Chemical Thermodynamics

Question 1

Two moles of an ideal gas undergo free expansion from 10 L to 100 L at 300 K. The values of ΔSsystem and ΔSsurroundings are (R is the universal gas constant)

Accepted Answer

ΔSsystem = 4.606 R; ΔSsurroundings = 0. Entropy is a state function, so the system's change is the same as for a reversible isothermal expansion: ΔS = nR ln(V₂/V₁) = 2R ln 10 = 4.606R. But free expansion is into a vacuum, so no heat crosses the boundary and the surroundings are left completely unchanged: ΔSsurroundings = 0. The total is positive, as an irreversible process requires.

Answer

ΔSsystem = 0; ΔSsurroundings = 0

Answer

ΔSsystem = 4.606 R; ΔSsurroundings = −4.606 R

Answer

ΔSsystem = 0; ΔSsurroundings = 4.606 R

Question 2

A protein undergoes reversible thermal denaturation from its initial state N to denatured state D according to N ⇌ D. At 60 °C, the concentrations of both N and D are equal at equilibrium, and the standard enthalpy change of denaturation is 666 kJ mol⁻¹. The standard entropy change (ΔS° in kJ K⁻¹ mol⁻¹) of the protein upon denaturation at 60 °C is closest to

Accepted Answer

2.0. Equal concentrations at equilibrium means K = 1, so ΔG° = −RT ln K = 0. With ΔG° = ΔH° − TΔS° = 0 the entropy change is simply ΔH°/T, and T must be in kelvin: 666/333 = 2.0 kJ K⁻¹ mol⁻¹.

Answer

2000.0

Answer

333.0

Answer

11.1

Question 3

The standard heat of formation, in kcal/mol, of Ba²⁺ is [Given: standard heat of formation of SO₄²⁻ ion (aq) = −216 kcal/mol, standard heat of crystallisation of BaSO₄(s) = −4.5 kcal/mol, standard heat of formation of BaSO₄(s) = −349 kcal/mol]

Accepted Answer

−128.5. Build the target equation by Hess’s law. Formation of solid BaSO₄ from its elements is −349 kcal/mol. That can be split into forming the two aqueous ions and then crystallising them, so ΔH_f(BaSO₄) = ΔH_f(Ba²⁺) + ΔH_f(SO₄²⁻) + ΔH_crystallisation. Substituting, −349 = ΔH_f(Ba²⁺) + (−216) + (−4.5), so ΔH_f(Ba²⁺) = −349 + 216 + 4.5 = −128.5 kcal/mol.

Answer

−133.0

Answer

+133.0

Answer

+220.5

Q1.Two moles of an ideal gas undergo free expansion from 10 L to 100 L at 300 K. The values of ΔSsystem and ΔSsurroundings are (R is the universal gas constant)

Q3.The standard heat of formation, in kcal/mol, of Ba²⁺ is [Given: standard heat of formation of SO₄²⁻ ion (aq) = −216 kcal/mol, standard heat of crystallisation of BaSO₄(s) = −4.5 kcal/mol, standard heat of formation of BaSO₄(s) = −349 kcal/mol]