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CH1.2 foundations of chemistry > s-block - hydrogen > Flashcards

s-block - hydrogen Flashcards

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1
Q

how does the bond dissociation of H2(g) ensure hydrogen is so stable in its diatomic elemental gaseous form?

A

both homolytic and heterolytic fission of hydrgogen are very costly
it is costly to make H+ ions due to their high charge density (lack of electrons means its just a nucleus)

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2
Q

can free H+ exist in every state?

A

no just gas state, it reacts/bonds with anything and everything

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3
Q

what drives bonding in H2?

A

we assume coulombic interactions dominate, however magnetic interactions start to dominate at very large magnetic fields, causing paramagnetic perpendicular bonding

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4
Q

give 4 methods for the synthesis of H2

A
  • steam reforming - heating of natural gas/crude oil with steam over nickel oxide catalyst
  • the shift reaction - oxidation of CO to CO2 by reacting with more steam + iron catalyst
  • electrolysis of water
  • reaction of an electropositive metal and dilute acid
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5
Q

give the equation for the steam reforming of methane

A

CH4g + H2Og –800C/Ni cat–> COg + 3H2g
this process is reversible + endothermic
methane is the main compound present in natural gas

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6
Q

give the equation for the shift reaction

A

COg + H2Og –300C/Fe cat–> CO2g + H2g
process is reversible + exothermic, can be done after steam reforming

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7
Q

give 1 advantage + 1 disadvantage of electrolysis of water to produce H2

A

it is a carbon free alternative
it is very expensive and only viable when electricity is cheaper than natural gas e.g. in HEP plants

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8
Q

give one other source of energy for the cleavage of water to form H2

A

the sun - this powers the same process photolysis of water in plants, step 1 of photosynthesis

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9
Q

give the equation for the reaction of an electropositive metal and acid to produce hydrogen

A

M(s) +2H3O+(aq) –> M2+(aq) + H2(g) + 2H2O(l)
where M = metal
H3O+ = form acid dissociation

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10
Q

give the 6 colour codes for hydrogen + what they mean

A

H2 is often colour coded depending on how its synthesised:
grey = via steam reform
blue = made in process involving CO2 capture
black/brown = from coal
red/purple = from nuclear energy
green = using renewable energy resources
yellow = made with solar power energy

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11
Q

what are the 2 largest applications of H2?

A

1- the haber process
2- the conversion of syngas to methanol

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12
Q

syngas definition

A

a mixture of H2 and CO generated by steam reforming

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13
Q

why is the haber process so important?

A

involves the fixation of atmospheric nitrogen to form NH3, which is essential for fertilisers, food and crops, necessary to sustain the worlds population

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14
Q

give 5 less common uses for H2

A
  • hydrogenation in petrochemical industry
  • desulfuration reactions
  • in synthetic organic chemistry
  • food industry to produce oils/fats
  • extraction of metals from ores
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15
Q

what is the biological hydrogen cycle?

A

the transformation of 2H+ + 2 electrons into/from H2
H2 can be used then as an oxidant or as a fuel, even in anaerobic conditions

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16
Q

how does the hydrogen cycle go?

A

it is catalysed by Ni//Fe contianing hydrogenase enzymes

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17
Q

can hydrogen be used as a fuel?

A

yes - it has a very high energy density, can generate power e.g. via hydrogen fuel cells, H2 + O2 -> H2O which produces energy

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18
Q

what is hydrogen fusion?

A

one of the most important reactions of hydrogen - at very high temperatures/pressures, hydrogen can combine with itself to form helium in a molecular fusion reaction

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19
Q

is hydrogen fusion endothermic or exothermic?

A

mass loss associated with hydrogen fusion means its highly exothermic (E = mc^2) - this powers stars

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20
Q

what oxidation states can hydrogen adopt?

A

+1 if its in compounds with more electronegative elements, usually covalent compounds
-1 if in compounds with less electronegative elements, usually ionic compounds

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21
Q

hydride definition

A

a compound of hydrogen and one other element, where H has an O.S of -1

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22
Q

what does it mean if a compound has an -ane ending or an -ide ending + exceptions?

