2.22 - Group 7

Question 1

What is the trend in Boiling point down group 7? Why?

Answer 1

• Increases down the group
• because size of atom increases as more electron shells and electrons create stronger van de waals forces of attraction between molecules
• therefore require more energy to overcome

Question 2

What is the trend in electronegativity down group 7? Why?

Answer 2

• decreases
• because more electrons shells, causing greater atomic radius so outer electrons are further from the positive charge of the nucleus
• lower force of attraction between the nucleus and electron pair in the covalent bond

Question 3

What do you use to test for halide ions?

Answer 3

• acidified AgNO₃ (silver nitrate)

Question 4

Why do you add HNO₃ before adding silver nitrate to the solution? Why not HCl?

Answer 4

• to remove ions which might interfere with the test
• adding HCl would add Cl‾ ions, giving a false positive result

Question 5

Result and equation for Cl- test?

Answer 5

• white precipitate
• Ag⁺ + Cl‾ → AgCl (s)

Question 6

What is the result and equation for the test for Br‾ ?

Answer 6

• cream precipitate
• Ag⁺ + Br‾ → AgBr (s)

Question 7

What is the result and equation for the test for I‾ ?

Answer 7

• yellow precipitate
• Ag⁺ + I‾ → AgI (s)

Question 8

What happens to each of the silver halide precipitates when dilute/conc NH₃ are added?

Answer 8

• AgCl dissolves in both dilute and conc
  AgCl (s) + 2NH₃ (aq) → [Ag(NH₃)₂]⁺ + Cl‾
• AgBr only dissolves in conc
  AgBr (s) + 2NH₃ (aq) → [Ag(NH₃)₂]⁺ (aq) + Br‾
• AgI will not dissolve in either

Question 9

What is the trend in oxidising ability down the group? Why?

Answer 9

• decreases down group (Cl best, I worst)
• because Cl has the fewest electron shells
• therefore greatest force of attraction between outer electrons and nucleus so easiest to gain electrons and be reduced
• therefore best oxidising agent

Question 10

Write the equation for Cl₂ oxidising 2I‾ ?

Answer 10

• Cl₂ + 2I‾ → 2Cl‾ + I₂

Question 11

What is the trend in reducing ability of the halides down the group? Why?

Answer 11

• increases down the group (Cl‾ worst, I‾ best)
• because I‾ has the most electron shells, so outer electron are further from the nucleus.
• meaning it has the weakest attraction between outer electrons and positive charge of nucleus
• therefore easiest to be oxidised and lose electrons = best reducing agent

Question 12

What products are formed when I‾ reduces H₂SO₄? Do equations for all 4.

Answer 12

• H₂SO₄ + 2I‾ → SO₄²‾ + 2HI
• H₂SO₄ + 2H⁺ + 2I‾ → SO₂ + I₂ + 2H₂O (SO₂ is a choking gas with a pungent odour)
• H₂SO₄ + 6H⁺ + 6I‾ → S + 3I₂ + 4H₂O (S is a yellow solid)
• H₂SO₄ + 8H⁺ + 8I‾ → H₂S + 4I₂ + 4H₂O (H₂S smells of rotten eggs)

Question 13

What are the products of Br‾ and H₂SO₄?

Answer 13

• HBr and SO₂

Question 14

Does Cl‾ reduce H₂SO₄?

Answer 14

• no
• its is not a powerful enough reducing agent, only HCl is formed

Question 15

Why is chlorine added to drinking water? Why is it safe?

Answer 15

• forms ClO‾ ions which oxidise (kill) all microorganisms in water
• once it has done its job, little remains, and the health benefits outweigh the risks of using it.

Question 16

What are potential risks of adding chlorine to drinking water?

Answer 16

• Chlorine is toxic and damages the respiratory system in large enough quantities
• can form carcinogens with hydrocarbons.

Question 17

Why is ozone not used to purify water in the UK?

Answer 17

• more expensive than chlorine
• evaporates from water more quickly

Question 18

What is the equation for the reaction of Cl₂ with water?

Answer 18

• Cl₂ (g) + H₂O (l) → HClO (aq) + HCl (aq)

Question 19

What type of reaction is the reaction of chlorine with water?

Answer 19

• disproportionation
• chlorine is both oxidised and reduced

Question 20

What are the two forms of the chlorate ion?

Answer 20

• ClO‾ is chlorate (I)
• ClO₃¯ is chlorate (V)