10.1 - 10.4 - Group 7

Question 1

Which Group 7 element does not follow most trends and why?

Answer 1

Fluorine:

• F-F bond very weak compared to the rest of the halogens
• Small size of fluorine = repulsion between non-bonding electrons since they are so close

Question 2

Describe the atomic radius trend in Group 7

Answer 2

Increases down the group:

• More electron shells
• So bigger atoms

Question 3

Describe the electronegativity trend in Group 7

Answer 3

Decreases down the group:

• Bigger atoms
• More shielding
• Weaker attraction between nucleus and the 2 electrons in the covalent bond

Question 4

Describe the boiling point trend in Group 7

Answer 4

Increases down the group:

• All simple covalent molecules
• Bigger molecules = More VdW = higher b.p

Question 5

Describe the first ionisation energy trend in Group 7

Answer 5

Decreases down the group:

• Atoms get bigger
• More shielding
• Therefore weaker attraction from nucleus to electron in outer shell

Question 6

Describe the trend in reducing power in Group 7

Answer 6

Increases down the group:

• More shells
• More shielding
• Outer electrons further away
• Easier to lose electrons

Question 7

Describe the reaction of sodium chloride and sodium fluoride and sulphuric acid

Answer 7

Steamy fumes of HCL/HF gas
It is an acid base reaction
NaCl + H₂SO₄ → NaHSO₄ + HCl (or replace Cl with F)

Question 8

Describe the reaction of sodium bromide and sulphuric acid

Answer 8

• Steamy fumes of HBr gas and brown fumes of Bromine

Acid-base reaction:
NaBr + H₂SO₄ → NaHSO₄ + HBr

Exothermic redox reaction:
2H⁺ + 2Br⁻ + H₂SO₄ → SO₂ + 2H₂O + Br₂

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

Question 9

Describe the reaction of sodium iodide and sulphuric acid

Answer 9

• Steamy fumes of hydrogen iodide
• Black sold iodide
• Bad egg smell of hydrogen sulphide gas

Acid-base reaction:
NaI + H₂SO₄ → NaHSO₄ + HI

Redox reaction:
8H⁺ + 8I⁻ + H₂SO₄ → H₂S + 4H₂O + 4I₂

Question 10

Describe how to test halide ions

Answer 10

Add silver nitrate and nitric acid

Question 11

Describe the result of the halide ions test

Answer 11

Cl⁻:

• White precipitate (AgCl)
• Precipitate dissolves in dilute NH₃
• Precipitate dissolves in concentrated NH₃

Br⁻:

• Cream precipitate (AgBr)
• Precipitate does not dissolve in dilute NH₃
• Precipitate dissolves in concentrated NH₃

I⁻:

• Yellow precipitate (AgI)
• Precipitate does not dissolve in dilute NH₃
• Precipitate does not dissolve in concentrated NH₃

Question 12

Describe the reaction of chlorine with cold water

Answer 12

Cl₂ + H₂O ⇌ ClO⁻ + Cl⁻ + 2H⁺
Cl2 + H2O ⇌ HClO + HCl

This is a disproportionation reaction and produces chlorate and chloride ions

Question 13

Describe the reaction of chlorine with cold water, in the presence of UV light

Answer 13

2Cl₂ + 2H₂O → 4HCl + O₂
Chlorine is used in small quantities in water treatment. The risk or chlorine being toxic is outweighed by the benefits of safe water.

Question 14

Describe the reaction of chlorine with cold, aqueous sodium hydroxide

Answer 14

2NaOH + Cl₂ → NaClO + NaCl + H₂O

This produces sodium hypochlorite/sodium chlorate(I) which is a key ingredient in bleach

Question 15

Describe the test for sulphate ions (SO₄²⁻)

Answer 15

• Test with BaCl₂

- Sulphate react to form a white precipitate

Question 16

Describe the test for hydroxide ions (OH⁻)

Answer 16

• Test with red litmus paper (red -> blue)

- Or test with universal indicator (colourless -> blue/purple)

Question 17

Describe the test for carbonate ions (CO₃²⁻)

Answer 17

• Test with acid (i.e HCl)
• Carbonate ions will effervesce
• Test these bubbles are CO₂ by bubbling through lime water (colourless -> cloudy)

Question 18

Describe the test for group 2 ions

Answer 18

Flame tests:
Calcium (Ca²⁺) = Brick red
Strontium (Sr²⁺) = Red
Barium (Ba²⁺) = Pale green

Question 19

Describe the test for ammonium ions (NH₄⁺)

Answer 19

• Ammonium ions give off ammonia gas (add NaOH to produce gas faster)
• Test by holding red litmus paper over test tube (red -> blue)

Question 20

Give the ionic equation for AgCl being dissolved in dilute NH3

Answer 20

AgCl(s) + 2NH3(aq) → Ag(NH3)2 + (aq) + Cl−(aq)

Question 21

Why is nitric acid used when testing for halides?

Answer 21

Removes hydroxide/carbonate ions that may give other ppts with silver nitrate