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Many MCAT questions hinge on knowing whether a force operates within a molecule or between molecules; that’s where the difference between inter and intra comes in.
Let’s break this down with crystal clarity, because if you get these two terms straight, a ton of MCAT questions get easier instantly.
These are the attractive forces that occur between separate molecules. They don’t involve any atoms sharing or transferring electrons. Instead, they're all about how molecules “stick” to each other. Think of them as social glue, holding molecules near one another, not changing their actual identity.
Types of Intermolecular Forces (from weakest to strongest):
Where they show up on the MCAT:
These are the actual chemical bonds that hold atoms together in a single molecule. If you break an intramolecular force, you're changing the substance itself; you’re doing chemistry.
Types of Intramolecular Forces:
Key difference:
Once these definitions are locked in, everything else, from boiling water to why fats and oils don’t mix well with water, starts to click.
Let’s use water (H₂O) as the perfect molecule to illustrate both intramolecular and intermolecular forces. It’s high-yield, intuitive, and guaranteed to show up in multiple MCAT sections.
A single water molecule is made up of two hydrogen atoms and one oxygen atom, connected by polar covalent bonds. These bonds involve an unequal sharing of electrons, oxygen is more electronegative, so it hogs the electrons a bit, making the molecule polar.
Thanks to its polarity, each water molecule can form hydrogen bonds with neighboring water molecules. One molecule's partially positive hydrogen is attracted to another's partially negative oxygen.
MCAT Tip: Boiling water involves breaking hydrogen bonds, not breaking the H–O covalent bonds. The molecules stay intact, they just separate from each other.
Water has unusually high boiling and melting points compared to molecules of similar size (like methane, CH₄) because of its strong hydrogen bonding network. This means it takes a lot of energy (heat) to separate the molecules and go from liquid to gas.
MCAT application: If you see a passage comparing boiling points of different compounds, ask:
Here’s the golden rule MCAT students must remember: Intramolecular forces are MUCH stronger than intermolecular forces.
If you mix these up, you’ll misread boiling point trends, misinterpret molecular behavior, and fall into traps on passage-based questions. Let’s break it down:
These involve the sharing or transfer of electrons to form actual chemical bonds. Breaking these bonds changes the substance itself, which is why it requires significantly more energy.
Relative strength (approximate):
Example:
These are much weaker and govern how molecules interact, not what they’re made of. These are responsible for physical properties, like whether something is a gas, liquid, or solid at room temp.
Relative strength:
Example:
Think of intramolecular forces as the nuts and bolts holding a chair together, and intermolecular forces as the Velcro connecting one chair to another. Velcro can be peeled apart easily (inter), but unscrewing a bolt (intra) takes serious work, and changes the structure.
High-Yield Reminder:
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The MCAT won’t always hand you the answer. Instead, you’ll be expected to infer intermolecular strength based on structure and bonding clues. Here’s how to break it down, fast.
Look for an unequal distribution of charge (e.g., oxygen pulling electrons away from hydrogen). If there’s a dipole moment, it’s polar.
High-yield hydrogen bonders:
Example: Hexane (C₆H₁₄) has stronger LDFs than methane (CH₄), even though neither is polar.
MCAT Pattern: If a question gives you a set of molecules and asks which has the highest boiling point or greatest surface tension, you’re being tested on intermolecular forces.
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Despite how simple the terms sound, “intermolecular” vs. “intramolecular” confuses tons of students. That’s because the names are similar, and both deal with attractions and bonds. Here's how to clear the fog once and for all.
You're reading a passage about DNA denaturation. The question asks:
"What type of interaction is disrupted when DNA strands separate?"
Correct answer: Hydrogen bonds between base pairs (intermolecular)Not: Covalent bonds in the sugar-phosphate backbone (intramolecular)
Think you're ready for the MCAT? Let's take a look at a quick passage first.
Researchers compared the boiling points of several compounds with similar molecular weights but differing intermolecular forces.
Their findings are summarized below:
Note: Only water and methanol are capable of hydrogen bonding.
Question: Which of the following best explains why water has the highest boiling point, despite having the lowest molecular weight?
A) Water contains strong covalent bonds that require more energy to break
B) Water molecules form hydrogen bonds, which are strong intermolecular forces
C) Water has the largest number of electrons, leading to strong dispersion forces
D) Water has the most symmetrical molecular structure, minimizing repulsion
Correct Answer: B
Why It’s B:
Test Strategy Reminder:
When you see questions about:
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