Group 7

Question 1

Trend in boiling points of group 7 elements

Answer 1

Increase as go down the group, as van der Waals’ forces increase in strength due to an increase in the number of electrons

Question 2

Trend in reactivity in group 7

Answer 2

Decreases as go down the group, as atoms become larger and electron shielding increases, resulting in decreasing ease of forming negative ions. Increasing nuclear charge is outweighed by these other effects

Question 3

Disproportionation

Answer 3

A reaction in which an element is simultaneously oxidised and reduced

Question 4

Chlorine + water

Answer 4

Cl(2)(g) + H(2)O(l) -> HClO(aq) + HCl(aq). Example of a disproportionation reaction.

Question 5

Chlorine + cold, dilute NaOH

Answer 5

Cl(2)(aq) + 2NaOH(aq) -> NaCl(aq) + NaClO)aq_ + H(2)O(l). Example of a disproportionation reaction

Question 6

Benefits and disadvantages of using chlorine in water treatement

Answer 6

• Kills bacteria: yay. When government of Peru stopped chlorinating water, there was a huge outbreak of cholera
• Risks of toxic chlorine gas and formation of chlorinated hydrocarbons - potential carcinogens

Question 7

Testing for chloride ions

Answer 7

• dissolve unknown halide in water
• add AgNO(3)(aq)
• if chloride ions present, white precipitate of AgCl(s) forms
• add aqueous ammonia
• if precipitate soluble in dilute and concentrated ammonia, chloride ions are present

Question 8

Testing for bromide ions

Answer 8

• dissolve unknown halide in water
• add AgNO(3)(aq)
• if bromide ions present, cream precipitate of AgBr(s) forms
• add aqueous ammonia
• if precipitate insoluble in dilute and soluble in concentrated ammonia, bromide ions are present

Question 9

Testing for iodide ions

Answer 9

• dissolve unknown halide in water
• add AgNO(3)(aq)
• if iodide ions present, yellow precipitate of AgI(s) forms
• add aqueous ammonia
• if precipitate insoluble in dilute and concentrated ammonia, iodide ions are present