Thermodynamics- 5 Flashcards
what is calorimetry
In
calorimetry, the process is carried out in a
vessel called calorimeter, which is immersed
in a known volume of a liquid. Knowing theheat capacity of the liquid in which calorimeter
is immersed and the heat capacity of
calorimeter, it is possible to determine the heat
evolved in the process by measuring
temperature changes. Measurements are
made under two different conditions:
i) at constant volume, qV
ii) at constant pressure, q
what does steel wall vessel ensure
The steel vessel is
immersed in water bath to ensure that no heat
is lost to the surroundings
explain how delta U is measures
A combustible
substance is burnt in pure dioxygen supplied
in the steel bomb. Heat evolved during the
reaction is transferred to the water around the
bomb and its temperature is monitored. Since
the bomb calorimeter is sealed, its volume does
not change i.e., the energy changes associated
with reactions are measured at constant
volume. Under these conditions, no work idone as the reaction is carried out at constant
volume in the bomb calorimeter. Even for
reactions involving gases, there is no work
done as ∆V = 0. Temperature change of the
calorimeter produced by the completed
reaction is then converted to qV
, by using the
known heat capacity of the calorimeter
expain how enthalpy is measured
Measurement of heat change at constant
pressure (generally under atmospheric
pressure) can be done in a calorimeter shown
in Fig. 6.8. We know that ∆ = H qp
(at
constant p) and, therefore, heat absorbed or
evolved, qp
at constant pressure is also called
the heat of reaction or enthalpy of reaction,∆rH.
In an exothermic reaction, heat is evolved,
and system loses heat to the surroundings.
Therefore, qp
will be negative and ∆rH will also
be negative. Similarly in an endothermic
reaction, heat is absorbed, qp
is positive and
∆rH will be positive.
define spontaneotiy
it means having the potential ot proceed without the assistance of any external agency
what is a sponatenous process
it is an irreversible process and that can be reversed only by some external agency. They are spontaneosus only in 1 direction
2types of spontaneous processes
(i) process which requires no initiation- dissolution of sgar in water
(ii) process which requires some intiation: combustion of a fuel
is decrease in enthalpy a criterion for spontaneity
In most cases, a decrease in enthalpy of products compared to reactants , it is a spontaneous processs. (ie exothermic reacns)
However in
C(graphite, s) + 2 S(l) → CS2
(l)
this is endothermic, ie products have more energy, but its still spontaneous
The diffusion of 2 gases in a closed vessel isolated from its surroundings is also a spontaneous process but dH=0
It was discovered that decrease in enthalpy is not afactor for spontaneoity. so entropy was intorduced
what is entropy
it is the degree of randomness or dosorder in an isolated system
why is dissolution of salt spontaneous
Becase randomness bertween ions increase. They were not free to move about but now they are free. so
In so2+o3—-2so3. The no of moles of products are less than reactants and hence entropy icnrease
state the 2nd law of thermodynamics
All spontaneous processes are thermodynamically irreversible.
When a sponatenous process occurs, the total entropy of the universe increases.
It is impossible to convert heat completely into an equivalent amount work without any toher change happeneing elsewhere.
in an isolated system, there is always a
tendency for the systems’ energy to become
more disordered or chaotic
what does change in enthalpy entropy depend upon
Whenever heat is added to the system, it
increases molecular motions causing
increased randomness in the system. Thus
heat (q) has randomising influence
A system
at higher temperature has greater randomness
in it than one at lower temperature. Thus,
temperature is the measure of average
chaotic motion of particles in the system.
Heat added to a system at lower temperature
causes greater randomness than when the
same quantity of heat is added to it at highertemperature
Entropy always increases with Temperature. But, the Change in Entropy at lower temperature will be always higher than the Change in Entropy at higher temperature.
what is gibbs free enrgy? it use?
Gibbs gree energy G refers to the capacity to do useful work. Absolute value of G cannot be measured.
G= H+ TS
It’s an extensive property. It helps us calculate whether or not a reaction will be spontaneous for open systems which dont have definite entropy or enthalpy
what are the conditions for spontaneous processes
∆G gives a criteria for spontaneity at
constant pressure and temperature.
(i) If ∆G is negative (< 0), the process is
spontaneous.
(ii) If ∆G is positive (> 0), the process is non
spontaneous.
(iii)If ∆G is 0, (=), then system is in equilibrium
why are reactions carried out in high temperature
: If a reaction has a positive enthalpy
change and positive entropy change, it can be
spontaneous when T∆S is large enough to
outweigh ∆H. This can happen in two ways;
(a) The positive entropy change of the system
can be ‘small’ in which case T must be large.
(b) The positive entropy change of the system
can be ’large’, in which case T may be small.
The former is one of the reasons why reactions
are often carried out at high temperature.
Table 6.4 summarises the effect of temperature
on spontaneity of reactions.
