Thermodynamics imp Topic

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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

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: 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)

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,

Solution 3:

We know that,

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:

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).

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