## Sunday, October 13, 2013

### Chemistry assignments that use Molecule Calculator (MolCalc)

1. One of the reviewers of our J. Chem. Ed. paper on MolCalc included the following tutorial: Molecular Orbital Calculations of Molecules I.Diatomics, Triatomics and Reactions

2.  n-Butane can exist in two different conformations called gauche and anti (Google butane and conformation).  Use Molecular Calculator to estimate the fraction of molecules in the gauche conformation at 25 $^\circ$C. $\Delta H^\circ$  can be computed as the difference in heat of formation.

3. Estimate $\Delta H^\circ$  the for the following reaction at 25 $^\circ$C

NH$_2$CHO + H$_2$O $\rightleftharpoons$ NH$_3$ + HCOOH

a. Using bond energies
b. Using Molecule Calculator

4. How does the molecular structure determine the rotational entropy?  Find out by constructing a molecule with the largest possible rotational entropy using Molecule Calculator.  The largest value I could find was 133 J/molK.  Can you beat that?

5. How well do the simple solvation models work?
a. Estimate the solvation energy of NH$_4^+$ using MolCalc?
b. What is the polar solvation energy of NH$_4^+$ in water at 25 $^\circ$C assuming that it is spherical?

6. Why do ionic compounds dissolve in water?  Use MolCalc to estimate $\Delta G^\circ$ at 25 oC for the following equilibrium

N(CH$_3$)$_4^+\cdot$Cl$^-$ $\rightleftharpoons$ N(CH$_3$)$_4^+$ + Cl$^-$

a. in the gas phase
b. in aqueous solution

7. Solvent screening: charge-charge interactions are weaker in aqueous solution than in the gas phase.  Compute the difference in G$^\circ$ at 25 $^\circ$C between these two molecules using MolCalc

a. in the gas phase
b. in aqueous solution

8. Build a molecule with a solvation energy that is as close to 0 as possible.  The closest I got is -1.3 kJ/  How close can you get?