3.2.3 Group 7

Question 1

How do you test for the halide ions?

Answer 1

Add acidified silver nitrate solution:
- Cl- forms white ppt
- Br- forms cream ppt
- I- forms white ppt

Add dilute ammonia:
- Cl- dissolves
- Br- partially dissolves
- I- does not dissolve

Add conc. ammonia:
- Cl- dissolves
- Br- dissolves
- I- does not dissolve

Question 2

Write a general equation for the reaction of a sodium halide with acidified silver nitrate solution.

Answer 2

AgNO3 + NaX → AgX + NaNO3

Question 3

Write a general equation for the reaction of a silver halide with ammonia solution.

Answer 3

AgX + 2NH3 → Ag(NH3)2^+ + X-

Question 4

How do boiling/melting points change down group 7 and why?

Answer 4

Boiling points decrease down group 7:
- Increased nuclear charge, more electrons
- Stronger Van der Waals’ forces between the molecules

Question 5

How does the appearance change down group 7?

Answer 5

Fluorine: yellow gas, yellow vapour
Chrlorine: green gas, green vapour
Bromine: brown liquid, brown vapour
Iodine: grey solid, purple vapour

Question 6

Which halogen reacts most rapidly to produce a silver halide ppt with acidified silver nitrate?

Answer 6

iodine

Question 7

What is the trend in oxidising ability down group 7? Explain why?

Answer 7

Oxidising ability decreases down group:
- Larger atomic radius, more shielding
- Weaker attraction between outershell and nucleus
- Becomes more difficult to gain electron

Question 8

Which halide ions can Cl2 oxidise?
Write an equation for each oxidation and state the observations (in organic solvent).

Answer 8

Cl2 + 2Br- → 2Cl- + Br2
Solution turns yellow (orange)

Cl2 + 2I- → 2Cl- + I2
Solution turns brown (purple)

Question 9

Which halide ions can Br2 oxidise?
Write an equation for each oxidation and state the observations (in organic solvent).

Answer 9

Br2 + 2I- → 2Br- + I2
Solution turns brown (purple)

Question 10

What colour is the solution when the following reactents are mixed and why:
Iodine and chloride ions

Answer 10

• Brown solution (purple in organic solvent)
• Iodine is a weaker oxidising agent then chlorine, no reaction occurs
• Iodine remains oxidised, chloride ions remain reduced

Question 11

What is the trend in reducing ability down group 7 and why?

Answer 11

Reducing ability increases down the group:
- Larger ionic radius, outer electrons further from nucleus
- Weaker attraction between outershell electrons and nucleus
- Easier to lose electron

Question 12

What are the products and respective observations when H2SO4 reacts with the follwoing halide ions:
a) Cl-
b) Br-
c) I-

Answer 12

a) HCl
b) HBr, SO2, Br2
c) HI, SO2, I2, H2S, S

HCl/HBr/HI = steamy fumes
SO2 = colourless choking gas
Br2 = brown liquid
I2 = black solid
H2S = rotten egg smell
S = yellow solid

Question 13

Write an equation for the reaction of NaBr and H2SO4.

Answer 13

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

Question 14

How does chlorine react with water?

Answer 14

Cl2 + H2O ⇌ HClO + HCl
(Cl2 + H2O ⇌ ClO- + Cl- + 2H+)

Question 15

How does chlorine react with cold, dilute NaOH?

Answer 15

Cl2 + 2NaOH → NaClO + NaCl + H2O

Question 16

Write an ionic equation to show how chlorine reacts with cold, dliute NaOH.

Answer 16

Cl2 + 2OH- → ClO- + Cl- + H2O

Question 17

How does chlorine react with hot NaOH?

Answer 17

3Cl2 + 6NaOH → NaClO3 + 5NaCl + 3H2O

Question 18

Why is water treated with chlorine?

Answer 18

HCl kills bacteria, sterilizing the water