Entry Test Preparation 2015, Chemistry BOOK 1 Chapter # 6 Thermochemistry , Theory and Key Concepts
Entry Test Preparation 2015
Chemistry
Chapter #6
THERMOCHEMISRY
Thermo Chemistry:
The study of heat changes
accompanying a chemical reaction is known as thermo-chemistry.
Chemical System:
A chemical system is usually
consisted of the substance(s) undergoing a physical or chemical change.
Surrounding:
The surrounding is everything in
the universe that is not part of the system.
Endothermic Reaction:
An endothermic reaction is one in
which the reactants have less energy than the products, and thus a net input of
energy, usually in the form of heat, is required.
In endothermic reactions, the
bond breaking energy is very high as compared with that of the bond forming
energy in products. That is why energy is absorbed during the endothermic
reactions.
Exothermic Reaction:
An exothermic reaction is one
that releases heat.
In an exothermic reaction, the
total energy absorbed in bond breaking is less than the total energy released
in bond making. In other words the energy needed for the reaction to occur is
less than the total energy provided. As a result to this, the extra energy is
released, usually in the form of heat.
Heat of Reaction:
The amount of heat evolved or
absorbed in a chemical reaction, when the molar quantities of the products and
the reactants being the same as presented in the chemical equation, is called
heat of reaction.
Spontaneous Process (Natural Process):
A process which takes place on
its own without any outside assistance and moves from a non equilibrium state
towards and equilibrium state is termed as spontaneous process or natural
process.
A reaction will also be called
spontaneous process if it needs energy to start with, but once it is started,
then it proceeds on its own. Common experience shows that spontaneous processes
proceed with a decrease in energy.
Examples:
Water flows from higher level to
the lower level. The flow cannot be reversed without some external aid.
Neutralization of a strong acid
with a strong base.
Internal Energy:
The sum of kinetic as well as the
potential energies of the particles contained in the system is called the
internal energy of the system.
·
The kinetic energy is due
to the translational, rotational and vibrational movements of particles.
·
The potential energy
accounts for all the types of attractive forces present in the system.
State Function:
A state function is a macroscopic
property of a system which has some definite values for initial and final
states and which is independent of the path adopted to bring about a change.
The change in internal energy of the system (
E ) is a state function.
E ) is a state function.
Heat:
Heat is not a property of a
system.
It is therefore
not a state function.
It is defined as the quantity of
energy that flows across the boundary of a system during a change in its state
due to the system and the surroundings. Heat evolved or absorbed by the system
is represented by a symbol q.
A stable
system has lowest energy as possible.
Some important units of energy
and their relationship with joule are given below:
Name
|
Symbol
|
In Joule
|
Calorie
|
4.184 J
|
|
Kilocalorie
|
K cal
|
4.184
 103 J |
BTU
|
BTU
|
1.055
103 J |
Kilowatt hour
|
KWh
|
3.6
106 J |
Work:
Work is also a form in which
energy is transferred from one system to another. It is defined as the product
of force and distance i.e. w = F S. Work is measured in
Joules which is its S.I unit.
S. Work is measured in
Joules which is its S.I unit.
First law of Thermodynamics:
The first law of thermodynamics
which is also called the law of conservation of energy, states that energy can
neither be created nor be destroyed but can be changes from one form to
another. Mathematically first law of thermodynamics is expressed by the
relation
q = E + W
E + W
·
E will be positive if the internal energy of the system
increases and vice versa.
E will be positive if the internal energy of the system
increases and vice versa.
·
Work is + ve when work is
done on the system and it is –ve when work is done by the system.
The form of first law of
thermodynamics at constant volume is given as E = qv where qv represents the heat
exchanged at the constant volume. It is important to remember that work done in
this case is zero.
E = qv where qv represents the heat
exchanged at the constant volume. It is important to remember that work done in
this case is zero.
Enthalpy
In general, enthalpy is equal to
the internal energy E plus the product of pressure and volume (PV). It is
represented by H. Enthalpy s a state function. It is measured in Joules.
H = E + PV
A change in enthalpy of a system
can be written as:
H = E + (PV)
Or H = E + VP + PV
H = E + VP + PV
At constant pressure, P = 0 then
P = 0 then H = E + PV
The given amount of energy of
constant pressure only changes the enthalpy of the system. It is represented as
H = qp
H = qp
Solids which
have more than one crystalline forms possess different Ho values.
Ho values.
Enthalpy of a Reaction (Hor):
Hor):
In an exothermic reaction, the
heat content or enthalpy of the products, H2, is less than that of
the reactants H1. Since the system has lost heat, we can say the
enthalpy change for the reactionH is negative. In an endothermic reaction, the enthalpy of
the change, H is positive. Enthalpy of reaction Hor for product of H2O(l)
from H2(g) and O2(g)
is – 285.8kJ mo-1.
H is negative. In an endothermic reaction, the enthalpy of
the change, H is positive. Enthalpy of reaction Hor for product of H2O(l)
from H2(g) and O2(g)
is – 285.8kJ mo-1.
