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WBJEE: Important Questions and Study Notes for Thermodynamics

Jan 29, 2018 17:53 IST
    WBJEE 2018: Thermodynamics
    WBJEE 2018: Thermodynamics

    Every aspirant wants admission in the best engineering college by securing good rank in engineering entrance examinations. Aspirants will have to focus on all subjects if they want to secure good rank in the examination. Students should try to prepare notes of all the chapters of all subjects during their preparation for the examination.

    To help aspirants to score good marks in coming WBJEE 2018, the engineering section bring to you the notes of chapter Thermodynamics. Students always get 1-2 questions from this chapter in the examination.

    Students can read the following topics here:

    1. System and Surroundings,

    2. Types of system (open, closed, isolated),

    3. State variables/functions,

    4. Thermodynamic processes (isothermal, adiabatic, isobaric, isochoric),

    5. Sign conventions,

    6. Extensive and Intensive properties,

    7. Enthalpy and Entropy,

    8. Gibbs free energy,

    9. First law of thermodynamics,

    10. Relationship between Cp and Cv,

    11. Standard enthalpy of reactions,

    12. Enthalpy changes during phase transformations,

    13. Properties of enthalpy changes,

    14. Hess's law of constant heat summation and

    15. Calorimetry

    About the notes:

    1. These notes are based on the latest syllabus and pattern of WBJEE entrance examination.

    2. These notes are prepared by Subject Experts of Chemistry.

    3. Some previous years’ solved questions are also included here which will help aspirants to get familiar with the difficulty level of the questions asked in the previous years’ paper of WBJEE test.

    Important Concepts:

    The System and the Surroundings

    • System: The part of universe in which observations are made is called system.
    • Surroundings: It includes everything other than the system.

    The Universe = The system + The surroundings

    Types of the System

    • Open System: Exchange of energy and matter occurs between system and surroundings

    Mathematically, ΔE ≠ 0 and Δm ≠ 0

    • Closed System: No exchange of matter is possible, but exchange of energy is possible

    Mathematically, ΔE ≠ 0 and Δm = 0

    • Isolated System: No exchange of energy or matter between the system and the surroundings is possible

    Mathematically, ΔE = 0 and Δm = 0

    State variables / State functions: The functions whose values depend only on the state of the system and not on how it is reached are called state functions.

    Thermodynamics Processes:

    • Isothermal Process: For isothermal process temperature is constant

    Mathematically, ΔT = 0

    • Adiabatic Process: For adiabatic process there is no exchange of heat with surrounding

    Mathematically, q = 0

    • Isobaric Process: It occurs at constant pressure

    Mathematically, Δp = 0

    • Isochoric Process: It occurs at constant volume

    Mathematically, ΔV = 0

    Sign Conventions

    • Anything given to system is taken positive.

    For ex: heat given to system or work done on system is taken as positive

    • Anything taken out of system is taken negative.

    For ex: heat given out by the system or work done by system is taken negative

    Extensive and Intensive Properties

    • Intensive property: This is a property whose value doesn’t depend on the quantity or size of matter present in the system.

    Example: temperature, density, pressure

    • Extensive property: This is a property whose value depends on the quantity or size of matter present in the system.

    Example: mass, total volume

    WBJEE Thermodynamics Concepts

    WBJEE Thermodynamics Concepts

    Standard enthalpy of reactions: It is the enthalpy change for a reaction when all the participating substances are in their standard states.

    Enthalpy changes during phase transformations:

    • Standard Enthalpy of Fusion: It is also called molar enthalpy of fusion and denoted as ΔfusH°. It is defined as enthalpy change that accompanies melting of one mole of a solid substance in standard state.
    • Standard Enthalpy of vaporization: It is also called molar enthalpy of vaporization and denoted as ΔvapH°. It is defined as amount of heat required to vaporize one mole of a liquid at constant temperature and under standard pressure.
    • Standard enthalpy of sublimation: It is denoted as ΔsubH°.It is defined as the change in enthalpy when one mole of a solid substance sublimes at a constant temperature and under standard pressure (1 bar).
    • Standard enthalpy of formation: It is denoted as ΔfH°. It is defined as the standard enthalpy change for the formation of one mole of a compound from its elements in their most stable states of aggregation. It can be positive or negative.
    • Standard enthalpy of combustion: It is denoted as ΔcH°. It is defined as the heat evolved when one mole of substance in standard state is oxidised completely. It is always negative.

