Molecules made of springs

Science · Chemistry · ages 19-20

Name ______________________ Date ____________

In a force field, what is a bond?
- A shared electron cloud
- A spring obeying force equals k times x
- A fixed steel rod
A force field can model a bond actually snapping apart.
Circle one: True False
A model bond has k of 300 newtons per metre and stretch of 0.1 metres. Type the restoring force in newtons.
Answer: ______________
A force field treats every bond like a tiny spring that pushes back when stretched. In a practice model, one bond has spring constant k = 200 N/m and is stretched by x = 0.15 m. How hard does the spring push back?
- 13 N
- 30 N
- 215 N
- 0.75 N
True or false: because a force field lets each bond spring stretch, a force field can model a bond actually snapping apart.
Circle one: True False
A force field stores energy in each stretched bond using the spring energy formula U = 0.5 times k times x squared. One bond has k = 400 N/m and is stretched by x = 0.10 m. How much energy is stored, in joules?
Answer: ______________
A double bond resists stretching harder than a single bond. What does that mean in the model?
- It gets a bigger spring constant k
- It gets a longer equilibrium length only
- It is deleted from the field
A lab mate has four jobs and one free force field. Which job is the force field actually a good tool for?
- Predicting the energy barrier for breaking a carbon-carbon bond in a reaction
- Ranking three plausible folded shapes of a small protein to see which is most stable
- Finding where the electrons are located in a molecule
- Calculating what color of light the molecule will absorb
You build one starting shape of a floppy molecule, relax it with energy minimization, and get a nice low-energy shape. Why might this still not be the shape the molecule really adopts?
- Because minimization can only roll downhill into the nearest energy dip, and other starting shapes may roll into a deeper dip
- Because the force field silently breaks bonds during minimization
- Because one minimization always explores every possible shape
- Because lower energy always means the shape is wrong
A stiffer double bond and a softer single bond can share one identical spring constant.
Circle one: True False

Answer key

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

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A spring obeying force equals k times x · The bond term is a Hooke's law spring standing in for the real bond.
False · Springs stretch forever and never break.
30 · Force is 300 times 0.1, which is 30.
30 N · This is the bond-stretch term of a force field: a spring obeying F = kx, standing in for the real bond.
False · A force field spring keeps pulling back harder the farther you stretch it. A real breaking bond is the opposite: the force drops toward zero as the atoms separate. Since the model has no electrons at all, nothing in it can rearrange to let a bond split.
2 · The 0.5 k x squared term is exactly the bond-stretch energy term in a force field. Each term stands in for one physical effect.
It gets a bigger spring constant k · Stiffer bonds get bigger k values.
Ranking three plausible folded shapes of a small protein to see which is most stable · A force field has fixed bonds and no electrons, so anything involving bond breaking or electrons is out. Comparing stable shapes only flexes springs, which is exactly what it can do.
Because minimization can only roll downhill into the nearest energy dip, and other starting shapes may roll into a deeper dip · Minimization is a local downhill slide. It finds the nearest dip in the energy landscape, not the deepest one. A conformational search starts from many shapes and keeps the lowest result, which is why one minimization is not enough.
False · Each bond type carries its own fitted stiffness.