Group 17

Question 1

Halogens?

Answer 1

Atoms can readily gain an electron to form halide ions X- or they can from covalent bonds. The electron gain enthalpies are all positive the reactions are highly exothermic all of the halogens are non metals
Exist as X2 molecules, all oxidising agents oxidising power decreases down group, I- can even be used as a reducing agent.
Fluorine is most oxidising all elements in periodic table all components have fluorine in -1 oxidation state. Other halogens all show -1 oxidation state but +1, +3, +5 and +7 also occur

Question 2

Fluorine?

Answer 2

Gas, naturally occurs in minerals such as fluorspar CaF2. Not possible to oxidise F- to F2 using a chemical oxidising agent so fluorine is obtained from its ores by electrolysis of HF in molten KF, fluorine gas produced at carbon anode
oxidation at carbon anode 2F- —> F2 + 2e-
reduction at steel cathode 2H+ + 2e- —> H2
Main use of fluorine in production of uranium hexafluorine UF6 and SF6
F2 is very reactive and reactions generally highly exothermic, values of the enthalpy changes of reaction are negative die to the low F-F bond dissociation enthalpy and high X-F bond enthalpies or high MFn lattice enthalpies

Question 3

Chlorine?

Answer 3

Gas, toxis, obtained from sodium chloride by electrolysis of aqueous sodium chloride in the chloroalkali process, used to prepare organochlorine compound such as chloroethane used to make PVC, also used to prepare inorganic chlorides. Chlorine usually prepared from oxidation of hydrochloric acid by manganese (IV) oxide
MnO2 + 4HCl —> MnCl2 + 2H2O+ Cl2

Question 4

Bromine and iodine?

Answer 4

Bromine liquid, iodine solid that readily sublimes, both elements obtained from sea water where the anions are present in high concentrations, sea water is concentrated then treated with chlorine gas explaining the fact that chorine is more oxidising than either bromine or iodine
2Br- + Cl2 —> Br2 + 2Cl-

Question 5

Astatine?

Answer 5

Occurs in trace amounts of uranium and thorium ores

Question 6

Bond dissociation enthalpies?

Answer 6

Bond dissociation enthalpies for the X2 molecules decrease down the group from chlorine to iodine because the overlap between the valence orbitals decreases as the atoms get larger. F2 is an expiation to this trends and it has a much weaker bond dissociation enthalpy than might be expected this is a consequence of the high electron electron repulsion caused by the short F-F bond

Question 7

Halogen oxides?

Answer 7

OX2 and O2X2, here higher oxidation state oxides which exist for chlorine bromine and iodine, since fluorine is more electronegative than oxygen higher oxidation state oxides are unknown for fluorine

Question 8

ClO2 and Cl2O6?

Answer 8

Chlorine dioxide ClO2 is yellow paramagnetic gas and the molecules have bent geometry, doesn’t dimerise as unpaired electron is delocalised over the whole molecules, used industrially to bleach paper, reacts with ozone to form dichloride hexaoxide Cl2O6 dark red liquid that explodes
2ClO2 + 2O3 —> Cl2O6 + 2O2
Cl2O6 reacts with water to from a mixture of oxyacids
Cl2O6 + H2O —> HClO3 + HClO4
In the solid state Cl2O6 exists in the ionic compounds ClO2+ClO4-

Question 9

Cl2O7?

Answer 9

Dichloride heptoxide is a colourless oily liquid both shock sensitive and explosive, formed by dehydrating perchloric acid HClO4 using concentrated phosphoric acid at -10 degrees. The reverse reaction is also possible and Cl2O7 reacts with water to regenerate perchloric acid

Question 10

Bromine oxides?

Answer 10

Less stable than chlorine oxides BrO2 is unstable above - 40 degrees

Question 11

Iodine oxides?

Answer 11

Forms most stable halogen oxides, I2O5 is made by heating ionic acid HIO3 to 200 degrees and is a solid under normal conditions reacts with water to reform HIO3
I2O5 + H2O —> 2HIO3

Question 12

Fluorine oxoacids?

Answer 12

Most acids cannot be isolated pure but can be used in aqueous solution. Fluorine forms only one oxyacids HOF as it is unable to from positive oxidation states HOF is formed form the fluorination of ice at low temperature
F2 + H2O —> HF + HOF
solid HOF decomposes spontaneously to HF and O2 at room temperature

Question 13

Chlorine oxoacids - hypochlorous acid?

Answer 13

Chlorine dissolves in water to form hypochlorous acid HOCl
Cl2 + 2H2O HOCl + H3O+ + Cl-
When chlorine is dissolved in alkaline solution HOCl is deprotonated and the equilibrium above is pushed to the right hand side, this allows compound containing the hypochlorite anion OCl- to be isolated
Cl2 + 2OH- OCl- + Cl- + H2O
Bleach consists of an alkaline solution of sodium hypochlorite NaOCl, hypochlorite is thermodynamically unstable with respect to disproportionation to give chlorate ions and chloride ion but the reaction only occurs very slowly
3OCl- —> ClO3 + 2Cl-
The stability of the hypochlorous acids HOX decreases down the group

Question 14

Chlorine oxoacids - chlorate ion?

Answer 14

ClO3- is formed in a disproportionation reactions when chlorine gas is dissolved in hot alkaline solution
3Cl2 + 6OH- —> 5Cl- + ClO3- + 3H2O
chlorate compounds are strong oxidising agents, chlorate ions can be oxidised o perchlorate in ClO4- electrochemically

Question 15

Chlorine oxoacids - perchlorate ion?

