2 - Bonding and structure Flashcards
Draw the structure of the dimer of aluminium chloride:
Draw a hydroxide ion:
Draw CO:
When is the octet rule broken?
-hydrogen only needs 2 valence electrons
-elements like boron and aluminium only need 6
-other elements past silicon can have expanded octets, as the extra electrons can occupy d-orbitals
How do you account for lone pairs when finding bond angles?
-figure out the shape as if it were a normal bonding area
-subtract 2.5 from the expected bond angle
eg on NH₃ it is 3+1, so treat it like it were 4 bonding pairs (109.5) and subtract 2.5 to get the actual angle (107)
Explain the shape and bond angle of NF₃:
-3+1 pairs, trig. pyramidal
-lone pairs repel more than bond pairs
-separates to 107°, the point of minimum repulsion (maximum separation)
Why can you get phosphorus pentachloride, but not nitrogen pentachloride?
-phosphorus can have an expanded octet (since it has its 3d orbital available)
-nitrogen can only have a maximum of 8 (since there are no 2d orbitals)
How are covalent and ionic bonds related?
-opposite ends of a spectrum of bonds
-non-polar covalent bonds occur when the EN of both atoms are the same
-as the difference in EN increases, the covalent bond becomes more polar
-once the difference even greater, an ionic bond forms since the electrons are so greatly attracted to the more EN atom
Describe the formation of London forces:
aka Van der Waals, dispersion forces
-temporary asymmetrical distribution of electrons around a molecule form a temporary dipole
-these produce induced dipoles in other molecules, forming an electrostatic force of attraction between them
Describe the bonding found between hydrogen chloride molecules:
-weak London forces acting between molecules (since all have it)
-permanent dipole-dipole interactions (since H-Cl is a polar bond, and the molecule is asymmetrical)
Why is the bond angle of hydrogen bonds 180° about the central hydrogen?
-2 bonds form around the central H
-forms a straight line due to it being the point of minimum repulsion (maximum separation)
Why does 2,2-dimethylpropane have a lower BPt than its isomer, pentane?
-2,2 DMP is branched
-results in fewer London forces forming between molecules due to it having a lower surface area/points of contact
ie they can’t pack as closely together
Explain the difference in volatility in alcohols and alkanes of similar length:
-alcohols have a polar -OH group, meaning they can form hydrogen bonds which are stronger than just having London forces like in alkanes
-requires more energy to break apart
-this results in alcohols having a lower volatility (+ higher BPt) than alkanes of similar length
Describe the trend in boiling points of the hydrogen halides:
-HCl has the lowest (small molecule, smallest London forces)
-increases up to HI
-HF has the highest (because of its hydrogen bonding)
What is an exception to the “like dissolves like” rule?
-halogenoalkanes
-they are polar, but cannot dissolve in water
Why are methanol and water soluble in all proportions?
Draw a diagram
-methanol can form hydrogen bonds with water
-the strength of the IM forces between methanol and water is roughly the same as those between methanol and water molecules separately
2 lone pairs on the oxygen, and 180° OHO bond angle
