LC CHEM - electronegatvity and intermolecular forces!

Define electronegativity

the relative attraction an atom has for a shared pair of electrons in a covalent bond

Pure covalent bond (Non-polar covalent)

two atoms share a pair of electrons EQUALLY

Polar covalent bond

two atoms share a pair of electrons unequally

types of covalent bonds

polar, non polar

Why are electrons shared unequally in polar covalent bonding

• the difference in electronegativity values is significant

• the shared pair of electrons are pulled closer to the more electronegative atom

Why are electrons shared equally in polar covalent bonding

• The difference in electronegativity values is 0, or so small

• the shared pair of electrons are not pulled significantly closer to either atom and are shared equally

What is the significance of a molecule having polar covalent bonds

• shared pair of electrons are pulled closer to the more electronegative atom

  • more electronegative atom has a negative dipole

  • less electronegative atom has a positive dipole

what electronegativity difference do pure covalent / non polar bonds have

• no or tiny difference

• between 0 - 0.39

what electronegativity difference do polar covalent bonds have

• significant difference

• 0.4 - 1.69

what electronegativity difference do ionic bonds have

• VERY large difference

• greater than 1.7

If a question uses the word “molecule” is the compound covalent, or ionic?

covalent !

Exceptions to the electronegativity stuff

• Lithium hydride (LiH)

• Sodium hydride (NaH)

• Potassium hydride (KH)

• Calcium hydride (CaH₂)

• Hydrogen flouride (HF)

• Boron Trifluoride (BF₃)

What is a polar molecule?

• molecule with an uneven distribution of electrical charge

• one end of the molecule is partially positive, and the other end is partially negative

(has a positive and negative dipole)

When is a molecule polar ?

• It has polar covalent bonds (EN difference is 0.4 - 1.69)

• the molecule is not symmetrical (v shaoed or pyramidal)

What is a non polar molecule

• molecule with an even distribution of electrical charge

• no overall positive of negative end

when is a molecule non polar

• if all bonds are pure covalent (ED DIFFERENCE 0.0 - 0.39)

• or if the molecule is symmetrical (linear, trigonal planer, tetrahedral)

How can a molecule have polar covalent bonding but still be non polar molecules

• If a molecule is symmetrical, the negative and positive dipole coincide with eachother and cancel out - there is no overall positive or negative ned

• if a molecule is symmetrical, it is completely non polar regardless of the type of bonding it has

Symmetrical shapes of molecules

• linear

• trig planar

• tetrahedral

Non symmetrical shapes of molecules

• pyramidal

• v shaped

shape of a molecule with two atoms

linear

shape of a molecule with three atoms and no lone pairs

linear

shape of a molecule with three atoms and lone pairs

v shaped

shape of a molecule with four atoms and no lone pairs

trigonal planer

shape of a molecule with four atoms and lone pairs

pyramidal

shape of a molecule with five atoms

tetrahedral

intramolecular bonds

type of bond INSIDE the molecule or compound (ionic, covalent)

intermolecular forces

forces of attraction existing BETWEEN one molecule and another (eg van der waals forces)

What are the vanderwaals forces

intermolecular forces of attractions that exist between all molecules

list the types of van der waals forces in order of weakest to strongest

london dispersion → dipole dipole force → ion dipole force

two types of dipole dipole forces

normal

hydrogen bonding

How are london dispersian forces caused?

• all molecules can form temporary dipoles at each end due to the movement of electrons

• when attractions form between the negative dipole of one molecule and the positive dipole of another molecule

Why are london dispersian forces so weak?

because the attraction is between temporary dipoles that only exist for an instant

what types of molecules do london dispersion forces come between

All types! (polar / non polar)

what type of molecules do dipole dipole forces exist between

polar molecules !

How are dipole dipole forces caused ?

• polar molecules have permanent dipoles at each end

• attractions that form between the permanent negative dipole of one molecule and the positive end of another molecule are dipole dipole forces

Why are dipole dipole forces stronger than london dispersion forces

because the attraction between the dipoles is permanent

what is hydrogen bonding

• type of dipole dipole force

• occurs between a POSITIVE HYDROGEN DIPOLE in one molecule

• and a NEGATIVE NITROGEN, OXYGEN OR FLUORINE in another

why does hydrogen bonding occur between molecules with negative nitrogen, oxygen and fluorine dipoles

• as they are highly electronegative elements

• when they bond to hydrogen

• it causes strong polarity in the bond

are hydrogen bonds strong or weak in comparison to normal dipole dipole forces

MUCH STRONGER

three effects of hydrogen bonding

explains

• why the three hydrides h2o, hf and nh₃ have much higher boiling points than other hydrides

• explains hwy h2o has a higher boiling point than other molecules of similar size/molecular mass

• explains surface tension on water, the water molecules on the surface are strongly attracted to eachother by hydrogen bonding

what are ion dipole forces

force that exists between an ion and polar molecule (permanant dipole)

if the molecules are non polar + non polar what is the most dominant intermolecular forces present ?

london dispersion

if the molecules are polar and polar what is the most dominant intermolecular forces present ?

dipole dipole forces

if the molecules are polar molecules with hydrogen bonded to nitrogen, oxygen or fluorine what is the most dominant intermolecular forces present ?

hydrogen bonds

if the molecules are an ion and a polar molecule what is the most dominant intermolecular forces present ?

ion dipole forces

if you are comparing boiling points of substances, what are your steps ?

1. type of msot dominant force formed (is it polar or non polar, which van der waals force is it)

2. check its MR (larger molecules have more/stronger london dispersion forces)

3. if the molecule is polar, how polar is the bond? (the more polar thee bond, the stronger the dipole dipole forces/hydrogen bonding will be)

What elements exist in twos rather than ones? + whats the acronym

hydrogen, nitrogen, flourine, oxygen, iodine, chlorine, berillium

HAVE NO FEAR OF ICE COLD BEER

what is needed, force wise, for a substance to dissolve in another

intermolecular forces need to form that are stronger/equal to the ones already present

When might ionic compounds dissolve

polar solvent = will dissolve

non polar solvent = will not dissolve

• in water (polar solvents)

  • ionic compounds are very soluble in polar solvents/water

• in cyclohexane (non polar solvent)

  • ionic compounds WILL NOT DISSOLVE in polar solvents

why do ionic compounds dissolve in water

• ion dipole forces form between the partially negative oxygen in polar water, and the postivie ion in the ionic compound

• ion dipole forces form between the partially positive hydrogen in polar water and the negative ion in the ionic compound

these ion dipole forces pull apart the crystal lattice structure of the ionic compound

why dont ionic compounds dissolve in water

• ion dipole forces cant form, the lattice structure cannot be pulled apart

• (due to no permanent dipoles being present)

what is the rule for the soluability of covalent compounds

LIKE DISSOLVES LIKE

• polar dissolves polar

• non polar dissolves non polar