A

-ane = it is a nearly non-polar compound
-ide = strongly δ+ character
exceptions are traditional compounds e.g. water, ammonia, methane, etc

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23
Q

how are group 1 hydrides made?

A

alkali metals combine with H2 at high temperatures
2Ms + H2 g–> 2MHs

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24
Q

what are the structures of group 1 hydrides?

A

all group 1 hydrides are largely ionic solids with NaCl structure

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25
which group 1 hydride is the most different?
LiH it has the most covalent character, this also means its predicted lattice enthalpy is >> actual lattice energy
26
how do group 1 hydrides react with water?
2MH(s) + H2O(l) -> 2MOH(aq) + H2(g) this reaction becomes more violent down group 1 as reactivity increases because of this reaction group 1 hydrides make good drying agents, but they need to be stored under inert gas/oil to ensure they cannot react with water from air
27
how can H2(g) be produces from group 1 hydrides?
- hydrogen evolution via reacting with water - electrolysis of solution made by hydride dissolving in molten alkali halide, forms H2(g) at the anode
28
what is the ionic radius of hydride ions?
the hydride ion H- is very polarisable, and easily deformed by cations with high charge densities, so the ionic radius of this ion varies a lot depending on cation
29
how are group 2 hydrides made?
group 2 metals react with H2 M + H2 --> MH2 the exception is Be, these are more complicated to make, must be done via decomposition of Be alkyl compounds pure group 2 hydrides can also be prepared similarly via thermal decomposition of MBu2
30
what are the structures of group 2 hydrides?
all except BeH2 are ionic, colourless solids with ionic structures MgH2 like LiH has large covalent character BeH2 has a covalent polymeric structure, making it very stable
31
how do group 2 hydrides react with water?
all group 2 hydrides react with water under H2 evolution MH2s + 2H2Ol --> M(OH)2aq + 2H2aq also good drying agents, e.g. CaH2 can be stored more easily in dry air making it much more convenient - exception BeH2 which is very stable so doesn't react with water
32
what group 13 elements form hydrides?
B mostly, Al also can no other elements easily form hydrides at room temperature
33
what structures do group 13 hydrides adopt?
tend to have the formula XH3 or X2H6 e.g. BH3 dimerises to form B2H6, and can form larger oligomers - this involved hydride bridges, 2 electrons shared between 2 bonds
34
how is BH3 made?
synthesised in industry via 2BF3 + 6NaH -> B2H6(g) + 6NaF - other boranes can be formed via controlled pyrolysis e.g. BnHn+4/BnHn+6
35
how does B2H6 react in air + water?
spontaneously flammable in air B2H6 + 3O2 --> 2B(OH)3 hydrolyses in water B2H6 + 6H2O --> 2B(OH)3 + 6H2 - both of these reactions produce boric acid
36
give 2 uses of B2H6
strong lewis acid + can form adducts with many other lewis acids, e.g. NaBH4, important organic reducing agent
37
what group 14 elements form hydrides?
carbon forms a limitless number of covalent and inert hydrides = hydrocarbons Si, Ge and Sn can also all form hydride gases which decompose upon heating Si and Ge can form some oligomers as well Pb hydrides are poorly characterised + quite unstable
38
describe the stability of group 14 hydrides
hydrides of group 14 become more unstable going down the group this can be seen in hydride decomposition temperatures, which become lower and lower
39
how are group 14 hydrides made?
prepared from chlorides, where X = group 14: XCl4 + LiAlH4 --> XH4 + LiCl + AlCl3
40
do group 14 compounds react acidically or basically?
group 14 marks a point where the electronegativity between group elements and H is very similar hydrides can react with strong bases acting as a source of H+, acidically
41
what are the structures of group 15 hydrides?
all group 15 elements form gases trihydrides XH3 with pyramidal structures
42
which group 15 hydride is most different?
NH3 is anomalous due to H-bonding it has a much higher m.p/b.p the H-X-H bond angle is larger due to increased strength of lone pair (the others have 90° angles) it is also much more stable
43
how stable are the group 15 hydrides?
quite unstable - all except NH3 are sensitive to decomposition
44
do group 15 hydrides behave acidically/basically?
