Thermochemistry

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Basic Concepts Of Thermochemistry

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Law of conservation of
energy

•There are various forms of energy:

1-Chemical

Energy

2- Light

Energy

3- Kinetic

Energy

4- Heat

Energy

5- Electrical

Energy

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• In spite of this different forms of energy

there is a relationship between all these forms,
As the energy can be converted from one form to
another,
this leads us to know the law of conservation of energy

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• law of conservation of energy

Energy is neither created nor destroyed but it can be transformed from one form
to another.

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• Thermodynamics : is the branch of science that study

the

Heat energy(

Thermo

) and how it

transfers(dynamic).

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So, what do you think which is boarder field of

science ?

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Thermodynamics

Thermochemistry

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Chemical changes(reactions)

• Change in chemical composition of the substance.
• Examples:-

Combustion(burning)

Reactants

New product

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Physical changes

• No Change in chemical composition of the substance.
• Examples:-

Melting of ice

Boiling of water

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• And its time to know how can we calculate the heat changes

accompanied by the chemical and physical changes.

• But first we have to know the Basic concepts includes :

1-system and surrounding
2- First law of thermodynamics
3-Temp. and Heat
4- Specific heat

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• System : Is the part of the universe chosen for study, in which Physical or

chemical change(chemical reaction)occurs.

• Surrounding : Is that part outside the system that exchanges Matter or

Energy(heat) with the system.

1) System and Surrounding

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• The Chemical reaction can be expressed as a system as

follows:

The medium surrounds
the reaction container

(The laboratory)

The reactants and

products

(The reaction medium)

The wall of container

(flask)

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 The relationship between chemical reaction and

Energy :-

•

• The chemical reactions are accompanied

with changes in Energy (Releasing or
absorbing ).

• This Energy Exchange occurs between the

reaction medium

(System) and its

Surrounding.

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Difference between system and surrounding.

System

Surrounding

It is part of universe in

which physical or

chemical changes occur

The part outside the
system that exchange

heat energy.

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All systems can be classified according to their ability to

exchange

energy (Heat) or matter with the surrounding

Open

Isolated

Closed

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Solid

•

• Matter: Yes
• Heat: Yes

Liquid

•Matter: NO
•Heat: Yes

Gas

•Matter: No
•Heat: No

Open

Closed

Isolated

Types of systems

The system Exchanges

The system Exchanges

The system Exchanges

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Q) Classify the following figures into 3 types of systems

a)

c)

b)

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• Energy is very important in our life.
• There are 3 Types of systems:

1. Open
2. Closed
3. Isolated

•
There is a difference between thermodynamics and
thermochemistry.

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The total energy of an isolated system is constant

even the system is changed from state to

another.

Universe = system + surrounding

EUniverse = Esystem + Esurrounding

∆Esystem = - ∆Esurrounding

2- First law of thermodynamics

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3- Heat and Temp.

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• Heat is a form of energy, the flow of heat from one position to
another depends on the difference in temperature between them.

• Temperature is the measurement of the average kinetic energy of
matter molecules (it is an indication for the hotness and coldness of
an object)

• The atoms or molecules of the substances are in a continuous
motion (vibration), but they differ in their speed in the same
substance, so it is preferred to express the speed of molecules by the
expression "the average speed of the substance molecules" .

• This increases the kinetic energy of the molecules resulting in a rise
of the temperature of the system and vice versa (direct relationship).

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Calorie

Joule

It is the quantity of heat needed to raise
the temperature of 1gm of water by 1℃

( 15 ℃ : 16 ℃ )

1 Calorie = 4.18 Joule

It is the quantity of heat needed to raise

the temperature of 1gm of water by

1

4.18℃

Measuring units of the quantity of heat

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Specific Heat ( C ): it’s the quantity of heat required to
raise the temp. of 1 g of the substance by 1 Celsius degree

qp= m.c . T

4- Specific Heat:

• Specific heat is a characteristic property for the substance, because

specific heat is a constant value for the substance, but it differs from
one substance to another.

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• Specific heat of water is higher than those of the other substances,
because the quantity of heat required to raise the temperature of 1 g
of water by 1°C is higher than that of the other substances.

• The specific heat of the same substance differs according to its
physical state, as observed in case of water vapor and liquid water.

• The substance which requires the absorption of large quantity of heat

for its temperature to be raised, its specific heat is high, and raising or
decreasing the temperature of this substance takes long time, and
vice versa.

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Calculation of the quantity of heat :-

qp= m.c . T

Q: Calculate the Specific heat of an unknown substance whose mass is 155 g and
its Temp. is raised from 25 Celsius to 40 Celsius when it absorbs an amount of heat
equals 5700 j ??

