Group 7: The Halogens

boiling points down group 7

• increases as you go down the group

• vdw’s forces between molecules increases due to an increase in size and relative mass of atoms

electronegativity down group 7

• decreases down the group

  • atoms gets larger

  • so distance between postive nucleus and electrons increases

  • shielding increases

oxidising power down the group

• less oxidising as you go down the group

• a halogen will displace a halide from a solution if the halide is lower in the periodic table

table for the displacement of halides

	KCl solution	KBr solution	KI solution
addition of chlorine water (Cl2) - colourless	no reaction	orange solution (Br2) Cl₂ + 2Br⁻ —> Br₂ + 2Cl⁻	brown solution (I2) Cl₂ + 2I⁻ —> I₂ + 2Cl⁻
addition of bromine water (Br2) - orange	no reaction	no reaction	brown solution (I2) Br₂ + 2I⁻ —> I₂ + 2Br⁻
addition of iodine water (I2) - brown	no reaction	no reaction	no reaction

equation for the reaction of chlorine with water

give a reason why chlorine is added to drinking water

suggest a disadvantage of treating water in this way

Cl2 + H2O ⥬ HCl + HClO

to kill bacteria

wasteful, bc most of potable water is not used for drinking but rather washing clothes etc.

production of bleach equation

Cl2 + 2NaOH ——> NaClO + NaCl + H2O

ionic equation for the production of bleach

Cl2 + 2OH- —> OCl- + Cl- + H2O

reducing power down the group

• increases down the group

  • ionic radius increases down the group

  • distance between positive nucleus and electron increases

  • increased shielding

reaction of H2SO4 with with NaF, including observations

NaF + H2SO4 ——> NaHSO4 + HF

• white steamy fumes (of HF)

reaction of H2SO4 with with NaCl, including observations

NaCl + H2SO4 ——> NaHSO4 + HCl

• white steamy fumes (of HCl)

reactions of H2SO4 with NaBr, including observations

H2SO4 + NaBr ——> NaHSO4 + HBr

• white steamy fumes (of HBr)

2H2SO4 + 2NaBr —-→ Na2SO4 + SO2 + H2O + Br2

• orange fumes (of Br2)

the reason 2 reactions occur:

Br- ions are stronger reducing agents than F- and Cl- , so therefore S changes oxidation state and is reduced from:

+6 in H2SO4 to +4 in SO2

reactions of H2SO4 with NaI, including observations

H2SO4 + NaI —> NaHSO4 + HI

• white steamy fumes

2H⁺ + 2I- + H2SO4 —> I2 + SO2 + 2H2O

• brown fumes (of I2)

6H⁺ + 6I- + H2SO4 ——> 3I2 + S + 4H2O

• yellow solid (S) produced

8H⁺ + 8I- + H2SO4 —> 4I2 + H2S + 4H2O

• rotten egg smell (H2S) produced

the reason 4 reactions occur:

I- is the strongest halide reducing agent so therefore S changes oxidation state and is reduced from:

+6 in H2SO4

+4 in SO2

0 in S

-2 in H2S

test for halides

Add dilute HNO3

• to react with any other ions that may give other precipitates with silver nitrate

Add silver nitrate solution

• AgNO3 produces ppt. with halides

To confirm the results, add dilute and conc NH3

• NH3 dissolves AgCl and AgBr

• AgCl + 2NH3 —> Ag(NH3)₂⁺ + Cl-

• AgBr + 2NH3 —> Ag(NH3)₂⁺ + Br-

suggest why, in an experiment, silver nitrate is added in excess

to ensure that all halide ions are removed from the solution

results for halides with silver nitrate solution

halide	result
fluorides	no ppt.
chlorides	white ppt. Ag+ + Cl- —> AgCl AgNO3 + NaCl —> AgCl + NaNO3
bromides	cream ppt. Ag+ + Br- —> AgBr AgNO3 + NaBr —> AgBr + NaNO3
iodides	pale yellow ppt. Ag+ + l- —> Agl AgNO3 + Nal —> Agl + NaNO3

results for halides with dilute and conc. NH3

	dilute NH3	conc. NH3
chlorides	ppt. dissolves AgCl + 2NH3 —> Ag(NH3)2+ + Cl-	ppt. dissolves AgCl + 2NH3 —> Ag(NH3)2+ + Cl-
bromides	insoluble	ppt. dissolves. AgBr + 2NH3 —> Ag(NH3)2+ + Br-
iodides	insoluble	insoluble

flame test for group 2 ions

1. Ca2+ : brick red

2. Sr2+ : red

3. Ba2+ : green

test for ammonium compounds

• add NaOH and gently heat

• if ammonium compounds present: ammonia gas produced

• use damp red litmus paper

• ammonia will dissolve in the water and turn the litmus paper blue

test for sulfates

• add hydrochloric acid to remove any carbonates which could give a false positive result

• add barium chloride

• if sulfate present: white ppt. produced

• Ba2⁺ + SO^2-₄ ——> Ba2SO4

the correct order to test for ions to prevent false positives

carbonates —> sulfates —> halides