all group 15 hydrides have lone pairs so can act as lewis bases, however only NH3 acts significantly as a base in water NH3 + H2O --> NH4+ + OH- they can also act as a source of H+ against strong bases and form XH2- - in liquid NH3 both process occur simultaneously in self ionisation
45
how are group 15 hydrides made?
NH3 is made via haber process all other hydrides made through chlorides XCl3 + 3LiAlH4 --> 4XH3 + 3LiCl + 3AlCl3
46
give 1 use of NH3
can be used as a solvent similar to water below -33C (= b.p)
47
how do group 15 hydrides react in oxygen?
all hydrides combust in oxygen NH3 requires a catalyst for this process
48
what are the structures of group 16 hydrides?
form XH2 hydrides with a bent structure all have ~ 90° bond angles except H2O, which is 104.5° due to strength of lone pair heavier group hydrides are also possible, e.g. H2O2
49
how do the physical properties of group 16 hydrides change going down the group?
m.p/b.p increasing down the group exception = H2O, as able to form hydrogen bonds so has an unusually high m.p/b.p stability with respect to the elements decreases down the group - H2O and H2S are the most stable as they have -ve enthalpies of formation, the others do not, and so are at risk of decomposition
50
do group 16 hydrides behave acidically/basically?
all have free lone pairs so can act as lewis bases, can also act as acids in water, self ionisation in the case of H2O
51
is H2O2 stable?
it is thermodynamically unstable, but kinetically stable at room temp breaks down via a disproportionation reaction to form water + O2
52
give 1 undesirable property of group 16 hydrides
except water, they tend to be toxic and have strong smells even at small concentration
53
why do NH3/H2O have such different geometries compared to other hydrides of their groups?
lone pairs, NH3 and H2O are sp3 hybridised, their bond angles fit this model more whereas heavier analogues have bond angles closer to 90°, no hybridisation
54
what are the structures of group 17 halides?
all halogens form HX halogen hydrides
55
how does m.p/b.p of group 17 hydrides change going down the group?
m.p/b.p increases down the group except for fluorine due to its hydrogen bonding ability
56
are group 17 hydrides stable?
mostly they are quite stable relative to their elements and have -ve enthalpy of formations, only HI is unstable and prone to decomposition
57
do group 17 hydrides behave acidically or basically?
all halogen halides are strong acids δ+ character of H in hydride decreases down the group alongside bond strength due to increasing bond distance resulting in poorer orbital overlap as halogens get larger, so dissociation is more favourable down the group therefore acidity increases down group
58
why is HF able to ionise?
is is a very weak acid therefore can self ionise can still strongly bind to Si or Ca, therefore can attack glass or bones H-F is the strongest halogen hydrogen bond
59
does group 18 form hydrides?
there is some computational + spectroscopic evidence that XeH2 may exist these compounds are possible and exist in the gas phase for all
60
why is hydrogen bonding possible?
as hydrogen has such a strong charge density and is so small with a non-existent ionic radius, as H+ is just a nucleus it forms very strong interactions with other elements
61
what elements can engage in hydrogen bonding?
mostly occurs with N, O or F, however it can sometimes occur with other more electronegative compounds e.g. S, P, halogens
62
when are hydrogen bonds symmetrical?
if the acceptor and the bonded atom to the donor at the same
63
give 2 physical impacts of hydrogen bondnig in compounds
- often compounds have higher m.p/b.ps - can result in an open network strucuture with large holes, this is seen in water and ice - ice is less dense than water therefore floats
64
cathrate hydrates definition
guest molecules that can fill the open spaces in the network structure
65
how do hydrogen isotopes differ from normal hydrogen?
isotopes of hydrogen differ only in mass, there is little change in electronic structure, however there is an indirect effect on electronic energy when bonded to heavier atoms, considering vibrations - this difference is only chemically significant for change in mass between hydrogen atoms

CH1.2 foundations of chemistry (41 decks)

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