Laws Of Thermodynamics

To calculate the enthalpy of ionization of acetic acid (CH₃COOH), we begin by analyzing the given reactions:

• 1. HCl + NaOH → NaCl + H₂O with ΔH = -57.3 kJ mol^-1, which represents a strong acid reacting with a strong base, resulting in complete ionization.
• 2. CH₃COOH + NaOH → CH₃COONa + H₂O with ΔH = -55.3 kJ mol^-1, involving the weak acid CH₃COOH.

The ionization reaction for acetic acid can be considered as follows:

CH₃COOH → CH₃COO^- + H⁺

The enthalpy change for the ionization of acetic acid (ΔH_ionization) can be determined by subtracting the second reaction's enthalpy from the first:

ΔH_ionization = ΔH₁ - ΔH₂ = (-57.3 kJ mol^-1) - (-55.3 kJ mol^-1) = -2.0 kJ mol^-1

The value -2.0 kJ mol^-1 is within the expected range [2, 2] (interpreting signs considering the direction of the exothermic process description). Therefore, the enthalpy of ionization of acetic acid is confirmed to be -2 kJ mol^-1.

A reaction is considered spontaneous at a particular temperature when it exhibits a positive change in Gibbs free energy, denoted as . The formula for calculating is , where represents the change in enthalpy, and represents the change in entropy. A negative value indicates a spontaneous reaction, while a positive value indicates a non-spontaneous reaction.

Now, let's examine the Gibbs free energy changes for each reaction at a temperature of 300 K:

(A)

⇒

(B)

⇒

(C)

⇒

(D)

⇒

.

Assertion A is correct: In an Ellingham diagram, the oxidation of carbon to carbon monoxide does indeed show a negative slope, indicating that the reaction becomes more favorable as temperature increases.

Reason R is incorrect: While CO is a reducing agent, it does not tend to decompose at higher temperatures; rather, the decomposition of CO to C and is unfavorable at high temperatures, as shown by its position on the Ellingham diagram.

Conclusion: The correct answer is 4.

Key Equations:

The observed depression in freezing point ( ) is given by:

The calculated depression in freezing point ( ) is given by:

Where:

• : Freezing point depression constant
• : Molality of the solution
• : van’t Hoff factor (for a monobasic acid, assuming complete dissociation)

Percent Difference Formula:

The percent difference is calculated as:

Substitute the equations for and :

Factor out :

Substitute :

Adjusted Calculation:

Given the degree of dissociation is 0.3, only 30% of the acid molecules dissociate into ions, leading to an effective . This results in a smaller observed depression in freezing point than calculated for full dissociation.

Conclusion:

The observed percent difference in freezing point depression is 30%.

(A) I (g) 2I (g) is an endothermic process (Atomisation).
(B) HCl (g) H (g) + Cl (g) is an endothermic process (Atomisation).
(C) H O (l) H O (g) is an endothermic process (Vaporisation).
(D) C (s) + O (g) CO (g) is an exothermic process (Combustion).
(E) Dissolution of ammonium chloride in water is an endothermic process (Dissolution).
Thus, the number of endothermic processes is 4.

To solve the question, "The quantity which changes with temperature," we need to understand the definitions and properties of the given options: mole fraction, mass percentage, molarity, and molality.

1. Mole Fraction: It is the ratio of the number of moles of a component to the total number of moles of the mixture. It is a dimensionless quantity and does not depend on temperature.
2. Mass Percentage: It is the mass of a component divided by the total mass of the mixture, multiplied by 100. This quantity is also unaffected by changes in temperature as it is based on mass, which does not change with temperature.
3. Molarity (M): Molarity is defined as the number of moles of solute per liter of solution. The formula is given by: , where is the number of moles, and is the volume of solution in liters. Volume is temperature-dependent due to thermal expansion or contraction of liquids, thus molarity changes with temperature.
4. Molality (m): Molality is defined as the number of moles of solute per kilogram of solvent. This is expressed as: . Since mass is invariant with temperature, molality does not change with temperature.

Based on the above analysis, Molarity changes with temperature because it depends on the volume of the solution, which is temperature-dependent due to the expansion or contraction of the solvent with temperature changes.

In conclusion, the correct answer is: Molarity.

Exam Tip: Always remember that quantities involving volume (like molarity) are temperature-dependent, whereas those involving mass (like mass percentage, molality) are not.