Question

In the text the equation ΔG = ΔG° + RT ln(Q) was derived for gaseous reactions where the quantities in Q were expressed in units of pressure. We also can use units of mol/L for the quantities in Q—specifically for aqueous reactions. With this in mind, consider the reaction HF(aq) equilibrium reaction arrow H +(aq) + F −(aq) for which Ka = 7.2 ✕ 10−4 at 25°C. Calculate ΔG for the reaction under the following conditions at 25°C. Based on the calculated ΔG values, in what direction will the reaction shift to reach equilibrium for each of the five sets of conditions?

Answer 1

Answer:

ΔGrxn(25°C) = 2113.839 KJ/mol

⇒ ΔG >> 0 and K << 1; then the reaction shift to the left to reach equilibrium

Explanation:

• HF(aq) ↔ H+(aq) + F-(aq)
• Ka = 7.2 E-4 = [H+]*[F-] / [HF]
• ΔG = ΔG° + RT Ln (Q)

∴ in the equilibrium: Q = Ka = 7.2 E-4 (25°C)

∴ ΔG°HF(aq)(25°C) = - 294.595 KJ/mol

∴ R = 8.3145 E-3 KJ/mol.K

∴ T = 25°C = 298.15 K

⇒ ΔGrxn(25°C) = {-294.595 KJ/mol + [(8.3145 E-3 KJ/.K.mol)*(298.15K)]} Ln(7.2 E-4)

⇒ ΔGrxn(25°C) = [ -295.595 KJ/mol + 2.4789 KJ/mol ] * (-7.2363)

⇒ ΔGrxn(25°C) = ( -292.116 KJ/mol ) * ( - 7.2363 )

⇒ ΔGrxn(25°C) = 2113.839 KJ/mol

∴ ΔG >> 0 and K << 1; then the reaction shift to the left to reach equilibrium