1.5.3Physical Chemistry
Types of Forces Between Molecules — AQA AS Chemistry Revision Notes & Topical Questions
5 min read 4 parts Lesson 29 of 102
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Part 1 of 4The Three Forces
Induced dipole–dipole
- Induced dipole–dipole Electron movement creates a temporary dipole in one molecule,
- which induces a dipole in a neighbour and gives a weak attraction between them.
Where they occur
- Where they occur Induced dipole–dipole forces (also called London forces) act between all molecules,
- including non-polar ones such as the noble gases and alkanes.
Permanent dipole–dipole
- Permanent dipole–dipole Polar molecules attract one another because the δ+ end of one lines up with the δ− end of another.
Hydrogen bonding
- Hydrogen bonding This is a strong dipole attraction between a δ+ hydrogen on N, O or F and a lone pair on a nearby N, O or F atom.
Relative strength
- Relative strength Induced dipole–dipole forces are generally weakest, permanent dipole–dipole stronger,
- and hydrogen bonds the strongest,
- but all are far weaker than covalent bonds.
Part 2 of 4Hydrogen Bond Requirements
Hydrogen Bond Requirements
Hydrogen attached
- Hydrogen attached The hydrogen atom must be covalently bonded to N, O or F, which are highly electronegative and leave hydrogen strongly δ+.
Lone pair
- Lone pair The second molecule must have a lone pair on N, O or F that the δ+ hydrogen can be attracted to.
Linear arrangement
- Linear arrangement The hydrogen bond lies in a straight line along the O–H···O direction for maximum strength.
Examples
- Examples Water, ammonia, hydrogen fluoride, alcohols and carboxylic acids can all form hydrogen bonds with themselves.
| Force | Present between | Relative strength |
|---|---|---|
| Induced dipole–dipole | All molecules | Weakest |
| Permanent dipole–dipole | Polar molecules | Stronger |
| Hydrogen bonding | Molecules with H on N, O or F | Strongest |
Part 3 of 4Naming the Force
1
Check first whether hydrogen is bonded to nitrogen, oxygen or fluorine, in which case hydrogen bonding is present.
2
If not, decide whether the molecule is polar by looking at bond polarity and shape.
3
A polar molecule has permanent dipole–dipole forces as well as induced dipole–dipole forces.
4
A non-polar molecule has only induced dipole–dipole forces between its molecules.
5
In your answer, name the force and describe the charges involved for a complete explanation.
Part 4 of 4Exam Checklist & Tips
1.5.3 – Key Points for the Exam
- Induced dipole–dipole force A weak attraction caused by temporary dipoles that arise from the movement of electrons.
- Permanent dipole–dipole force An attraction between the permanent δ+ and δ− ends of polar molecules.
- Hydrogen bond A strong intermolecular attraction involving hydrogen bonded to N, O or F and a lone pair on another such atom.
- Intermolecular force An attraction between separate molecules rather than within a molecule.
Common Mistakes
- Calling a hydrogen bond a covalent bond; it is an intermolecular attraction.
- Saying hydrogen bonds occur whenever a molecule contains hydrogen, such as in methane.
- Forgetting that polar molecules also have induced dipole–dipole forces in addition to permanent dipoles.
- Stating that London forces are absent in polar molecules.
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