S-Block Flashcards
(30 cards)
describe the trends of ionisation energies in group 1
in group 1 the first ionisation energies are all very low making them very reactive, decrease in IE down the group due to increase in atomic radius
what is the trends in chemical properties in group 1
Li+ has high degree of covalent character in its bonding due to its high charge density, the heavier elements have ionic character.
lithium’s behaviour is analogous to Mg- diagonal relationships
Why are group 1 elements strong reducing agents?
can loose first electron very easily so they can be oxidised easily
Why are group 1 elements poor complexing agents?
low charge of +1 with a large atomic radius so enthalpy of hydration (complexation) is low, weak bonds formed
does group 1 elements form nitrides?
only lithium forms the nitride from the element and N2 gas
do group 1 elements form hydrides?
all from ionic hydrides when reacted with hydrogen, these salts react vigorously with water. these can be used as reagents, non-nucleophilic bases and reductants
explain the properties of Group 1 halides
colourless crystalline solids, enthalpies of formation are large and negative. with fluorides enthalpies of formation become less negative down group, other halide enthalpies of formation become more negative down group. all are water soluble (except LiF) to give ionic solutions
why cant LiF dissolve in water?
the lattice enthalpy is too large as Li is the smallest
bigger UL, the more stable
How do group 1 metals react with oxygen? which arent so stable?
all group 1 metals react vigorously with oxygen but the products differ
can form oxides, peroxides or superoxides
all can be prepared for all g1 metals but LiO2 and Li2O2 are unstable and decompose
how can oxides be prepared? how does stability change with oxides/peroxides?
by heating the metal with limited O2 or by thermal decomposition of peroxide/superoxide
stability of peroxides and superoxides wrt thermal decomposition decreases down group
How ae ozonides formed? what is special about them?
formed by buring M with ozone. very unstable and will explode violently, KO3 contains paramagnetic O3- ion
describe the coordination chemistry of group 1
(are they acids? what ligands form?)
group 1 cations (Li+ to K+) are hard lewis acids so form with hard donors (O/N)
often H2O ligands coordinate cation often forming octahedral complexes but Li forms tetrahedral
What are chelating ligands?
ligands that bind tightly to
metal ion by forming multiple bonds as they contain 2 or more donor atoms
what are crown ethers
type of ligand which forms very stable complexes with alkali metals, theyre cyclic polyethers,
very stable due to the fact the cavities in middle fit well with the atomic radius of alkaline metals.
cavity size is not fixed as some are flexible
what is the macrocyclic effect?
combination of an entropic effect as seen in the chelate effect with an additional energetic contribution from preorganised nature of ligands
What is the chelate effect?
where chelating ligands form more stable complexes with metal ions comapred to non-chelating ligands. the key driving force behind the chelate effect is entropy
what are cryptands?
special class of macrocyclic ligands that form more stronger complexes with alkali metals than crown ethers.
cryptands show peak selectivity as their more rigid so cannot constrict sufficiently to bind to cations that are too small for cavity (also cannot expand to fit larger cations into cavity)
What are alkalides?
chemical compounds where the alkalide metals are anions with -1 charge. this makes the M- radius a lot larger
explain the organometallic chemistry of group 1 metals (soluble/bonding character)
the alkali metals form a number of organometallic compounds, often unstable to water, the bonding between M-C is ionic and increases down the group as metals become larger and less polarising
how can we determine the C NMR for organometallic compounds? give an example involving a dimer
we can use the 2ni+1 rule, for example with a dimer, n=2 so 2(2)(1)+1=5 so youd expect to see 5 peaks
Why are nBuLi and tBuLi important in lithium/halogen exchange reactions?
nBuLi + R-X = nBuX + R-Li
two equivalents of nBuLi needed, one for exchange of Li and X ad the second one reacts with nBuX to give isobutene, isobutane and Li-X
what happens to the ionic character of ionic organometallics down group 1
as you go down the group, more ionic character the compounds have so becomes more like a salt
why do group 2 metals have a higher melting point than group 1?
higher m.p that group 1 as stronger metallic bonds formed with group 2 elements have they have more electrons available for bonding
explain the trend for ionisation energies down group 2
IE increases down group and so elements become more reactive/ electropositive