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Chapter 4:
Heat content

Sum of energies stored in 1 mole of the substance

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Energy stored in the substance in
the form of:

In the atom

In the molecule

Intermolecular force

Sum of potential & kinetic
energies of the electrons
around the nucleus

The energy of the bond in the
molecule
(ionic >covalent)

Forces between molecules like
hydrogen bond
(Vander vaals forces)

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Change in heat content

• Difference between the heat content of products & reactants

CH4 + 2O2 --- CO2 + 2H2O

Bond breaking Bond formation

absorbs energy release energy
Endothermic exothermic

(+) (-)

∆H = (H)products - (H)reactants

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Change in standard Heat content :

•

=

−𝑞

𝑛(𝑚𝑜𝑙𝑒)

Q)

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•If energy released (exo) > energy absorbed
(endo) -----> (∆H = -ve) exothermic reaction

•If energy released (exo) < energy absorbed
(endo) -----> (∆H = +ve) endothermic reaction

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Exothermic

Endothermic

Produce heat

Absorbs heat

Heat content of the reactant more than product

Heat content of the reactant less than product

∆H is –ve sign

∆H is +ve sign

More stable product

Less stable product

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P.O.C
Exothermic
Endothermic

Change in Temp.

+ve

-ve

Quantity of heat (q)

+ve
-ve

Change in heat content

(Enthalpy)
-ve

+ve

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Bond energy: is the amount of energy absorbed to
break a bond OR released Energy during formation of a
new bond in one mole of a substance.

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•Ex.: Calculate the heat of the following reaction &
Determine if it is exothermic or endothermic:

CH4 + 2O2 ---------- CO2 + 2H2O

(C=O) 803,
(O-H) 467,
(C-H) 413,
(O=O) 498

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Heat changes accompanying physical changes
:-

1- Standard heat of solution
2- Standard heat of dissolution

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1) Standard heat of solution :-

By dissolving
Sodium hydroxide (NaOH) in water, the
temperature of the solution increases

By dissolving
Ammonium nitrate (NH4NO3) in water, the
temperature of the solution decreases

The solution is called
Exothermic solution , and its H has a negative sign

The solution is called
Endothermic solution , and its H has a positive sign

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Heat of solution:- H Sol

Standard heat of solution H

Is the quantity of heat absorbed or released on dissolving one mole of solute in
a certain amount of solvent to obtain a saturated solution under standard
conditions.

Is the quantity of heat released or absorbed which is accompanying the dissolving
process to obtain a saturated solution.

Sol

𝑞𝑝 = 𝑚 𝑐 Δ𝑇

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Molar heat of solution Is the heat change resulting from dissolving one mole
of the solute in an amount of solvent to form one liter of the solution.

• If the amount of solute does not equal 1 mol, we can calculate the molar heat

of solution from the relation :

𝜟𝑯 = −𝒒𝒑

𝒏
Sol

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EX: By dissolving 80 g of NaOH in an amount of water to
produce 1 L of a solution, the temperature increased from 20°C
to 44.4°C :
[Na = 23 ,0=16,H=1]
(1) Calculate the heat of solution.
(2) Calculate the molar heat of solution.
(3) Is this solution exothermic or endothermic ?

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Dissolving process is affected by three forces:-

• Attraction forces between particles (molecules) of solvent.(H1)
• Attraction forces between particles (molecules) of solute. (H2)
• Attraction forces between particles (molecules) of solvent and
solute (H3)

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1) It is an endothermic process, because an amount of energy is absorbed
to overcome the attraction forces between the particles of the solvent with
each other (H1 has positive sign)
2) It is an endothermic process, because an amount of energy is absorbed
to overcome the attraction forces between the particles of the solute with
each other (H2 has positive sign)
3) It is an exothermic process, because an amount of energy is released
when solvent particles are combined with the particles of solute ( has
negative sign)
• The change in heat content of this process is represented by
The value of heat of solution ( Hsol) equals the sum of the change in heat
content of the three steps :
= AH1+ AH2 + AH3

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2) Standard heat of dilution :-

• Standard heat of dilution (H diL) is the quantity of released or

absorbed heat for each one mole of solute upon diluting the solution
from a high concentration to another lower concentration with the

condition of being in its standard state.

• Example:-

When 1 mol of sodium hydroxide NaOH(s) is dissolved in different amounts of
water H2O() , the heat of the solution differs with the difference of the amount of
water, as in the following equations :

NaOH(s) + 5H20(e) Na0H(aq) AH1 = - 37.8 kJ/mol
NaOH(s) + 200H20(l) Na0H(aq) AH2 = - 42.3 kJ/mol

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Conclusion :-

• It is observed that AH2 value > AH1 value

• It is concluded that the amount of released or absorbed energy increases by
adding another amount of water (solvent).

