Intermolecular Bonding Questions Flashcards
Explain the term electronegativity.
The ability of an atom to attract the bonding electrons towards itself in a covalent bond.
Why are water molecules polar overall?
They contain two polar O-H bonds (as oxygen and hydrogen have a large difference in electronegativity) and two lone pairs on the central O atom which creates an unsymmetrical bent V shape molecule so the two dipoles do not cancel
Ice has a higher melting point than ammonia. Why?
The O-H bond in water has a stronger dipole than in the N-H bond in ammonia as O is more electronegative than N which creates stronger hydrogen bond interactions between neighbouring water molecules, which require more energy to break, raising it’s melting point relative to ammonia
Describe the intermolecular bonding in BF3
A temporary dipole is formed in one molecule due to the instantaneous uneven distribution of electrons which induces a dipole in a neighbouring BF3 molecule. A dipole-dipole attraction is formed between the 2 BF3 molecules, known as a Van der Waals’ force.
State and explain the trend in electronegativity down Group 6.
Electronegativity decreases down the group because the bonding pair of electrons is increasingly distant from the attraction of the nucleus. The outer electrons involved in bonding are further from the nucleus (atomic radius increases) and more shielded by completed shells which outweighs the increase in nuclear charge
Is sulphur dioxide polar? Explain.
It is polar as it is unsymmetrical and the two dipoles in the S=O bonds do not cancel out
The above graph shows the trends in boiling points for the Group 4 and Group 6 hydrides, mainly determined by the Van der Waals’ forces. Continue the trend for the Group 6 top member oxygen (as H2O). Explain the position you choose.
H2O has a higher boiling point than the trend in Van der Waals’ forces alone would predict as it also has the strongest intermolecular force holding its molecules together – hydrogen bonds.
State and explain two properties of ice that are a direct result of hydrogen bonding.
Higher than expected boiling/melting point
A significant amount of energy is needed to overcome the relatively strong hydrogen bonds between the H2O molecules
Ice is less dense than water
The hydrogen bonds space the H2O molecules further apart in ice than in water
NH3 is a polar molecule whereas BF3 is a non-polar molecule. Why?
The NH3 molecules are unsymmetrical - dipole across each N-H bond do not cancel out
BF3 molecules are symmetrical - dipole across each B-F bond do cancel out
Nitrogen can also form a fluoride, NF3, which has a permanent dipole. Explain why NF3 has a permanent dipole.
It is a non symmetrical molecule to the dipoles do not cancel out
The halogen molecules F2, Cl2 and Br2 all contain Van der Waals’ forces. What is the trend in boiling point down group 7? Explain.
Tredn - boiling point increases down group 7
Molcules become larger so more shells so more electrons
London forces between the molecules become stronger
Need more energy to over the london forces in order to separate the molecule to allow boiling
