CHAPTER 7. THERMOCHEMISTRY F.Sc Chemistry 1st Year short question
CHAPTER 7. THERMOCHEMISTRY F.Sc Chemistry 1st Year
Q.1 Define the following terms and give three
examples of each.
Ans: System:
The substance which is under experiment or under observation is
called as system. Examples:
(i) Pb(NO3)2 in
decomposition of Pb(NO3)2.
(ii) Zn and CuSO4 solution, the
reaction mixture in the vessel.
(iii) CaCO3 in thermal decomposition
of CaCO3
(b) Surroundings:
Everything around the system which
is not a
part of system
is called surroundings. For example
During the reaction between Zn
and CuSO4 solution
vessel and
air etc are surroundings.
(c) State
function: A macroscopic property
of a system
which has some
definite value for initial
and final state
and independent of
the path
followed e.g. (i)
Pressure (ii) Temperature
(iii) Internal energy.
Note: Heat is not a state function.
Q.2 Describe the units of energy.
Ans. Mostly Joule and calorie are used for the measurement of energy.
Calorie: The amount
of energy required
to raise the
temperature of one gram of water from 14.5oC to 15.5oC is
called one calorie.
Joule: It is SI
unit of energy and defined as energy
expanded when a force of
one Newton moves
a body through
one meter in the
direction in which force is applied.
Joule =
Force x distance
1J = 1 N x 1 m
Q.3 What are thermochemical reactions?
Ans: Exothermic
reaction: Those
thermochemical reactions in
which heat is
evolved as a result of reaction are called as exothermic
reactions.
H2(g) + O2(g)
H2O(l)
H = – 285.5 kJ/mole
N2
+ 3H2(g)
2NH3(g)
H = – 41.6 kJ/mole
Endothermic
reactions: Those
thermochemical reactions in
which heat is absorbed
as a
result
of reaction are called as endothermic reaction.
N2 (g) + O2(g
2NO(g)
H = + 180.51 kJ/mole
H2O(l)
H2(g)
+ O2(g)
H = + 285.58 kJ/mole
H2(g) + I2(g)
2H I
H = + 52.96 kJ/mole
Q.4 Differentiate between
internal energy and
enthalpy of a system?
Ans: Internal energy: The total
of all
kinds of K.E
and P.E of
all the particles
of a
system
is called as internal energy. It is denoted by “E’ e.g., kinetic
energy
may be in the form of translation, vibrational and rotational
motion and
potential energy is
intermolecular and intramolecular
forces
of attraction. It is a sate function of system.
E= K.E + P.E
Enthalpy of the
system:
The total
heat contents of
a system and
denoted by H. The
increase
in the internal energy of a system plus work done is called
as
enthalpy i.e. H = E +
PV
Q.5 Define the followings:
(i)
Enthalpy of reaction (ii)
Enthalpy of combustion
Ans:
Standard Enthalpy of reaction:
The enthalpy changes when no. of moles of reactants as indicated
by
the balanced chemical
equation react completely
together to
give
the products under the standard conditions.
Standard
Enthalpy of Combustion:
The enthalpy change when one mole
of a substance is completely
burnt
in excess of oxygen under standard conditions.
` C2H5OH(l)
+ 3O2(g)
2CO2(g)
+ 3H2O(l)
H = – 1368 kJ/m
C(s) + O2(g)
CO2(g)
H = – 393.7 kJ/m
2Al(s) + O2(g)
Al2O3(s)
H = – 1675.7 kJ/m
Q.6 Why it is
essential to mention the physical states of reactants and products in a
thermochemical equation?
Ans.
The heat
of reaction depends
upon the physical
states of the reactants
and products, heat
of reaction is
different in different physical states therefore, while
writing a thermochemical equation
it is
essential to mention
the physical states
of the reactants
and products. E.g
H2(g) + 1/2O2(g) → H2O(g) ΔH
= -241.5 KJ mol-1
H2(g)
+ 1/2O2(g) → H2O(l) Δ H=
-285.8 KJ mol-1
Q.7 Differentiate between
spontaneous and non–spontaneous reaction?
Ans. The
process which takes
place on its
own without any
outside help and moves from a
non–equilibrium state
to equilibrium state is
called spontaneous or natural
process. It is real, unidirectional and irreversible
e.g. water flows
from higher level
to low level, reaction between acid and base
etc. There are
certain reactions which need
energy to start but once they start they proceed their own for example burning of candle.
The
process which does not take place
on its own and does not occur in
nature is called
as
non–spontaneous. It
is reverse of the
spontaneous process i.e.
pumping of water
uphill, flow of
heat from colder to hotter region etc.
Q.8 Prove that change in enthalpy is equal to heat
of reaction? or prove that
Δ H =
qp ?
Ans. We know
that enthalpy is
equal to the
internal energy plus
product of pressure and volume.
H
= E + PV
A change in enthalpy of system is
written as
Δ H = ΔE + Δ(PV)
Δ H = ΔE + ΔPV
+ PΔV
Since the gas I kept at constant
pressure. ΔP= 0
Hence Δ H = ΔE
+ PΔV ………eq…1 (for solids & liquids ΔV =0
so, Δ H = ΔE )
According
to first law of the thermodynamics
ΔE =
q + w
If w is pressure-volume work done by
system, then, W = - PΔV
ΔE =
q - PΔV
Putting this value of ΔE in Equation no..1
Δ H
= q - PΔV + PΔV
Δ
H =
q
since the pressure is kept constant, then
Δ H
= qp
This
shows that enthalpy
change is equal
to amount of
heat absorbed at constant pressure.
Q.9. Heat
is evolved in
exothermic reactions and
absorbed in endothermic
relations.
Ans. When
bond formation energy is
greater than the
bond breaking energy then
the excess of
energy is evolved
making the reaction exothermic. When
the bond breaking
energy is greater than
the bond formation energy
then the difference
of energy is
supplied from surrounding making the reaction is endothermic.
Q.10. How would
you explain that
change in enthalpy
is a state function?
Ans. As
H = E + PV
E,
P and v
are state functions
as they are
independent of path and
depend only on
the initial and
final state of
the system therefore enthalpy
of a system
is also a
state function because
enthalpy
depends on E, P and V.
Q11.
What is difference between heat and temperature?
Or Write a mathematical relationship between these two
parameters?
Ans: Heat:
The measure
of total energy of
a substance is called
heat. It is property of a body which flows
from a body at higher temperature
to a body at lower
temperature. It is
denoted by ‘q’.
It depends upon
the quantity of a substance.
It is measured
by calorimeter. It is not
a state function.
Temperature: It
is measure of
average K.E of the
molecules in the system. It
is denoted by ‘T’.
It
is independent of the
quantity of a substance. It is measured by thermometer.
It is a state function.
Relationship: q= m x S x Δ
T
Q15. What is enthalpy of neutralization? or
Why enthalpy of neutralization of strong acid
and base is always -57.4 KJ mol-1 ?
Ans: It is the amount of heat evolved or absorbed
when one mole of H+ ions from an acid reacts with one mole of OH- from
a base to form one mole
of H2O. Under standard conditions
it is called
standard
enthalpy
of neutralization, and it is denoted by Δ Hn.
The
heat of neutralization of strong acid or base is always -57.4 KJ mol-1
because strong acid or base is completely ionized and
when acid and base is mixed no bond has to be broken.
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