Chemistry 30

Calculate ΔG at 25°C for the following reaction, by first calculating ΔH and ΔS. Once you've found ΔH and ΔS, solve for ΔG using the formula:

ΔG = ΔH - T ΔS

Also - will this reaction be spontaneous at this temperature?

CH₃CO₂H _(l) + 2 O_{2 (g)} → 2 CO_{2 (g)} + 2 H₂O _(g)

Step 1 - Calculate ΔH

Step 2- Calculate ΔS

Step 3 - Calculate ΔG Be sure to convert 25°C into K and ΔS into kJ/K

K	= C + 273
	= 25 + 273
K	= 298 K

 

ΔG	= ΔH - T ΔS
	= -786.1 - 298.0(0.235)
ΔG	= -856.1 kJ

Because ΔG is negative, the reaction is spontaneous at this temperature.

Again find ΔG at 25°C for the reaction

CH₃CO₂H _(l) + 2 O_{2 (g)} → 2 CO_{2 (g)} + 2 H₂O _(g)

This time using the Table of Thermochemical Data and the formula

ΔG = ΣΔG^° _products - ΣΔG^° _reactants 

For the reaction Fe₂O_{3 (s)} + 3 CO _(g) → 2 Fe _(s) + 3 CO_{2 (s)}

ΔG° = -31.3 kJ. Calculate the standard free energy of formation of the ferric oxide, Fe₂O₃, if ΔG°_f of CO = -137 kJ/mol and ΔG°_f of CO₂ = -394 kJ/mol.

This time we are given the value ΔG°for the entire reaction, and need to find ΔG°_f for one of the reaction participants, Fe₂O₃. Let's let that unknown equal x:

Next, set up our formula for ΔG° and substitute in the values we know, then solve for x:

Answer - ΔG°_f for Fe₂O₃ = -740 kJ/mol