Answer 15

Commonly used as anions in synthetic chemistry as they interact very poorly with the metal, perchlorates are strong oxidising agents but the reactions are often very slow, perchlorate compounds can be explosive

Question 16

Bromine oxoacid?

Answer 16

Bromic acid is most stable HBrO3

Question 17

Iodine oxoacid?

Answer 17

Although periodate ions IO4- are known iodine (VII) tends to form ions with higher coronation number such as I2O94-

Question 18

p block fluorine halides?

Answer 18

Fluorine reacts with all of the elements except helium, neon and argon to form fluorides.

Question 19

p block chlorine halides?

Answer 19

Less reactive than fluorine but still reacts with all of the elements with the exception of carbon, nitrogen and oxygen and the group 18 elements

Question 20

p block halides?

Answer 20

Compounds of group 17 elements with metals typically form extended structures with high ionic character when the metals are in low oxidation states however in high oxidation states many metals form covalently bonded molecular halide. for examples SnCl4, al2Br6, and UF6 are all molecular solids

Question 21

Covalent compounds?

Answer 21

For compounds with non metals contain covalent bonding, fluorine is the most oxidising of the halogens and it stabilises high oxidation states, for the covalent compounds the bond enthalpies decrease down the group due to the poorer overlap of the atomic orbitals

Question 22

Ionic compounds?

Answer 22

Sizes of the halogens increase down the group as a consequence the lattice enthalpies are highest for the fluorides so the fluorides tend to be less soluble than the other halides

Question 23

Stability of p block halides?

Answer 23

For a p block element the stability of the halides generally decreases with increasing size of the halogens so the fluoride is more stable than the chloride which in turn is more stable than the bromide, you can use an enthalpy cycle of AXn where A is p block element and X is halogen to find the enthalpy of formation for different halogens
The enthalpy change of formation of atom A does not depend on the halogen, the enthalpy change of formation of the halogen atom is notably lower for fluorine than for other halogens due to the weakness of the
F-F bond. The A-X bond enthalpies decrease with the increasing size of the halogens due to both decreased orbital overlap and the decreasing ionic character of the bonds, enthalpy change of formation gets less exothermic with increasing halogen size

Question 24

Stabilities and bond enthalpies?

Answer 24

In addition to forming more stable compounds fluorine stabilises higher oxidation states than the other halogens, the A-X bonds enthalpies generally decrease with the increasing oxidation state of A, the decrease bond enthalpy with increasing oxidation state is much greater for the P-Cl bond than the P-F bond
The stability of higher oxidation fluorides means that fluorides in lower oxidation state are often unstable to disproportionation
Eg S2F2 disproportionates to form sulfur and SF4
S2F2 —> 3S8 + 8SF4

Question 25

Interhalogen compounds?

Answer 25

Compounds formed between the halogen elements are known as interhalogens. inttrhaofens of the general formula XY, XY3, XY5 and XY7
Interhalogens are prepared by direct reactions between the elements with the products depending mainly on stoichiometry
Cl2 + 3F2 —> 2ClF3
Cl2 + SF2 —> 2ClF5
In addition to the natural inter halogens compounds man interhalogen cations and anions are known

Question 26

ClF3?

Answer 26

Chlorine trifluoride is extremely reactive and will even spontaneously ignite asbestos, it converts most oxides and chlorides to fluorides

Question 27

Interhalogen cations?

Answer 27

Formed by removal of a fluoride ion from a neutral inter halogen compound by reacting it with a fluoride ion acceptor such as AsF5
ClF3 + AsF5 —> ClF2+AsF6-
Formed by reaction of a neutral interhalogen compound with a fluoride ion acceptor like AsF5. The reaction involves transfer of a fluoride ion from the interhalogen molecule to AsF5 so the starting material to form IF4+ is IF5
IF5 + AsF5 —> IF4+AsF6-

Question 28

Interhalogen anions?

Answer 28

Formed by addition of a fluoride ion

ClF3 + CsF —-> Cs+ClF4-

Question 29

Polyiodide anions?

Answer 29

Related to inter halogen anions and the most common polyiodide is the linear anions I3- iodides is not very soluble in water but in the presence of iodides it forms the more soluble I3- ion
I2 + I- I3-
Larger polyiodide ions such I5- and I9- are also known

Question 30

Bonds?

Answer 30

F-F bonds is small so lots of lone pair lone pair repulsion makes F-F weaker than anticipated, Cl is bigger so Cl-Cl distance is larger as ortolans are more diffuse also the repulsion is lessened bond strength is at a maximum for the group, as we get to bigger elements the repulsions between lone pairs drops off as the length of the bed become the important thing

Question 31

Hydrogen halides?

Answer 31

As we go down the group acidity increases as the bond strength decreases, [HF2]-

Question 32

HF?

Answer 32

Expect HF to hydrogen bond like O and N in H2O and NH3, indeed F does form hydrogen bonds, HF H bonding is unidirectional, in water H bonding goes in two directions, H bonding is retained even upon eating HF, boiling point is lowered as not all of H bonding has to be removed strong H bonding might expect boiling point of HF itself to be comparable or great than that of H2O, however HF boils at 19 degrees compared to 100 for H2O since in HF bonding is directional some H bonding is retained even upon heating to 60 degrees, not all of the interactions need to be broken to boil HF

Question 33

Weak strength of HF as an acid?

Answer 33

H-F [H3O+.F-] H3O+ + F-
lone pair held together in complex by H bonding ion pair favourable in its formation
HF bond is strong lower dissociation, trapped in the ion pair stage, [HF] lowered by bifluoride formation
weak strength of aqueous HF, contrast that to HCl, weaker bond for a weaker ion pair formation of HCl2-, less favourable than HF2-