Free Energy vs Nernst Equation

Test Preparation AP Test Preparation
1

<p>General equation for dG from dGstandard is</p>

<p>dGstandard + RTln(Q), where Q is reaction quotient.</p>

<p>General equation for E from Estandard (where E is maximum cell potential in V) is</p>

<p>E = Estandard - (RT/nF)ln(Q) where Q is reaction quotient.</p>

<p>For the first equation, can Q only be in terms of Pressure?
Similarly, for the Nernst equation, should only Q in terms of concentration be used?</p>

<p>Because Zumdahl derives the Nernst equation from the first equation, but the first equation was derived with Q being the pressure reaction quotient. However, the book then goes on to use the Nernst equation with Q based on concentrations. </p>

<p>From my knowledge, aren’t the two quotients completely different? Q<em>pressure = Q</em>concentration (RT)^n where n is the difference in coefficient sums from reactants to products.</p>

2

<p>dG=dGstandard + RTln(Q) can be rearranged to
dG=-RTln(K) + RTln(Q) which can be further rearranged to
dG= RT[ln(Q)-ln(K)=RTln(Q/K)
Note, with the ratio Q/K, it doesn’t matter whether Q and K are in atm or mol/L, as long as they match. (Any conversion factors would be used on top and bottom and cancel each other out.)</p>

<p>In general, the difference between two logs is the same as the log of the ratio of the two terms. Therefore, when a formula uses the difference between two logs, units don’t matter as long as they are consistent. You may have noticed in the kinetics chapter radioactive decay problems often only provide the gram amounts - no volume is provide because molarities aren’t necessary. The first order integrated rate law of ln[A]-ln[A]0 can use grams directly. Conversion to concentration is unnecessary because the conversion factors cancel.</p>

3

<p>BTW, if you are using Zumdahl’s 6th edition, the fact that molarities can be used in the dG equation is buried in a note in problem #71 on p.823.</p>

4

<p>Umm…this stuff isn’t on the AP. Is it? In all the AP Chem tests I took in class, we never had to sue such a complex equation. the most complex equation would probably be the reaction order equations.</p>

5

<p>Thermodynamics- ΔG, ΔS, and ΔH-are most definitely on the AP Exam.</p>

6

<p>Yeah, I know that, but I’m talking about dG= RT[ln(Q)-ln(K)=RTln(Q/K).</p>

<p>I already know dE=q+w and dG=dH-TdS and dblah=dfin-dinitial</p>

7

<p>oh gosh you guys confused me a bit. i saw all the d’s and i freaked a little, because i thought they were derivatives. i was like “no! there’s calculus on the ap exam??” but then i realized they were deltas. i’m a dumdum. but yeah, afruff23, that stuff (deviations from standard E and G) is most def on there because i was working some past free responses with that. it’s not too bad, considering the formula is right there…</p>

8

<p>heh…don’t feel so bad. I thought the same thing when I first saw that on these boards(I’m still in pre-calc).</p>

<p>On the multiple choice part of the exam, you don’t get that sheet. Sorry…</p>

<p>Oh and a correction to the last equation:
dblah=fin-initial</p>

9

<p>The nernst equation and the dG=dGstandard + RTlnQ only show up on the free response section where you have the equation sheet. Since 1999, when they took calculators away on the MC part, the only equations you’ve really needed to know for MC is dG=dH-TdS and PV=nRT.</p>