Group 15

Question 1

Elements?

Answer 1

All contain lone pairs electrons so electron rich and lewis basic. Nitrogen (different to other elements in the group 2nd period) and phosphorous can form trianions N3- and P3- for which the oxidation state is -3, all of the elements form covalent compounds with the most common oxidation states +3 and +5, bismuth also forms trianions Bi3+, with the exception of the NF4+ cation nitrogen (V) is only observed in compounds containing multiple bonds such as the nitrate ion and in these cases the nitrogen atom has a formal charge of +1
Nitrogen and phosphorous are typical non metals and the metallic character of the elements increases down the group arsenic and antimony are often described as metalloids whereas bismuth is a metal

Question 2

Nitrogen?

Answer 2

N2 form main component of atmosphere, colourless odourless gas can be obtained by fractional distillation of liquid air, bond dissociation energy of N2 very high due to strong triple bond so is chemically unreactive Other factors in the low reactivity are a large HOMO LUMO gap which prevents oxidation or reduction and low polarisability which brevets attack by electrophiles or nucleophiles
Reacts with oxygen
N2 + O2 –> 2NO highly endothermic and only proceeds at high temperature. Pentanitrogen cation N5+ v shaped structure, oxide N3- is linear

Question 3

Phosphorous?

Answer 3

Occurs naturally in phosphate containing minerals such as apatites and calcium phosphate. Elemental phosphorous is obtained by heating phosphorous with coke and SiO2 in an electric furnace
2Ca3(PO4)2 + 10C + 6SiO2 –> 6CaSiO3 + 10CO + P4
phosphorus vapour collected and condense to get a white waxy solid consisting of P4 tetrahedra called white phosphorous. At temperature above 800 degrees C P4 partially dissociates into P2 molecules which contain triple bonds

Question 4

Red phosphorous?

Answer 4

If white phosphorous is heated to 300 degrees C in the absence of air it is converted to red phosphorous, has a amorphous structure consists of P4 units linked into chains red phosphorous is much less reactive than white phosphorous because of the reduces strain but it can ignited by friction

Question 5

Black phosphorous?

Answer 5

Heating phosphorous at higher pressures leads to black phosphorous there are several different forms of black phosphorous all of which have more extended structures such as the layer structure. Black phosphorous is even less reactive than red phosphorous so the reactivity of phosphorous decreases with the increasing connectivity with the structure

Question 6

Nitrogen dioxide?

Answer 6

NO in +2 oxidation state and NO2 in + 1 oxidation state, are radicals and so contain unpaired electrons all the nitrogen oxides contain multiple bonds between nitrogen and oxygen and none of the heavier group 15 elements form oxides like them with the expiation of N2O the nitrogen oxides dissolves in water to from acidic solutions. All have positive enthalpy changes of formation. Dinitrogen oxide is prepared by heating ammonium nitrate
NH4NO3 –> N2O + 2H2O
this is a comproportionation reaction in which the NH4+ ion is oxidised and the NO3- ion is reduced
NO2 exists in equilibrium with its dimer N2O4 in the +4 oxidation state

Question 7

Nitrogen monoxide?

Answer 7

Nitrogen monoxide is a radical and dimerises at low temperature, prepared by oxidation of ammonia
4NH3 + 5O2 –> 4NO + 6H2O
This is the first in the manufacture of nitric acid
2NO + O2 –> 2NO2
which is then treated with water to form HNO3
3NO2 + H2O –> HNO3 + NO

Question 8

N2O3?

Answer 8

NO2 exists almost exclusively as N2O4, when NO and NO2 are mixes at low temperatures they react to form N2O3 in + 3 oxidation state, N2O3 exists only as the solid or liquid both of which are blue. In the has phase N2O3 dissociates init NO and NO2

Question 9

N2O5?

Answer 9

N2O5 exists as covalent molecules in the gas phase in the +5 oxidation state but as the ionic compounds NO2+NO3- in the solid. It is formed by dehydrating nitric acid with phosphorous (V) oxide

Question 10

Nitrogen oxoacids and oxoanions?

Answer 10

Nitric acid HNO3 and the nitrate ion NO3-
Nitrous acid HNO2 and the nitrite ion NO2-
Hyponitrous acid H2N2O2 and hyponitrite ion N2O22-
nitric acid important in fertilisers and explosives, nitrate ion has trigonal planar structure with the three bond lengths identical with some partial double bond character. Nitrate salts such KNO3 used as oxidising agents. Most nitrates soluble in water. Nitrous acid not stable as pure compound but is used as a reagent in aqueous solution, nitrite ion is stable

Question 11

Phosphrous oxides?

