Internal energy heat and Work [Thermodynamics]

Internal energy  heat and Work

(1) Internal energy (E):

“Every system has a quantity of matter is associated with a definite amount of energy. This energy is known as internal energy.”
E= Etranslational+ERotational +EVibrational +EBonding +EElectronic
(i) Characteristics of internal energy
a)      Internal energy (E) of a system is an extensive property.
b)      Internal energy (E) is a state property.
c)       The internal energy (E) does not depend on the path by which the process occurs. It is depend on initial and final state of the process
d)      The internal energy in a cyclic process remains constant that means there is no change in internal energy of system in cyclic process.
e)      The internal energy (E) of an ideal gas is a function of temperature only.
f)        Internal energy (E) of a system depends upon the amount/quantity of substance, its chemical nature, temperature, volume and pressure.
The unit of Internal Energy (E) is joules in SI and ergs in CGS
1 Joule = 107ergs


(ii) Change in internal energy (△E) :

 It is neither possible nor necessary to calculate the absolute value of internal energy of a system then,
△E = Efinal,-Eintial
E is positive if Efinal > Eintial, and
E negative if Efinal <Eintial.

(2) Heat (q) and work (w):

The energy of a system may vary either increase or decrease in various ways but the common ways are heat (q) and work (w).
Heat (q) is a form of energy. The flow of heat occurs from one system to another due to the difference in temperature between them. Heat flows is take place from higher temperatures to lower temperatures. Therefore, it is regarded as energy on the move.
Work is said to be done if the point of application of force is displaced in the direction of the force. It is calculated to the force multiplied by the displacement (distance through which the force acts).

The three main types of work that we usually come across are following
a                   Gravitational work
Mechanical work = Force x displacement = F.d
Electrical work
Electrical work = potential difference x charge = V.q
Mechanical work
Gravitational work = mgh

(i) Units of heat and work : 
The heat changes are measured in calories (cal), Kilo calories (kcal), joules (J) or kilo joules (kJ).
The S.I. unit of heat is joule (J) or kilojoule. The Joule (J) is also expressed as  Newton-metre (1 J=1 Nm). Work done is measured in terms of ergs or joules. The S.I. unit of work is Joule
1 cal = 4.184 J
SI Unit= Joules(J)
1 Joule = 107 ergs = 0.2390 cal.
1kcal = 4.184kJ
CGS Unit =erg
1 Joule = 107 ergs = 0.2390 cal.
1 J   =   1 Nm
1cal > 1 joule > 1 erg

(ii) Sign conventions for heat and work

Heat absorbed by the system = q positive(+ve)
Heat evolved by the system = q negative(-ve)
Work done on the system = w positive(+ve)
Work done by the system = w negative. (-ve)

Test What You Learned

Here are some problem with Internal energy  heat and Work thermodynamics for practice

1.      Conditions of standard state used in thermochemistry is
(a) 0°C and 1 atm
(b) 20″C and 1 atm
(c) 25°C and 1 atm
(d) 0° K and 1 atm
2.     The temperature of the system decreases in an [KCET 2005]
(a) Adiabatic compression
(b) Isothermal compression
(c) Isothermal expansion
(d) Adiabatic expansion
3.     Changes in a system from an initial state to the final state were made by a different manner that AH remains same but q changes because [J & K CET 2010]
(a) △H is a path function and g is a state function.
(b) △H is a state function and q is a path function
(c) Both△H and q are state functions
(d) Both △H and q are path functions
4.     If a refrigerator’s door is opened, then we get [CPMT 1980]
(a) Room heated
(b) Room cooler
(c) More amount of heat is passed out
(d) No effect on room
5.     The cooling in refrigerator is due to [KCET 2005]
(a) Reaction of the refrigerator gas
(b) Expansion of ice
(c) The expansion of the gas in the refrigerator
(d) The work of the compressor
6.     Point out the wrong statement in relation to enthalpy
(a) It is a state function
(b) It is an intensive property
(c) It is independent of the path followed for the change
(d) Its value depends upon the amount of substance in the system
7.      Which of the following is zero for an isochoric process
(a) dP
(b) dV
(c) dT
(d) dE
8.     Mark the correct statement [MP PET 1997]
(a) For a chemical reaction to be feasible, AG should be zero
(b) Entropy is a measure of order in a system
(c) For a chemical reaction to be feasible, AG should be
(d) Total Energy of an isolated system is constant
9.     Among them intensive property is
(a) Mass
(b) Volume
(c) Surface tension
(d) Enthalpy
10. It is general principle that the less energy a system contains it is ,[MH CET 1999
(a) Less stable
(b) More stable
(c) Unstable
(d) More unstable
11.   Internal energy of an ideal gas depends on
(a) Volume
(b) Temperature
(c) Pressure
(d) None of these
 
Answers 
  1.  (c) 25°C and 1 atm  
  2.  (d) Adiabatic expansion
  3. (b) △H is a state function and q is a path function
  4. (a) Room heated
  5. (c) The expansion of the gas in the refrigerator
  6. (b) It is an intensive property
  7. (b) dV
  8. (d) Total Energy of an isolated system is constant
  9. (c) Surface tension
  10. (d) More unstable
  11. (b) Temperature