৪৯তম বিসিএস ⎯ প্রাণ রসায়ন [৬০১]

Correct Answer: A) The total energy of the universe is constant.
Explanation: The first law of thermodynamics, also known as the law of conservation of energy, states that energy cannot be created or destroyed, only transferred or transformed.

Correct Answer: B) It represents the heat absorbed or released in a reaction at constant pressure.
Explanation: Enthalpy change (ΔH) is the heat exchanged with the surroundings at constant pressure. A negative ΔH indicates an exothermic reaction (releasing heat), and a positive ΔH indicates an endothermic reaction (absorbing heat).

Correct Answer: C) The entropy of the universe always increases in a spontaneous process.
Explanation: The second law of thermodynamics states that for any spontaneous process, the total entropy of the universe (system + surroundings) must increase (ΔS universe >0).

Correct Answer: B) Negative
Explanation: A process is spontaneous when the Gibbs free energy change (ΔG) is negative. This indicates that the system is moving towards a lower, more stable energy state.

Correct Answer: D) Joules per mole per Kelvin (J/mol K)
Explanation: Entropy (S) is a measure of the disorder of a system. The standard units for entropy are Joules per mole per Kelvin (J/molK).

Correct Answer: A) ΔH>0
Explanation: An endothermic reaction absorbs heat from the surroundings, resulting in a positive change in enthalpy (ΔH>0).

Correct Answer: B) ΔG=ΔH−TΔS
Explanation: The Gibbs-Helmholtz equation is ΔG=ΔH−TΔS, where T is the absolute temperature. This equation is fundamental for predicting the spontaneity of a reaction.

This equation links thermodynamics (ΔG∘) with chemical equilibrium (Keq), where R is the ideal gas constant and T is the absolute temperature.

 The Gibbs-Helmholtz equation is ΔG=ΔH−TΔS. If ΔH is negative and ΔS is positive, the term −TΔS will be negative, making ΔG negative regardless of the temperature, so the reaction is always spontaneous.

The standard state for most thermodynamic calculations is defined as 298K (25∘C) and a pressure of 1 bar.

Explanation: An adiabatic process is one in which no heat is transferred into or out of the system. Therefore, the change in heat (q) is zero.

By convention, the standard free energy of formation of any element in its most stable form (e.g., O2 gas, C(graphite)) is defined as zero.

A gas expanding into a vacuum increases the volume and disorder of the system, leading to a positive change in entropy. The other options involve a decrease in disorder.

A cyclic process returns to its initial state. Since internal energy is a state function, its change is zero for a complete cycle.

When ΔG = 0, the forward and reverse reaction rates are equal, and there is no net change in the concentrations of reactants and products. The system is in a state of chemical equilibrium.

Enthalpy (H) is defined as H=U+PV. Therefore, at constant pressure, the change in enthalpy is ΔH=ΔU+PΔV.

Using the equation ΔG=ΔH−TΔS, if ΔH is negative and ΔS is positive, then ΔG will always be negative, making the reaction spontaneous at any temperature.

The biochemical standard state is similar to the standard state but with a concentration of H⁺ at 10⁻⁷M (pH 7), which is a more relevant condition for biological systems.

A dilute solution has a low concentration of solute relative to the solvent.

Colligative properties depend only on the number of solute particles in a solution, not on their identity. Surface tension is a property of the substance itself and is not a colligative property.

Osmosis is the net movement of solvent molecules across a semi-permeable membrane from a region of higher solvent concentration to a region of lower solvent concentration.

The osmotic pressure (Π) is given by the formula Π=iMRT, where M is the molarity of the solution.

Freezing point depression is a colligative property. CaCl₂ dissociates into three ions (Ca²⁺ and 2Cl⁻), while NaCl dissociates into two ions. Glucose and urea do not dissociate. The greater the number of particles, the greater the freezing point depression.

The presence of a non-volatile solute lowers the vapor pressure of the solvent, requiring a higher temperature to reach the boiling point.

Explanation: The equation Π = iMRT shows a direct relationship between osmotic pressure (Π) and molarity (M). Since molarity is moles/volume and moles = mass/molecular weight, we can determine the molecular weight from osmotic pressure measurements.

A solution is a homogeneous mixture where the components are uniformly distributed. Suspensions and colloids are heterogeneous mixtures.

Reverse osmosis is the process of forcing a solvent from a region of high solute concentration through a semi-permeable membrane to a region of low solute concentration by applying external pressure.

The presence of a non-volatile solute occupies some of the surface area of the liquid, reducing the number of solvent molecules that can escape into the vapor phase, thereby lowering the vapor pressure.

Brass is an alloy, which is a solid solution of zinc dissolved in copper. Saltwater and vinegar are liquid solutions, and air is a gaseous solution.

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