Wednesday 12 September 2012

CBSE Class 11 - Chemistry - Thermodynamics

(Short Q & A)

Q1: Define Thermodynamics

Answer: It is a physical science that deals with quantitative relation
between heat and mechanical energy. It is mainly based on three laws of thermodynamics.

Q2: Which law of thermodynamics evaluate thermodynamic parameters?

Answer: Third Law of Thermodynamics

Q3: Which Law of Thermodynamics deals with equivalence of different forms of energies?

Answer: First Law of Thermodynamics

Q4: State Zeroth Law.

Answer: It states "Two systems separately in thermal equilibrium with the third
system are said to be in thermal equilibrium with each other."

If system A and system B separately are in thermal equilibrium with another system,
then system A and system B are also in thermal equilibrium with each other.

Q5: What is thermochemistry?

Answer: Thermochemistry is a branch of thermodynamics which deals with the relationships
between chemical reactions and corresponding energy changes. It is based on 1st law of thermodynamics.

Q6: Name two laws of thermochemistry.

Answer: (i)  Lavosier and Laplace Law
        (ii) Hess's Law

Q7:Which Law of thermodynamics tells about direction of chemical change?

Answer: 2nd Law of thermodynamics.

Q8: What are the limitations of Laws of Thermodynamics?

  1. Laws are applicable at macroscopic level i.e. only to matter in bulk. It does not apply to individual molecules or atoms.
  2. It does not tell anything about the rate of process will change nor tells about the time taken to reach equilibrium.
  3. Concerns with the states (initial and final) but does not concern about the path.
  4. It fails to explain to which extent heat will be converted to work and vice versa.

Q9: What is a System? What are its different types?

Answer: The part of the universe under thermodynamic study is called a system. It is isolated from
the rest of the universe by with a bounding surface.
Types of System are:
1. Open System
2. Closed System
3. Isolated System

Q10: Identify from the following which can be considered as open, closed or isolated system
i. ice in a closed beaker
ii. lime kiln
iii. Animals and Plants
iv. ice in thermos flask for first one hour.

i. ice in a closed beaker : Closed System
ii. lime kiln : Open System
iii. Animals and Plants: Open System
iv. ice in thermos flask.: Isolated System

Q11(MCQ): Which of the following is the intensive property?

(a) temperature
(b) viscosity
(c) density
(d) all of these

Answer: (d) all of these

Q12(MCQ): Which of the folloing is an example of extensive property?
(a) temperature
(b) density
(c) internal energy
(d) surface tension

Answer: (c) internal energy

Q13: Define intensive property. Give its examples.

Answer: A property which is independent of the amount of substance(s) present in the system
is called intensive property

Examples are: temperature, pressure, density, viscosity, refractive index, concentration,
specific heat, surface tension etc.

Q14: Which law of thermodynamics describes entropy?

Answer: Second Law.

Q15: Which law of thermodynamics is about the absolute zero temperature?

Answer: Third Law of Thermodynamics

Q16: What are state functions? Give examples of state functions?

Answer: The thermodynamic parameter which depend on the initial and final states of the system and are independent of how the change is accomplished are called state functions.

Examples are:
  • Internal Energy (U)
  • Enthalpy (H)
  • Entropy (S)
  • Pressure (P)
  • Potential Energy (PE)
  • Temperature (T)
  • Volume (V)
Q17(NCERT/NCERT): Choose the correct answer. A thermodynamic state function is a quantity(i) used to determine heat changes
(ii) whose value is independent of path
(iii) used to determine pressure volume work
(iv) whose value depends on temperature only.

Answer: (ii) whose value is independent of path

Q18: What are path functions?

Answer: The system variables which depend upon the path of the system are called path functions or process functions.
Examples are: work, heat and length.

Q19: Why is potential energy a state function while work is not?

Answer: An object at a particular height has fixed potential energy (mgh), irrespective of the fact how that object is brought to that height. Therefore potential energy depends on the final state and is a state function. Work done to bring that object to that height may be different i.e. whether it is picked up by lift or brought by a slope. Work is a path function.