Standard Enthalpy:
The standard enthalpy of a
reaction is defined as the enthalpy change which occurs when the number of
moles of reactants as indicated by the balanced chemical equation, react
together completely to give the products under standard condition, i.e. 25oC
and one atmospheric pressure. All the reactants and products must be in their
standard physical states.
Enthalpy of Formation (Hor):
The standard enthalpy of
formation of a compound is defined as the change of enthalpy (H when one mole of the compound is formed from is elements.
It is denoted by Hof Hof Hof
H when one mole of the compound is formed from is elements.
It is denoted by
Enthalpy of Atomization (Hoat):
Hoat):
The standard enthalpy of
atomization Hoat of an element is defined as the
enthalpy change when one mole of gaseous atoms is formed from the element under
standard conditions. For example, the standard enthalpy of atomization of
hydrogen is Hoat = 218 kJ mol-1.
Hoat of an element is defined as the
enthalpy change when one mole of gaseous atoms is formed from the element under
standard conditions. For example, the standard enthalpy of atomization of
hydrogen is Hoat = 218 kJ mol-1.
Enthalpy of Neutralization (Hoa):
Hoa):
The standard enthalpy of
neutralization Hoa is the amount of heat evolved when
one mole of hydrogen ions H+, from an acid, reacts with one mole of
hydroxide ions from a base to form one mole of water, For example, the enthalpy of neutralization of sodium
hydroxide by hydrochloric acid is – 57.4 kJ mol-1. Enthalpy of
neutralization for any strong acid with a strong base is approximately the same
i.e. – 57.4 Kj mol-1.
Hoa is the amount of heat evolved when
one mole of hydrogen ions H+, from an acid, reacts with one mole of
hydroxide ions from a base to form one mole of water, For example, the enthalpy of neutralization of sodium
hydroxide by hydrochloric acid is – 57.4 kJ mol-1. Enthalpy of
neutralization for any strong acid with a strong base is approximately the same
i.e. – 57.4 Kj mol-1.
Enthalpy of combustion (Hoc):
Hoc):
The standard enthalpy of
combustion of substance, Hoc is defined as the enthalpy change
when one mole of a substance is completely burnt in excess of oxygen under
standard conditions. For example, standard enthalpy of combustion of ethanol Hoc is – 1368 kJ mol-1.
Hoc is defined as the enthalpy change
when one mole of a substance is completely burnt in excess of oxygen under
standard conditions. For example, standard enthalpy of combustion of ethanol Hoc is – 1368 kJ mol-1.
Enthalpy of Solution (Hosol):
Hosol):
The standard enthalpy of a
solution is the amount of heat absorbed or evolved when one mole of a substance
is dissolved in, so much solvent that further dilution results in no detectable
heat change. For example, enthalpy of solution (Hsol) of ammonium chloride is + 15.1 kJ mol-1
and that of sodium carbonate is – 25.0 kJ mol-1.
Hsol) of ammonium chloride is + 15.1 kJ mol-1
and that of sodium carbonate is – 25.0 kJ mol-1.
MEASUREMENT OF ENTHALPY
Exothermic and endothermic
reactions can easily be detected by observing the temperature of the reaction
vessel before and after the reaction, as long as the heat of reaction evolved
or absorbed is considerable. More accurate values of H can be determined by using calorimeters.
H can be determined by using calorimeters.
Glass Calorimeter:
The quantity of heat q evolved or
a absorbed during the reaction can be determined by the relation q = m s T where m = mass of reactants, s = specific heat of the
reaction mixture and T is the change in temperature.
s T where m = mass of reactants, s = specific heat of the
reaction mixture and T is the change in temperature.
Specific heat capacity:
The amount of energy required to
raise the temperature of one kilogram of the substance by one Kelvin. e.g. for
water it is 4.184 kj kg-1 k-1
Bomb calorimeter:
Knowing the heat capacity of the
whole system (calorimeter, water, thermometer and strirrer), the rise in
temperature and the heat generated by ignition, it is possible to calculate the
heat of combustion as follows.
Q = m s T
s T
Where m is the number of moles of
the substance burnt, s is the specific heat capacity of the whole system (i.e.
calorimeter, water etc.) and T is the rise in the temperature.
T is the rise in the temperature.
Hess’s Law of constant Heat Summation:
The law states that “because
enthalpy is a state function, the enthalpy change of a reaction is the same
regardless of what pathway is taken to achieve the products”. In other words,
only the start and end states matter to the reaction, not the individual steps
between.
Mathematically 
H (cycle) = 0 Hess’s law can also be used for the indirect
measurement of the enthalpy change for a chemical
H (cycle) = 0 Hess’s law can also be used for the indirect
measurement of the enthalpy change for a chemical
The Born-Haber Cycle:
This is the special application
of Hess’s law to binary ionic compounds such as M+X-.
Physical state of a substance at
25oC (298K) and on atmospheric pressure (760 mm Hg) is called
standard state.
Thermodynamic Equation:
The chemical equation which includes value of H and physical states is called thermodynamic equation.
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