19. A certain reaction has an enthalpy value of 225 kJ mol. Assume this value re

Question

19. A certain reaction has an enthalpy value of 225 kJ mol. Assume this value remains constant in the temperature range being studied. At 125 °C, the equilibrium constant has a value of 2.2 x 10-2. At what temperature will the equilibrium constant have a value of 5.9 x 102 1. 436 °C 2. 366 °C 3. 163 °C 4. 93 °C 20. Nitrous acid, HNO2, has a Ki of 7.0x 10 at 25 °C. What is the standard change in Gibbs free energy for the ionization of nitrous acid at 25 °C? 1. 1.5 kJ mol 2. 18 kJ mol 3. 1.5 x 103 kJ mol 4. 1.8 x 10 kJ mol

Explanation / Answer

Ans 19

Enthalpy H = - 225 kJ/mol x 1000J/kJ = - 225000 J/mol

Temperature T1 = 125 + 273 = 398 K

Equilibrium constant K1 = 2.2 x 10^-2

K2 = 5.9 x 10^-5

T2 =?

Gas constant R = 8.314 J/mol·K

From the Arrhenius equation

ln (k2/k1) = H/R (1/T1 - 1/T2)

ln ( 5.9 x 10^-5 / 2.2 x 10^-2) = - (225000/8.314)(1/398 - 1/T2)

-5.92 = - 27062.78 (1/398 - 1/T2)

5.92 = 67.996 - 27062.78/T2

62.076 = 27062.78/T2

T2 = 435.95 K

T2 = 435.95 - 273 = 162.95 °C = 163 °C

Option 3 is the correct answer

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