Answer 11

In +3 and +5 oxidation state, phosphorous (III) oxide P4O6 and phosphorous (V) oxide P4O10.
P4 + 5O2 —> P4O10
Phosphorous pentoxide is a dehydrating agent.
Both compound have structures based on P4 tetrahedra but with bridging oxygen atoms inserted between the phosphorous atoms. For P4O10 the phosphorous atoms are also bonded to terminal oxygen atoms. Mixed oxidation state compounds such as P4O7 in which only some of the phosphorous atoms are bonded to terminal oxygen atoms are known

Question 12

Acids in water?

Answer 12

P4O6 with P in the (III) oxidation state and P4O10 are formed by burning phosphorous in air P4O6 in limited oxygen dissolves in water to give phosphonic acid H3PO3 and P4O10 in excess oxygen, dissolves in water to give phosphoric acid H3PO4. Both compounds are acidic
P4O10 + 6H2O –> 4H3PO4 (phosphoric(V) acid)
P4O6 + 6H2O –> 4H3PO3 (phosphoric acid)

Question 13

Phosphorous oxoacids?

Answer 13

Phosphoric acid - produced industrially on a large scale in fertilisers and detergents, tribasic acid with all hydrogen atoms ionisable.
disphosphoric acid
cyclo triphosphoric acid - both examples of condensed oxoacids they are derived from condensation reactions between two or more molecules of phosphoric acid
hypodisphosphoric - contain a P-P bond
phosphonic acid - dibasic
phosphinic acid - monobasic
Basicity is related to number of OH groups present

Question 14

Halides?

Answer 14

AX3 trigonal pyramidal with exception of nitrogen pentahalide of general formula AX5, not known but may be possible to isolate NF5
Dihalides of formula A2X4 containing an A-A bond between group 15 atoms have been prepared.
N2F4 has a double bond but analogous compounds are unknown for the heavier group 15 elements due to the decreasing strength of pi bonds with increasing size

Question 15

NF3?

Answer 15

Nitrogen trifluoride NF3 colourless and thermodynamically stable gas and have trigonal pyramidal geometry. Does not react with acids, water or alkalis. less basic than ammonia due to high polarity of bonds which remove electron density from the N atom
NF3 converted into dinitrogen tetrafluoride N2F4 by heating with copper
2NF3 + Cu –> N2F4 + CuF2

Question 16

N2F4?

Answer 16

N2F4 exists in equilibrium with the paramagnetic monomer NF2 which is blue
N2F4 2NF2
N2F4 reacts with AlCL3 to give dinitrogen difluoride N2F2 this has a double bond between the N atoms and exists as cis and trans isomer with the cis isomer being more stable

Question 17

NCl3?

Answer 17

More reactive than NF3 because of less polarisation, explosive liquid reacts with water to form ammonia and hypochlorous acid
NCl3 + 3H2O –> NH3 + 3HOCl

Question 18

PF3?

Answer 18

Phosphorous trihalides have trigonal pyramidal geometries, PF3 is gas under normal conditions it hydrolysis only very slowly in water through the reaction is faster in alkaline solutions
PF3 + 3OH- –> H3PO3 + 3F-
PF3 highly toxic bonds to haemoglobin

Question 19

PCl3?

Answer 19

reacts vigorously with water to form phosphoric acid and HCl
PCl3 + 3H2O –> H3PO3 + 3HCl
and with oxygen to form phosphorous oxytrichloride
2PCl3 + O2 –> 2POCl3
The reaction with PCl3 with water is very different from that of NCl3 this is due to the difference in the polarity of bonds, nitrogen has a similar electronegativity to chlorine and in NCl3 it is the chlorine atoms that are attacked by water molecules to form HOCl. In contrast chlorine is more electronegative than phosphorous so in PCl3 the phosphorous atom carrying a partial positive charge and is this that is attacked by the water

Question 20

Electronegativity on bond angles in phosphorous trihalides?

Answer 20

X-P-X angle increases from PF3 (96degrees) to PCl3 (100 degree) PBr3 (101 degrees) and PI3 (102 degrees). With the more electronegative fluorine atoms the bonding pairs are located further from the phosphorous atom so the steric effect of these electron pairs are reduced. The size of the halogen is another factor in the observed trend

Question 21

Phosphorous pentahalides?

