Homework Help: Free energy and temperature graphs

1. Dec 29, 2015

brake4country

1. The problem statement, all variables and given/known data
My questions are based on a graph. I am trying to figure out the standard ΔH change for the graphed reaction and the standard entropy change.

(1) What is the standard enthalpy change for the graphed reaction?
(A) -31 KJ/mol
(B) 0
(C) +12 KJ/mol
(D) +11 KJ/mol

(2) What is the standard entropy change for the graphed reaction?
(A) -35 KJ/mol K
(B) 0
(C) +12 KJ/mol K
(D) +35 KJ/mol K

2. Relevant equations
ΔG = ΔH-TΔS

3. The attempt at a solution
I rearranged the Gibb's free energy equation to resemble a linear function. I know that the enthalpy is the y-intercept and the slope is the entropy but my answers are not matching any of possibilities. I included an attachment graph. I apologize in advance if the format is not standard for this site. Thanks in advance!

Last edited by a moderator: Dec 30, 2015
2. Dec 30, 2015

theodoros.mihos

Multiple choice them may not need calculations.
If (1) and (2) works together what can be the only possibility?

3. Dec 30, 2015

Staff: Mentor

Can't see any plot in your attachment, just a reaction equation between silver and oxygen, and yes - docx is not a reasonable format. Try just an image - jpg, gif, png.

4. Dec 30, 2015

Staff: Mentor

I presume your graph shows a plot of the equilibrium constant as a function of temperature (or 1/T). Is that correct? Or, is it a plot of $\Delta G^0$ as a function of temperature?

5. Dec 30, 2015

brake4country

All the instructions say is: Refer to the following graph, which shows the temperature dependence of the standard free energy change for the reaction:
1/2O2+ 2Ag---> Ag2O

I have attached a jpg of the graph. I also noticed that the x-axis is in Celcius, not Kelvin. Some calculations are probably required to convert. For example, I noticed that the gibb's free energy equation is linear and by rearranging: ΔG = -TΔS + ΔH. So, entropy is slope and enthalpy is y-intercept?

Please let me all know what you think of this graph and the two questions I posted in the earlier thread. Thanks!

Attached Files:

• 20151230_163134[1].jpg
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6. Dec 30, 2015

Staff: Mentor

The actual relationship is going to be curved, but this graph describes the behavior over a limited range of temperatures, so it can be used to determine the local values of the standard changes in H and S. Your approach is correct. First write out the equation for ΔG as a linear function of centigrade temperature TC. Then substitute T = TC + 273. Then determine the slope and intercept of the resulting equation to get ΔH and ΔS .

Chet

7. Dec 30, 2015

brake4country

Ok. So for the standard enthalpy change, we have to look at the values in Kelvin. Since at 0K, ΔG = ΔH, we have to look at the value for -273 C on the graph, which is approx. -31 kJ/mol. Similarly, the entropy change is slope, which was a bit tricky because picking two points on the line, the y values have to be converted to kelvin. I get -0.035 kJ/mol K, which converts to the right answer: -35 J/mol K. I hope my rationale is correct here!