Intermolecular Forces Practice- Comprehensive Worksheet
What You Actually Need to Know About Intermolecular Forces
Intermolecular forces (IMF) are the attractions and repulsions between molecules. They're not the same as chemical bonds — those happen inside molecules. IMF happens between them.
This distinction trips up more students than you'd think. If you're mixing these up, stop now and go back to your textbook. Everything else in this unit depends on getting this right.
The Three Forces You Actually Need to Master
Most general chemistry courses focus on three main types. Learn these cold.
London Dispersion Forces (LDF)
Every molecule has them. Even noble gases. They're caused by temporary shifts in electron density that create instantaneous dipoles.
Bigger molecules = stronger LDF because they have more electrons to create these temporary imbalances.
Dipole-Dipole Interactions
These happen between molecules that already have permanent dipoles — molecules with polar bonds and asymmetric shapes. The positive end of one molecule attracts the negative end of another.
These are stronger than LDF but still weak compared to the next one.
Hydrogen Bonding
Not actually a bond. It's an especially strong dipole-dipole interaction that happens when hydrogen is bonded to N, O, or F and interacts with a lone pair on another N, O, or F.
This is why water boils at 100°C instead of what it should based on its molecular weight. Remember this example — it shows up constantly.
Comparing the Three Forces
| Force Type | What Causes It | Strength | Examples |
|---|---|---|---|
| London Dispersion | Temporary electron shifts | Weakest | All molecules, noble gases |
| Dipole-Dipole | Permanent dipoles interacting | Intermediate | HCl, SO2, CH3Cl |
| Hydrogen Bonding | H bonded to N/O/F + lone pair | Strongest | H2O, NH3, HF |
Why You Need Practice Problems
Reading about intermolecular forces isn't enough. You have to apply the concepts to different molecules and scenarios.
A good practice worksheet tests three things:
- Identifying what forces exist in a given molecule
- Ranking compounds by boiling/melting point based on their IMFs
- Predicting physical properties (solubility, viscosity, vapor pressure)
If your worksheet doesn't cover these three areas, it's not comprehensive enough.
What Makes This Worksheet Different
Most worksheets give you a molecule and ask you to identify forces. That's step one. The useful stuff starts when you have to compare multiple molecules and justify your answers.
Look for worksheets that include:
- Lewis structures for each compound
- Reasoning sections where you explain your ranking
- Questions about molecules with competing forces
- Real-world applications (why does antifreeze work?)
How to Use This Worksheet Effectively
Don't just write answers. Show your work.
For every ranking question, your process should look like this:
- Draw the Lewis structure
- Identify all IMFs present
- Consider molecular size/molecular weight
- Weigh the forces against each other
- Make your ranking with justification
Skipping steps 1-4 is why students get these questions wrong. The answer without reasoning is worthless.
Common Mistakes That Cost You Points
Confusing intramolecular and intermolecular forces. Breaking ionic bonds into ions isn't an IMF. Covalent bond breaking isn't an IMF. IMFs are forces between molecules.
Forgetting that LDF exists in every molecule. Even water has London dispersion forces. They just get overshadowed by hydrogen bonding.
Ranking by molecular weight alone. This works when molecules have similar forces, but fails completely when comparing molecules with different dominant forces. A small molecule with hydrogen bonding beats a large molecule with only LDF every time.
Missing hydrogen bonding criteria. Hydrogen must be bonded directly to N, O, or F. The H in CH bonds isn't hydrogen bonding — it's just a polar bond.
Getting Started
Download a comprehensive worksheet. Work through it twice.
First pass: answer what you can without looking at notes. Mark anything uncertain.
Second pass: go back to the marked questions. Draw the structures. Identify the forces. Walk through the ranking process step by step.
If you're still stuck on questions after two passes, that's where your studying needs to focus.
The goal isn't to finish the worksheet. The goal is to nail the reasoning process so it works when you see similar questions on the exam.