Intermolecular forces and physical properties
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All 12 Flashcards — Intermolecular forces and physical properties
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Question
What is an intermolecular force?
Answer
A force of attraction **between** separate molecules — much weaker than the covalent bonds **inside** a molecule.
Question
What sets the boiling point of a molecular substance?
Answer
The strength of its **intermolecular forces** — stronger IMFs need more energy, so a **higher** boiling point.
Question
Order the three IMFs by increasing strength.
Answer
**London (dispersion) < dipole–dipole < hydrogen bonding.**
Question
What are London (dispersion) forces?
Answer
Forces from **temporary, instantaneous dipoles**; present between **all** molecules and the **only** force in non-polar ones.
Question
What makes London forces stronger?
Answer
**More electrons** (a larger, more polarisable molecule) — so they increase **down a group** and with molecular size.
Question
When does a molecule have dipole–dipole forces?
Answer
When it is **polar** — it has a **permanent dipole** (δ+ and δ− ends) from an electronegativity difference.
Question
What is hydrogen bonding?
Answer
The **strongest** IMF: a very δ+ H bonded to **N, O or F** is attracted to a lone pair on the N, O or F of a neighbour.
Question
Hydrogen bonding only occurs with which atoms?
Answer
Hydrogen bonded directly to **N, O or F** ('H bonds to NOF').
Question
Why does NH_{3} boil much higher than PH_{3}?
Answer
NH_{3} has **hydrogen bonding** (H on N); PH_{3} has only weaker dipole–dipole/London forces.
Question
Why do alkane/alkene boiling points rise along the series?
Answer
Larger molecules have **more electrons → stronger London forces → higher boiling point**.
Question
Does boiling water break the O–H bonds?
Answer
**No** — boiling only **separates the molecules** by overcoming intermolecular forces; the covalent bonds stay intact.
Question
Why is hydrogen bonding stronger than ordinary dipole–dipole?
Answer
N, O and F are very electronegative, so the H is very δ+ and the attraction to a lone pair is especially strong.
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Topic 2.2 hub
The covalent model
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