    Properties of Enthalpy Change:

    • It is an extensive property.
    • It is a state function.
    • On reversing the chemical reaction the sign ΔH of also reverses.

    WBJEE Thermodynamics Concepts

    WBJEE: Important Questions and Preparation Tips – Atomic Structure

    Some previous years' solved questions are given below:

    Question 1:

    For the reaction X2Y4(I) → 2 XY2(g) at 300 K the values of ΔU and ΔS are 2 kCal and 20 Cal K–1 respectively. The value of ΔG for the reaction is

    (A) – 3400 Cal

    (B) 3400 Cal

    (C) – 2800 Cal

    (D) 2000 Cal

    Solution 1:

    We have the reaction,

    X2Y4(I) → 2 XY2(g)

    WBJEE Thermodynamics Solution 1

    Hence, the correct option is (C).

    WBJEE: Important Questions and Preparation Tips – Basic Concepts of Chemistry

    Question 2:

    The condition for a reaction to occur spontaneously is

    (A) ΔH must be negative

    (B) ΔS  must be negative

    (C) (ΔH-TΔS) must be negative

    (D) (ΔH+TΔS) must be negative

    Solution 2:

    When ΔG<0 then the reaction occur spontaneously.

    Also, ΔG= ΔH-TΔS

    So, for spontaneous reaction ΔH-TΔS should be negative.

    Hence, the correct option is (C).

    Question 3:

    Which of the following plots represent an exothermic reaction ?

    For example,

    WBJEE Thermodynamics Question 3a

    WBJEE Thermodynamics Question 3b

    Solution 3:

    We know that,

    WBJEE Thermodynamics Question 3b

    Hence, the correct option is (D).

    WBJEE: Important Questions and Preparation Tips – States of Matter

    Students can also have a look on the following questions and solutions to check their preparation.

    Question 1:

    WBJEE Thermodynamics Question 4

    Solution 1:

    The criterion for spontaneity of any process the change in entropy is (ΔSsystem + ΔSsurrounding ) > 0.

    Hence, the correct option is (D).

    Question 2:

    The species which by definition has zero standard molar enthalpy of formation at 298 K is           

    (A) Br2(g)                    

    (B) Cl2(g)                    

    (C) H2O(g)                  

    (D) CH4(g)

    Solution 2:

    Elements in its standard state have zero enthalpy of formation. Since, Cl2 is gas at room temperature, therefore ΔH° of Cl2(g) is zero.

    Hence, the correct option is (B).

    Question 3:

    Which of the following statements is false?

    (A) Work is a state function

    (B) Temperature is a state function

    (C) Change in the state is completely defined when the initial and final states are specified

    (D) Work appears at the boundary of the system

    Solution 3:

    Work is not an example of state function.

    Hence, the correct option is (A).

    Question 4:

    In thermodynamics, a process is called reversible when

    (A) Surroundings and system change into each other

    (B) There is no boundary between system and surroundings

    (C) The surroundings are always in equilibrium with the system

    (D) The system changes into the surroundings spontaneously

    Solution 4:

    When the surroundings are always in equilibrium with the system, then the process is called reversible process.

    Hence, the correct option is (C).


    Students can easily understand this chapter as the content given here is arranged in a systematic manner. Students can also track their progress by attempting the questions given here. They can also refer the solutions if they stuck somewhere while attempting the paper.

    WBJEE 2018: Notification, Application, Dates, Eligibility, Exam Pattern, Syllabus

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