Flashcards in Section A: Non-Covalent Interactions Deck (19)
Differences between covalent and non-covalent interactions
1. Covalent have a shorter range
2. Covalent are more highly directional
3. Covalent more long-lasting
If more than 1 bond in a molecule is polar then
The overall molecular dipole, u, must be considered
The overall average interaction energy between freely rotating dipolar molecules is dependant on
The distance between them : E ~ - 1/r^6
and inversely dependant on temperature
How are non-polar molecules held together?
The electrons in a molecule can be polarised due to the influence of a polar molecule. The permanent dipole and induced dipole can then attract.
Fluctuating electron density can cause a instantaneous dipole in non-polar molecules that induce a dipole in neighbouring molecule. These induced dipoles attract
The size of a permanent dipole
Measured experimental values that determine how well a material stores electrical energy when exposed to an electric field -> property of the bulk substance
Generally more polar molecules have
Higher dielectric constants
When an ion is surrounded by polar molecules,
Many of their dipoles align in the field of the ion which lowers self-energy of ion
Why are H-bonds generally short ranged?
The formation of H bonds involves overlap of AO's
Why are the properties of water anomalous?
Each water molecule can form up to 4 H-bonds with other water molecules, 2 as acceptors 2 as donors
The Grotthuss Mechanism
Conduction of H+ through water is an order of magnitude faster than conduction of similarly charged ions. This is because H bonds can become covalent bonds (visa versa) allowing transfer of charge from one location to another without diffusion of atoms.
In TLC, the mobile phase is driven by
In TLC, affinity to stationary phase is dependent on
Strength of non-covalent interactions
Why do systems with H-bonds have high entropy?
There are many ways of arranging the H bonding network, each of the same energy
Can't form H-bonds -> reduces entropy
The hydrophobic effect
Clustering of non-polar molecules due to entropically unfavourable interaction with water
Both hydrophobic and hydrophilic e.g. lipids