FC10: Intermolecular forces Flashcards
What is meant by electro negativity?
A measure of the relative ability for an atom to attract the bonding pair(s) of electrons in a covalent bond.
Trend in electro negativity across a period
Increases because nuclear charge increases and so greater number of protons and so greater attraction to the binding pair of electrons
Trend in electro negativity down a group
Electronegativity decreases due to greater number of shells and so greater shielding effect between nucleus and the bonding pair of electrons and so less attraction to the bonding pair of electrons. GREATER SHIELDING EFFECT OUTWEIGHS GREATER NUCLEAR CHARGE
Permanent dipole
2 atoms with significantly different electronegativity values are covalently bonded together, the more electronegative atom would attract the bonding pair of electrons towards it. Forms uneven distribution of electrons along the covalent bond, called a permanent dipole
Polar?
Molecules with an overall dipole is called polar
how do you phrase whether a molecule is polar or not regarding its shape and dipoles
There are x dipoles within the y molecule, y is highly/is not completely symmetrical therefore the dipoles within the molecule cancel/do not cancel each other out completely and the molecule has no/has an overall dipole. Therefore y is non polar/polar.
Permanent dipole-dipole forces
Forces of attraction between opposite partial charges on adjacent molecules with permanent dipoles
What are the two factors on measuring electronegativity
Nuclear charge and number of electron shells
What is a dipole?
The partial negative and partial positive charges along a covalent bond
what is an intermolecular force
forces between molecules
what is an intramolecular force
forces within molecules
how does the size of a dipole along a molecule affect the strength of the pd-d forces between molecules
the bigger the dipoles are, the stronger the pd-d forces
how do london forces/induced dipole-dipole forces form
use iodine as an example
electrons in an I2 molecule are constantly moving around the molecule, at any one time there may be an uneven distribution of electrons along the molecule, this forms a temporary/instantaneous dipole. The partial negative charge of this temporary dipole repels the electrons on an adjacent iodine molecule, causing an induced dipole. Opposite partial charges on these adjacent molecules attract to form an induced dipole-dipole forces
what are the two factors that affect the strength of id-d forces
number of electrons in the molecule, greater the number = stronger id-d forces
surface area for contact between molecules, more branching -> smaller surface area for contact between its molecules.
And so the London forces between the molecules are weaker than those between molecules with less branching
What is hydrogen bonding and compare its strength to london and pd-d forces for molecules of similar size
Forces that can only form between H-F, H-O, H-N. AND where the F,O and N have lone pairs available.
Are stronger than London and pd-d forces for molecules of similar size
STATE the anomalous properties of water due to Hydrogen bonding
1- Relatively high melting point and boiling point for the size of molecule
2 - Solid water (ice) has a lower density than liquid water
Explain why water has a higher melting and boiling point for the size of its molecule
Water has a higher than expected bpt/mpt for the small size of its molecule, this is due to hydrogen bonding between water molecules, H-bonding is stronger than pd-d forces and id-d forces for molecules of similar size so more energy is needed to separate water molecules
Explain why water as ice has a lower density than water
Water molecules are on average further apart when water is solid due to the fact that hydrogen bonding is directional and the maximum number of H-bonds are formed when water freezes to give an open lattice structure with greater spacing between water molecules
What does miscible mean
same as mixable, but of liquids
what must happen between a substance and a solvent for the substance to be soluble and give an example
the substance’s particles must form strong interactions with those of the solvent, for example, ammonia is highly soluble in water as it can form h-bonding with water
What do intermolecular forces determine
melting and boiling points
what do intramolecular forces determine
bond enthalpy, reactivity
