Sunday, September 18, 2016

Why is there no standard state temperature?

The standard state pressure is 1 bar, why is there no standard temperature?

The short answer
The standard state pressure is not an experimental condition, while the temperature is.

The long answer
The main reason the standard state is defined is because it leads to this very useful equation
$$K_p = e^{-\Delta G^\circ/RT}$$
Say you have this reaction: $A \rightleftharpoons B + C$ One way to use this equation is to compute the free energy of 1 mol of $A$, $B$, and $C$ at 1 bar using equations derived for an ideal gas, compute $\Delta G^\circ = G^\circ (B) + G^\circ (C) - G^\circ (A)$, and use that value to predict $K_p$.
If the gasses behave like ideal gasses "in real life" then the measured  $K_p$ will match the $K_p$ computed from $\Delta G^\circ$.  You can do the measurement at any pressure you want, not just at 1 bar.* The standard state refers to the pressure you use when computing $\Delta G^\circ$.  The only thing it has to do with the experimental measurement is that it defines the units you should use for your partial pressures when computing $K_p$

$\Delta G^\circ$ does also depend on temperature, but the temperature you chose should be the same as the experimental conditions.  So the temperature is not part of the standard state definition.

But what about "Standard temperature and pressure (STP)?"
Standard temperature and pressure (STP) refers to STP conditions under which $K_p$ is measured, not the pressure used to compute $\Delta G^\circ$. I know, they couldn't have made it more confusing if they tried when they named these things.

*Of course if you do the measurements at very high pressures or low temperatures, then the assumption that the gasses behave ideally will be less valid and the measured $K_p$ will differ more from the $K_p$ computed from Equation 1.  However, that is a separate issue unrelated to the standard state because the $K_p$ in Equation 1 refers to the $K_p$ you would measure if the gasses behaved ideally at the pressure and temperature used in the experiment.

This work is licensed under a Creative Commons Attribution 3.0 Unported License.

Saturday, September 17, 2016

Why I tweet and blog

Update: here is the audio.  Something wen't wrong with Google Hangouts so the slides are missing. Despite the fact that I did a few practice runs yesterday ... and have a PhD in theoretical quantum chemistry.  WTH Google!

Update 2: the trick (in addition to this tip) is to start the Powerpoint slideshow before you start streaming.

On Tuesday I am giving presentations on tweeting and blogging in a Scientific Writing course.  Here are my slides and the message to the students

Dear Scientific Writing students

On Tuesday I will give two presentations: one on tweeting and one on blogging.  You can find the slides below.

You'll also do some writing so please bring a laptop and make sure you can get on Eduroam.

In preparation for Tuesday, please find one science related blogpost and twitter account you think looks interesting and share them on the discussion forum I created on Absalon.

Finally, I may try to live broadcast my talk using Google's Hangout On Air.  I've never tried it, so I am not sure if I can get it to work by Tuesday.  If you are uncomfortable with this, just send me an email and I won't do it.

See you Tuesday!


This work is licensed under a Creative Commons Attribution 3.0 Unported License.