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## 18 Comments

Hide/Show Comments## Hannah Q. Karp

I noticed that on the Exam 1 Equations snapshot there were no equations for half life. We should still expect half life problems or problems that incorporate half life though, right?

## Kevin P. Rice

Hi Hannah, all the half life equations can be easily derived from the integrated rate laws. Honestly, the only one we really use is the 1st order version (the others were mostly for comparison). So you can either quickly derive the 1st order half life equation or memorize it.

## Hannah Q. Karp

Hi Prof. Rice, I was wondering how to do problem 85 from the chapter 14 exercises section. I don't understand how to manipulate the Arrhenius equation to find the difference in the activation energies. Thanks! Hannah

## Kevin P. Rice

You can derive this version of the Arrhenius equation... let me know is this doesn't make sense!

## htang

Hi Prof. Rice - can you post the solutions for the "hard" kinetics problems? Thanks!

## Kevin P. Rice

Sorry... they're there now.

## Alessandra B. Grossman

Hi, I had a question from our class notes on organic acids. Do resonance structures generally make molecules more or less acidic? Thanks!

## Kevin P. Rice

Hi Alessandra,

When molecules have the capacity to form resonance structures it tends to give electrons "more places to hide". So, for example, a delocalized Pi system like benzene, when attached to a carboxylic acid (benzoic acid), would act as an electron withdrawing group and make the acid stronger. If an amine was attached to benzene (aniline), it would make the lone pair less basic. The other application of resonance we talked about was for conjugate bases... One of the reasons a carboxylic acid molecule (like acetic acid, CH3COOH) is acidic where an alcohol (like ethanol, CH3CH2OH) is not is because the carboxylate anion has resonance structures that can distribute the 'extra' electrons around and make the anion more stable than it otherwise would have been. I hope this helps!

Kevin

## Hannah Q. Karp

Hi Prof. Rice,

How do you do 17.45 in the textbook? I am trying to use H-H for b-e, but c-f are not working because I can't take the log of a negative number... I wanted to convert the A/HA values into moles instead of concentration so that I could just subtract the added NaOH from the HA portion, and add the NaOH to the A portion of my fraction. Thanks!

## Kevin P. Rice

Hi Hannah, I don't have my book at home, but I'll get back to you on this tomorrow morning.

## Kevin P. Rice

You can ONLY use the H.H. equation (and thus the initial conc. = eq. conc. assumption) for part (b). All the other parts are outside the 'buffer zone'.

## Hannah Q. Karp

So, would you find the difference of what the buffer can't neutralize and take the pOH of the [OH-], then do 14-pOH = pH?

(I tried that, but I got the wrong answer...)

## Kevin P. Rice

If you're dealing with a weak acid, no. You have to use the ICE table and the equilibrium constant expression.

## Kevin P. Rice

If you're dealing with a weak acid, no. You have to use the ICE table and the equilibrium constant expression.

## Alessandra B. Grossman

Hi, I had a question from the chapter 20 homework. The question is:

A concentration cell is constructed from two hydrogen electrodes, both with PH2=1.00. One electrode is immersed in pure H20, and the other in 6.0M hydrochloric acid. What is the emf generated by the cell and what is the identity of the electrode that is immersed in hydrochloric acid (either cathode or anode)?

I tried to use the equation relating E cell to K (standard state because pressure of H2 is 1.00), but I was not getting one of the answers given.

Thanks!

## Kevin P. Rice

Hi Alessandra, that's the wrong equation actually. What is being described to you is outside the standard state. 6M HCl is 6M H+, which you'd plug in the Nernst Eq.

## bphall20

On the exam, you noted how there will be an equation sheet. However, are there any equations that the ACS expects us to know? For example, we were expected to know the Henderson-Hasselbalch equation for Exam 2. Are we expected to memorize equations such as these for the ACS one?

Thanks!

## Kevin P. Rice

Hi Brian,

The ACS exam includes a list of equations and constants that's similar in scope to what I've given you.

Best,

Kevin