2. Thermodynamics class 11

What is thermochemistry?

The branch of chemistry which studies thermochemical reactions is called thermochemistry.

Thermodynamics

Name of Lesson

Summary

1. Subject(s):- Chemistry

2. Topic or Unit of Study:- Thermochemistry

3. Grade/Level:- 11th

4. Based on:- NCERT BOOK

NCERT PAGES TIP: Think to relax and start learning and reading. If you’re a student you can write in your notebook as notes.

Thermochemistry

Define thermochemistry?

The branch of chemistry which studies thermochemical reactions is called thermochemistry.

Thermochemical equations

A chemical reaction in which the value of delta H is written is called a thermochemical equation.

C + O2   —>    CO2 chemical equation

C + O2–> CO2  ∆H=-393kj/mole[thermochemical equation]

∆H=(+): endothermic (evaporation)

∆H=(-): Exothermic (CaO + H2O)

∆H= 0: No thermic (mixing of gas)

Thermochemistry is based on the following law:-

1. FLOT:- (First law of Thermodynamics)

• The total energy is conserved. It means it is based on the conservation of energy. The total energy of the reactant side is equal to the total energy of the product.

R(100) – P(150)    •°• Q= -50

∆H = Hp – HR 

2. What is Lavoisier and Laplace law?

• Thermochemical equations follow the algebra rule. it means if the reaction is the reverse value of delta H changes their sign. if the reaction is multiplied by the same number.

C + O2➡  CO2 ∆H = -393

• CO2  ➡  C+O2  ∆H=393

• 4C + 4O2  ➡4CO2  ∆H=-393×4

• C/4 + O2/4➡  CO2/4  ∆H=-393/4

This is Lavoisier and Laplace law

3. What is Hess’s law?

Enthalpy change depends upon the initial reactant and the final product does not depend upon the intermediate product. Or, in another word, enthalpy change of single-stage equal to the sum of enthalpy change of multiple stages. Because enthalpy change is a state function.

What is the Type of enthalpy?

1. Enthalpy of combustion

Enthalpy change takes place when 1-mole of fuel is burned in a sufficient amount of oxygen. It is an exothermic process. it is denoted by ∆CH

i) ∆CH=enthalpy of combustion

ii) Pressure=1bar=75.2cmHg

Temperature=25°C=298K

∆–CH=standard enthalpy of combustion

What is the Formula of combustion?

CXHY + (X + Y/4)O2      ➡    XCO2 + Y/2H2O

Given:- C4H10 ( L.P.G)

C4H10 + (4 + 10/4)O2   ➡ 4CO2 + 10/2H2O

How does Enthalpy changes during phase transformation?

1. Enthalpy of fusion:- Enthalpy change take place when 1mole of solid change into liquid without temperature change is called enthalpy of fusion. H2O (ice)⟶   H2O(liquid)  ∆H(fusion)=x Kj/mole

2. Enthalpy of vaporization

Enthalpy change takes place when 1mole of liquid chachangesto Vapour without change in their temperature 🌡️ is called enthalpy of vaporization.

H2O(liquid)   ⟶ H2O(g)       ∆H(vaporization) =y Kj/mole

3. Enthalpy of sublimation

Enthalpy change takes place when 1 Mole of solid changes into Vapour without change in their temperature 🌡️ is called enthalpy of sublimation.

NH4Cl ( solid) ⟶  NH4Cl (gas)  ∆H=z Kj/mole

     2.   Bond enthalpy

Enthalpy change takes place when 1 Mole of molecule change into an atom is called bond enthalpy.

In the case of diatomic molecule bind enthalpy is also called enthalpy of Atomization.

But, in the case of multiple bonds and polyatomic molecule bind enthalpy is taken as average bond enthalpy which equals to enthalpy change by total no. Of bond.

H2(molecule)(HH) ➡ 2H(atom) ∆H(bond)=∆H(Atomization)

         3. Lattice enthalpy

Enthalpy change takes place when 1 Mole of ionic solid dissociate into their component ion in its gaseous state. This is called lattice enthalpy.

NaCl (solid) ➡  Na+(gas) + Cl–(gas)   ∆H=lattice

What is Born Haber Cycle?

It is not calculated by the direct method. It is calculated by the indirect method. This indirect method is called Born Haber Cycle.

∆H(formation)=∆H+I.P+{∆H(bond)×1/2}+∆H(E.g. enthalpy) + ∆H(lattice)

E.g=electron gaining

4. Hydration enthalpy:- When ions dissolve in water then enthalpy change is called hydration enthalpy. It is denoted by ∆H(hydration).

For solubility of ionic compounds in water hydration energy must be equal to or greater than lattice enthalpy. For solubility of an ionic compound in water∆H (hydration) is greater than ∆H(lattice).

5. Enthalpy of solution:- Enthalpy change take place when 1 mole of a solute dissolving a sufficient amount of solvent is called enthalpy of Solution. It is denoted by∆H(solution).

Solute + solvent= solution    ∆H(solution)

NaCl + H2O = solution

∆H(solution) = ∆H(hydration) + ∆H(lattice)

6. Enthalpy of neutralization:- when 1 Mole equivalent of acid reacts with the base then salt and water form this reaction is called enthalpy of neutralization. It is denoted by∆H(neut.).

For strong acid and strong base, the value is 57.2 Kj/mole.

Exception:- If acid is HF(hydrofluoric acid), its value is more than 57.2.

What are the demerits of FLOT{ First law of Thermodynamics}?

Demerits of FLOT(First law of Thermodynamics)

• It does not explain the direction of the flow of energy.

• It does not explain the spontaneity of the reaction.

• Not contradict 100% efficient engine.