Thermodynamics

Which of the following is not correct?

(i) ∆G is zero for a reversible reaction

(ii) ∆G is positive for a spontaneous reaction

(iii) ∆G is negative for a spontaneous reaction

(iv) ∆G is positive for a non-spontaneous reaction

Enthalpy of sublimation of a substance is equal to

(i) Enthalpy of fusion + enthalpy of vapourisation

(ii) Enthalpy of fusion

(iii) Enthalpy of vapourisation

(iv) Twice the enthalpy of vapourisation

The enthalpies of elements in their standard states are taken as zero. The enthalpy of formation of a compound

(i) Is always negative

(ii) Is always positive

(iii) May be positive or negative

(iv) Is never negative

Consider the reactions given below. On the basis of these reactions find out which of the algebric relations given in options (i) to (iv) is correct?

(a) C (g) + 4H (g) → CH₄ (g); ∆r H = x kJ/ mol

(b) C (graphite,s) + 2H₂ (g) → CH₄ (g); ∆rH = y kJ /mol

(i) x = y

(ii) x = 2y

(iii) x > y

(iv) x < y

On the basis of thermochemical equations (a), (b) and (c), find out which of the algebric relationships given in options (i) to (iv) is correct.

(a) C (graphite) + O₂ (g) → CO₂ (g) ; ∆_rH = x kJ/ mol

(b) C (graphite) + $\frac{1}{2}$ O₂ (g) → CO (g) ; ∆_rH = y kJ/ mol

(c) CO (g) $\frac{1}{2}$ +  O2 (g) → CO₂ (g) ; ∆_rH = z kJ/ mol

(i) z = x + y

(ii) x = y – z

(iii) x = y + z

(iv) y = 2z – x

The entropy change can be calculated by using the expression ∆S = . When water freezes in a glass beaker, choose the correct statement amongst the following:

(i) ∆S (system) decreases, but ∆S (surroundings) remains the same.

(ii) ∆S (system) increases, but ∆S (surroundings) decreases.

(iii) ∆S (system) decreases, but ∆S (surroundings) increases.

(iv) ∆S (system) decreases, and ∆S (surroundings) also decreases.

The pressure volume work for an ideal gas can be calculated by using the expression w = - ${\int }_{{\mathit{V}}_{\mathit{i}}}^{{\mathbf{V}}_{\mathbf{f}}}{\mathit{P}}_{\mathit{e}\mathit{x}}$ dv. The work can also be calculated from the pV−plot by using the area under the curve within the specified limits. When an ideal gas is compressed (a) reversibly or (b) irreversibly from volume Vi to Vf. Choose the correct option.

(i) w (reversible) = w (irreversible)

(ii) w (reversible) < w (irreversible)

(iii) w (reversible) > w (irreversible)

(iv) w (reversible) = w (irreversible) + p_ex.?V

In an adiabatic process, no transfer of heat takes place between system and surroundings. Choose the correct option for free expansion of an ideal gas under adiabatic condition from the following.

(i) q = 0, ∆T ≠ 0, w = 0

(ii) q ≠ 0, ∆T = 0, w = 0

(iii) q = 0, ∆T = 0, w = 0

(iv) q = 0, ∆T < 0, w 0 V_f

∆_fU $°$ of formation of CH₄ (g) at certain temperature is –393 kJ/mol. The value of ∆_fH $°$ is

(i) zero

(ii) < _f U $°$

(iii) > ∆_f U $°$

(iv) Equal to ∆_fU $°$

During complete combustion of one mole of butane, 2658 kJ of heat is released. The thermochemical reaction for above change is

(i) 2C₄H₁₀(g) + 13O₂(g) → 8CO₂(g) + 10H₂O(l) ∆_c H = –2658.0 kJ/mol

(ii) C₄H₁₀(g) $\frac{13}{2}$ +  O₂ (g) → 4CO₂ (g) + 5H₂O (g) ∆_c H = –1329.0 kJ/mol

(iii) C₄H₁₀(g) $\frac{13}{2}$ +  O₂ (g) → 4CO₂ (g) + 5H₂O (l) ∆_c H = –2658.0 kJ/mol

(iv) C₄H₁₀ (g) $\frac{13}{2}$ + O₂ (g) → 4CO₂ (g) + 5H₂O (l) ∆_c H = +2658.0 kJ/mol