Putting the Solvent Into a Calculation · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.
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Adding the solvent to your calculation

Science · Chemistry · ages 23-24
Name ______________________   Date ____________

  1. Your solute forms a hydrogen bond to the solvent. What must you add to the calculation?

    • Those partners as real molecules
    • A larger cavity around the solute
    • A higher dielectric constant

  2. What does a continuum model put in place of the liquid?

    • One real water molecule
    • A vacuum with nothing in it
    • A smooth polarisable medium

  3. A continuum model can show individual hydrogen bonds to the solute.

    Circle one:   True   False

  4. A solute holds hydrogen bonds to a few first shell partners. Which molecules do you include explicitly?

    • Only those partners, inside the medium
    • Every molecule in the beaker
    • None, since the medium covers such bonds

  5. In a test calculation, a continuum model misses the measured hydration energy of a chloride ion by 60 kJ/mol. Adding four water molecules hydrogen-bonded to the ion, with the continuum kept around them, brings it within 5 kJ/mol. What was the continuum alone missing?

    • A higher dielectric constant for the bulk
    • The specific first-shell hydrogen bonds
    • A larger vacuum region around the ion

  6. A polar solvent lowers a barrier when the transition state holds more separated charge than the reactants.

    Circle one:   True   False

  7. A computed ion solvation energy is minus 300 kilojoules per mole, and the measured value is minus 340. By how many kilojoules per mole do the two differ? Type the number.

    Answer: ______________

  8. Two ions react, and the gas phase barrier disagrees badly with experiment. What is missing and how do you fix it?

    • More electron correlation, still in the gas phase
    • The solvent: add a medium and key molecules
    • A longer run with the same vacuum forces
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Adding the solvent to your calculation W1-mt_3SCFfcvoei-s1

  1. Those partners as real molecules · Specific bonds need real molecules, with the medium kept around them.
  2. A smooth polarisable medium · The continuum is a smooth medium set by the dielectric constant.
  3. False · No real molecules sit inside it, so specific bonds are absent.
  4. Only those partners, inside the medium · You include the molecules that do specific work and let the medium cover the rest.
  5. The specific first-shell hydrogen bonds · Only adding hydrogen-bonded water molecules closed the gap, so the smooth medium had been missing those specific first-shell bonds.
  6. True · Tracking charge tells you the direction the barrier moves.
  7. 40 · Take the gap between the values: 340 minus 300 is 40.
  8. The solvent: add a medium and key molecules · Ions in vacuum miss solvation, so the medium and bonded partners must enter.
Worksheet · LightMySky