Answer 21

Trigonal bipyramidal structures in the gas phase but their solid state structures are more complicated, PF5 is a gas expect to see 2 F signals in the NMR but only observe one due to rapid Berry pseudo rotation, solid PCl5 has an ionic structure containing tetrahedral [PCl4]+ cations and octahedral [PCl6]- anions. PBr5 and PI5 are also ionic but have structure [PBr4]+Br- and [PI4]+I- respectively. AsF5 is similar to PF5

Question 22

Fluxional molecules?

Answer 22

axial and equatorial positions in a trigonal bipyramidal are different and a molecule such as PF5 has two different P-F bond lengths, molecules are not static in solution and bonds constantly vibrating and rotating, square pyramidal structure of PF5 is only slightly higher in energy than the trigonal bypyramidal structure, as there is a low energy barrier between the two geometries they interconvert in solution, scrambles axial and equatorial positions fluxional molecules, berry pseudorotation.

Question 23

Ionic nitride compounds?

Answer 23

Containing N3- ion are only stable with small cations such as Li+ and Mg2+, nitrogen forms a range of sulphides, reactions between SCl2 and NH3 leads to the compound S4N4 tetrasulfur tetra nitride which has a structure containing an 8 membered ring, S4N4 is stabilised by a degree of pi bonding it reacts with silver to form S2N2 which also has a ring structure

Question 24

Phosphorous sulphide?

Answer 24

P4S10 has an analogous structure to P4O10, P4S3 has a structure derived from the P4 tetrahedron with three P-P bonds and three bridging sulfur atoms

Question 25

NBr3 and NI3?

Answer 25

Mor reactive, NI3 is a contact explosive, more reactive because of weak bonds, diffuse halides N-X poor overlap of orbitals

Question 26

NF5?

Answer 26

Not known N cannot achieved the +5 oxidation state with halides

Question 27

SbF5?

Answer 27

Liquid F bridges even in gas phase, cyclic tetramer increased coordination number because of increased metallic character extended structures due to atomic size of Sb linked to catenation, ring/chain formation
SbF6- the ability of SbF5 to accept X- leads to their use in superacids
SbF5 + 2HSO3F (fluorosulfonic acid) —> [SbF6O3]-H2SO3F+ SFO3H + SbF5

Question 28

BiF5?

Answer 28

Linear chains with trans bridges

Question 29

What do superacids do to methane?

Answer 29

Protonate it

CH4 + H+ —> CH5+ —> CH3+ + H2 —–> (CH3)C+ + 3H2

Question 30

NH3?

Answer 30

Hydrogen bonding raises the boiling point

Question 31

PH3 to BiH3?

Answer 31

Van der Waals forces only, weakening of the E-H bond strength, this weakening accounts for the fact PH5 does not exist
PH5 —> PH3 + H2
exothermic due to strong dihydrogen bond
Compare to PF5
PF5 —> PF3 + F2
This does not happen because the F2 bond is weak because of electron electron repulsion

Question 32

Acidity down the group?

Answer 32

Weakening due to incompatible overlap of E and H1s orbitals, decreasing bond strength leads to increasing acidity easier release of H+

Question 33

Bond angles in NH3 compared to PH3?

Answer 33

Poorer overlap of E-H orbitals down the group results in long bonds therefore less electron electron repulsion in comparison to NH3

Question 34

Why less sp mixing down group?

Answer 34

Energy difference between s and p orbitals increases down the group so less sp mixing lone pairs are held in orbitals closer to the nucleus, s type lone pairs unavailable for reactions heavy EH3 compounds are less basic (unable to abstract a proton) PH3 more likely to release H+ while NH3 more likely to abstract H +

Question 35

Second row anomalies?

Answer 35

Greater element-element bond stability
Increase catenation covalent bonding of 2 or more atoms of the same element to one another
Increase allotropy the distinct forms of an element in the same physical sate
Greater stability of multiple bonds double and triple bonds are more common in period two sing 2p orbitals, which overlap effectively. The nppi overallap diminishes with increasing atomic size, the larger more diffuse orbitals give poorer overlap and therefore weaker bonds
octet rule is general obeyed
(CF4 but not CF62-, SiF4 and SiF62- are not stable)
maximum coordination number tends to be 4
(BF3.NH3 but not BF3.2NH3)
(ALlF3.2NH3 is known and stable)
Lower reactivity of compounds (CCl4 vs SiCl4)

Question 36

Reasons for period 2 anomalies?

Answer 36

2nd row elements especially small
High electronegativity - strong bonding
Absence of extensive pi bonding in heavier elements, more diffuse orbitals so weaker overlap
Access to sigma*/low lying anti bonding MOs, this plus their small size limits oxidation number NF3 vs PF3, as well as coordination number is limited to 4 (one s and 3p orbitals)