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Dilution process takes place in two opposite steps according to the

energy, which are :

1) Separation energy
It is an endothermic process Because separating the ions or particles of the solute
from each other in the concentrated solution needs absorbing an amount of
energy
2) Attaching energy
It is an exothermic process Because the ions or particles of solute are attached to a
greater number of molecules of the solvent, which leads to releasing an amount of
energy

The heat of dilution is the sum of those two energies

(separation and attaching).

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Heat changes accompanying Chemical
changes :-

1- Standard heat of Combustion
2- Standard heat of Formation

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1-Heat of Combustion :-

• Combustion of a substance is the fast combination between

this substance and oxygen (an oxidation — reduction
reaction).

• The complete combustion of some elements and compounds

releases a large amount of energy in the form of heat or light
or both.

• The quantity of heat released when any substance combusts

complete in an excess amount of oxygen is known as
(Heat of combustion) Hc

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• If the combustion takes place at standard conditions, it is called Standard
heat of combustion

Hc

• The combustion of most of organic substances (like fuel and glucose)

produces :
- Water (H20) whether liquid or vapor.
- Carbon dioxide (CO2).
- Heat energy.

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Examples of Combustion reactions :-

1) Combustion of stove gas (Butagas) :

• Stove gas (Butagas) is a mixture of :
- Propane C3H8 - Butane C4H10
• This combustion reaction produces a large amount of heat which is used in
cooking food and has other uses.
• The following equation and the opposite energy diagram represent the

complete combustion of propane gas :

C3H8(g) + 502(g) 3C02(g) + 4H2O9(l) HC= —2323.7 kJ/mol

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Combustion of propane gas:-

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2) Combustion of glucose inside the body of the living organism

•It is one of the very important combustion reactions as it
provides the living organisms with the needed energy to
perform their vital processes.

C6H1206(s) + 602(g) 6CO2(g)+6H2O(l)
Hc = –2808 kJ/mol

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Calculate the mass of glucose which is burnt to raise
the temperature of 100 g of water from 20°C to 25°C
(assuming no heat is lost).
according to the equation :

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2-Standard heat of formation HF

• The heat change accompanying the formation of the compound from
its pure constituent elements is called Heat of formation Hf

• If the constituent elements are in their standard state, which is the
most stable state of matter at the standard conditions, therefore the
heat change which accompanies the formation of the compound is
called

Standard heat of formation Hf

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• Graphite is considered the standard state of carbon.
Because it represents the most stable state of carbon at the standard conditions.

• Standard heat of formation of glucose.

6C(s) + 6H2(g) + 3O2(g)
C6H12O6(s) H = -1260 kJ/mol

Applications based on Heat of formation:-

Note : The Heat of formation of any element is supposed to be zero in
the standard conditions.

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• Calculation of the change in heat content (the enthalpy) All
of the reactions in terms of the standard heat of formation H

The change in heat content = Heat content of the products — Heat content of

the reactants

• The standard heat of formation of a compound equals its heat content.

• ΔH = [the sum of heat of formation – the sum of heat of formation]

of the products

of the reactants

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Thermally stable compounds Thermally unstable compounds

Thermally stable at room temperature and
don't tend to dissociate to their constituent

elements

Thermally unstable at room temperature and

tend to dissociate spontaneously to their

constituent elements

The Heat of formation

Has a negative sign

The Heat of formation

Has a Positive sign

Their heat content is lower than the

heat content of their constituent

elements

Their heat content is higher than the

heat content of their constituent

elements

Relation between Heat of formation and the stability of the compound:-

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• By decreasing of the value of the standard heat of formation of the
Compound, its thermal stability increases and vice versa.

• Most of the reactions tend to proceed in the direction of the formation of the

compounds with lower value of heat of formation (higher stability).

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Hess’s Law

Scientists usually prefer to use indirect methods to calculate heat of reactions.
This is due to many reasons such as :

• A mixture of reactants or products with other substances.
• Some reactions occur very slowly and need a long time like the formation of iron

rust.

• Being dangerous to measure the heat of some reactions experimentally.
• Difficulties of measuring the heat of reaction in normal conditions of pressure

and temperature.

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In order to calculate the heat changes in these kinds of reactions, Hess proposed a
law which is known as Hess's law of constant heat summation and it states that
the heat of reaction is a constant amount in standard conditions, whether the
reaction took place in one step or a number of steps.

• Hess's law is one of the forms of the first law of thermodynamics, because it
considers the chemical reaction as an isolated system whose heat is a constant
amount.

• Hess's law is dealing with the chemical equations as if they were algebraic
equations which can be added together or subtracted from each other, and their
coefficients can be multiplied by constant numerical values.

• The mathematical relation of Hess's law is : H = H1 + H2 + H3 +…………….