Group 7

Question 1

natural appearance & properties of halogens:

Answer 1

Fluorine (F2): very pale yellow gas. It is highly reactive
Chlorine : (Cl2) greenish, reactive gas, poisonous in high concentrations
Bromine (Br2) : orange brown liquid
Iodine (I2) : black/grey solid & brown in (aq)

Question 2

oxidising strength down G7:

Answer 2

The oxidising strength decreases down the group.
Oxidising agents are electron acceptors.

Question 3

displacement

The colour of the solution in the test tube shows which free halogen is present in solution.

Answer 3

Chlorine = very pale green solution (often colourless),
Bromine = yellow solution
Iodine = brown solution (sometimes black solid present)

Question 4

natural state of halogens:

Answer 4

f2 & cl2: gasses
br2: liquid
i2: solid

Question 5

disproportionation reaction:

Answer 5

one substance is both reduced and oxidised simultaneously

Question 6

reaction of Cl2 with water:

hint: alkali + acid

Answer 6

cl2 + h2o <—> hcl + hclo

hclo kills bacteria in water preventing cholera

Question 7

why no sunlight when cl2 reacts with water?

Answer 7

cl2 will decomopose to form hcl and o2

so no clo (I) ions so no benefits

Question 8

testing for halide ions

How to test for halides?

Answer 8

HNO3 & AgNO3:
AgF: no visible change
AgCl: white ppt
AgBr: cream ppt
AgI: yellow ppt

Question 9

when testing for halide ions why acidify?

Answer 9

to remove any other ions that could give a ppt with silver nitrate (aq)

if CO3- present, effervescence

would prod. false +ve white ppt

Question 10

how to differentiate betw/ white (cl-) ppt and cream (br-) ppt

Answer 10

• use dilute NH3 solution, if Cl- then ppt should dissolve
• with con. NH3 solution, Br- ppt dissolves
• no visible change with I-

Question 11

test for SO2

Answer 11

filter paper soaked in k2cr2o7
changes from orange to green

prod. when h2so4 reacts with NaBr

Question 12

products formed when I- reduces S in H₂SO₄

very strong reducing agent

Answer 12

• S (S gets reduced, gaining 6 e-)
• H₂S (eggy smell, reduced by 8 e-)

Question 13

test for CO₂

Answer 13

• bubble through limewater
• white ppt forms

Question 14

reaction for silver chloride with NH3 (to differentiate between cream and white solution)

Answer 14

AgCl(s) + 2 NH₃ (aq) –> [Ag(NH₃ )₂]+ + Cl-(aq)

Question 15

testing for halides: method overview

Answer 15

• make a solution of a solid halide salt
• acidify with dilute nitric acid
  -> prevents the precipitation of other insoluble silver salts such as Ag₂CO3
• add a few drops of silver nitrate solution
• treat any precipitate with dilute ammonia solution
• if a precipitate still exists, add concentrated ammonia solution

Question 16

why can’t F- ions be tested for using Ag?

Answer 16

AgF is soluble in water (solubility decreases down the group) so
* no observable change

Question 17

A colourless solution contains a mixture of sodium chloride and sodium bromide.
Using aqueous silver nitrate and any other reagents of your choice, develop a procedure to prepare a pure sample of silver bromide from this mixture.
Explain each step in the procedure and illustrate your explanations with equations, where appropriate.
(6 marks)

Answer 17

Stage 1: formation of precipitates
*Add silver nitrate
*to form precipitates of AgCl and AgBr
*AgNO₃ + NaCl → AgCl + NaNO₃
*AgNO₃ + NaBr → AgBr + NaNO₃

Stage 2: selective dissolving of AgCl
*Add excess of dilute ammonia to the mixture of precipitates
*the silver chloride precipitate dissolves
*AgCl + 2NH → [Ag(NH₃)2]+ + Cl−

Stage 3: separation and purification of AgBr
*Filter off the remaining silver bromide precipitate
*Wash to remove soluble compounds
*Dry to remove water

Question 18

NaCO₃ +2HNO₃

Answer 18

—> CO₂ +H₂O +2NaNO₃

Question 19

Separate unlabelled solid samples of three anhydrous sodium compounds are provided for a student to identify.
These compounds are known to be sodium carbonate, sodium fluoride and sodium chloride but it is not known which sample is which.
Outline a logical sequence of test-tube reactions that the student could carry out to identify each of these compounds.
Include the observations the student would expect to make. Give equations, including state symbols, for any reactions that would take place.

Answer 19

• Add nitric acid to all three
• add water to make solution
• add AgNo3.
  …
• Na2CO3 will fizz with acid
• NaCl gives white precipitate with AgNO3
• NaF shows no visible change (no ppt formed).
  ….
• Na2CO3(s)+ 2HNO3(aq)→ 2NaNO3 + CO2 (g)+ H2O(l)
• AgNO3(aq) + NaCl(s) → AgCl(s) + NaNO3(aq)

Question 20

Write equation of reaction betw/ cold Cl2 + dilute NaOH.
Give two uses of resulting solution. (3)

Answer 20

Cl 2 + NaOH ⟶ NaC l + NaClO3 + H2O
* bleach
* disinfectant / kills bacteria

Question 21

observation for reaction forming gaseous hydrogen halides

Answer 21

steamy fumes

Question 22

reaction between potassium bromide and sulfuric acid

redox

Answer 22

KBr + H2SO4 –> KHSO4 + HBr + Br2 + SO2 (steamy fumes)

Question 23

products formed when H2SO4 reacted with Br- and I- :

Answer 23

• Br- : only strong enough to reduce to SO₂
• I- : can reduce to SO₂, or S or H₂S

Question 24

Give the equation for solid NaBr with H2SO4

Answer 24

Acid- base step: NaBr(s) + H2SO4(l) ⟶ NaHSO4(s) + HBr(g)
Redox step: 2H+ + 2Br - + H2SO4 ⟶ Br2(g) + SO2(g) + 2 H2O(l)

Overall equation: combining two steps above:
2 NaBr + 3 H2SO4 ⟶ 2 NaHSO4 + SO2 + Br2 + 2 H2O