Another short Quiz on Chemical Equilibrium

True/False

The change in standard free energy for a reaction:

equals the heat released by the system to the surroundings at constant pressure
predicts the speed of the reaction
equals H + T S
depends on the standard state chosen

An endothermic reaction is one for which H < 0

If G < 0 for a reaction, then the reaction is said to be spontaneous in the forward direction

All endothermic reactions must have a positive change in entropy to be spontaneous

Consider the melting and sublimation of a pure substance:

S_fus⁰ - S_sub⁰ = 0
S_fus⁰ = 144.3 J/mol-K, S_sub⁰ = 9.0 J/mol-K
S_fus⁰ = 9.0 J/mol-K, S_sub⁰ = -144.3 J/mol-K
H⁰ for both processes is less than zero
S_fus⁰ = 9.0 J/mol-K, S_sub⁰ = 144.3 J/mol-K

The maximum quantity of energy available for useful work at constant temperature and pressure is the change in Gibbs Free Energy for the reaction

The standard free energy change for a reaction is -298.0 kJ/mol and the actual free energy change for the reaction conditions is zero. The reaction is therefore

at equilibrium
has a large equilibrium constant
will be shifted from equilibrium if the temperature is changed

Consider the following reaction:

C(g) + 2H₂(g) <--> CH₄(g)

at a temperature where

S⁰ [C(g)] = 5.69 J/mol^.K
S⁰ [H_2(g)] = 130.58 J/mol^.K
S⁰ [CH₄(g)] = 186.19 J/mol^.K

What is S⁰ for the above reaction at the same temperature in J/mol^.K?

ANSWER: S⁰ = -80.7 J/K^.mol

For

CO(g) + H (g) -> H₂CO(g) ,

Hrxn⁰ = -5.36 kJ/mol
Srxn⁰ = -109.8 J/mol^.K

What is G⁰ in kJ/mol at 300 K?

ANSWER: 27.6 kJ/mol

What is the equilibrium constant for the reaction?

ANSWER: 1.58 x 10^-5

Of the following materials, predict the ordering of the molar entropy

pure water
powdered sugar
salt water
salt

Predict whether S_rxn is positive or negative for each of the following processes.

H_{2 (g)} + 1/2O_{2 (g)} -> H₂O_(g)

2Cl₂O₇(g) -> 2Cl₂(g) + 7O₂(g)

H₂O(g) -> H₂O(s)

Sugar_(s) -> Sugar_(aq)

For the reaction

CO(g) + 2H₂(g) -> CH₃OH(g)

K_p = 91.4 at 350 K and K_p = 2.05 x 10^-4 at 298 K.
What is the value of H_rxn⁰ ?

ANSWER: 216.9 kJ/mol
ln(K₂/K₁) = -H/R (1/T₂ - 1/T₁)

Consider the reaction:

AB(g) <--> A(g) + B(g)

K_eq = 0.729 at 20 ^oC and H⁰ = 45.9 kJ/mol
What is K_eq for this reaction at 377 K?

ANSWER: K_eq = 48.5
ln(K₂/K₁) = -H/R (1/T₂ - 1/T₁)

PJ Brucat // University of Florida