group 7 - halogens

Question 1

what’s the trend in bpt down group 7? why?

Answer 1

• increases down the group
• as size of atom increases as more occupied electron shells —> stronger van der waals forces of attraction between molecules, take more energy to break.

Question 2

what’s the trend in electronegativity down group 7? why?

Answer 2

• decreases
• as more occupied electron shells —> greater atomic radius + outer electrons further from positive charge of nucleus —> lower force of attraction between nucleus and electron pair in covalent bond .

Question 3

why does reactivity of halogens decrease?

Answer 3

• outer electrons further away from nucleus so attraction between nucleus and outer electron shell decreases .
• more electron shielding, further decreases the nuclear charge .
• harder to grab hold of additional electron and place in outermost shell, resulting in reduction in reactivity from fluorine to iodine

Question 4

what do

Question 5

how can you work out the order of halogen reactivity ?

Answer 5

• carrying out a displacement reaction.
• any element period 7: able to displace any halogen that is beneath it in periodic table.
• 2Kl + Br2 —> 2KBr + I2
• 2I- + Br2 —> 2Br- + I2

Question 6

what are the colours of each halogen that will appear as in organic solvent ?

Answer 6

• chlorine = pale yellow-green
• Bromine = orange
• Iodine = purple .

Question 7

what do you use to test halide ions?

Answer 7

• Acidified AgNO3

Question 8

why do you need to add HNO3? why not HCl?

Answer 8

• to remove CO3^2-
• adding HCl would add Cl- ions, giving a false positive result

Question 9

why do you need to add nitric acid and silver nitrate ?

Answer 9

• nitric acid removed any impurities that might insure the result
• silver nitrate = reacts with any halide ions to form coloured silver halide precipitates .

Question 10

result and equation for Cl- test?

Answer 10

• white ppt
• Ag+ (aq) + Cl- (aq) —> AgCl (s)

Question 11

what’s the result and equation for the test for Br?

Answer 11

• cream ppt
• Ag+(aq) + Br- (aq) —> AgBr (s)

Question 12

what’s the result and equation for the test for I-?

Answer 12

• yellow ppt
• Ag+ (aq) + I- (aq) —> AgI (s)

Question 13

what are the three precipitates that have different solubilities in aqueous ammonia?

Answer 13

1) silver chloride dissolves in dilute ammonia solution (and conc)

—> AgCl (s) + 2NH3 (aq) —> (Ag(NH3)2)+ (aq) + Cl-

2) silver bromide dissolves in concentrated ammonia solution

3)silver iodide dissolves in dilute or concentrated ammonia solution (insoluble)

Question 14

what is the trend in oxidising ability down the group? why?

Answer 14

• decreases down the group (Cl best, I worst)
• as , Cl has fewest occupied electron shells, greatest force of atttaction between outer electrons and nucleus
• easiest to gain electrons and be reduced = body oxidising agent .

Question 15

what does disproportionation mean?

Answer 15

• when the same element is simultaneously oxidised and reduced .

(e.g = chlorine gas mixed with cold, dilute aqueous sodium hydroxide . = sodium hypochlorite (NaClO) / bleach formed with sodium and chloride and water )

Question 16

what’s the equation for making bleach?

Answer 16

• Cl2 (g) + 2NaOH (aq) —> NaClO (aq) + NaCl (aq) + H2O (l)

Question 17

what are the uses of chlorine ?

Answer 17

• used to make bleach, for disinfectant
• added to water to make safe to drink = kills any microorganisms in water + prevents people suffering from cholera + typhoid

Question 18

what’s the equation which shows what happens when chlorine is added to water.

Answer 18

• Cl2 + H2O ⇌ HCl + HClO

Question 19

what is the toxicity of chlorine?

Answer 19

• kill humans if concentrated enough
• chlorine gas is toxic + can irritate respiratory system if inhaled
• liquid chlorine burn the skin + irritate the eyes —> causes serious injures .
• when reacts with hydrocarbons - forms chlorinated hydrocarbons = cause cancer if into human body : carcinogenic

Question 20

write the equation for Cl2 oxidising 2I- ?

Answer 20

• Cl2 + 2I- —> 2Cl- + I2

Question 21

what’s the trend in reducing ability of the halides down the group? why?

Answer 21

• increases down group (Cl- worst, I- best)
• I- has most occupied electron shells , so outer electrons further from nucleus , weakest force of attraction between outer electrons + positive charge of nucleus

—> easiest to be oxidised and lose electrons —> best reducing agent .

Question 22

what products are formed when I- reduces H2SO4?

Answer 22

1) H2SO4 + 2I- —> SO4^2- + 2HI

2) H2SO4 + 2H^+ + 2I- —> SO2 + I2 + 2H2O (SO2 is a choking gas with pungent odour )

3) H2SO4 + 6H^+ + 6I- —> S + 3I2 + 4H2O (S is yellow solid)

4) H2SO4 + 8H+ + 8I- —> H2S + 4I2 + 4H2O (H2S smells of bad/rotten eggs )

Question 23

what’s the products of Br- + H2SO4?

Answer 23

• HBr and SO2

Question 24

Does Cl- reduce H2SO4?

Answer 24

• No, not a powerful enough reducing agent; only HCl is formed.

Question 25

why is ozone not used to purify water in UK?

Answer 25

• more expensive than chlorine, evaporates from water more quickly .

Question 26

what’s the two forms of chlorate ion?

Answer 26

• ClO- is chlorate (I)
• ClO3^- is chlorate (V)

Question 27

give the equation for reaction of chlorine and water in presence of sunlight.

Answer 27

• 2Cl2 + 2H2O —> 4HCl + O2 (g)

Question 28

what is desalination?

Answer 28

• converts saltwater into clean, potable water
• either by reverse osmosis (using smart membrane) or by vaccum distillation at low pressure and low temp.