Q20: How will you define thermodynamic process? What are its types?

Answer: The conversion of state of system from one to another is called a process. Various thermodynamic processes are:
  • Isothermal
  • Adiabatic
  • Isobaric
  • IsoChoric
  • Cyclic
In addition, a process may be classified as:
  • Reversible Process
  • Irreversible Process
Q21(MCQ): Thermodynamics is applicable to
(a) Only to microscopic systems
(b) Only to macroscopic systems
(c) Only to homogeneous systems
(d) Only to heterogeneous systems

Answer: (b) Only to macroscopic systems

Q22: What is a Isothermal Processes?

Answer: The processes in which the temperature remains fixed, are termed isothermal processes.
The isothermal system is usually achieved by keeping the system in contact with a thermostat (i.e. constant temperature bath).
In isothermal process, ΔT = 0 and ΔU = 0.

Q23: Define Adiabatic Process?

Answer: The process in which no heat can flow into or out of the system, is called adiabatic process.  It can be achieved by running the process in an insulated container like thermos-flask.
For an adiabatic process Δq = 0

Q24(NCERT/MCQ): For the process to occur under adiabatic conditions, the correct condition is:

(i) ΔT = 0
(ii) ΔP = 0
(iii) q = 0
(iv) w = 0

Answer: (iii) q = 0
Under adiabatic conditions no heat transfer takes place, ∴ q = 0.

Q25: Define Isobaric Processes?

Answer: Isobaric  process is the one at which the process is carries out at constant pressure. e.g. water heating takes places in an open vessel under atmospheric pressure is an isobaric process.
In an Isobaric process ΔP = 0

Q26: What is an Isochoric Process?

Answer: The process in which the volume remains constant is known as Isochoric process. 
e.g. a substance is heated in a non-expanding container will have an isochoric syste,
In Isochoric process ΔV = 0 and ΔW = 0.

Q27: What factors determine the state of a thermodynamic system?

Answer: The factors are:
  • Absolute Pressure (P) in pascals.
  • Temperature (T) in Kelvins
  • Volume (V) in cubic meters
  • Number of moles (n) of working gas.

Q28: What is internal energy?

Answer: The internal energy (U or E) of a system is the total of all kinds of energy possessed by
the particles that make up the system.
⇒ It is the sum of its translational, vibrational, rotational energies, chemical bond energy, nuclear energy possessed by sub-atomic particles, electronic energy and its potential energy due to its interaction with neighbouring molecules.

It is a state property  and its absolute value cannot be determined. We can determine the change in internal energy only.

Q29: What are the factors an internal energy of the system depends upon?

  1. Quantity of the gas or substance
  2. Its chemical nature
  3. Temperature, Pressure and Volume.

Q30: How can we say internal energy is a state function?

Answer: Consider 1 mole of water is heated from 273K to 373K, the change in energy is 7.52KJ. It is the same regardless of the form in which this energy is transferred to the water i.e. either by heating or by performing work or by another means.

Q31: Is internal energy is extensive or intensive property ?

Answer: The value of internal energy of a system depends on the mass of the matter contained in a
system, it is classed as an extensive property.

Q32: How work is defined in thermodynamics?

Answer: In thermodynamics, in general the work is done in expansion (or compression) of a gas. This is known as pressure-volume work or PV work or expansion work.

For expansion work, W = - P × V joules
For compression, work W =  P × V joules

Q33: What are the sign conventions used for Heat and Work?

  1. Heat flows into the system, q is +ve
  2. Heat flows out of the system, q is –ve
  3. Work is done on the system, w is +ve
  4. Work is done by the system, w is –ve
Q34: Define First law of thermodynamics.

Answer: The total energy of an isolated system remains constant though it may change from
one form to another.
Mathematically,  when work is done on the system,  ΔU = q + w
And when work is done on  by the system, ΔU = q - w

(In Progress...)


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