Hydrides Flashcards
What are ionic hydrides and how do they react? Give some examples
Ionic hydrides are hydrides formed with the most electropositive elements and they react as H- (powerful Bronstead base). Examples: LiH, NaH, CaH
Covalent hydrides can be split into three groups, what are these three groups?
Electron precise, electron deficient and electron rich
How do covalent hydrides of electro+ve elements react?
They react as H- which is a powerful Bronstead base
How do covalent hydrides of electro-ve elements react?
They react as H+ which is a very powerful Bronstead acid
How do the E-H bond enthalpies change down the group?
As you go down the group the E-H bond enthalpies decrease. This is because of the difference in orbital size between the element and hydrogen.
How do the E-H bond enthalpies change with electronegativity of E
The more electronegative E is, the stronger the bond (higher bond enthalpy) this is because the bond has a higher ionic contribution
What is special about the bonding in Diborane?
It is electron deficient, it has 8xB-H bonds but only 12 valence electrons, therefore it has 3-centre-2-electron bonding between the B-H-B bond
What type of hydrides do the group 13 elements form?
Electron deficient hydrides
What type of hydrides do the group 14 elements form?
Electron precise hydrides
Describe and explain the difference in reactivity between CH4 and SiH4
SiH4 (silane) is much more reactive than methane. It reacts explosively with air and reacts spontaneously with water unlike methane. This is due to a combination of factors. Silicon can be hypervalent - can interact w/ lone pair on O. The SiH bond is less polar than the CH bond and it is a weaker bond (larger atom,lower down the group)
What type of hydrides do the group 15 elements form?
Electron rich hydrides
Describe the boiling point trends in the p block hydrides
Anomalously high for NH3, HF and H20 because of hydrogen bonding the steady increase in Bpt. down the group due to stronger vdw between bigger atoms
Why do carboxylic acids form dimers?
Because they form hydrogen bonds with each other
What is methane hydrate and how does it form?
Methane hydrate forms when H2O crystallises around methane. It is stable at low temp and high pressures (can cause blockages in gas pipelines). The structure is 20 x H20 molecules per CH4 molecule, arranged in a pentagonal dodecahedron
Describe the evidence that points to hydrogen bonding
- higher melting point of H20 than e.g. H2S
- Higher than expected molecular weight (dimers of carboxylic acids)
- Broadening of OH stretch (and lowering of frequency) in IR spectrum