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Explore the forces that act between molecules and hold matter together.
Why can a tiny water strider walk across a pond without sinking, while a pebble drops straight to the bottom? The answer lies in the invisible 'handshakes' between molecules that hold our world together.
To understand chemistry, we must distinguish between two types of forces. Intramolecular bonds are the 'super glue' inside a molecule that keep atoms together, like the covalent bonds in . These are very strong. On the other hand, Intermolecular Forces (IMF) are the 'velcro' attractions between separate molecules. While IMFs are much weaker than chemical bonds, they determine whether a substance is a solid, liquid, or gas. Think of it this way: an intramolecular bond is like the glue holding a single Lego brick together, while an intermolecular force is the friction that holds two stacked bricks together.
Quick Check
If you boil water, do you break the bonds inside the molecule or the forces between the molecules?
Answer
You break the intermolecular forces between the molecules; the molecules stay intact as steam.
The strongest type of intermolecular force is the Hydrogen Bond. This occurs when a Hydrogen atom is bonded to a very 'greedy' (electronegative) atom like Fluorine (F), Oxygen (O), or Nitrogen (N). This creates a highly polar molecule. The partial positive charge on the Hydrogen is attracted to the partial negative charge on a neighboring molecule's lone pair. This 'FON' rule (Fluorine, Oxygen, Nitrogen) is why water has such unique properties. Without hydrogen bonding, water would be a gas at room temperature, and life as we know it would be impossible!
Let's look at Ammonia () and Methane (). 1. In , Hydrogen is bonded to Nitrogen. Nitrogen is part of the 'FON' group, so molecules will form Hydrogen Bonds with each other. 2. In , Hydrogen is bonded to Carbon. Carbon is not electronegative enough, so molecules only have very weak attractions and cannot form hydrogen bonds.
Quick Check
Which molecule would have a higher boiling point: or ?
Answer
, because it can form strong Hydrogen bonds (Hydrogen + Fluorine), whereas cannot.
Intermolecular forces dictate how liquids behave. Surface tension is the 'skin' on a liquid's surface caused by molecules pulling on each other. Water has high surface tension because its hydrogen bonds are so strong. Similarly, IMFs affect evaporation. For a molecule to leave a liquid and become a gas, it must 'break free' from the attractions of its neighbors. If the IMFs are strong, the liquid evaporates slowly. If the IMFs are weak (like in rubbing alcohol), the molecules fly away easily, leading to fast evaporation and a cooling sensation on your skin.
Imagine placing a drop of Water () and a drop of Acetone (, nail polish remover) on a table. 1. Water molecules are held by strong Hydrogen bonds. 2. Acetone molecules are held by weaker Dipole-Dipole forces. 3. Result: The Acetone evaporates in seconds because its 'velcro' is easier to pull apart, while the water drop remains for much longer.
Consider two substances: Ethanol () and Dimethyl Ether (). They have the same atoms but different shapes. 1. Ethanol has an group, allowing for Hydrogen bonding. 2. Dimethyl Ether has an Oxygen in the middle (), so it cannot form H-bonds with itself. 3. Prediction: Ethanol will have a much higher boiling point () than Dimethyl Ether () because more energy is required to overcome the stronger H-bonds.
Which of the following is an example of an intermolecular force?
Why does water () have a much higher boiling point than Hydrogen Sulfide ()?
A liquid with very weak intermolecular forces will likely evaporate faster than a liquid with strong intermolecular forces.
Review Tomorrow
In 24 hours, try to list the three elements (F, O, N) that allow Hydrogen to form H-bonds and explain how they affect surface tension.
Practice Activity
Place a drop of water and a drop of vegetable oil on a piece of wax paper. Observe which one beads up more (higher surface tension) and try to explain why based on their molecular attractions.