TD 1 Flashcards
types of boundary
(i) Boundaries can be conducting (diathermic) or non-conducting. (Adiabatic)
(ii) Boundaries can be rigid or non-rigid. (Flexible)
(iii) Boundaries can be real or imaginary.
what are state functions
State Function :
* Those state variables which depend only upon initial and final state of the system but doesn’t depend upon the
path or mechanism followed by the system are called state function.
* State functions are denoted by capital letters.
* State functions are not absolute in nature. So we talk about change in state function like DE, DH, DS, DG etc.
Ex. E, H, S, G, T etc.
Path function :
* Properties of the system which depend upon the initial and final state of the system as well as the path or
mechanism followed by the system is called path function.
* Path functions are denoted by small letters.
Eg. Work done (w), heat (q)
what are reversible reactions
(1) Process in which all changes occuring at any part of the system are exactly reversed when small changes
in variables are carrried out in opposite direction.
(2) Driving force should be infinitesimally greater than opposing force.
(3) Process takes place in infinitesimal small steps or in many steps and takes infinite time to complete the
process.
(4)
Pext
is variable.
(5) It is an ideal process.
(6) Work obtained is maximum.
(7) System is in virtual equilibrium at any state.
(8)
Pint = Pext ± dP
what is heat
Heat is defined as the energy that flow in or out of a system because of a difference in temperature between the
thermodynamic system and its surroundings. It is a path function.
internal energy
We can only calculate the change in internal energy of the system (DE) by using an instrument which is called as
Bomb calorimeter. In Bomb calorimeter reactions are carried out at constant volume.
spontaneous and non spontaneous processes
(i) Spontaneous process :
* The process which has a natural tendency to occur in a particular direction either of its own or after
proper initiation under the given set of conditions.
* All natural processes are ir-reversible and spontaneous processes. The natural processes take place
of their own in one direction only.
(ii) Non–spontaneous process :
* The process which does not occur of its own in a particular direction i.e. a process which does not
have a natural tendency to occur in a particular direction is called as non-spontaneous process.
* Non-spontaneous process may be made to occur when energy from some external source is supplied
continuously throughout the process.
criteria for spontaneity
(I) Tendency to attain minimum energy
(II) Tendency to attain maximum randomness.
SOME FAMOUS OR EXTRA ORDINARY EXAMPLES OF ENTROPY CHANGE
(i) Entropy of graphite > Entropy of diamond.
(ii) In rusting of iron its entropy increases.
(iii) During dissolution of salt entropy increases.
Ex. NH4
Cl(s) + aq ¾® NH4
+
In this process NH4
+
(aq) + Cl–
(aq)
and Cl– ions are free to move in solution where as they are not free to move
in solid NH4Cl. Hence DS is positive for this type of dissolution process.
(iv) During precipitation and crystallisation entropy decreases.
Ex. On addition of HCl in the aqueous solution of Ag+
(v)
(vi)
ions entropy decreases due to precipitation of AgCl.
On boiling of egg : Denaturation of proteins occur. Thus entropy increases.
Stretching of rubber : During stretching of rubber band its long flexible macromolecules get uncoiled.
The uncoiled form has more specific geometry and more ordered arrangement. Thus entropy decreases.
what is third law of td
At zero kelvin (absolute zero temperature), the entropy of pure perfect
crystalline solid is taken as zero.
Exceptions :
(i)
(ii)
NO, N2O
CO, CO2
(iii) Mixture of isotopes
(iv)
Ice
what is a thermochemical eqn
THERMOCHEMICAL REACTION :
The chemical reaction which gives information about the physical state of reactants & products and heat change
is called as thermo chemical reaction.
enthalpy of reaction
The amount of heat evolved or absorbed when number of moles of the reactant according to the balanced
chemical reaction had completely reacted is called as heat of reaction.
(i) Reaction condition :
The chemical reactions are carried out at constant temperature with
either pressure or volume constant.
At constant pressure qp = DHreaction
At constant volume
DH = DE + Dng· RT
(ii) Quantity of reactant :
Example : H2(g) + O2(g) H2O(g)
2H2(g) + O2(g) 2H2O(g)
DH1 = –x Kcal/mol
DH2 = 2 × (–x Kcal)
If equation is multiplied by coefficient then value of DH is also
multiplied by that coefficient.
Physical state of products and reactants :For H2O (liq.), DH is more negative in comparison to the
formation of H2O(vap.) because when vapours convert into liquid some
heat is released.
(iv) Allotropic form : (Physical nature of reactant)
enthalpy of formation
It is the enthalpy change when one mole of a substance is formed
from its elements in their most abundant naturally occurring form or
in their standard and stable state form (also called reference states).
enthalpy of combustion
Amount of heat evolved when 1 mole of substance is completely burnt (or oxidised) in excess of oxygen.
calorific value
The amount of heat evolved when 1 g of a substance (food or fuel) is completely burnt (or oxidised)
Calorific value =
DHcomb
Molecular weight
Unit :- kJ g–1
or kCal g–1
Special Points :
(I) Maximum value of calorific value = Maximum efficiency or best fuel
(II)
H2 has the highest calorific value (150 kJ/g) but it is not used as domestic or industrial fuel due to some
technical problems
