1.Compute the standard free energy change for the following reaction. N 2 (g) +

Question

1.Compute the standard free energy change for the following reaction.

N2(g) + 3H2(g) -> 2NH3(g)

Enter your answer in kJ.

2.Estimate the free energy change (?Grxn) for the following reaction at 298 K, if each gaseous substance is present at p = 0.10 atm.

N2(g) + 3H2(g) <-or-> 2NH3(g)

Enter your answer in kJ.

3.In the upper atmosphere, ozone is produced from oxygen gas in the following reaction.

3O2(g) -> 2O3(g) Where delta H=285 delta S=-141 delta G=326

Assume an atmosphere where p(O2) = 0.060 atm, and where T = 298 K. Below what pressure of O3 will ozone production be spontaneous? (Enter your answer in atm).

4. Breathalyzers determine the alcohol content in a person's breath by the following (unbalanced) redox reaction:

C2H5OH + Cr2O72? ? CH3CO2H + Cr3+ (acidic solution)

If analysis of a breath sample generates 8.90 x 10-4 M Cr3+ ions in 25.0 mL, how many mg of alcohol did it contain?

Explanation / Answer

1. The equation to calculate Gibbs free energy, ?G, looks like this:

?G = ?H - T?S
It is the *changes* in enthalpy and entropy which give us Gibbs free energy, ?G, and from which we can determine if a reaction is spontaneous or nonspontaneous at a particular temperature.

We need to compute the enthalpy change, ?H. ?Hrx = ??Hf(prod) - ??Hf(react)
?Hrx = 2mol(-46.1 kJ/mol) = -92.2 kJ .... the sum of the heats of formation of the reactants are zero because they are in the elemental state.

?Srx = ??Sf(prod) - ??Sf(react) = 2 mol(0.1923 kJ/Kmol) - (3 mol(0.1306 kJ/Kmol) + 1mol(0.1915 kJ/Kmol)) = -0.1987 kJ/K

?G = ?H - T?S
?G = -92.2 kJ - 573K(-0.1987 kJ/k)
?G = +21.7 kJ

2. ?G=?G

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