔHc
respectively
Example: combustion of methane
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Combustion is a chemical change
Therefore ΔHc is the quantity of energy
produced in the combustion of one mole of
methane
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Some types of changes have more
specific names
Example: Melting Ice
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Melting is a physical change that we call
fusion
ΔHp therefore becomes ΔHfusion
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Example: Dissolving solutes in solvents
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The molar heat of this reaction is called the
molar heat of dissolution and uses the
symbol ΔHd
This shows the quantity of energy involved in
dissolving one mole of solute in the solvent
(which is usually water)
Expressed in kilojoules per mole (kJ/mol) of
dissolved solute
In this reaction heat can be either released
or absorbed
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The interaction between a solute and a
solvent is characterized by the strength
with which the solvent particles attract
each other
*
Miss Mullin’s Go-To Answer for
Inorganic Chemistry in CEGEP
(not a real title)
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Water is what we call a polar molecule
A polar molecule is one where there is
some physical separation between the
chemical bonds, so that one part is
slightly positive and the other slightly
negative
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In water the force of attraction occurs
between the hydrogen atoms of one
water molecule and the oxygen atoms of
another nearby water molecule
This type of bond is a special bond
called…
 HYDROGEN
BONDING
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Hydrogen bonds keep all the molecules
of liquid water together over its surface
It is because of hydrogen bonds that we
have the specific surface tension of water
that allow some insects to move on the
surface of the water, appearing to “bend
it”
Ease of Dissolution
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The reaction of a dissolution depends on
what type of bonds exists between solid
solute molecules
If the solid is a molecular compound:
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Forces between molecules usually hydrogen
bonds
Dissolution is a molecular dissolution
No dissociation, no ions produced
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If the solid is an ionic compound:
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Forces between particles are ionic bonds
After dissolution, interactions form between
the polar water molecules and the solid ions
and the aqueous particles
The molecules dissociate produces oppositely
charged ions
Example: Silver Chloride
AgCl(s) -->Ag+(s) + Cl-(aq)
Types of Solvent-Solute
Interactions
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There are different interactions between
the solute and solvent
Taking this into account allows us to
calculate the energy involved
The Three Stages of Dissolution
Stage 1
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Particles of solvent that are attracted to
each other through solvent-solvent
interactions must separate
Stage 2
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The particles of solute, which are
attracted through solute-solute interacts
must separate
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In both these stages energy is absorbed
as necessary to break these bonds
The enthalpy change for stages 1 and 2
is therefore positive
ΔH1 > 0 and ΔH2 > 0
Stage 3
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This consists of the rearrangement of the
particles of solute and solvent to now
make the solution
This stage is usually exothermic
(ΔH3 < 0)
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The molar heat of dissolution (ΔHd) is the
energy balance between energy
absorbed and energy released
ΔHd = ΔH1 + ΔH2 + ΔH3
Example
Dissolve 6.69 g of lithium chloride (LiCl) in 100
mL of water at 24.2°C in a calorimeter. The
final temperature of the water is 37.4°C
a) What is the molar heat of dissolution (ΔHd)
of lithium chloride
b) Write the thermochemical equation for the
dissolution of LiCl, incorporating the calculated
value of ΔHd
Example
Dissolve 4.25g of sodium nitrate (NaNO3) in
100 mL of water at 23.4 °C. Given that the
molar heat of dissolution of NaNO3 is 21.0
kJ/mol, what will the final temperature of
the water be?
Molar Heat of Neutralization
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The molar heat of neutralization (ΔHn) is
the quantity of energy that is absorbed
or released in the neutralization of one
mole of an acid or base.
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An acid-base neutralization is a chemical
reaction in which an acid reacts with a
base to form a salt and water
Acid + Base → Salt + Water
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We can observe this reaction by uses an
indicator.
The most common is phenolphthalein.
Phenolphthalein turns pink in the
presence of a base
An acid-base neutralization is complete
when the indicator changes colour
permanently
Arrhenius Theory
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In a typical acid-base neutralization the
ions that react are hydrogen (H+) ions
from the acid in aqueous solution and
hydroxide (OH-) ions from the base in
aqueous solution
These ions combine to form water (H2O)
The other ions do not participate; called
spectator ions
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Neutralization reactions are accompanied
by a heat transfer from the interactions
of the ions
The heat at the moment of neutralization
of ONE MOLE of a substance (acid or
base) is called the MOLAR HEAT OF
NEUTRALIZATION (ΔHn) and expressed
in kJ/mol
Dilute solutions: Use a calorimeter and
assume density and specific heat capacity
equal to water
Questions for Practice
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TxBk pg 195-196
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3, 6 and 11