For the gaseous reaction of carbon monoxide and chlorine to form phosgene, (COCl2), calculate each of the following.
(a) Calculate S° at 298 K
I got -136.76 J/K using delta S = sum of delta S of products minus sum of delta S of reactants which is the right answer.
But I can't figure out the second part:
(b) Assuming that S° and H° change little with temperature, calculate G° at 426 K.
In the standard thermodynamics values appendix it says the deltaH of COCl2 is -220 kJ/mol.
I am just not sure how to do this.
First, this is at 426K and not 298 K; therefore, any number you obtain by dGo = dHo - TdSo will not agree with 220 kJ/mol. So you substitute the values you have for dHo and dSo along with 426K for T and calculate dGo. Remember dH is given in kJ/mol and dS in J/mol.
DeltaG=DeltaH-DeltaS*T
Where
DeltaH=-220kJ/mol
DeltaS=-136.76kJ/K
T=426K
and
Delta=????
The key part of the equation states that ASSUME that DeltaS and DeltaH DOESN'T Change much, meaning that it is safe to plug and chug.
***To the questioner: change -136.76J/K to kJ, which should be -0.13676kJ/K
After reading Devron's reply I realize I read your initial question wrong. His response and mine agree; the only problem with mine is I thought you were quoting dG as -220 kJ/mol where you were quoting dH. Therefore, that part I wrote about "not agreeing with" make s no sense.
To the questioner: I apologize about the typos.
I meant to say solve for Delta*G and not just Delta. My post should have said the following:
DeltaG=DeltaH-DeltaS*T
Where
DeltaH=-220kJ/mol
DeltaS=-136.76J/K=-0.13676kJ/K
T=426K
and
DeltaG=???
I initially made the same mistake, which confused me a little. I reread the question and saw that the questioner and I, both, may have initially overthought the question.
--Best
To calculate G° at 426 K, we can use the equation:
ΔG° = ΔH° - TΔS°
where ΔG° is the change in Gibbs free energy, ΔH° is the change in enthalpy, ΔS° is the change in entropy, and T is the temperature in Kelvin.
We already have the value for ΔH° (-220 kJ/mol) and the value for ΔS° (-136.76 J/K) at 298 K.
To convert the units of ΔH° from kJ/mol to J/mol, we multiply by 1000:
ΔH° = -220 kJ/mol × 1000 J/kJ = -220,000 J/mol
To find ΔG° at 426 K, we need to use the assumption that ΔH° and ΔS° change little with temperature. This implies that the values at 426 K are approximately the same as those at 298 K.
Hence, we can set ΔH° = -220,000 J/mol and ΔS° = -136.76 J/K for our calculations at 426 K.
Now, plug in the values into the equation:
ΔG° = -220,000 J/mol - (426 K)(-136.76 J/K)
To perform this calculation, we need to convert the temperature to Kelvin:
426 °C + 273.15 = 699.15 K
Substituting the values:
ΔG° = -220,000 J/mol - (699.15 K)(-136.76 J/K)
Simplifying:
ΔG° = -220,000 J/mol + 95,603 J
Therefore, the calculated value of ΔG° at 426 K is approximately -124,397 J/mol.